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Sample records for fluorite structure first-principles

  1. Investigating the Electronic Structure of Fluorite Oxides: Comparsion of EELS and First Principles Calculations

    SciTech Connect

    Aguiar, J; Asta, M; Gronbech-Jensen, N; Perlov, A; Milman, V; Gao, S; Pickard, C; Browning, N

    2009-06-05

    Energy loss spectra from a variety of cubic oxides are compared with ab-initio calculations based on the density functional plane wave method (CASTEP). In order to obtain agreement between experimental and theoretical spectra, unique material specific considerations were taken into account. The spectra were calculated using various approximations to describe core-hole effects and electronic correlations. All the calculations are based on the local spin density approximation to show qualitative agreement with the sensitive oxygen K-edge spectra in ceria, zirconia, and urania. Comparison of experimental and theoretical results let us characterize the main electronic interactions responsible for both the electronic structure and the resulting EEL spectra of the compounds in question.

  2. Investigating the electronic structure of fluorite-structured oxide compounds: comparison of experimental EELS with first principles calculations

    SciTech Connect

    Aguiar, Jeff; Ramasse, Q. M.; Asta, Mark D.; Browning, Nigel D.

    2012-06-27

    Energy loss spectra from fluorite-structured ZrO2, CeO2, and UO2 compounds are compared with theoretical calculations based on density functional theory (DFT) and its extensions, including the use of Hubbard-U corrections (DFT + U) and hybrid functionals. Electron energy loss spectra (EELS) were obtained from each oxide using a scanning transmission electron microscope (STEM). The same spectra were computed within the framework of the full-potential linear augmented plane-wave (FLAPW) method. The theoretical and experimental EEL spectra are compared quantitatively using non-linear least squares peak fitting and a cross-correlation approach, with the best level of agreement between experiment and theory being obtained using the DFT + U and hybrid computational approaches.

  3. First-principles calculations of Mg2X (X = Si, Ge, Sn) semiconductors with the calcium fluorite structure

    NASA Astrophysics Data System (ADS)

    Sandong, Guo

    2015-05-01

    The electronic structures of Mg2X (X = Si, Ge, Sn) have been calculated by using generalized gradient approximation, various screened hybrid functionals, as well as Tran and Blaha's modified Becke and Johnson exchange potential. It was found that the Tran and Blaha's modified Becke and Johnson exchange potential provides a more realistic description of the electronic structures and the optical properties of Mg2X (X = Si, Ge, Sn) than else exchange-correlation potential, and the theoretical gaps and dielectric functions of Mg2X (X = Si, Ge, Sn) are quite compatible with the experimental data. The elastic properties of Mg2X (X = Si, Ge, Sn) have also been studied in detail with the generalized gradient approximation, including bulk modulus, shear modulus, Young's modulus, Poisson's ratio, sound velocities, and Debye temperature. The phonon dispersions of Mg2X (X = Si, Ge, Sn) have been calculated within the generalized gradient approximation, suggesting no structural instability, and the measurable phonon heat capacity as a function of the temperature has been also calculated. Project supported by the Fundamental Research Funds for the Central Universities (No. 2013QNA32) and the National Natural Science Foundation of China (No. 11404391).

  4. Interface Structure Prediction from First-Principles

    SciTech Connect

    Zhao, Xin; Shu, Qiang; Nguyen, Manh Cuong; Wang, Yangang; Ji, Min; Xiang, Hongjun; Ho, Kai-Ming; Gong, Xingao; Wang, Cai-Zhuang

    2014-05-08

    Information about the atomic structures at solid–solid interfaces is crucial for understanding and predicting the performance of materials. Due to the complexity of the interfaces, it is very challenging to resolve their atomic structures using either experimental techniques or computer simulations. In this paper, we present an efficient first-principles computational method for interface structure prediction based on an adaptive genetic algorithm. This approach significantly reduces the computational cost, while retaining the accuracy of first-principles prediction. The method is applied to the investigation of both stoichiometric and nonstoichiometric SrTiO3 Σ3(112)[1¯10] grain boundaries with unit cell containing up to 200 atoms. Several novel low-energy structures are discovered, which provide fresh insights into the structure and stability of the grain boundaries.

  5. First Principles Study of Carbyne Structural Stability

    NASA Astrophysics Data System (ADS)

    Kwon, Kevin; Holmes, Colin; Kim, Ki Chul; Jang, Seung Soon

    Carbyne is composed of linear sp-hybridized carbon bonds and yields promising results to surpass graphene's mechanical and electrical properties. Carbyne has two semi-stable conformations: Polyyne (alternating triple and single bonds) and Polycumulene (repeating double bonds). This study investigated the stability of these forms at infinite chain lengths by using periodic boundary conditions. Geometric optimization was performed via DFT calculations using DMoL3 and PBE GGA functional group. Each configuration's chain was stretched or compressed until the most stable form - lowest energy - was obtained. After comparing the energies, the most stable form alternated between Polyyne and Polycumulene as the number of carbon atoms within each boundary increased. Polyyne was the most stable form for odd number of carbons and Polycumulene was the most stable for even number of carbons. Finally, K-point sampling was increased in the direction of the chain axis to obtain a more accurate depiction of structural stability. As the number of k-points increased, the Polycumulene structure became more stable compared to Polyyne. School of Materials Science and Engineering, Georgia Institute of Technology.

  6. Chemical expansion affected oxygen vacancy stability in different oxide structures from first principles calculations

    SciTech Connect

    Aidhy, Dilpuneet S.; Liu, Bin; Zhang, Yanwen; Weber, William J.

    2015-01-21

    We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO2|ZrO2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. Furthermore, these results could serve as guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.

  7. Chemical expansion affected oxygen vacancy stability in different oxide structures from first principles calculations

    DOE PAGESBeta

    Aidhy, Dilpuneet S.; Liu, Bin; Zhang, Yanwen; Weber, William J.

    2015-01-21

    We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO2|ZrO2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. Furthermore, these results could serve asmore » guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.« less

  8. Chemical expansion affected oxygen vacancy stability in different oxide structures from first principles calculations

    SciTech Connect

    Aidhy, Dilpuneet S.; Liu, Bin; Zhang, Yanwen; Weber, William J.

    2015-03-01

    We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO2|ZrO2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. These results could serve as guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.

  9. Giant Mechanocaloric Effects in Fluorite-Structured Superionic Materials.

    PubMed

    Cazorla, Claudio; Errandonea, Daniel

    2016-05-11

    Mechanocaloric materials experience a change in temperature when a mechanical stress is applied on them adiabatically. Thus, far, only ferroelectrics and superelastic metallic alloys have been considered as potential mechanocaloric compounds to be exploited in solid-state cooling applications. Here we show that giant mechanocaloric effects occur in hitherto overlooked fast ion conductors (FIC), a class of multicomponent materials in which above a critical temperature, Ts, a constituent ionic species undergoes a sudden increase in mobility. Using first-principles and molecular dynamics simulations, we found that the superionic transition in fluorite-structured FIC, which is characterized by a large entropy increase of the order of 10(2) JK(-1) kg(-1), can be externally tuned with hydrostatic, biaxial, or uniaxial stresses. In particular, Ts can be reduced several hundreds of degrees through the application of moderate tensile stresses due to the concomitant drop in the formation energy of Frenkel pair defects. We predict that the adiabatic temperature change in CaF2 and PbF2, two archetypal fluorite-structured FIC, close to their critical points are of the order of 10(2) and 10(1) K, respectively. This work advocates that FIC constitute a new family of mechanocaloric materials showing great promise for prospective solid-state refrigeration applications. PMID:27070506

  10. Two Dimensional Ice from First Principles: Structures and Phase Transitions.

    PubMed

    Chen, Ji; Schusteritsch, Georg; Pickard, Chris J; Salzmann, Christoph G; Michaelides, Angelos

    2016-01-15

    Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here, we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression, the pentagonal structure becomes the most stable and persists up to ∼2  GPa, at which point the square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and the width. PMID:26824547

  11. Two Dimensional Ice from First Principles: Structures and Phase Transitions

    NASA Astrophysics Data System (ADS)

    Chen, Ji; Schusteritsch, Georg; Pickard, Chris J.; Salzmann, Christoph G.; Michaelides, Angelos

    2016-01-01

    Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here, we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression, the pentagonal structure becomes the most stable and persists up to ˜2 GPa , at which point the square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and the width.

  12. 2D ice from first principles: structures and phase transitions

    NASA Astrophysics Data System (ADS)

    Chen, Ji; Schusteritsch, Georg; Pickard, Chris J.; Salzmann, Christoph G.; Michaelides, Angelos

    Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression the pentagonal structure becomes the most stable and persists up to ca. 2 GPa at which point square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. We also find a double layer AA stacked square ice phase, which clarifies the difference between experimental observations and earlier force field simulations. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and width.

  13. Structural instabilities in strontium titanate from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Lasota, Christopher Andrew

    For some time now, first-principles calculation methods have proven to be an effective tool for investigating the physics of condensed matter systems. The additional use of density functional theory (DFT) and the local density approximation (LDA) has permitted even complex materials to be studied on desktop workstations with remarkable success. The incorporation of linear response theory into these methods has extended their power, allowing investigation of important dynamical properties. Contained within the following pages are the results of a first-principles study of SrTiO3. This transition metal oxide is often grouped with ferroelectric materials, due to its similar composition and perovskite structure. Although it behaves as if it were to become ferroelectric, it fails to do so, even at the lowest temperatures. Using the LAPW method for bulk materials, the ground-state equilibrium properties for the cubic phase were found. Additional linear response calculations produced the phonon frequencies throughout the Brillouin zone. Imaginary values for these frequencies revealed two distinct regions of reciprocal space corresponding to structural instabilities of the ferroelectric (FE) and antiferrodistortive (AFD) types. A cell-doubling AFD transition to tetragonal structure is observed experimentally, so subsequent calculations were continued in this phase. Total energy calculations were performed for both FE and AFD distortions in this new phase, and it was found that the AFD instability is enhanced with decreasing lattice parameter, while the FE instability is diminished. Furthermore, these calculations suggest that this material is marginally stable against FE distortions, even at the 105 K volume.

  14. First-principles study of structural properties of alkaline earth metals methanides A2C(A = Be,Mg)

    NASA Astrophysics Data System (ADS)

    Paliwal, U.; Trivedi, D. K.; Galav, K. L.; Joshi, K. B.

    2013-06-01

    The structural properties of alkaline earth binary carbides A2C(A = Be,Mg) are evaluated using first-principles periodic linear combination of atomie orbitals method based on density functional theory implemented in the CRYSTAL06 code. The total energy is computed for the two binary carbides considering the anti-Fluorite structure. The computed total energy is coupled with the Murnaghan equation of states to report the equilibrium lattice constant and bulk modulus of the compounds. The cohesive energy and density are also reported for the two compounds.

  15. Electronic structure and ionicity of actinide oxides from first principles

    NASA Astrophysics Data System (ADS)

    Petit, L.; Svane, A.; Szotek, Z.; Temmerman, W. M.; Stocks, G. M.

    2010-01-01

    The ground-state electronic structures of the actinide oxides AO , A2O3 , and AO2 ( A=U , Np, Pu, Am, Cm, Bk, and Cf) are determined from first-principles calculations, using the self-interaction corrected local spin-density approximation. Emphasis is put on the degree of f -electron localization, which for AO2 and A2O3 is found to follow the stoichiometry, namely, corresponding to A4+ ions in the dioxide and A3+ ions in the sesquioxides. In contrast, the A2+ ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidation and reduction in the actinide dioxides is discussed, and it is found that the dioxide is the most stable oxide for the actinides from Np onward. Our study reveals a strong link between preferred oxidation number and degree of localization which is confirmed by comparing to the ground-state configurations of the corresponding lanthanide oxides. The ionic nature of the actinide oxides emerges from the fact that only those compounds will form where the calculated ground-state valency agrees with the nominal valency expected from a simple charge counting.

  16. Predicted novel hydrogen hydrate structures under pressure from first principles

    NASA Astrophysics Data System (ADS)

    Qian, Guangrui; Lyakhov, Andriy; Zhu, Qiang; Oganov, Artem; Dong, Xiao

    2014-03-01

    Gas hydrates are systems of prime importance. In particular, hydrogen hydrates are potential materials of icy satellites and comets, and may be used for hydrogen storage. We explore the H2O-H2 system at pressures in the range 0 ~ 100 GPa with ab initio variable-composition evolutionary simulations. According to our calculation and previous experiments, the H2O-H2 system undergoes a series of transformations with pressure, and adopts the known open-network clathrate structures (sII, C0), dense ``filled ice'' structures (C1, C2) and two novel hydrogen hydrate phases. One of these structures is based on the hexagonal ice framework and has the same H2O:H2 ratio (2:1) as the C0 phase at low pressures and similar enthalpy (we name this phase Ih-C0). The other newly predicted hydrate phase has a 1:2 H2O:H2 ratio and structure based on cubic ice. This phase (which we name C3) is predicted to be thermodynamically stable above 38 GPa when including van der Waals interactions and zero-point vibrational energy. This is the hydrogen-richest hydrate and this phase has the highest gravimetric densities (18 wt.%) of extractable hydrogen among all known materials. We thank the DARPA (Grants No. W31P4Q1310005 and No. W31P4Q1210008), National Science Founda- tion (EAR-1114313, DMR-1231586), AFOSR (FA9550- 13-C-0037), DOE (DE-AC02-98CH10886), CRDF Global (UKE2-7034-KV-11) for financial support. We thank Purdue University Teragrid for providing computational resources and technical support for this work (Charge No.: TG-DMR110058).

  17. Two-dimensional boron nitride structures functionalization: first principles studies.

    PubMed

    Ponce-Pérez, R; Cocoletzi, Gregorio H; Takeuchi, Noboru

    2016-09-01

    Density functional theory calculations have been performed to investigate two-dimensional hexagonal boron nitride (2D hBN) structures functionalization with organic molecules. 2x2, 4x4 and 6x6 periodic 2D hBN layers have been considered to interact with acetylene. To deal with the exchange-correlation energy the generalized gradient approximation (GGA) is invoked. The electron-ion interaction is treated with the pseudopotential method. The GGA with the Perdew-Burke-Ernzerhoff (PBE) functionals together with van der Waals interactions are considered to deal with the composed systems. To investigate the functionalization two main configurations have been explored; in one case the molecule interacts with the boron atom and in the other with the nitrogen atom. Results of the adsorption energies indicate chemisorption in both cases. The total density of states (DOS) displays an energy gap in both cases. The projected DOS indicate that the B-p and N-p orbitals are those that make the most important contribution in the valence band and the H-s and C-p orbitals provide an important contribution in the conduction band to the DOS. Provided that the interactions of the acetylene with the 2D layer modify the structural and electronic properties of the hBN the possibility of structural functionalization using organic molecules may be concluded. PMID:27566317

  18. Unfolding method for first-principles LCAO electronic structure calculations

    NASA Astrophysics Data System (ADS)

    Lee, Chi-Cheng; Yamada-Takamura, Yukiko; Ozaki, Taisuke

    2013-08-01

    Unfolding the band structure of a supercell to a normal cell enables us to investigate how symmetry breakers such as surfaces and impurities perturb the band structure of the normal cell. We generalize the unfolding method, originally developed based on Wannier functions, to the linear combination of atomic orbitals (LCAO) method, and present a general formula to calculate the unfolded spectral weight. The LCAO basis set is ideal for the unfolding method because the basis functions allocated to each atomic species are invariant regardless of the existence of surface and impurity. The unfolded spectral weight is well defined by the property of the LCAO basis functions. In exchange for the property, the non-orthogonality of the LCAO basis functions has to be taken into account. We show how the non-orthogonality can be properly incorporated in the general formula. As an illustration of the method, we calculate the dispersive quantized spectral weight of a ZrB2 slab and show strong spectral broadening in the out-of-plane direction, demonstrating the usefulness of the unfolding method.

  19. Unfolding method for first-principles LCAO electronic structure calculations.

    PubMed

    Lee, Chi-Cheng; Yamada-Takamura, Yukiko; Ozaki, Taisuke

    2013-08-28

    Unfolding the band structure of a supercell to a normal cell enables us to investigate how symmetry breakers such as surfaces and impurities perturb the band structure of the normal cell. We generalize the unfolding method, originally developed based on Wannier functions, to the linear combination of atomic orbitals (LCAO) method, and present a general formula to calculate the unfolded spectral weight. The LCAO basis set is ideal for the unfolding method because the basis functions allocated to each atomic species are invariant regardless of the existence of surface and impurity. The unfolded spectral weight is well defined by the property of the LCAO basis functions. In exchange for the property, the non-orthogonality of the LCAO basis functions has to be taken into account. We show how the non-orthogonality can be properly incorporated in the general formula. As an illustration of the method, we calculate the dispersive quantized spectral weight of a ZrB2 slab and show strong spectral broadening in the out-of-plane direction, demonstrating the usefulness of the unfolding method. PMID:23912816

  20. First principles based multiparadigm modeling of electronic structures and dynamics

    NASA Astrophysics Data System (ADS)

    Xiao, Hai

    Electronic structures and dynamics are the key to linking the material composition and structure to functionality and performance. An essential issue in developing semiconductor devices for photovoltaics is to design materials with optimal band gaps and relative positioning of band levels. Approximate DFT methods have been justified to predict band gaps from KS/GKS eigenvalues, but the accuracy is decisively dependent on the choice of XC functionals. We show here for CuInSe2 and CuGaSe2, the parent compounds of the promising CIGS solar cells, conventional LDA and GGA obtain gaps of 0.0-0.01 and 0.02-0.24 eV (versus experimental values of 1.04 and 1.67 eV), while the historically first global hybrid functional, B3PW91, is surprisingly the best, with band gaps of 1.07 and 1.58 eV. Furthermore, we show that for 27 related binary and ternary semiconductors, B3PW91 predicts gaps with a MAD of only 0.09 eV, which is substantially better than all modern hybrid functionals, including B3LYP (MAD of 0.19 eV) and screened hybrid functional HSE06 (MAD of 0.18 eV). The laboratory performance of CIGS solar cells (> 20% efficiency) makes them promising candidate photovoltaic devices. However, there remains little understanding of how defects at the CIGS/CdS interface affect the band offsets and interfacial energies, and hence the performance of manufactured devices. To determine these relationships, we use the B3PW91 hybrid functional of DFT with the AEP method that we validate to provide very accurate descriptions of both band gaps and band offsets. This confirms the weak dependence of band offsets on surface orientation observed experimentally. We predict that the CBO of perfect CuInSe2/CdS interface is large, 0.79 eV, which would dramatically degrade performance. Moreover we show that band gap widening induced by Ga adjusts only the VBO, and we find that Cd impurities do not significantly affect the CBO. Thus we show that Cu vacancies at the interface play the key role in

  1. Crystal Structure Prediction from First Principles: The Crystal Structures of Glycine

    PubMed Central

    Lund, Albert M.; Pagola, Gabriel I.; Orendt, Anita M.; Ferraro, Marta B.; Facelli, Julio C.

    2015-01-01

    Here we present the results of our unbiased searches of glycine polymorphs obtained using the Genetic Algorithms search implemented in Modified Genetic Algorithm for Crystals coupled with the local optimization and energy evaluation provided by Quantum Espresso. We demonstrate that it is possible to predict the crystal structures of a biomedical molecule using solely first principles calculations. We were able to find all the ambient pressure stable glycine polymorphs, which are found in the same energetic ordering as observed experimentally and the agreement between the experimental and predicted structures is of such accuracy that the two are visually almost indistinguishable. PMID:25843964

  2. Crystal structure prediction from first principles: The crystal structures of glycine

    NASA Astrophysics Data System (ADS)

    Lund, Albert M.; Pagola, Gabriel I.; Orendt, Anita M.; Ferraro, Marta B.; Facelli, Julio C.

    2015-04-01

    Here we present the results of our unbiased searches of glycine polymorphs obtained using the genetic algorithms search implemented in MGAC, modified genetic algorithm for crystals, coupled with the local optimization and energy evaluation provided by Quantum Espresso. We demonstrate that it is possible to predict the crystal structures of a biomedical molecule using solely first principles calculations. We were able to find all the ambient pressure stable glycine polymorphs, which are found in the same energetic ordering as observed experimentally and the agreement between the experimental and predicted structures is of such accuracy that the two are visually almost indistinguishable.

  3. First principles study of structural, electronic and magnetic properties of magnesium

    NASA Astrophysics Data System (ADS)

    Abdel Rahim, G. P.; Rodríguez M, J. A.; Moreno-Armenta, M. G.

    2016-02-01

    We investigated the structural, electronic, and magnetic properties of Mg, in the CS (simple cubic), NiAs (Nickel arsenide), FCC (rock-salt), R (Rhombohedral), Diamond and WZ (wurtzite) phases. Calculations were performed using the first-principles pseudo-potential method within the framework of spin-density functional theory (DFT).

  4. Conformational structures of a decapeptide validated by first principles calculations and cold ion spectroscopy.

    PubMed

    Roy, Tapta Kanchan; Kopysov, Vladimir; Nagornova, Natalia S; Rizzo, Thomas R; Boyarkin, Oleg V; Gerber, R Benny

    2015-05-18

    Calculated structures of the two most stable conformers of a protonated decapeptide gramicidin S in the gas phase have been validated by comparing the vibrational spectra, calculated from first- principles and measured in a wide spectral range using infrared (IR)-UV double resonance cold ion spectroscopy. All the 522 vibrational modes of each conformer were calculated quantum mechanically and compared with the experiment without any recourse to an empirical scaling. The study demonstrates that first-principles calculations, when accounting for vibrational anharmonicity, can reproduce high-resolution experimental spectra well enough for validating structures of molecules as large as of 200 atoms. The validated accurate structures of the peptide may serve as templates for in silico drug design and absolute calibration of ion mobility measurements. PMID:25721337

  5. A genetic algorithm for first principles global structure optimization of supported nano structures

    SciTech Connect

    Vilhelmsen, Lasse B.; Hammer, Bjørk

    2014-07-28

    We present a newly developed publicly available genetic algorithm (GA) for global structure optimisation within atomic scale modeling. The GA is focused on optimizations using first principles calculations, but it works equally well with empirical potentials. The implementation is described and benchmarked through a detailed statistical analysis employing averages across many independent runs of the GA. This analysis focuses on the practical use of GA’s with a description of optimal parameters to use. New results for the adsorption of M{sub 8} clusters (M = Ru, Rh, Pd, Ag, Pt, Au) on the stoichiometric rutile TiO{sub 2}(110) surface are presented showing the power of automated structure prediction and highlighting the diversity of metal cluster geometries at the atomic scale.

  6. Structure of the (111) surface of bismuth: LEED analysis and first-principles calculations

    SciTech Connect

    Moenig, H.; Wells, J.; Hofmann, Ph.; Sun, J.; Pohl, K.; Koroteev, Yu.M.; Bihlmayer, G.; Chulkov, E.V.

    2005-08-15

    The surface structure of Bi(111) was investigated by low-energy electron diffraction (LEED) intensity analysis for temperatures between 140 and 313 K and by first-principles calculations. The diffraction pattern reveals a (1x1) surface structure and LEED intensity versus energy simulations confirm that the crystal is terminated with a Bi bilayer. Excellent agreement is obtained between the calculated and measured diffraction intensities in the whole temperature range. The first interlayer spacing shows no significant relaxation at any temperature while the second interlayer spacing expands slightly. The Debye temperatures deduced from the optimized atomic vibrational amplitudes for the two topmost layers are found to be significantly lower than in the bulk. The experimental results for the relaxations agree well with those of our first-principles calculation.

  7. First principles study of structural, electronic and mechanical properties of alkali nitride-KN

    SciTech Connect

    Murugan, A.; Rajeswarapalanichamy, R. Santhosh, M.; Iyakutti, K.

    2015-06-24

    The structural, electronic and elastic properties of alkali- metal nitride (KN) is investigated by the first principles calculations based on density functional theory as implemented in Vienna ab-initio simulation package. At ambient pressure KN is stable in the ferromagnetic state with NaCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that the KN is half metallic ferromagnet at normal pressure. A pressure-induced structural phase transition from NaCl to ZB phase is observed in KN. Half metallicity and ferromagnetism is maintained at all pressures.

  8. First-principles calculation of the structural stability of 6d transition metals

    SciTech Connect

    Oestlin, A.; Vitos, L.

    2011-09-15

    The phase stability of the 6d transition metals (elements 103-111) is investigated using first-principles electronic-structure calculations. Comparison with the lighter transition metals reveals that the structural sequence trend is broken at the end of the 6d series. To account for this anomalous behavior, the effect of relativity on the lattice stability is scrutinized, taking different approximations into consideration. It is found that the mass-velocity and Darwin terms give important contributions to the electronic structure, leading to changes in the interstitial charge density and, thus, in the structural energy difference.

  9. High-pressure crystal structures of TaAs from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Lu, Mingchun; Guo, Yanan; Zhang, Miao; Liu, Hanyu; Tse, John S.

    2016-08-01

    In this work, we systematically studied the phase transition of TaAs under high pressures and reported three high-pressure structures P-6m2 (hexagonal, stable at 13-32 GPa), P21/c (monoclinic, stable at 32-103 GPa) and Pm-3m (cubic, stable above 103 GPa), by using particle swarm optimization in combination with first principles electronic structure methodology. All predicted structures are dynamically stable, since there is no imaginary mode to be found in the whole Brillouin zone. At high pressures, the TaAs was found to become superconductor with the superconducting critical temperature of ~1 K at about 100 GPa.

  10. First principle study of band structure of SrMO3 perovskites

    NASA Astrophysics Data System (ADS)

    Daga, Avinash; Sharma, Smita

    2016-05-01

    First principle study of band structure calculations in the local density approximations (LDA) as well as in the generalized gradient approximations (GGA) have been used to determine the electronic structure of SrMO3 where M stands for Ti, Zr and Mo. Occurrence of band gap proves SrTiO3 and SrZrO3 to be insulating. A small band gap is observed in SrMoO3 perovskite signifies it to be metallic. Band structures are found to compare well with the available data in the literature showing the relevance of this approach. ABINIT computer code has been used to carry out all the calculations.

  11. Mechanical properties of the interface structure of nanodiamond composite films: First-principles studies

    NASA Astrophysics Data System (ADS)

    Zhang, Suhui; Liu, Xuejie; Jiang, Yongjun; Ren, Yuan; Li, Suozhi

    2016-02-01

    The elastic properties of the interface structure of nanodiamond composite films are investigated using first-principles calculations. The nanodiamond grains in the films are surrounded by a monolayer heterogeneous interface. The interface phase comprises B, Si, P, and Ge. The elastic constants, bulk, shear and Young's modulus of the interface structures are all obtained with first principle calculations. Calculated elastic constants of the diamond (0 0 1) interface are larger than those of the (1 1 1) interface. For the B, Si, P, and Ge interface structures, as the average atomic distance increases, the average Young's modulus decrease, which follows the sequence EbarB>EbarSi >EbarP > EbarGe , with corresponding values of 927.05, 843.841, 840.152, and 819.805 GPa. The ductility and plasticity, as well as the anisotropy values (A and AU) of the interface structures were discussed based on the obtained mechanical parameters. The results show that P interface structures demonstrate ductile property when stressed longitudinally, whereas the other interface structures are all brittle. Then the visualization of the directional dependence of the Young's modulus are also presented. These reflected an interesting results. For the B, Si, and Ge interface structures, whether they show isotropy or anisotropy depends on the crystal structure, while it depends on the direction of the applied strain for the P interface structures.

  12. First-principles study of electronic structures of graphene on Y2O3

    NASA Astrophysics Data System (ADS)

    Kaneko, Tomoaki; Ohno, Takahisa

    2016-06-01

    We investigate the structures, stability and electronic properties of graphene adsorbed on Y2O3(111) using first-principles calculations based on density functional theories. When the interface of Y2O3(111) is terminated by an Y-layer, graphene is chemisorbed on Y2O3, resulting in the strong modification of electronic band structures. When the Y2O3(111) surface is terminated with O atoms and extra O atoms, on the other hand, graphene is physisorbed on Y2O3(111). Therefore, an O-rich environment is preferable for the graphene and Y2O3 interface.

  13. A comparative first-principles study of structural and electronic properties among memantine, amantadine and rimantadine

    NASA Astrophysics Data System (ADS)

    Middleton, Kirsten; Zhang, G. P.; Nichols, Michael R.; George, Thomas F.

    2012-05-01

    Memantine, amantadine and rimantadine are structurally derived from the same diamondoid, adamantane. These derivatives demonstrate therapeutic efficacy in human diseases: memantine for Alzheimer's disease and amantadine and rimantadine for influenza. In order to better understand some of the properties that distinguish these three compounds, we conduct first-principles calculations on their structure and electronic properties. Our results indicate that protonation has a significant effect on the dipole moment, where the dipole moment in protonated memantine is over eight times larger than in the deprotonated form.

  14. First-principles study of the electronic and molecular structure of protein nanotubes

    NASA Astrophysics Data System (ADS)

    Okamoto, Hajime; Takeda, Kyozaburo; Shiraishi, Kenji

    2001-09-01

    The electronic and molecular structures of protein nanotubes (PNT's) have been investigated theoretically by first-principles electronic structure calculations. The results have been discussed in comparison to those of the polypeptide open chains (POC's) and polypeptide closed rings (PCR's) in order to give a systematic understanding. Focusing on the intra-ring and inter-ring hydrogen bonds (HB's), we also investigate the PCR stacking mechanism. The present calculation reveals that PNT's are semiconductors and that an extra proton in the tube interior has the potential to be an electron acceptor.

  15. First-principles calculations on the structural evolution of solid fullerene-like CP x

    NASA Astrophysics Data System (ADS)

    Gueorguiev, G. K.; Furlan, A.; Högberg, H.; Stafström, S.; Hultman, L.

    2006-08-01

    The formation and structural evolution of fullerene-like (FL) carbon phosphide (CP x) during synthetic growth were studied by first-principles calculations. Geometry optimizations and comparison between the cohesive energies suggest stability for solid FL-CP x compounds. In comparison with fullerene-like carbon nitride, higher curvature of the graphene sheets and higher density of cross-linkages between them is predicted and explained by the different electronic properties of P and N. Cage-like and onion-like structures, both containing tetragons, are found to be typical for fullerene-like CP x. Segregation of P is predicted at fractions exceeding ˜20 at.%.

  16. Study of mercury thiogallate in defect stannite structure: A first-principle approach

    NASA Astrophysics Data System (ADS)

    Nayak, Vikas; Verma, U. P.

    2016-05-01

    Quantum mechanical based first principle calculations have been employed to obtain the unit cell lattice parameters of mercury thiogallate (HgGa2S4) in defect stannite structure for the first time. For this, we treated HgGa2S4 in two different types of site symmetries in the same space group. In both the cases obtained unit cell parameters are same, which shows the accuracy of present approach. The electronic band structures show the semiconducting behavior in both the cases. The density of states plot are also studied and discussed.

  17. First-principles prediction of the equation of state for TcC with rocksalt structure

    NASA Astrophysics Data System (ADS)

    Sun, Xiao-Wei; Chu, Yan-Dong; Liu, Zi-Jiang; Song, Ting; Tian, Jun-Hong; Wei, Xiao-Ping

    2014-10-01

    The equation of state of TcC with rocksalt structure is investigated by means of first-principles density functional theory calculations combined with the quasi-harmonic Debye model in which the phononic effects are considered. Particular attention is paid to the predictions of the compressibility, the isothermal bulk modulus and its first pressure derivative which play a central role in the formulation of approximate equations of state for the first time. The properties of TcC with rocksalt structure are summarized in the pressure range of 0-80 GPa and the temperature up to 2500 K.

  18. The structural and electronic properties of amorphous HgCdTe from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Zhao, Huxian; Chen, Xiaoshuang; Lu, Jianping; Shu, Haibo; Lu, Wei

    2014-01-01

    Amorphous mercury cadmium telluride (a-MCT) model structures, with x being 0.125 and 0.25, are obtained from first-principles calculations. We generate initial structures by computation alchemy method. It is found that most atoms in the network of amorphous structures tend to be fourfold and form tetrahedral structures, implying that the chemical ordered continuous random network with some coordination defects is the ideal structure for a-MCT. The electronic structure is also concerned. The gap is found to be 0.30 and 0.26 eV for a-Hg0.875Cd0.125Te and a-Hg0.75Cd0.25Te model structures, independent of the composition. By comparing with the properties of crystalline MCT with the same composition, we observe a blue-shift of energy band gap. The localization of tail states and its atomic origin are also discussed.

  19. Superconductivity in compressed sulfur hydride: Dependences on pressure, composition, and crystal structure from first principles

    NASA Astrophysics Data System (ADS)

    Akashi, Ryosuke

    The recent discovery of high-temperature superconductivity in sulfur hydride under extreme pressure has broken the long-standing record of superconducting transition temperature (Tc) in the Hg-cuprate. According to the isotope effect measurement and theoretical calculations, the superconducting transition is mainly ascribed to the conventional phonon-mediated pairing interaction. It is, however, not enough for understanding the high-Tc superconductivity in the sulfur hydride. To elucidate various possible effects on Tc with accuracy, we have analyzed Tc with first-principles methods without any empirical parameters. First, for various pressures and theoretically proposed crystal structures, we calculated Tc with the density functional theory for superconductors (SCDFT) to examine which structure(s) can explain experimentally measured Tc data [Akashi et al., PRB 91, 224513 (2015)]. We next solved the Eliashberg equations without introducing the renormalized Coulomb parameter mu*, which is the Green-function-based counterpart of the SCDFT, and evaluated the effects of rapidly varying electron density of states, atomic zero-point motion, and phonon anharmonic corrections on Tc [Sano et al., in preparation]. In the talk, we review these results and discuss the dominant factors for the Tc and their relation to the experimental results. We also report some crystal structures that we recently found with first-principles calculations, which could have a key role for the pressure-induced transformation to the high-Tc phase.

  20. Pressure induced structural phase transition of OsB{sub 2}: First-principles calculations

    SciTech Connect

    Ren Fengzhu; Wang Yuanxu; Lo, V.C.

    2010-04-15

    Orthorhombic OsB{sub 2} was synthesized at 1000 deg. C and its compressibility was measured by using the high-pressure X-ray diffraction in a Diacell diamond anvil cell from ambient pressure to 32 GPa [R.W. Cumberland, et al. (2005)]. First-principles calculations were performed to study the possibility of the phase transition of OsB{sub 2}. An analysis of the calculated enthalpy shows that orthorhombic OsB{sub 2} can transfer to the hexagonal phase at 10.8 GPa. The calculated results with the quasi-harmonic approximation indicate that this phase transition pressure is little affected by the thermal effect. The calculated phonon band structure shows that the hexagonal P 6{sub 3}/mmc structure (high-pressure phase) is stable for OsB{sub 2}. We expect the phase transition can be further confirmed by the experimental work. - Abstract: Graphical Abstract Legend (TOC Figure): Table of Contents Figure Pressure induced structural phase transition from the orthorhombic structure to the hexagonal one for OsB{sub 2} takes place under 10.8 GPa (0 K), 10.35 GPa (300, 1000 K) by the first-principles predictions.

  1. First-principles simulation of Raman spectra and structural properties of quartz up to 5 GPa

    NASA Astrophysics Data System (ADS)

    Liu, Lei; Lv, Chao-Jia; Zhuang, Chun-Qiang; Yi, Li; Liu, Hong; Du, Jian-Guo

    2015-12-01

    The crystal structure and Raman spectra of quartz are calculated by using first-principles method in a pressure range from 0 to 5 GPa. The results show that the lattice constants (a, c, and V) decrease with increasing pressure and the a-axis is more compressible than the c axis. The Si-O bond distance decreases with increasing pressure, which is in contrast to experimental results reported by Hazen et al. [Hazen R M, Finger L W, Hemley R J and Mao H K 1989 Solid State Communications 725 507-511], and Glinnemann et al. [Glinnemann J, King H E Jr, Schulz H, Hahn T, La Placa S J and Dacol F 1992 Z. Kristallogr. 198 177-212]. The most striking changes are of inter-tetrahedral O-O distances and Si-O-Si angles. The volume of the tetrahedron decreased by 0.9% (from 0 to 5 GPa), which suggests that it is relatively rigid. Vibrational models of the quartz modes are identified by visualizing the associated atomic motions. Raman vibrations are mainly controlled by the deformation of the tetrahedron and the changes in the Si-O-Si bonds. Vibrational directions and intensities of atoms in all Raman modes just show little deviations when pressure increases from 0 to 5 GPa. The pressure derivatives (dνi/dP) of the 12 Raman frequencies are obtained at 0 GPa-5 GPa. The calculated results show that first-principles methods can well describe the high-pressure structural properties and Raman spectra of quartz. The combination of first-principles simulations of the Raman frequencies of minerals and Raman spectroscopy experiments is a useful tool for exploring the stress conditions within the Earth. Project supported by the Key Laboratory of Earthquake Prediction, Institute of Earthquake Science, China Earthquake Administration (CEA) (Grant No. 2012IES010201) and the National Natural Science Foundation of China (Grant Nos. 41174071 and 41373060).

  2. Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations

    SciTech Connect

    Zhao, Xin; Ke, Liqin; Nguyen, Manh Cuong; Wang, Cai-Zhuang Ho, Kai-Ming

    2015-06-28

    The structures and magnetic properties of Co-Zr-B alloys near the composition of Co{sub 5}Zr with B at. % ≤6% were studied using adaptive genetic algorithm and first-principles calculations. The energy and magnetic moment contour maps as a function of chemical composition were constructed for the Co-Zr-B magnet alloys through extensive structure searches and calculations. We found that Co-Zr-B system exhibits the same structure motif as the “Co{sub 11}Zr{sub 2}” polymorphs, and such motif plays a key role in achieving strong magnetic anisotropy. Boron atoms were found to be able to substitute cobalt atoms or occupy the “interruption” sites. First-principles calculations showed that the magnetocrystalline anisotropy energies of the boron-doped alloys are close to that of the high-temperature rhombohedral Co{sub 5}Zr phase and larger than that of the low-temperature Co{sub 5.25}Zr phase. Our calculations provide useful guidelines for further experimental optimization of the magnetic performances of these alloys.

  3. Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations

    DOE PAGESBeta

    Zhao, Xin; Ke, Liqin; Nguyen, Manh Cuong; Wang, Cai -Zhuang; Ho, Kai -Ming

    2015-06-23

    The structures and magnetic properties of Co-Zr-B alloys near the composition of Co5Zr with B at. % ≤6% were studied using adaptive genetic algorithm and first-principles calculations. The energy and magnetic moment contour maps as a function of chemical composition were constructed for the Co-Zr-B magnet alloys through extensive structure searches and calculations. We found that Co-Zr-B system exhibits the same structure motif as the “Co11Zr2” polymorphs, and such motif plays a key role in achieving strong magnetic anisotropy. Boron atoms were found to be able to substitute cobalt atoms or occupy the “interruption” sites. First-principles calculations showed that themore » magnetocrystalline anisotropy energies of the boron-doped alloys are close to that of the high-temperature rhombohedral Co5Zr phase and larger than that of the low-temperature Co5.25Zr phase. As a result, our calculations provide useful guidelines for further experimental optimization of the magnetic performances of these alloys.« less

  4. Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations

    SciTech Connect

    Zhao, Xin; Ke, Liqin; Nguyen, Manh Cuong; Wang, Cai -Zhuang; Ho, Kai -Ming

    2015-06-23

    The structures and magnetic properties of Co-Zr-B alloys near the composition of Co5Zr with B at. % ≤6% were studied using adaptive genetic algorithm and first-principles calculations. The energy and magnetic moment contour maps as a function of chemical composition were constructed for the Co-Zr-B magnet alloys through extensive structure searches and calculations. We found that Co-Zr-B system exhibits the same structure motif as the “Co11Zr2” polymorphs, and such motif plays a key role in achieving strong magnetic anisotropy. Boron atoms were found to be able to substitute cobalt atoms or occupy the “interruption” sites. First-principles calculations showed that the magnetocrystalline anisotropy energies of the boron-doped alloys are close to that of the high-temperature rhombohedral Co5Zr phase and larger than that of the low-temperature Co5.25Zr phase. As a result, our calculations provide useful guidelines for further experimental optimization of the magnetic performances of these alloys.

  5. First-principles study of the structural and electronic properties of ultrathin silver nanowires

    NASA Astrophysics Data System (ADS)

    Ma, Liang-Cai; Ma, Ling; Lin, Xue-Ling; Yang, You-Zhen; Zhang, Jian-Min

    2015-12-01

    By using first-principles calculations based on density-functional theory, we have systematically investigated the equilibrium structure, stability and electronic properties of silver nanowires (AgNWs) with dimer, triangular, square, pentagonal and hexagonal cross-section. It is found that, using the string tension criterion, for the triangular and square AgNWs with small diameters the preferred structures should be the hollow one with staggered configuration, while for the pentagonal and hexagonal AgNWs with bigger diameters the preferred structures should be the staggered ones which contain a linear chain along the wire axis passes through the center of the polygons, where each chain atom is just located at a point equidistant from the planes of polygons. Electronic band structures and density of states calculations show that the AgNWs with different structures exhibit metallic behavior. Charge density contours show that there is an enhanced interatomic interaction in AgNWs compared with Ag bulk.

  6. Structural, electronic and mechanical properties of rare earth nitride-ErN: A first principles study

    SciTech Connect

    Murugan, A.; Rajeswarapalanichamy, R. Santhosh, M.; Priyanga, G. Sudha; Kanagaprabha, S.; Iyakutti, K.

    2015-06-24

    The structural, electronic and mechanical properties of rare earth nitride ErN is investigated by the first principles calculations based on density functional theory using the Vienna ab-initio simulation package. At ambient pressure ErN is stable in the ferromagnetic state with NaCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that ErN is half metallic at normal pressure. A pressure-induced structural phase transition from NaCl (B1) to CsCl (B2) phase is observed in ErN. Ferromagnetic to non magnetic phase transition is predicted in ErN at high pressure.

  7. Structural predictions based on the compositions of cathodic materials by first-principles calculations

    NASA Astrophysics Data System (ADS)

    Li, Yang; Lian, Fang; Chen, Ning; Hao, Zhen-jia; Chou, Kuo-chih

    2015-05-01

    A first-principles method is applied to comparatively study the stability of lithium metal oxides with layered or spinel structures to predict the most energetically favorable structure for different compositions. The binding and reaction energies of the real or virtual layered LiMO2 and spinel LiM2O4 (M = Sc-Cu, Y-Ag, Mg-Sr, and Al-In) are calculated. The effect of element M on the structural stability, especially in the case of multiple-cation compounds, is discussed herein. The calculation results indicate that the phase stability depends on both the binding and reaction energies. The oxidation state of element M also plays a role in determining the dominant structure, i.e., layered or spinel phase. Moreover, calculation-based theoretical predictions of the phase stability of the doped materials agree with the previously reported experimental data.

  8. Vibrational and mechanical properties of single layer MXene structures: a first-principles investigation

    NASA Astrophysics Data System (ADS)

    Yorulmaz, Uğur; Özden, Ayberk; Perkgöz, Nihan K.; Ay, Feridun; Sevik, Cem

    2016-08-01

    MXenes, carbides, nitrides and carbonitrides of early transition metals are the new members of two dimensional materials family given with a formula of {{{M}}}n+1 X n . Recent advances in chemical exfoliation and CVD growth of these crystals together with their promising performance in electrochemical energy storage systems have triggered the interest in these two dimensional structures. In this work, we employ first principles calculations for n = 1 structures of Sc, Ti, Zr, Mo and Hf pristine MXenes and their fully surface terminated forms with F and O. We systematically investigated the dynamical and mechanical stability of both pristine and fully terminated MXene structures to determine the possible MXene candidates for experimental realization. In conjunction with an extensive stability analysis, we report Raman and infrared active mode frequencies for the first time, providing indispensable information for the experimental elaboration of MXene field. After determining dynamically stable MXenes, we provide their phonon dispersion relations, electronic and mechanical properties.

  9. First-principles study of structural, elastic, and electronic properties of chromium carbides

    NASA Astrophysics Data System (ADS)

    Jiang, Chao

    2008-01-01

    Using first-principles calculations, we systematically studied the structural, elastic, and electronic properties of the technologically important chromium carbides: Cr3C2, Cr7C3, Cr23C6, Cr3C, and CrC. Our calculations show that the ground state structure for Cr7C3 is hexagonal, not orthorhombic. We further predict WC to be the energetically most stable structure for CrC. Our results indicate that all chromium carbides considered in this study are metallic and mechanically stable under the ambient condition. Among all chromium carbides, WC-type CrC exhibits the highest bulk and shear moduli and the lowest Poisson's ratio, and is a potential low-compressibility and hard material.

  10. Vibrational and mechanical properties of single layer MXene structures: a first-principles investigation.

    PubMed

    Yorulmaz, Uğur; Özden, Ayberk; Perkgöz, Nihan K; Ay, Feridun; Sevik, Cem

    2016-08-19

    MXenes, carbides, nitrides and carbonitrides of early transition metals are the new members of two dimensional materials family given with a formula of [Formula: see text] X n . Recent advances in chemical exfoliation and CVD growth of these crystals together with their promising performance in electrochemical energy storage systems have triggered the interest in these two dimensional structures. In this work, we employ first principles calculations for n = 1 structures of Sc, Ti, Zr, Mo and Hf pristine MXenes and their fully surface terminated forms with F and O. We systematically investigated the dynamical and mechanical stability of both pristine and fully terminated MXene structures to determine the possible MXene candidates for experimental realization. In conjunction with an extensive stability analysis, we report Raman and infrared active mode frequencies for the first time, providing indispensable information for the experimental elaboration of MXene field. After determining dynamically stable MXenes, we provide their phonon dispersion relations, electronic and mechanical properties. PMID:27377143

  11. Lithium halide monolayers: Structural, electronic and optical properties by first principles study

    NASA Astrophysics Data System (ADS)

    Safari, Mandana; Maskaneh, Pegah; Moghadam, Atousa Dashti; Jalilian, Jaafar

    2016-09-01

    Using first principle study, we investigate the structural, electronic and optical properties of lithium halide monolayers (LiF, LiCl, LiBr). In contrast to graphene and other graphene-like structures that form hexagonal rings in plane, these compounds can form and stabilize in cubic shape interestingly. The type of band structure in these insulators is identified as indirect type and ionic nature of their bonds are illustrated as well. The optical properties demonstrate extremely transparent feature for them as a result of wide band gap in the visible range; also their electron transitions are indicated for achieving a better vision on the absorption mechanism in these kinds of monolayers.

  12. First-principles study on oxidation of Ge and its interface electronic structures

    NASA Astrophysics Data System (ADS)

    Ono, Tomoya; Saito, Shoichiro; Iwase, Shigeru

    2016-08-01

    We review a series of first-principles studies on the defect generation mechanism and electronic structures of the Ge/GeO2 interface. Several experimental and theoretical studies proved that Si atoms at the Si/SiO2 interface are emitted to release interface stress. In contrast, total-energy calculation reveals that Ge atoms at the Ge/GeO2 interface are hardly emitted, resulting in the low trap density. Even if defects are generated, those at the Ge/GeO2 interface are found to behave differently from those at the Si/SiO2 interface. The states attributed to the dangling bonds at the Ge/GeO2 interface lie below the valence-band maximum of Ge, while those at the Si/SiO2 interface generate the defect state within the band gap of Si. First-principles electron-transport calculation elucidates that this characteristic behavior of the defect states is relevant to the difference in the leakage current through the Si/SiO2 and Ge/GeO2 interfaces.

  13. First-principles study of structure and properties of the cyclic pentamer of formaldehyde

    NASA Astrophysics Data System (ADS)

    Sreepad, H. R.; Ravi, H. R.; Hembram, K. P. S. S.; Waghmare, Umesh V.

    2012-06-01

    Structure of the cyclic pentamer of formaldehylde - Pentaxecane has been studied using first-principles. The structural parameters have been compared with the x-ray data available in the literature. The binding energy per monomer and per atom turn out to be to 2240 kJ/mole/monomer and 560 kJ/mole/atom respectively. Electronic density of states (EDOS) have been calculated which gives a value of 5.64 eV as the band gap. Phonon frequencies have been calculated at the Gamma point. Phonon modes show wave numbers ranging from 22cm-1 to 2995cm-1. Values of dielectric constant along different axes have also been calculated. The dielectric constant has also been determined experimentally and compared.

  14. Band structure and optical properties of amber studied by first principles

    NASA Astrophysics Data System (ADS)

    Rao, Zhi-Fan; Zhou, Rong-Feng

    2013-03-01

    The band structure and density of states of amber is studied by the first principles calculation based on density of functional theory. The complex structure of amber has 214 atoms and the band gap is 5.0 eV. The covalent bond is combined C/O atoms with H atoms. The O 2p orbital is the biggest effect near the Fermi level. The optical properties' results show that the reflectivity is low, and the refractive index is 1.65 in visible light range. The highest absorption coefficient peak is at 172 nm and another higher peak is at 136 nm. These convince that the amber would have a pretty sheen and that amber is a good and suitable crystal for jewelry and ornaments.

  15. First-principles study of the structure and stability of oxygen defects in zinc oxide

    NASA Astrophysics Data System (ADS)

    Erhart, Paul; Klein, Andreas; Albe, Karsten

    2005-08-01

    A comparative study on the structure and stability of oxygen defects in ZnO is presented. By means of first-principles calculations based on local density functional theory we investigate the oxygen vacancy and different interstitial configurations of oxygen in various charge states. Our results reveal that dumbbell-like structures are thermodynamically the most stable interstitial configurations for neutral and positive charge states due to the formation of a strongly covalent oxygen-oxygen bond. For negative charge states the system prefers a split-interstitial configuration with two oxygen atoms in almost symmetric positions with respect to the associated perfect lattice site. The calculated defect formation energies imply that interstitial oxygen atoms may provide both donor- and acceptor-like defects.

  16. Novel phases of lithium-aluminum binaries from first-principles structural search

    NASA Astrophysics Data System (ADS)

    Sarmiento-Pérez, Rafael; Cerqueira, Tiago F. T.; Valencia-Jaime, Irais; Amsler, Maximilian; Goedecker, Stefan; Romero, Aldo H.; Botti, Silvana; Marques, Miguel A. L.

    2015-01-01

    Intermetallic Li-Al compounds are on the one hand key materials for light-weight engineering, and on the other hand, they have been proposed for high-capacity electrodes for Li batteries. We determine from first-principles the phase diagram of Li-Al binary crystals using the minima hopping structural prediction method. Beside reproducing the experimentally reported phases (LiAl, Li3Al2, Li9Al4, LiAl3, and Li2Al), we unveil a structural variety larger than expected by discovering six unreported binary phases likely to be thermodynamically stable. Finally, we discuss the behavior of the elastic constants and of the electric potential profile of all Li-Al stable compounds as a function of their stoichiometry.

  17. First principle study of transport properties of a graphene nano structure

    NASA Astrophysics Data System (ADS)

    Kumar, Naveen; Sharma, Munish; Sharma, Jyoti Dhar; Ahluwalia, P. K.

    2013-06-01

    The first principle quantum transport calculations have been performed for graphene using Tran SIESTA which calculates transport properties using nonequilibrium Green's function method in conjunction with density-functional theory. Transmission functions, electron density of states and current-voltage characteristic have been calculated for a graphene nano structure using graphene electrodes. Transmission function, density of states and projected density of states show a discrete band structure which varies with applied voltage. The value of current is very low for applied voltage between 0.0 V to 5.0 V and lies in the range of pico ampere. In the V-I characteristic current shows non-linear fluctuating pattern with increase in voltage.

  18. Novel phases of lithium-aluminum binaries from first-principles structural search

    SciTech Connect

    Sarmiento-Pérez, Rafael; Cerqueira, Tiago F. T.; Botti, Silvana; Marques, Miguel A. L.; Valencia-Jaime, Irais; Amsler, Maximilian; Goedecker, Stefan; Romero, Aldo H.

    2015-01-14

    Intermetallic Li–Al compounds are on the one hand key materials for light-weight engineering, and on the other hand, they have been proposed for high-capacity electrodes for Li batteries. We determine from first-principles the phase diagram of Li–Al binary crystals using the minima hopping structural prediction method. Beside reproducing the experimentally reported phases (LiAl, Li{sub 3}Al{sub 2}, Li{sub 9}Al{sub 4}, LiAl{sub 3}, and Li{sub 2}Al), we unveil a structural variety larger than expected by discovering six unreported binary phases likely to be thermodynamically stable. Finally, we discuss the behavior of the elastic constants and of the electric potential profile of all Li–Al stable compounds as a function of their stoichiometry.

  19. Electronic structures and optical spectra of BaO from first principles

    SciTech Connect

    Wu, Chang-Wei; Pan, Bo; Wang, Neng-Ping

    2015-08-21

    We present the results of first-principles study for the electronic structure and optical absorption spectrum of the alkaline-earth metal oxide BaO. The quasiparticle band structure is evaluated within the Hedin's GW approximation [Phys. Rev. 139, A796 (1965)]. Thereafter, the electron-hole interaction is taken into consideration and the Bethe-Salpeter equation for the electron-hole two-particle Green function is solved. The calculated quasiparticle band gap of BaO is 4.1 eV, which is in good agreement with the experimental result. The calculated optical absorption spectrum of BaO is also in agreement with the experimental data. In particular, the calculated excitation energy for the lowest exciton peak in the optical absorption spectrum of BaO reproduces very well the corresponding experimental result.

  20. Pressure induced structural phase transition of OsB 2: First-principles calculations

    NASA Astrophysics Data System (ADS)

    Ren, Fengzhu; Wang, Yuanxu; Lo, V. C.

    2010-04-01

    Orthorhombic OsB 2 was synthesized at 1000 °C and its compressibility was measured by using the high-pressure X-ray diffraction in a Diacell diamond anvil cell from ambient pressure to 32 GPa [R.W. Cumberland, et al. (2005)]. First-principles calculations were performed to study the possibility of the phase transition of OsB 2. An analysis of the calculated enthalpy shows that orthorhombic OsB 2 can transfer to the hexagonal phase at 10.8 GPa. The calculated results with the quasi-harmonic approximation indicate that this phase transition pressure is little affected by the thermal effect. The calculated phonon band structure shows that the hexagonal P 6 3/ mmc structure (high-pressure phase) is stable for OsB 2. We expect the phase transition can be further confirmed by the experimental work.

  1. Solvation Structure and Dynamics of Ni(2+)(aq) from First Principles.

    PubMed

    Mareš, Jiří; Liimatainen, Helmi; Laasonen, Kari; Vaara, Juha

    2011-09-13

    The aqueous solution of Ni(2+) was investigated using first principles molecular dynamics (FPMD) simulation based on periodic density-functional theory (DFT) calculations. The experimental structural parameters of the Ni(aq) complex are reproduced well by the simulation. An exchange event of the water molecule in the first solvation shell is observed, supporting the proposed dissociative mechanism of exchange. The calculated dynamic characteristics of the surrounding water molecules indicate too slow translational diffusion in comparison to experimental results, in agreement with other FPMD studies employing a similar level of theory. We also find that the reorientational dynamics of water are an order of magnitude slower as compared to experimental data. On the other hand, the angular momentum dynamics are in better agreement with the experimental data than the previously reported results from MD simulations employing empirical force fields. The obtained MD trajectory can supply accurate structures for the calculation of magnetic properties. PMID:26605483

  2. Electronic structures and optical spectra of BaO from first principles

    NASA Astrophysics Data System (ADS)

    Wu, Chang-Wei; Pan, Bo; Wang, Neng-Ping

    2015-08-01

    We present the results of first-principles study for the electronic structure and optical absorption spectrum of the alkaline-earth metal oxide BaO. The quasiparticle band structure is evaluated within the Hedin's GW approximation [Phys. Rev. 139, A796 (1965)]. Thereafter, the electron-hole interaction is taken into consideration and the Bethe-Salpeter equation for the electron-hole two-particle Green function is solved. The calculated quasiparticle band gap of BaO is 4.1 eV, which is in good agreement with the experimental result. The calculated optical absorption spectrum of BaO is also in agreement with the experimental data. In particular, the calculated excitation energy for the lowest exciton peak in the optical absorption spectrum of BaO reproduces very well the corresponding experimental result.

  3. First-principles calculation of electronic structure and optical absorption of BN ZnO

    NASA Astrophysics Data System (ADS)

    Zhang, Xiao; Schleife, Andre

    2015-03-01

    The α-BN structure of ZnO, a nonequilibrium phase with a transition pressure of 25 GPa, has been found in nano structures of ZnO. The structural difference between the BN structure and the equilibrium wurtzite structure can play an important role for applications of nanostructured ZnO. In order to understand the difference, first principles calculations have been performed on both phases. The electronic structure is computed using the GW method based on Density Functional Theory and HSE hybrid functional calculations. The GW method includes the quasiparticle effects due to the screened electron-electron interaction which gives an accurate description of the electronic band structure and density of states. After that, by solving the Bethe-Salpeter Equation for the optical polarization function, which take excitonic effects into account, we have achieved an accurate description of optical absorption spectra for both structures. We find a good agreement with experimental and previous computational results for WZ structure, and predict the absorption for the BN structure. The BN structure shows a larger band gap and we found a very large optical anisotropy: The gap for extraordinary light polarization is almost 0.7eV larger than that for ordinary light polarization.

  4. Structure, electronic and electrochemical properties of Li-rich metal phosphate by first-principles study

    NASA Astrophysics Data System (ADS)

    Lin, Zhiping; Zhao, Yu-Jun; Zhao, Yanming; Xu, Jiantie

    2014-01-01

    We present a first-principles investigation for the structure, electronic properties, and average potentials of Li9M3(P2O7)3(PO4)2 (M = V, Fe, Cr) compounds. The calculated Wyckoff coordinates are in good agreement with experimental observations. All the studied compounds show semiconductor characteristics, with band gaps between 1.89 eV and 2.55 eV. It is found that the Li-ion extraction is in the order of Li1(2b), Li2(12g), and Li3(4d) based on the calculated formation enthalpies of Li vacancies. Consequently, the calculated average potentials versus the number of Li ions are in good agreement with experiment.

  5. First principles DFT investigation of yttrium-doped graphene: Electronic structure and hydrogen storage

    SciTech Connect

    Desnavi, Sameerah; Chakraborty, Brahmananda; Ramaniah, Lavanya M.

    2014-04-24

    The electronic structure and hydrogen storage capability of Yttrium-doped grapheme has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom prefers the hollow site of the hexagonal ring with a binding energy of 1.40 eV. Doping by Y makes the system metallic and magnetic with a magnetic moment of 2.11 μ{sub B}. Y decorated graphene can adsorb up to four hydrogen molecules with an average binding energy of 0.415 eV. All the hydrogen atoms are physisorbed with an average desorption temperature of 530.44 K. The Y atoms can be placed only in alternate hexagons, which imply a wt% of 6.17, close to the DoE criterion for hydrogen storage materials. Thus, this system is potential hydrogen storage medium with 100% recycling capability.

  6. First-principles determination of the structural, vibrational, and thermodynamic properties of Methylammonium Lead Iodide Perovskite

    NASA Astrophysics Data System (ADS)

    Saidi, Wissam; Wissam Saidi Team

    Intrinsic energy-loss processes in solar cells ultimately increase the operational temperature, which can have profound effect on the power conversion efficiency of solar cells. Here I report investigations on the temperature effects on structural and mechanical properties of CH3NH3PbI3 using well-converged first-principles calculations with van der Waals dispersion corrections. The computed lattice parameters for cubic and tetragonal phases at finite temperature are found within 1% of experimentally measured values. Furthermore, the finite-temperature potential energy surface shows how the mechanical properties of the cubic and tetragonal phases of CH3NH3PbI3 evolve with temperature. Finally, I discuss the implications of these calculations on the nature of the tetragonal-to-cubic phase transition, and show that the underpinnings of this transition can be largely attributed to the phonons associated with methylammonium cations.

  7. Structural stability and electronic properties of InSb nanowires: A first-principles study

    NASA Astrophysics Data System (ADS)

    Zhang, Yong; Tang, Li-Ming; Ning, Feng; Wang, Dan; Chen, Ke-Qiu

    2015-03-01

    Using first-principles calculations, we investigate the structural stability and electronic properties of InSb nanowires (NWs). The results show that, in contrast to the bulk InSb phase, wurtzite (WZ) NWs are more stable than zinc-blende (ZB) NWs when the NW diameter is smaller than 10 nm. Nonpassivated ZB and WZ NWs are found to be metallic and semiconducting, respectively. After passivation, both ZB and WZ NWs exhibit direct-gap semiconductor character, and the band gap magnitude of the NWs strongly depends on the suppression of surface states by the charge-compensation ability of foreign atoms to surface atoms. Moreover, the carrier mobility of the NW can be strengthened by halogen passivation.

  8. The structural, electronic and phonon behavior of CsPbI3: A first principles study

    NASA Astrophysics Data System (ADS)

    Bano, Amreen; Khare, Preeti; Parey, Vanshree; Shukla, Aarti; Gaur, N. K.

    2016-05-01

    Metal halide perovskites are optoelectronic materials that have attracted enormous attention as solar cells with power conversion efficiencies reaching 20%. The benefit of using hybrid compounds resides in their ability to combine the advantage of these two classes of compounds: the high mobility of inorganic materials and the ease of processing of organic materials. In spite of the growing attention of this new material, very little is known about the electronic and phonon properties of the inorganic part of this compounds. A theoretical study of structural, electronic and phonon properties of metal-halide cubic perovskite, CsPbI3 is presented, using first-principles calculations with planewave pseudopotential method as personified in PWSCF code. In this approach local density approximation (LDA) is used for exchange-correlation potential.

  9. First-principles study of the electronic structure and magnetism of CaIrO3

    NASA Astrophysics Data System (ADS)

    Subedi, Alaska

    2012-01-01

    I study the electronic structure and magnetism of postperovskite CaIrO3 using first-principles calculations. The density functional calculations within the local density approximation without the combined effect of spin-orbit coupling and on-site Coulomb repulsion show the system to be metallic, which is in disagreement with the recent experimental evidences that show CaIrO3 to be an antiferromagnetic Mott insulator in the Jeff=1/2 state. However, when spin-orbit coupling is taken into account, the Ir t2g bands split into fully filled Jeff=3/2 bands and half-filled Jeff=1/2 bands. I find that spin-orbit coupling along with a modest on-site Coulomb repulsion opens a gap leading to a Mott insulating state. The ordering is antiferromagnetic along the c axis with total moments aligned antiparallel along the c axis and canted along the b axis.

  10. Structural stability and electronic properties of InSb nanowires: A first-principles study

    SciTech Connect

    Zhang, Yong; Tang, Li-Ming Ning, Feng; Chen, Ke-Qiu; Wang, Dan

    2015-03-28

    Using first-principles calculations, we investigate the structural stability and electronic properties of InSb nanowires (NWs). The results show that, in contrast to the bulk InSb phase, wurtzite (WZ) NWs are more stable than zinc-blende (ZB) NWs when the NW diameter is smaller than 10 nm. Nonpassivated ZB and WZ NWs are found to be metallic and semiconducting, respectively. After passivation, both ZB and WZ NWs exhibit direct-gap semiconductor character, and the band gap magnitude of the NWs strongly depends on the suppression of surface states by the charge-compensation ability of foreign atoms to surface atoms. Moreover, the carrier mobility of the NW can be strengthened by halogen passivation.

  11. Structural, elastic, and lattice dynamic stability of yttrium selenide (YSe) under pressure: A first principle study

    SciTech Connect

    Sahoo, B. D. Joshi, K. D.; Gupta, Satish C.

    2014-11-21

    Structural, elastic, and lattice dynamical stability of YSe has been investigated as a function of pressure through first principles electronic band structure calculations. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of pressure suggests that the B1 phase will transform to B2 structure at ∼32 (30 GPa at 300 K obtained from comparison of Gibbs free energy at 300 K). The transition is identified to be of first order in nature with a volume discontinuity of ∼6.2% at the transition pressure. Furthermore, the theoretically determined equation of state has been utilized to derive various physical quantities, such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus. The single crystal elastic constants have been predicted at various pressures for both the B1 and B2 structures using the energy strain method. The activation barrier between B1 and B2 phases calculated at transition point is ∼19.7mRy/formula unit. Our lattice dynamic calculations show that both the B1 as well as B2 structures are lattice dynamically stable not only at ambient pressure but also at transition pressure. The B1 phase becomes lattice dynamically unstable at ∼112 GPa, i.e., much beyond the transition pressure. The effect of temperature on volume and bulk modulus of the YSe in B1 phase has also been examined.

  12. First-principles assessment of potential ultrafast laser-induced structural transition in Ni

    NASA Astrophysics Data System (ADS)

    Bévillon, E.; Colombier, J. P.; Stoian, R.

    2016-06-01

    The possibility to trigger ultrafast solid-to-solid transitions in transition metals under femtosecond laser irradiation is investigated by means of first-principles calculations. Electronic heating can drastically modify screening, charge distribution and atomic binding features, potentially determining new structural relaxation paths in the solid phase, before thermodynamic solid-to-liquid transformations set in. Consequently, we evaluate here the effect of electronic excitation on structural stability and conditions for structural transitions. Ni is chosen as a case study for the probability of a solid transition, and the stability of its FCC phase is compared to the non-standard HCP structure while accounting for the heating of the electronic subsystem. From a phonon spectra analysis, we show that the thermodynamic stability order reverses at an electronic temperature of around 104 K. Both structures exhibit a dynamic stability, indicating they present a metastability depending on the heating. However, the general hardening of phonon modes with the increase of the electronic temperature points out that no transformation will occur, as confirmed by the study of a typical FCC to HCP diffusionless transformation path, showing an increasing energy barrier. Finally, based on electronic density of states interpretation, the tendency of different metal categories to undergo or not an ultrafast laser-induced structural transition is discussed.

  13. Thermodynamic ground state of MgB{sub 6} predicted from first principles structure search methods

    SciTech Connect

    Wang, Hui; Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 ; LeBlanc, K. A.; Gao, Bo; Yao, Yansun; Canadian Light Source, Saskatoon, Saskatchewan S7N 0X4

    2014-01-28

    Crystalline structures of magnesium hexaboride, MgB{sub 6}, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB{sub 6}. The energy of the Cmcm structure is significantly lower than the theoretical MgB{sub 6} models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B{sub 6} octahedra and extended B{sub ∞} ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB{sub 6} maintains a semiconducting state with permanent dipole moments. MgB{sub 6} is estimated to have much weaker electron-phonon coupling compared with that of MgB{sub 2}, and therefore it is not expected to be able to sustain superconductivity at high temperatures.

  14. Structural, electronic and magnetic properties of Fe2-based full Heusler alloys: A first principle study

    NASA Astrophysics Data System (ADS)

    Dahmane, F.; Mogulkoc, Y.; Doumi, B.; Tadjer, A.; Khenata, R.; Bin Omran, S.; Rai, D. P.; Murtaza, G.; Varshney, Dinesh

    2016-06-01

    Using the first-principles density functional calculations, the structural, electronic and magnetic properties of the Fe2XAl (X=Cr, Mn, Ni) compounds in both the Hg2CuTi and Cu2MnAl-type structures were studied by the full-potential linearized augmented plane waves (FP-LAPW) method. The exchange and correlation potential is treated by the generalized-gradient approximation (GGA) where the results show that the Cu2MnAl-type structure is energetically more stable than the Hg2CuTi-type structure for the Fe2CrAl and Fe2MnAl compounds at the equilibrium volume. The full Heusler compounds Fe2XAl (X=Cr, Mn) are half-metallic in the Cu2MnAl-type structure. Fe2NiAl has a metallic character in both CuHg2Ti and AlCu2Mn-type structures. The total magnetic moments of the Fe2CrAl and Fe2MnAl compounds are 1.0 and 2.0 μB, respectively, which are in agreement with the Slater-Pauling rule Mtot=Ztot- 24.

  15. An Insight into Sodiation of Antimony from First-Principles Crystal Structure Prediction

    NASA Astrophysics Data System (ADS)

    Caputo, Riccarda

    2016-02-01

    Elemental antimony has recently become an attractive anode material for potential application in rechargeable sodium-ion batteries. I present a first-principles study of the structure-composition dependence of the Na-Sb system for both sodiation and desodiation processes. The enthalpy of reaction of x moles of sodium with the hexagonal structure of antimony reveals several stable crystal structures for 0 < x ≤ 3, with variable composition states for 1.25 < x < 2.75. The direct and reverse reactions pass through similar states in terms of enthalpy of formation and symmetry representation of the corresponding optimized structures, in particular for x = 1 and x = 3, confirming the two known phases, namely NaSb and Na3Sb. The calculations suggest that the optimal composition range for reversible sodiation of antimony is 1 < x ≤ 3, thus avoiding the global minimum at x = 1. This can help to rationalize the structure-composition dependence of the electrochemical performance of antimony in Na-ion batteries.

  16. Prediction of Stable Ruthenium Silicides from First-Principles Calculations: Stoichiometries, Crystal Structures, and Physical Properties.

    PubMed

    Zhang, Chuanzhao; Kuang, Xiaoyu; Jin, Yuanyuan; Lu, Cheng; Zhou, Dawei; Li, Peifang; Bao, Gang; Hermann, Andreas

    2015-12-01

    We present results of an unbiased structure search for stable ruthenium silicide compounds with various stoichiometries, using a recently developed technique that combines particle swarm optimization algorithms with first-principles calculations. Two experimentally observed structures of ruthenium silicides, RuSi (space group P2(1)3) and Ru2Si3 (space group Pbcn), are successfully reproduced under ambient pressure conditions. In addition, a stable RuSi2 compound with β-FeSi2 structure type (space group Cmca) was found. The calculations of the formation enthalpy, elastic constants, and phonon dispersions demonstrate the Cmca-RuSi2 compound is energetically, mechanically, and dynamically stable. The analysis of electronic band structures and densities of state reveals that the Cmca-RuSi2 phase is a semiconductor with a direct band gap of 0.480 eV and is stabilized by strong covalent bonding between Ru and neighboring Si atoms. On the basis of the Mulliken overlap population analysis, the Vickers hardness of the Cmca structure RuSi2 is estimated to be 28.0 GPa, indicating its ultra-incompressible nature. PMID:26576622

  17. Crystal Structures, Stabilities, Electronic Properties, and Hardness of MoB2: First-Principles Calculations.

    PubMed

    Ding, Li-Ping; Shao, Peng; Zhang, Fang-Hui; Lu, Cheng; Ding, Lei; Ning, Shu Ya; Huang, Xiao Fen

    2016-07-18

    On the basis of the first-principles techniques, we perform the structure prediction for MoB2. Accordingly, a new ground-state crystal structure WB2 (P63/mmc, 2 fu/cell) is uncovered. The experimental synthesized rhombohedral R3̅m and hexagonal AlB2, as well as theoretical predicted RuB2 structures, are no longer the most favorite structures. By analyzing the elastic constants, formation enthalpies, and phonon dispersion, we find that the WB2 phase is thermodynamically and mechanically stable. The high bulk modulus B, shear modulus G, low Poisson's ratio ν, and small B/G ratio are benefit to its low compressibility. When the pressure is 10 GPa, a phase transition is observed between the WB2-MoB2 and the rhombohedral R3̅m MoB2 phases. By analyzing the density of states and electron density, we find that the strong covalent is formed in MoB2 compounds, which contributes a great deal to its low compressibility. Furthermore, the low compressibility is also correlated with the local buckled structure. PMID:27387577

  18. Structural stabilities, elastic and electronic properties of chromium tetraboride from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Xu, C.; Li, Q.; Liu, C. M.; Duan, M. Y.; Wang, H. K.

    2016-05-01

    First-principles calculations are employed to investigate the structural and elastic properties, formation enthalpies and chemical bonding features as well as hardness values of chromium tetraboride (CrB4) with different structures. The lattice parameters, Poisson’s ratio and B/G ratio are also derived. Our calculations indicate that the orthorhombic structure with Pnnm symmetry is the most energetically stable one for CrB4. Except for WB4P63/mmc structure with imaginary frequencies, another six new structures are investigated through the full phonon dispersion calculations. Their mechanical and thermodynamic stabilities are also studied by calculating the elastic constants and formation enthalpies. Our calculations show that the thermodynamic stabilities of all these CrB4 phases can be enhanced under high pressure. The large shear moduli, Young’s moduli and hardness values indicate that these CrB4 phases are potential hard materials. Analyses of the densities of states (DOSs) and electron localization functions (ELFs) provide further understandings of the chemical and physical properties of these CrB4 phases. It is observed that the large occupations and high strengths of the B-B covalent bonds are important for the stabilities, incompressibility and hardnesses of these CrB4 phases.

  19. First principles investigations of electronic structure and transport properties of graphitic structures and single molecular junctions

    NASA Astrophysics Data System (ADS)

    Owens, Jonathan R.

    In this work, we first present two powerful methods for understanding the electronic, structural, conducting, and energetic properties of nano-materials: density functional theory (DFT) and quantum transport. The basics of the theory and background of both methods are discussed thoroughly. After establishing a firm foundation, we turn our attention to using these tools to solve practical problems, often in collaboration with experimental colleagues. The first two projects pertain to nitrogen doping in graphene nanoribbons (GNRs). We study nitrogen doping in two different schema: concentration-based (N_x-doped) and structural based (N_2. {AA}-doped). Concentration based doping is explored in the context of experimental measurements of IV curves on GNRs with differing dopant concentrations. These results show a shift towards semi-conducting behavior with an increase in dopant concentration. We combine first principles calculations (DFT) and transport calculations in the Landauer formalism to compute the density-of-states (DOS) and transport curves for various dopant concentrations (0.46%, 1.39%, 1.89%, and 2.31%), which corroborate the experimental observations. The N_2. {AA}-doped GNR study was inspired by experimental observation of an atomically precise nitrogen doping scheme in bulk graphene. Experimental STM images, combined with simulated STM images, revealed that the majority (80%) of doping sites consist of nitrogen atoms on neighboring sites of the same sublattice (A) in graphene, hence N_2. {AA} doping. We examine this doping scheme applied to zigzag and armchair GNRs under different orientations of the dopants. We present spin-resolved charge densities, energetics, transport, DOS, and simulated STM images for all four systems studied. Our results show the possibility of spin-filtered devices and the STM images provide an aid in helping experimentalist identify the dopant patterns, if these GNRs are fabricated. We next venture to explain different observed

  20. Structural stability and electronic properties of β-tetragonal boron: A first-principles study

    SciTech Connect

    Hayami, Wataru

    2015-01-15

    It is known that elemental boron has five polymorphs: α- and β-rhombohedral, α- and β-tetragonal, and the high-pressure γ phase. β-tetragonal (β-t) boron was first discovered in 1960, but there have been only a few studies since then. We have thoroughly investigated, using first-principles calculations, the atomic and electronic structures of β-t boron, the details of which were not known previously. The difficulty of calculation arises from the fact that β-t boron has a large unit cell that contains between 184 and 196 atoms, with 12 partially-occupied interstitial sites. This makes the number of configurations of interstitial atoms too great to calculate them all. By introducing assumptions based on symmetry and preliminary calculations, the number of configurations to calculate can be greatly reduced. It was eventually found that β-t boron has the lowest total energy, with 192 atoms (8 interstitial atoms) in an orthorhombic lattice. The total energy per atom was between those of α- and β-rhombohedral boron. Another tetragonal structure with 192 atoms was found to have a very close energy. The valence bands were fully filled and the gaps were about 1.16 to 1.54 eV, making it comparable to that of β-rhombohedral boron. - Graphical abstract: Electronic density distribution for the lowest-energy configuration (N=192) viewed from the 〈1 0 0〉 direction. Left: isosurface (yellow) at d=0.09 electrons/a.u.{sup 3} Right: isosurface (orange) at d=0.12 electrons/a.u.{sup 3}. - Highlights: • β-tetragonal boron was thoroughly investigated using first-principles calculations. • The lowest energy structure contains 192 atoms in an orthorhombic lattice. • Another tetragonal structure with 192 atoms has a very close energy. • The total energy per atom is between those of α- and β-rhombohedral boron. • The band gap of the lowest energy structure is about 1.16 to 1.54 eV.

  1. First-principles study of structural, elastic, and thermodynamic properties of ZrHf alloy

    NASA Astrophysics Data System (ADS)

    Wei, Zhao; Zhai, Dong; Shao, Xiao-Hong; Lu, Yong; Zhang, Ping

    2015-04-01

    Structural parameters, elastic constants, and thermodynamic properties of ordered and disordered solid solutions of ZrHf alloys are investigated through first-principles calculations based on density-functional theory (DFT). The special quasi-random structure (SQS) method is used to model the disordered phase as a single unit cell, and two lamella structures are generated to model the ordered alloys. Small strains are applied to the unit cells to measure the elastic behavior and mechanical stability of ZrHf alloys and to obtain the independent elastic constants by the stress-strain relationship. Phonon dispersions and phonon density of states are presented to verify the thermodynamic stability of the considered phases. Our results show that both the ordered and disordered phases of ZrHf alloys are structurally stable. Based on the obtained phonon frequencies, thermodynamic properties, including Gibbs free energy, entropy, and heat capacity, are predicted within the quasi-harmonic approximation. It is verified that there are no obvious differences in energy between ordered and disordered phases over a wide temperature range. Project supported by the National Natural Science Foundation of China (Grant No. 51102009) and the Long-Term Subsidy Mechanism from the Ministry of Finance and the Ministry of Education of China.

  2. Determination of structure and properties of molecular crystals from first principles.

    PubMed

    Szalewicz, Krzysztof

    2014-11-18

    CONSPECTUS: Until recently, it had been impossible to predict structures of molecular crystals just from the knowledge of the chemical formula for the constituent molecule(s). A solution of this problem has been achieved using intermolecular force fields computed from first principles. These fields were developed by calculating interaction energies of molecular dimers and trimers using an ab initio method called symmetry-adapted perturbation theory (SAPT) based on density-functional theory (DFT) description of monomers [SAPT(DFT)]. For clusters containing up to a dozen or so atoms, interaction energies computed using SAPT(DFT) are comparable in accuracy to the results of the best wave function-based methods, whereas the former approach can be applied to systems an order of magnitude larger than the latter. In fact, for monomers with a couple dozen atoms, SAPT(DFT) is about equally time-consuming as the supermolecular DFT approach. To develop a force field, SAPT(DFT) calculations are performed for a large number of dimer and possibly also trimer configurations (grid points in intermolecular coordinates), and the interaction energies are then fitted by analytic functions. The resulting force fields can be used to determine crystal structures and properties by applying them in molecular packing, lattice energy minimization, and molecular dynamics calculations. In this way, some of the first successful determinations of crystal structures were achieved from first principles, with crystal densities and lattice parameters agreeing with experimental values to within about 1%. Crystal properties obtained using similar procedures but empirical force fields fitted to crystal data have typical errors of several percent due to low sensitivity of empirical fits to interactions beyond those of the nearest neighbors. The first-principles approach has additional advantages over the empirical approach for notional crystals and cocrystals since empirical force fields can only be

  3. First principle molecular dynamics simulation of hydrous modal basalt melt structure

    NASA Astrophysics Data System (ADS)

    Karki, B. B.; Bajgain, S. K.

    2012-12-01

    We have performed the first principle molecular dynamics simulation of hydrous model basalt to investigate its structural properties over wide ranges of pressure (0-100 GPa) and temperature (2200-6000 K) ranges. Our initial results show that all partial radial distribution functions represent well-defined peaks with decreased amplitudes compared to those in pure basalt liquid. The mean Si-O (Al-O) coordination number at the reference volume of 3422.47 Å3 is ~ 3.9 (4.8) at the ambient pressure and 3000 K. The coordination increases with increasing pressure but varies only a little with temperature though the abundances of various coordination species are highly sensitive to both pressure and temperature. We find that isolated structures of water component dominate in lower pressure, which consists of hydroxyl, water molecule, O-H-O bridging, and four-atom (O-H-O-H and H-O-H-O) groups. At higher pressures, extended structures (five or more O and H atoms) are formed. The effects of water on melt depolymerization, compressibility and dynamical properties will also be examined.

  4. First-principles material modeling of solid-state electrolytes with the spinel structure.

    PubMed

    Mees, Maarten J; Pourtois, Geoffrey; Rosciano, Fabio; Put, Brecht; Vereecken, Philippe M; Stesmans, André

    2014-03-21

    Ionic diffusion through the novel (AlxMg1-2xLix)Al2O4 spinel electrolyte is investigated using first-principles calculations, combined with the Kinetic Monte Carlo algorithm. We observe that the ionic diffusion increases with the lithium content x. Furthermore, the structural parameters, formation enthalpies and electronic structures of (AlxMg1-2xLix)Al2O4 are calculated for various stoichiometries. The overall results indicate the (AlxMg1-2xLix)Al2O4 stoichiometries x = 0.2…0.3 as most promising. The (AlxMg1-2xLix)Al2O4 electrolyte is a potential candidate for the all-spinel solid-state battery stack, with the material epitaxially grown between well-known spinel electrodes, such as LiyMn2O4 and Li4+3yTi5O12 (y = 0…1). Due to their identical crystal structure, a good electrolyte-electrode interface is expected. PMID:24503944

  5. First principles investigation of the structure, elasticity, and vibrational property of the serpentine minerals. (Invited)

    NASA Astrophysics Data System (ADS)

    Tsuchiya, J.; Tsuchiya, T.

    2011-12-01

    Serpentine is formed by reaction between peridotite and water which is released from hydrous mineral in subducting slab under pressure. Partially serpentinized peridotite may be a significant reservoir for water in the subducted cold slab and is considered to play an important role in subduction zone processes such as generation of arc magmatism. Precise determination of structure, vibrational and elastic properties of serpentine become the basis for understanding the transporting processes of water into deep Earth interior. Here we investigate by first principles calculation, the detailed structures, vibrational and elastic properties of lizardite, chlorite, and antigorite which are major hydrous minerals in the serpentinized peridotite. We found a very sudden softening of the elastic constants at high pressure condition. This anomaly is associated with a slight change in the compressibility of the c axis which corresponds to the layer normal direction. The calculated OH stretching frequencies also increase suddenly associated with the anomaly and these vibrational behaviors are consistent with the previous Raman measurements. Since other hydrous phyllosilicates such as clay minerals, and mica have similar crystal structures to these hydrous minerals, these anomalous softening is also expected in these minerals under pressure. Research supported in part by special coordination funds for promoting science and technology (Supporting Young Researchers with Fixed-term Appointments) and Grants-In-Aid for Scientific Research from the Japan Society for the Promotion of Science (Nos. 21740380, 20103005, and 24740357).

  6. Design of BAs-AlN monolayered honeycomb heterojunction structures: A first-principles study

    NASA Astrophysics Data System (ADS)

    Camacho-Mojica, Dulce C.; López-Urías, Florentino

    2016-04-01

    BAs and AlN are semiconductor materials with an indirect and direct gap respectively in the bulk phase. Recently, electronic calculations have demonstrated that a single-layer or few layers of BAs and AlN exhibit a graphite-like structure with interesting electronic properties. In this work, infinite sheets single-layer heterojunction structures based on alternated strips with honeycomb BAs and AlN layers are investigated using first-principles density functional theory calculations. Optimized geometries, density of states, band-gaps, formation energies, and wave functions are studied for different strip widths joined along zigzag and armchair edges. Results in optimized heterojunction geometries revealed that BAs narrow strips exhibit a corrugation effect due to a lattice mismatch. It was found that zigzag heterojunctions are more energetically favored than armchair heterojunctions. Furthermore, the formation energy presents a maximum at the point where the heterojunction becomes a planar structure. Electronic charge density results yielded a more ionic behavior in Alsbnd N bonds than the Bsbnd As bonds in accordance with monolayer results. It was observed that the conduction band minimum for both heterojunctions exhibit confined states located mainly at the entire AlN strips whereas the valence band maximum exhibits confined states located mainly at BAs strips. We expect that the present investigation will motivate more experimental and theoretical studies on new layered materials made of III-V semiconductors.

  7. First-principles investigation of the structural changes in Li-rich cathode composites

    NASA Astrophysics Data System (ADS)

    Dixit, Hemant; Zhou, Wu; Nanda, Jagjit; Idrobo, Juan-Carlos; Cooper, Valentino; Materials Science and Technology Division Collaboration

    2014-03-01

    Lithium ion batteries have high energy densities and are widely used in consumer electronics. However, it is essential to improve their power rate and cycle life for long-term usage. Cathode materials containing Li-excess layered oxide compounds, xLi2MnO3(1- x)LiMO2, (where M=Mn, Co, Ni and x= 0.2-0.7) have two times higher capacities than the conventional cathode material but during cycling a decrease in energy density and a concomitant development of a low voltage plateau are often observed. Furthermore, recent experimental studies have observed the formation and clustering of the anti-site defects near the surface. Thus a detailed understanding of the structural changes at the atomic scale of these Li-rich composites is essential to establish the correlation between the structural and electrochemical property. We present first-principles density functional theory study of the structural and electronic properties in Li-rich cathode composites. These cathode composites are modelled as solid solutions of the LiMnO2 (R 3 m) and Li2MnO3 (C2m) phases. We discuss the stability of the proposed model, the diffusion energy barriers of Li+ ions calculated using nudged-elastic band method and the formation energies of the antisite defects.

  8. First principles calculations of interlayer exchange coupling in bcc Fe/Cu/Fe structures

    SciTech Connect

    Kowalewski, M.; Heninrich, B.; Schulthess, T.C.; Butler, W.H.

    1998-01-01

    The authors report on theoretical calculations of interlayer exchange coupling between two Fe layers separated by a modified Cu spacer. These calculations were motivated by experimental investigations of similar structures by the SFU group. The multilayer structures of interest have the general form: Fe/Cu(k)/Fe and Fe/Cu(m)/X(1)/Cu(n)/Fe where X indicates one AL (atomic layer) of foreign atoms X (Cr, Ag, or Fe) and k, m, n represent the number of atomic layers of Cu. The purpose of the experimental and theoretical work was to determine the effect of modifying the pure Cu spacer by replacing the central Cu atomic layer with the atomic layer of foreign atoms X. The first principles calculation were performed using the Layer Korringa-Kohn-Rostoker (LKKR) method. The theoretical thickness dependence of the exchange coupling between two semi-infinite Fe layers was calculated for pure Cu spacer thicknesses in the range of 0 < k < 16. The effect of the foreign atoms X on the exchange coupling was investigated using the structure with 9 AL Cu spacer as a reference sample. The calculated changes in the exchange coupling are in qualitative agreement with experiment.

  9. First principles investigations of electronic structure and transport properties of graphitic structures and single molecular junctions

    NASA Astrophysics Data System (ADS)

    Owens, Jonathan R.

    In this work, we first present two powerful methods for understanding the electronic, structural, conducting, and energetic properties of nano-materials: density functional theory (DFT) and quantum transport. The basics of the theory and background of both methods are discussed thoroughly. After establishing a firm foundation, we turn our attention to using these tools to solve practical problems, often in collaboration with experimental colleagues. The first two projects pertain to nitrogen doping in graphene nanoribbons (GNRs). We study nitrogen doping in two different schema: concentration-based (N_x-doped) and structural based (N_2. {AA}-doped). Concentration based doping is explored in the context of experimental measurements of IV curves on GNRs with differing dopant concentrations. These results show a shift towards semi-conducting behavior with an increase in dopant concentration. We combine first principles calculations (DFT) and transport calculations in the Landauer formalism to compute the density-of-states (DOS) and transport curves for various dopant concentrations (0.46%, 1.39%, 1.89%, and 2.31%), which corroborate the experimental observations. The N_2. {AA}-doped GNR study was inspired by experimental observation of an atomically precise nitrogen doping scheme in bulk graphene. Experimental STM images, combined with simulated STM images, revealed that the majority (80%) of doping sites consist of nitrogen atoms on neighboring sites of the same sublattice (A) in graphene, hence N_2. {AA} doping. We examine this doping scheme applied to zigzag and armchair GNRs under different orientations of the dopants. We present spin-resolved charge densities, energetics, transport, DOS, and simulated STM images for all four systems studied. Our results show the possibility of spin-filtered devices and the STM images provide an aid in helping experimentalist identify the dopant patterns, if these GNRs are fabricated. We next venture to explain different observed

  10. Prediction of new high pressure structural sequence in thorium carbide: A first principles study

    SciTech Connect

    Sahoo, B. D. Joshi, K. D.; Gupta, Satish C.

    2015-05-14

    In the present work, we report the detailed electronic band structure calculations on thorium monocarbide. The comparison of enthalpies, derived for various phases using evolutionary structure search method in conjunction with first principles total energy calculations at several hydrostatic compressions, yielded a high pressure structural sequence of NaCl type (B1) → Pnma → Cmcm → CsCl type (B2) at hydrostatic pressures of ∼19 GPa, 36 GPa, and 200 GPa, respectively. However, the two high pressure experimental studies by Gerward et al. [J. Appl. Crystallogr. 19, 308 (1986); J. Less-Common Met. 161, L11 (1990)] one up to 36 GPa and other up to 50 GPa, on substoichiometric thorium carbide samples with carbon deficiency of ∼20%, do not report any structural transition. The discrepancy between theory and experiment could be due to the non-stoichiometry of thorium carbide samples used in the experiment. Further, in order to substantiate the results of our static lattice calculations, we have determined the phonon dispersion relations for these structures from lattice dynamic calculations. The theoretically calculated phonon spectrum reveal that the B1 phase fails dynamically at ∼33.8 GPa whereas the Pnma phase appears as dynamically stable structure around the B1 to Pnma transition pressure. Similarly, the Cmcm structure also displays dynamic stability in the regime of its structural stability. The B2 phase becomes dynamically stable much below the Cmcm to B2 transition pressure. Additionally, we have derived various thermophysical properties such as zero pressure equilibrium volume, bulk modulus, its pressure derivative, Debye temperature, thermal expansion coefficient and Gruneisen parameter at 300 K and compared these with available experimental data. Further, the behavior of zero pressure bulk modulus, heat capacity and Helmholtz free energy has been examined as a function temperature and compared with the experimental data of Danan [J

  11. Unusual structural and electronic properties of porous silicene and germanene: insights from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Ding, Yi; Wang, Yanli

    2015-01-01

    Using first-principles calculations, we investigate the geometric structures and electronic properties of porous silicene and germanene nanosheets, which are the Si and Ge analogues of α-graphyne (referred to as silicyne and germanyne). It is found that the elemental silicyne and germanyne sheets are energetically unfavourable. However, after the C-substitution, the hybrid graphyne-like sheets (c-silicyne/c-germanyne) possess robust energetic and dynamical stabilities. Different from silicene and germanene, c-silicyne is a flat sheet, and c-germanyne is buckled with a distinct half-hilled conformation. Such asymmetric buckling structure causes the semiconducting behaviour into c-germanyne. While in c-silicyne, the semimetallic Dirac-like property is kept at the nonmagnetic state, but a spontaneous antiferromagnetism produces the massive Dirac fermions and opens a sizeable gap between Dirac cones. A tensile strain can further enhance the antiferromagnetism, which also linearly modulates the gap value without altering the direct-bandgap feature. Through strain engineering, c-silicyne can form a type-II band alignment with the MoS 2 sheet. The combined c-silicyne/MoS 2 nanostructure has a high power conversion efficiency beyond 20% for photovoltaic solar cells, enabling a fascinating utilization in the fields of solar energy and nano-devices.

  12. Unusual structural and electronic properties of porous silicene and germanene: insights from first-principles calculations.

    PubMed

    Ding, Yi; Wang, Yanli

    2015-01-01

    Using first-principles calculations, we investigate the geometric structures and electronic properties of porous silicene and germanene nanosheets, which are the Si and Ge analogues of α-graphyne (referred to as silicyne and germanyne). It is found that the elemental silicyne and germanyne sheets are energetically unfavourable. However, after the C-substitution, the hybrid graphyne-like sheets (c-silicyne/c-germanyne) possess robust energetic and dynamical stabilities. Different from silicene and germanene, c-silicyne is a flat sheet, and c-germanyne is buckled with a distinct half-hilled conformation. Such asymmetric buckling structure causes the semiconducting behaviour into c-germanyne. While in c-silicyne, the semimetallic Dirac-like property is kept at the nonmagnetic state, but a spontaneous antiferromagnetism produces the massive Dirac fermions and opens a sizeable gap between Dirac cones. A tensile strain can further enhance the antiferromagnetism, which also linearly modulates the gap value without altering the direct-bandgap feature. Through strain engineering, c-silicyne can form a type-II band alignment with the MoS 2 sheet. The combined c-silicyne/MoS 2 nanostructure has a high power conversion efficiency beyond 20% for photovoltaic solar cells, enabling a fascinating utilization in the fields of solar energy and nano-devices. PMID:25852311

  13. First-principles study of the crystal and electronic structures of {alpha}-tetragonal boron

    SciTech Connect

    Hayami, Wataru; Otani, Shigeki

    2010-07-15

    The crystal and electronic structures of {alpha}-tetragonal ({alpha}-t) boron were investigated by first-principles calculation. Application of a simple model assuming 50 atoms in the unit cell indicated that {alpha}-t boron had a metallic density of state, thus contradicting the experimental fact that it is a p-type semiconductor. The presence of an additional two interstitial boron atoms at the 4c site made {alpha}-t boron semiconductive and the most stable. The cohesive energy per atom was as high as those of {alpha}- and {beta}-rhombohedral boron, suggesting that {alpha}-t boron is produced more easily than was previously thought. The experimentally obtained {alpha}-t boron in nanobelt form had about two interstitial atoms at the 8i sites. We consider that the shallow potential at 8i sites generates low-energy phonon modes, which increase the entropy and consequently decrease the free energy at high temperatures. Calculation of the electronic band structure showed that the highest valence band had a larger dispersion from {Gamma} to Z than from {Gamma} to X; this indicated a strong anisotropy in hole conduction. - Graphical abstract: Calculated electron densities of B{sub 50} and B{sub 50}+2B at site 4c (configuration B).

  14. New crystal structure and physical properties of TcB from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Zhang, Gang-Tai; Bai, Ting-Ting; Yan, Hai-Yan; Zhao, Ya-Ru

    2015-10-01

    By combining first-principles calculations with the particle swarm optimization algorithm, we predicted a hexagonal structure for TcB, which is energetically more favorable than the previously reported WC-type and Cmcm structures. The new phase is mechanically and dynamically stable, as confirmed by its phonon and elastic constants calculations. The calculated mechanical properties show that it is an ultra-incompressible and hard material. Meanwhile, the elastic anisotropy is investigated by the shear anisotropic factors and ratio of the directional bulk modulus. Density of states analysis reveals that the strong covalent bonding between Tc and B atoms plays a leading role in forming a hard material. Additionally, the compressibility, bulk modulus, Debye temperature, Grüneisen parameter, specific heat, and thermal expansion coefficient of TcB are also successfully obtained by using the quasi-harmonic Debye model. Project supported by the Science Foundation of Baoji University of Arts and Sciences of China (Grant No. ZK11061) and the Natural Science Foundation of the Education Committee of Shaanxi Province, China (Grant Nos. 2013JK0637, 2013JK0638, and 2014JK1044).

  15. Elastic stability and electronic structure of tantalum boride investigated via first-principles density functional calculations

    NASA Astrophysics Data System (ADS)

    Chen, Hai-Hua; Bi, Yan; Cheng, Yan; Ji, Guangfu; Cai, Lingcang

    2012-10-01

    The elastic properties, electronic structure and thermodynamic behavior of the TaB have been investigated for the first time in this work. Using first-principles plane-wave ultrasoft-pseudopotential density functional theory (DFT), the ground state properties and equation of state of TaB have been obtained. The average zero-pressure bulk modulus of TaB is 302 GPa. By analyzing the elastically anisotropic behavior and the relative structure parameters of TaB, we found that the crystal cell along the b-axis was more compressible than along the a and c axes. The calculated ratio of bulk modulus and shear modulus (B/G) for TaB is 1.58, demonstrating that TaB is rather brittle. From the elastic stiffness constants, we found that TaB in the Cmcm phase is mechanically stable. The calculated hardness of TaB is 28.6 GPa which is close to the previous data. Moreover, using the Gibbs 2 model, the thermodynamic properties such as the thermal expansion and Debye temperature of TaB have been obtained firstly. At the ambient temperature, the Debye temperatures of TaB are 792 K and 845 K from GGA calculation and LDA calculation, respectively.

  16. Defects in ion-implanted hcp-titanium: A first-principles study of electronic structures

    NASA Astrophysics Data System (ADS)

    Raji, Abdulrafiu T.; Mazzarello, Riccardo; Scandolo, Sandro; Nsengiyumva, Schadrack; Härting, Margit; Britton, David T.

    2011-12-01

    The electronic structures of hexagonal closed-packed (h.c.p) titanium containing a vacancy and krypton impurity atoms at various insertion sites are calculated by first-principles methods in the framework of the density-functional theory (DFT). The density of states (DOS) for titanium containing a vacancy defect shows resonance-like features. Also, the bulk electron density decreases from ˜0.15/Å 3 to ˜0.05/Å 3 at the vacancy centre. Electronic structure calculations have been performed to investigate what underlies the krypton site preference in titanium. The DOS of the nearest-neighbour (NN) titanium atoms to the octahedral krypton appears to be less distorted (relative to pure titanium) when compared to the NN titanium atoms to the tetrahedral krypton. The electronic density deformation maps show that polarization of the titanium atoms is stronger when the krypton atom is located at the tetrahedral site. Since krypton is a closed-shell atom, thus precluding any bonding with the titanium atoms, we may conclude that the polarization of the electrons in the vicinity of the inserted krypton atoms and the distortion of the DOS of the NN titanium atoms to the krypton serve to indicate which defect site is preferred when a krypton atom is inserted into titanium. Based on these considerations, we conclude that the substitutional site is the most favourable one, and the octahedral is the preferred interstitial site, in agreement with recent DFT calculations of the energetics of krypton impurity sites.

  17. A first-principle study of Os-based compounds: Electronic structure and vibrational properties

    NASA Astrophysics Data System (ADS)

    Arıkan, N.; Örnek, O.; Charifi, Z.; Baaziz, H.; Uğur, Ş.; Uğur, G.

    2016-09-01

    The electronic structure, elastic, and phonon properties of OsM (M=Hf, Ti, Y and Zr) compounds are studied using first-principles calculations. Elastic constants of OsY and specific heat capacity of OsM (M=Hf, Ti, Y, and Zr) are reported for the first time. The predicted equilibrium lattice constants are in excellent agreement with experiment. The calculated values of bulk moduli are considerably high but are much smaller than that of Osmium, which is around 400 GPa. The phase stability of the OsM (M=Hf, Ti, Y and Zr) compounds were studied by DOS calculations and the results suggest that OsY is unstable in the B2 phase. The brittleness and ductility properties of OsM (M=Hf, Ti, Y and Zr) are determined. OsM (M=Hf, Ti, Y and Zr) compounds are predicted to be ductile materials. The electronic structure and phonon frequency curves of OsM (M=Hf, Ti, Y and Zr) compounds are obtained. The position of Fermi level of these systems was calculated and discussed in terms of the pseudo gaps. The finite and small DOS at the Fermi level 0.335, 0.375, 1.063, and 0.383 electrons/eV for OsHf, OsTi, OsY, and OsZr, respectively, suggest that OsM (M=Hf, Ti, Y and Zr) compounds are weak metals.

  18. First-principles study of electronic structures and phase transitions of lithiated molybdenum disulphide

    NASA Astrophysics Data System (ADS)

    Li, Jun; Chen, Xiaobo

    2012-02-01

    By first-principles calculations, electronic structures of MoS2, intercalation-induced 2H to 1T phase transition and reversibility are investigated. It is revealed that change of interlayer stacking from 2H to 3R imposes negligible influence on the band structure and stability of MoS2. In contrast, the change of intralayer stacking from 2H to 1T changes the character of p-d repulsion, resulting in a semiconductor-to-metal transition. We demonstrate that the Kohn-Sham band energy, rather than the coulomb repulsion energy, plays dominant roles in both the phase stabilization and transition during Li intercalation. It is found that the evolution of 1T phase is crucially determined by chemical hardness, which underlies the cycle irreversibility. Due to the charge-density-wave (CDW) phase, Li extraction is impeded by the enhancement of Li-host binding. It is indicated that the cycle reversibility can be improved by electron-donor doping in MoS2, because the resultant pre-reduction of Mo and S eliminates the electron transfer from Li to host.

  19. First-principles investigations of elastic stability and electronic structure of cubic platinum carbide under pressure

    NASA Astrophysics Data System (ADS)

    Sun, Xiao-Wei; Chen, Qi-Feng; Chen, Xiang-Rong; Cai, Ling-Cang; Jing, Fu-Qian

    2011-11-01

    The authors have presented a detailed investigation on the phase stabilities and electronic properties of ideal stoichiometric platinum carbide (PtC) in the rock-salt (RS) and zinc-blende (ZB) structures under high pressure. Theoretical calculations are performed using the first-principles pseudopotential density functional method, in which we employ the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof form and local density approximation (LDA) of Ceperly and Adler parameterized by Perdew and Zunger together with plane-wave basis sets for expanding the periodic electron density. Through a series of tests, such as the total energy as a function of volume, the Gibbs free energy as a function of pressure, the P-V equation of states, the elastic stabilities, and the electronic band structures of PtC with ZB and RS phases, we have confirmed that the recently synthesized compound PtC is crystallized in the ZB structure at zero pressure and that the RS structure is a high-pressure phase; the phase transition studied from the usual condition of equal enthalpies occurs at the pressures of 46.6 and 46.5 GPa for GGA and LDA calculations, respectively. Our conclusions are consistent with the theoretical prediction obtained from the full-potential linearized augmented plane-wave method, but are reversed with the DAC experimental results and other pseudopotential plane-wave theoretical results. Therefore, the experimental observation of the RS structure in PtC remains a puzzle, and our study indicates that further experimental and theoretical investigations need to be carried out to find the cause of the stability of the PtC.

  20. Hydration thermodynamics of pyrochlore structured oxides from TG and first principles calculations.

    PubMed

    Bjørheim, Tor S; Besikiotis, Vasileios; Haugsrud, Reidar

    2012-11-21

    In this contribution we investigate trends in the defect chemistry and hydration thermodynamics of rare-earth pyrochlore structured oxides, RE(2)X(2)O(7) (RE = La-Lu and X = Ti, Sn, Zr and Ce). First principles density functional theory (DFT) calculations have been performed to elucidate trends in the general defect chemistry and hydration enthalpy for the above-mentioned series. Further, to justify the use of such theoretical methods, the hydration properties of selected compositions were studied by means of thermogravimetric measurements. Both DFT calculations and TG measurements indicate that the hydration enthalpy becomes less exothermic with decreasing radii of RE ions within the RE(2)X(2)O(7) series (X = Ti, Sn, Zr and Ce), while it is less dependent on the X site ion. The observed hydration trends are discussed in connection with trends in the stability of both protons and oxygen vacancies and changes in the electronic density of states and bonding environment through the series. Finally, the findings are discussed with respect to existing correlations for other binary and ternary oxides. PMID:23001186

  1. First Principles Calculations of the Electronic Structure of ZrN Allotropes

    NASA Astrophysics Data System (ADS)

    Yin, Li-Chang; Saito, Riichiro

    2011-11-01

    The atomic structures and electronic properties of different ZrN allotropes, including face-centered cubic ZrN (B1 ZrN), hypothetic wurtzite (w) ZrN, and hypothetic two-dimensional (2D) and three-dimensional (3D) layered hexagonal (h) ZrN, are investigated by systematic first-principles calculations. Although the cohesive energy calculation indicates that the B1 ZrN is more stable than the hypothetic w-ZrN and h-ZrN, we suggest that the monolayer h-ZrN may be stable on some substrates. Charge population analysis shows that the polar, covalent bonding character appears between N atoms and Zr atoms for all ZrN allotropes involved in this paper. A Van Hove singularity (VHS) with a high density of states (DOS) locating at 0.2 eV above the Fermi level appears for monolayer h-ZrN, which results from a saddle point of the partially occupied Zr-dz^{2 energy bands due to lack of interlayer interaction. Such a VHS observed in the monolayer h-ZrN indicates that this hypothetic monolayer material might be a potential candidate for new superconducting material by electron doping.

  2. Substrate-induced structures of bismuth adsorption on graphene: a first principles study.

    PubMed

    Lin, Shih-Yang; Chang, Shen-Lin; Chen, Hsin-Hsien; Su, Shu-Hsuan; Huang, Jung-Chun; Lin, Ming-Fa

    2016-07-28

    The geometric and electronic properties of Bi-adsorbed monolayer graphene, enriched by the strong effect of a substrate, are investigated by first-principles calculations. The six-layered substrate, corrugated buffer layer, and slightly deformed monolayer graphene are all simulated. Adatom arrangements are thoroughly studied by analyzing the ground-state energies, bismuth adsorption energies, and Bi-Bi interaction energies of different adatom heights, inter-adatom distance, adsorption sites, and hexagonal positions. A hexagonal array of Bi atoms is dominated by the interactions between the buffer layer and the monolayer graphene. An increase in temperature can overcome a ∼50 meV energy barrier and induce triangular and rectangular nanoclusters. The most stable and metastable structures agree with the scanning tunneling microscopy measurements. The density of states exhibits a finite value at the Fermi level, a dip at ∼-0.2 eV, and a peak at ∼-0.6 eV, as observed in the experimental measurements of the tunneling conductance. PMID:27354143

  3. Structure of hydrophobic hydration of benzene and hexafluorobenzene from first principles

    SciTech Connect

    Allesch, M; Schwegler, E; Galli, G

    2006-10-23

    We report on the aqueous hydration of benzene and hexafluorobenzene, as obtained by carrying out extensive (>100 ps) first principles molecular dynamics simulations. Our results show that benzene and hexafluorobenzene do not behave as ordinary hydrophobic solutes, but rather present two distinct regions, one equatorial and the other axial, that exhibit different solvation properties. While in both cases the equatorial regions behave as typical hydrophobic solutes, the solvation properties of the axial regions depend strongly on the nature of the {pi}-water interaction. In particular, {pi}-hydrogen and {pi}-lone pair interactions are found to dominate in benzene and hexafluorobenzene, respectively, which leads to substantially different orientations of water near the two solutes. We present atomic and electronic structure results (in terms of Maximally Localized Wannier Functions) providing a microscopic description of benzene- and hexafluorobenzene-water interfaces, as well as a comparative study of the two solutes. Our results point at the importance of an accurate description of interfacial water in order to characterize hydration properties of apolar molecules, as these are strongly influenced by subtle charge rearrangements and dipole moment redistributions in interfacial regions.

  4. First-principle studies of electronic structure and magnetic excitations in FeSe monolayer

    NASA Astrophysics Data System (ADS)

    Bazhirov, Timur; Cohen, Marvin L.

    2013-03-01

    Recent experimental advances made it possible to study single-layered superconducting systems of iron-based compounds. The results show evidence of significant enhancement of superconducting properties compared to the bulk case. We use first-principle pseudopotential density functional theory techniques and the local spin-density approximation to study the electronic properties of an FeSe monolayer in different spin configurations. The results show that the experimental shape of the Fermi surface is best described by a checkerboard antiferromagnetic (AFM) spin arrangement. To explore the underlying pairing mechanism, we study the evolution of the non-magnetic to the AFM-ordered structures under constrained magnetization, and we estimate the electronic coupling to magnetic excitations involving transfer and increase of iron magnetic moments and compare it to the electron-phonon coupling. Finally, we simulate the substrate-induced interaction by using uniform charge doping and show that the latter can lead to an increase in the density of states at the Fermi level and possibly produce higher superconducting transition temperatures. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at Lawrence Berkeley National Laboratory's NERSC facility

  5. Electronic, structural, and elastic properties of metal nitrides XN (X = Sc, Y): A first principle study

    NASA Astrophysics Data System (ADS)

    Ekuma, Chinedu E.; Bagayoko, Diola; Jarrell, Mark; Moreno, Juana

    2012-09-01

    We utilized a simple, robust, first principle method, based on basis set optimization with the BZW-EF method, to study the electronic and related properties of transition metal mono-nitrides: ScN and YN. We solved the KS system of equations self-consistently within the linear combination of atomic orbitals (LCAO) formalism. It is shown that the band gap and low energy conduction bands, as well as elastic and structural properties, can be calculated with a reasonable accuracy when the LCAO formalism is used to obtain an optimal basis. Our calculated, indirect electronic band gap (E^Γ -X_g) is 0.79 (LDA) and 0.88 eV (GGA) for ScN. In the case of YN, we predict an indirect band gap (E^Γ -X_g) of 1.09 (LDA) and 1.15 eV (GGA). We also calculated the equilibrium lattice constants, the bulk moduli (Bo), effective masses, and elastic constants for both systems. Our calculated values are in excellent agreement with experimental ones where the latter are available.

  6. FIRST-PRINCIPLES APPROACHES TO THE STRUCTURE AND REACTIVITY OF ATMOSPHERICALLY RELEVANT AQUESOUS INTERFACES

    SciTech Connect

    Mundy, C; Kuo, I W

    2005-06-08

    successfully applied to studying the complex problems put forth by atmospheric chemists. To date, the majority of the molecular models of atmospherically relevant interfaces have been comprised of two genres of molecular models. The first is based on empirical interaction potentials. The use of an empirical interaction potential suffers from at least two shortcomings. First, empirical potentials are usually fit to reproduce bulk thermodynamic states, or gas phase spectroscopic data. Thus, without the explicit inclusion of charge transfer, it is not at all obvious that empirical potentials can faithfully reproduce the structure at a solid-vapor, or liquid-vapor interface where charge rearrangement is known to occur (see section 5). One solution is the empirical inclusion of polarization effects. These models are certainly an improvement, but still cannot offer insight into charge transfer processes and are usually difficult to parameterize. The other shortcoming of empirical models is that, in general, they cannot describe bond-making/breaking events, i.e. chemistry. In order to address chemistry one has to consider an ab initio (to be referred to as first-principles throughout the remaining text) approach to molecular modeling that explicitly treats the electronic degrees of freedom. First-principles modeling also give a direct link to spectroscopic data and chemistry, but at a large computational cost. The bottle-neck associated with first-principles modeling is usually determined by the level of electronic structure theory that one chooses to study a particular problem. High-level first-principles approaches, such as MP2, provide accurate representation of the electronic degrees of freedom but are only computationally tractable when applied to small system sizes (i.e. 10s of atoms). Nevertheless, this type of modeling has been extremely useful in deducing reaction mechanisms of atmospherically relevant chemistry that will be discussed in this review (see section 4). However

  7. Structure and mechanical properties of tantalum mononitride under high pressure: A first-principles study

    PubMed Central

    Chang, Jing; Zhao, Guo-Ping; Zhou, Xiao-Lin; Liu, Ke; Lu, Lai-Yu

    2012-01-01

    The structure and mechanical properties of tantalum mononitride (TaN) are investigated at high pressure from first-principles using the plane wave pseudopotential method within the local density approximation. Three stable phases were considered, i.e., two hexagonal phases (ε and θ) and a cubic δ phase. The obtained equilibrium structure parameters and ground state mechanical properties are in excellent agreement with the experimental and other theoretical results. A full elastic tensor and crystal anisotropy of the ultra-incompressible TaN in three stable phases are determined in the wide pressure range. Results indicated that the elastic properties of TaN in three phases are strongly pressure dependent. And the hexagonal θ-TaN is the most ultraincompressible among the consider phases, which suggests that the θ phase of TaN is a potential candidate structure to be one of the ultraincompressible and hard materials. By the elastic stability criteria, it is predicted that θ-TaN is not stable above 53.9 GPa. In addition, the calculated B/G ratio indicated that the ε and δ phases possess brittle nature in the range of pressure from 0 to 100 GPa. While θ phase is brittleness at low pressure (below 8.2 GPa) and is strongly prone to ductility at high pressure (above 8.2 GPa). The calculated elastic anisotropic factors for three phases of TaN suggest that they are elastically highly anisotropic and strongly dependent on the propagation direction. PMID:23185097

  8. First principle study of structural, electronic and magnetic properties of zigzag boron nitride nanoribbon: Role of vacancies

    SciTech Connect

    Kumar, Arun; Bahadur, Amar; Mishra, Madhukar; Vasudeva, Neena

    2015-05-15

    We study the effect of vacancies on the structural, electronic and magnetic properties of zigzag boron nitride nanoribbon (ZBNNR) by using first principle calculations. We find that the shift of the vacancies with respect to the ribbon edges causes change in the structural geometry, electronic structure and magnetization of ZBNNR. These vacancies also produce band gap modulation and consequently results the magnetization of ZBNNR.

  9. First-Principles Study of Lattice Dynamics, Structural Phase Transition, and Thermodynamic Properties of Barium Titanate

    NASA Astrophysics Data System (ADS)

    Zhang, Huai-Yong; Zeng, Zhao-Yi; Zhao, Ying-Qin; Lu, Qing; Cheng, Yan

    2016-08-01

    Lattice dynamics, structural phase transition, and the thermodynamic properties of barium titanate (BaTiO3) are investigated by using first-principles calculations within the density functional theory (DFT). It is found that the GGA-WC exchange-correlation functional can produce better results. The imaginary frequencies that indicate structural instability are observed for the cubic, tetragonal, and orthorhombic phases of BaTiO3 and no imaginary frequencies emerge in the rhombohedral phase. By examining the partial phonon density of states (PDOSs), we find that the main contribution to the imaginary frequencies is the distortions of the perovskite cage (Ti-O). On the basis of the site-symmetry consideration and group theory, we give the comparative phonon symmetry analysis in four phases, which is useful to analyze the role of different atomic displacements in the vibrational modes of different symmetry. The calculated optical phonon frequencies at Γ point for the four phases are in good agreement with other theoretical and experimental data. The pressure-induced phase transition of BaTiO3 among four phases and the thermodynamic properties of BaTiO3 in rhombohedral phase have been investigated within the quasi-harmonic approximation (QHA). The sequence of the pressure-induced phase transition is rhombohedral→orthorhombic→tetragonal→cubic, and the corresponding transition pressure is 5.17, 5.92, 6.65 GPa, respectively. At zero pressure, the thermal expansion coefficient αV, heat capacity CV, Grüneisen parameter γ, and bulk modulus B of the rhombohedral phase BaTiO3 are estimated from 0 K to 200 K.

  10. First Principles Investigations of Technologically and Environmentally Important Nano-structured Materials and Devices

    NASA Astrophysics Data System (ADS)

    Paul, Sujata

    In the course of my PhD I have worked on a broad range of problems using simulations from first principles: from catalysis and chemical reactions at surfaces and on nanostructures, characterization of carbon-based systems and devices, and surface and interface physics. My research activities focused on the application of ab-initio electronic structure techniques to the theoretical study of important aspects of the physics and chemistry of materials for energy and environmental applications and nano-electronic devices. A common theme of my research is the computational study of chemical reactions of environmentally important molecules (CO, CO2) using high performance simulations. In particular, my principal aim was to design novel nano-structured functional catalytic surfaces and interfaces for environmentally relevant remediation and recycling reactions, with particular attention to the management of carbon dioxide. We have studied the carbon-mediated partial sequestration and selective oxidation of carbon monoxide (CO), both in the presence and absence of hydrogen, on graphitic edges. Using first-principles calculations we have studied several reactions of CO with carbon nanostructures, where the active sites can be regenerated by the deposition of carbon decomposed from the reactant (CO) to make the reactions self-sustained. Using statistical mechanics, we have also studied the conditions under which the conversion of CO to graphene and carbon dioxide is thermodynamically favorable, both in the presence and in the absence of hydrogen. These results are a first step toward the development of processes for the carbon-mediated partial sequestration and selective oxidation of CO in a hydrogen atmosphere. We have elucidated the atomic scale mechanisms of activation and reduction of carbon dioxide on specifically designed catalytic surfaces via the rational manipulation of the surface properties that can be achieved by combining transition metal thin films on oxide

  11. First Principles Modeling of Phonon Heat Conduction in Nanoscale Crystalline Structures

    SciTech Connect

    Sandip Mazumder; Ju Li

    2010-06-30

    of optical phonons, and (2) by developing a suite of numerical algorithms for solution of the BTE for phonons. The suite of numerical algorithms includes Monte Carlo techniques and deterministic techniques based on the Discrete Ordinates Method and the Ballistic-Diffusive approximation of the BTE. These methods were applied to calculation of thermal conductivity of silicon thin films, and to simulate heat conduction in multi-dimensional structures. In addition, thermal transport in silicon nanowires was investigated using two different first principles methods. One was to apply the Green-Kubo formulation to an equilibrium system. The other was to use Non-Equilibrium Molecular Dynamics (NEMD). Results of MD simulations showed that the nanowire cross-sectional shape and size significantly affects the thermal conductivity, as has been found experimentally. In summary, the project clarified the role of various phonon modes - in particular, optical phonon - in non-equilibrium transport in silicon. It laid the foundation for the solution of the BTE in complex three-dimensional structures using deterministic techniques, paving the way for the development of robust numerical tools that could be coupled to existing device simulation tools to enable coupled electro-thermal modeling of practical electronic/optoelectronic devices. Finally, it shed light on why the thermal conductivity of silicon nanowires is so sensitive to its cross-sectional shape.

  12. Combined First Principles Electronic Structure Calculations and Thermodynamic Study of Binary Alloys

    NASA Astrophysics Data System (ADS)

    Guo, Xiaoqing

    entropy and thus, in principle, all thermodynamic quantities. Illustrative results for the Al-Li alloys show: (i) structural properties versus concentration in very good agreement with experiment and (ii) features on the Al-rich side of the phase diagram of the fcc solid solution which are important for alloy formation. This thermodynamic model shows promise for studying alloy phase diagrams entirely from first principles.

  13. First-Principles Study of the Geometric and Electronic Structures of Zinc Ferrite with Vacancy Defect

    NASA Astrophysics Data System (ADS)

    Yao, Jinhuan; Li, Yanwei; Li, Xuanhai; Zhu, Xiaodong

    2016-04-01

    The effects of Zn-vacancy (Zn7Fe16O32), Fe-vacancy (Zn8Fe15O32), and O-vacancy (Zn8Fe16O31) on the geometric and electronic structures of normal spinel ZnFe2O4 (Zn8Fe16O32) are studied by using a first-principles method based on density functional theory (DFT) at a generalized gradient approximation (GGA) level. Compared with perfect ZnFe2O4, the lattice parameters of ZnFe2O4 with Zn-vacancy or Fe-vacancy increase slightly, while the lattice parameters of ZnFe2O4 with O-vacancy decrease significantly. All the vacancy defects induce the distortion of the unit cell structure, especially for the O-vacancy. Zn-vacancy, Fe-vacancy, and O-vacancy in ZnFe2O4 cannot be formed spontaneously, but Zn-vacancy is the most prone to form, followed by Fe-vacancy and O-vacancy under the condition of external energy supply. Zn-vacancy, Fe-vacancy, and O-vacancy change the properties of ZnFe2O4 from a semiconducting character to a metallic character. Either ZnFe2O4 or ZnFe2O4 has various vacancy defects, the strength of the O-Zn bond is stronger than that of the O-Fe bond, and both of them have a covalent bond character. Zn-vacancy enhances the strength of O-Fe bonds and slightly weakens the strength of O-Zn bonds around Zn-vacancy. Fe-vacancy induces a significant increase of the strength of O-Fe bonds and O-Zn bonds around Fe-vacancy. O-vacancy leads to a significant decrease in the strength of O-Zn bonds and to a slight increase in the strength of O-Fe bonds around O-vacancy.

  14. First-Principles Study of the Geometric and Electronic Structures of Zinc Ferrite with Vacancy Defect

    NASA Astrophysics Data System (ADS)

    Yao, Jinhuan; Li, Yanwei; Li, Xuanhai; Zhu, Xiaodong

    2016-07-01

    The effects of Zn-vacancy (Zn7Fe16O32), Fe-vacancy (Zn8Fe15O32), and O-vacancy (Zn8Fe16O31) on the geometric and electronic structures of normal spinel ZnFe2O4 (Zn8Fe16O32) are studied by using a first-principles method based on density functional theory (DFT) at a generalized gradient approximation (GGA) level. Compared with perfect ZnFe2O4, the lattice parameters of ZnFe2O4 with Zn-vacancy or Fe-vacancy increase slightly, while the lattice parameters of ZnFe2O4 with O-vacancy decrease significantly. All the vacancy defects induce the distortion of the unit cell structure, especially for the O-vacancy. Zn-vacancy, Fe-vacancy, and O-vacancy in ZnFe2O4 cannot be formed spontaneously, but Zn-vacancy is the most prone to form, followed by Fe-vacancy and O-vacancy under the condition of external energy supply. Zn-vacancy, Fe-vacancy, and O-vacancy change the properties of ZnFe2O4 from a semiconducting character to a metallic character. Either ZnFe2O4 or ZnFe2O4 has various vacancy defects, the strength of the O-Zn bond is stronger than that of the O-Fe bond, and both of them have a covalent bond character. Zn-vacancy enhances the strength of O-Fe bonds and slightly weakens the strength of O-Zn bonds around Zn-vacancy. Fe-vacancy induces a significant increase of the strength of O-Fe bonds and O-Zn bonds around Fe-vacancy. O-vacancy leads to a significant decrease in the strength of O-Zn bonds and to a slight increase in the strength of O-Fe bonds around O-vacancy.

  15. Cesium stability in a typical mica structure in dry and wet environments from first-principles

    NASA Astrophysics Data System (ADS)

    Suehara, Shigeru; Yamada, Hirohisa

    2013-05-01

    Cesium ion stability in a typical mica structure in various environments of solid salts (XCl; X = Cs, K, and Na), metals (X) and saltwaters (XCl aqueous liquids) was investigated using first-principles density-functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE) functional as well as a van der Waals (vdW) corrected functional (vdW-DFC09x). We specifically examined interlayer ion-exchange in bulk phlogopite-type mica, which is expected to produce a well-defined benchmark in a thermodynamic equilibrium state. In general, theoretical models have well reproduced the experimental and theoretical data found in the literature from the viewpoints of structure, heat capacity, and entropy. The vdW-DFC09x lattice parameters of the mica appear to be better reproducible than the PBE parameters are. However, the vdW correction calculations of the thermodynamic properties with the harmonic approximation using the phonon frequencies showed poor results in some cases, whereas the PBE calculations yielded robust and reasonable results in terms of structure and thermodynamic properties. The isotope effect of the 137Cs atom appears to be confined in thermodynamic properties such as entropy, heat capacity, and ion-exchange energy, although the theoretical infrared spectra showed a small redshift ca. 1 cm-1 in the far-infrared region of 50-75 cm-1. The calculated RDF and the coordination number for X-O (i.e., X-H2O) for the saltwater model indicated that the Cs, K, and Na ions with respective hydrated radii of 0.323, 0.284, and 0.238 nm were surrounded, respectively, by 6.5, 4.5, and 4.0 of H2O molecules in a water solution. Ion-exchange energy values based on free-energy calculations around ambient temperatures derived using the PBE functional and a harmonic approximation suggest that the cesium ion in mica interlayer phlogopite is stable in an environment consisting of KCl, NaCl, K, and Na solids, and in NaCl saltwater as well. However, it can be exchanged competitively by

  16. Tuning the band structures of single walled silicon carbide nanotubes with uniaxial strain: a first principles study

    SciTech Connect

    Wang, Zhiguo; Zu, Xiaotao T.; Xiao, H. Y.; Gao, Fei; Weber, William J.

    2008-05-09

    Electronic band structures of single-walled silicon carbide nanotubes are studied under uniaxial strain using first principles calculations. The band structure can be tuned by mechanical strain in a wide energy range. The band gap decreases with uniaxial tensile strain, but initially increases with uniaxial compressive strain and then decreases with further increases in compressive strain. These results may provide a way to tune the electronic structures of silicon carbide nanotubes, which may have promising applications in building nanodevices.

  17. First principles predictions of van der Waals bonded inorganic crystal structures: Test case, HgCl2

    SciTech Connect

    Cooper, Valentino R; Donald, Kelling J

    2015-01-01

    We study the crystals structure and stability of four possible polymorphs of HgCl2 using first principles density functional theory. Mercury (II) halides are a unique class of materials which, depending on the halide species, form in a wide range of crystal structures, ranging from densely packed solids to layered materials and molecular solids. Predicting the groundstate structure of any member of this group from first principles, therefore, requires a general purpose functional that treats van der Waals bonding and covalent/ionic bonding adequately. Here, we demonstrate that the non-local van der Waals density functional paired with the C09 exchange functional meets this bar for HgCl2. In particular, this functional is able to predict the correct groundstate among the structures tested as well as having extremely good agreement with the experimentally known crystal structure. These results highlight the maturity of this functional and open the door to using this method for truly first principles crystal structure predictions.

  18. Experimental and first principle studies on electronic structure of BaTiO{sub 3}

    SciTech Connect

    Sagdeo, Archna Ghosh, Haranath Chakrabarti, Aparna Kamal, C. Ganguli, Tapas Deb, S. K.; Phase, D. M.

    2014-04-24

    We have carried out photoemission experiments to obtain valence band spectra of various crystallographic symmetries of BaTiO{sub 3} system which arise as a function of temperature. We also present results of a detailed first principle study of these symmetries of BaTiO{sub 3} using generalized gradient approximation for the exchange-correlation potential. Here we present theoretical results of density of states obtained from DFT based simulations to compare with the experimental valence band spectra. Further, we also perform calculations using post density functional approaches like GGA + U method as well as non-local hybrid exchange-correlation potentials like PBE0, B3LYP, HSE in order to understand the extent of effect of correlation on band gaps of different available crystallographic symmetries (5 in number) of BaTiO{sub 3}.

  19. Experimental and first principle studies on electronic structure of BaTiO3

    NASA Astrophysics Data System (ADS)

    Sagdeo, Archna; Ghosh, Haranath; Chakrabarti, Aparna; Kamal, C.; Ganguli, Tapas; Phase, D. M.; Deb, S. K.

    2014-04-01

    We have carried out photoemission experiments to obtain valence band spectra of various crystallographic symmetries of BaTiO3 system which arise as a function of temperature. We also present results of a detailed first principle study of these symmetries of BaTiO3 using generalized gradient approximation for the exchange-correlation potential. Here we present theoretical results of density of states obtained from DFT based simulations to compare with the experimental valence band spectra. Further, we also perform calculations using post density functional approaches like GGA + U method as well as non-local hybrid exchange-correlation potentials like PBE0, B3LYP, HSE in order to understand the extent of effect of correlation on band gaps of different available crystallographic symmetries (5 in number) of BaTiO3.

  20. First-Principles Study of the Li-Na-Ca-N-H System: Compound Structures and Hydrogen-Storage Properties

    NASA Astrophysics Data System (ADS)

    Teeratchanan, Pattanasak; Zhou, Fei; Michel, Kyle; Ozolins, Vidvuds

    2012-02-01

    Mixed-metal amides and imides are being widely investigated as potential hydrogen storage materials. Using a combination of first-principle DFT calculations, grand-canonical linear programming, and prototype electrostatic ground state (PEGS) approaches, we predict hydrogen storage reactions in the Li-Na-Ca-N-H system. The enthalpies, entropies, static, zero-point, and T0K vibrational energies of known compounds together with our predictions of some incomplete experimental crystal structures are presented.

  1. First principles study on the electronic structures and stability of Cr 7C 3 type multi-component carbides

    NASA Astrophysics Data System (ADS)

    Xiao, B.; Feng, J.; Zhou, C. T.; Xing, J. D.; Xie, X. J.; Chen, Y. H.

    2008-06-01

    First principles calculations were conducted to investigate the stabilities of six multi-component carbides of Cr 7C 3 by calculating the cohesive energy and formation enthalpy of them. The theoretical predictions were compared with the experimental results and they were in agreement with each other. The electronic structures of the six carbides were also calculated in order to provide more information about the relationship between the stability and crystal compositions at atomic scale.

  2. First-principles study of monolayer and bilayer honeycomb structures of group-IV elements and their binary compounds

    NASA Astrophysics Data System (ADS)

    Pan, L.; Liu, H. J.; Wen, Y. W.; Tan, X. J.; Lv, H. Y.; Shi, J.; Tang, X. F.

    2011-01-01

    By using first-principles pseudopotential method, we investigate the structural, vibrational, and electronic properties of monolayer and bilayer honeycomb structures of group-IV elements and their binary compounds. It is found that the honeycomb structures of Si, Ge, and SiGe are buckled for stabilization, while those of binary compounds SiC and GeC containing the first row elements C are planar similar to a graphene sheet. The phonon dispersion relations and electronic band structures are very sensitive to the number of layers, the stacking order, and whether the layers are planar or buckled.

  3. Investigation of structural stability and elastic properties of Zrh and Zrh{sub 2}: A first principles study

    SciTech Connect

    Kanagaprabha, S.; Rajeswarapalanichamy, R. Sudhapriyanga, G. Murugan, A. Santhosh, M.; Iyakutti, K.

    2014-04-24

    The electronic, structural and mechanical properties of ZrH and ZrH{sub 2} are investigated by means of first principles calculation based on density functional theory as implemented in VASP code with generalized gradient approximation. The calculated ground state properties are in good agreement with previous experimental and other theoretical results. Among the six crystallographic structures considered for ZrH, ZB phase is found to be the most stable phase, whereas ZrH{sub 2} is energetically stable in tetragonal structure at ambient condition. A structural phase transition from ZB→NaCl at a pressure 10 GPa is predicted for ZrH.

  4. First-Principles Study of Carbon and Vacancy Structures in Niobium

    SciTech Connect

    Ford, Denise C.; Zapol, Peter; Cooley, Lance D.

    2015-04-03

    The interstitial chemical impurities hydrogen, oxygen, nitrogen, and carbon are important for niobium metal production, and particularly for the optimization of niobium SRF technology. These atoms are present in refined sheets and can be absorbed into niobium during processing treatments, resulting in changes to the residual resistance and the performance of SRF cavities. A first-principles approach is taken to study the properties of carbon in niobium, and the results are compared and contrasted with the properties of the other interstitial impurities. The results indicate that C will likely form precipitates or atmospheres around defects rather than strongly bound complexes with other impurities. Based on the analysis of carbon and hydrogen near niobium lattice vacancies and small vacancy chains and clusters, the formation of extended carbon chains and hydrocarbons is not likely to occur. Association of carbon with hydrogen atoms can, however, occur through the strain fields created by interstitial binding of the impurity atoms. In conclusion, calculated electronic densities of states indicate that interstitial C may have a similar effect as interstitial O on the superconducting transition temperature of Nb.

  5. Electronic Structure and Carrier Mobilities of Arsenene and Antimonene Nanoribbons: A First-Principle Study

    NASA Astrophysics Data System (ADS)

    Wang, Yanli; Ding, Yi

    2015-06-01

    Arsenene and antimonene, i.e. two-dimensional (2D) As and Sb monolayers, are the recently proposed cousins of phosphorene (Angew. Chem. Int. Ed., 54, 3112 (2015)). Through first-principle calculations, we systematically investigate electronic and transport properties of the corresponding As and Sb nanoribbons, which are cut from the arsenene and antimonene nanosheets. We find that different from the 2D systems, band features of As and Sb nanoribbons are dependent on edge shapes. All armchair As/Sb nanoribbons keep the indirect band gap feature, while the zigzag ones transfer to direct semiconductors. Quantum confinement in nanoribbons enhances the gap sizes, for which both the armchair and zigzag ones have a gap scaling rule inversely proportional to the ribbon width. Comparing to phosphorene, the large deformation potential constants in the As and Sb nanoribbons cause small carrier mobilities in the orders of magnitude of 101-102 cm2/Vs. Our study demonstrates that the nanostructures of group-Vb elements would possess different electronic properties for the P, As, and Sb ones, which have diverse potential applications for nanoelectronics and nanodevices.

  6. First-Principles Study of Carbon and Vacancy Structures in Niobium

    DOE PAGESBeta

    Ford, Denise C.; Zapol, Peter; Cooley, Lance D.

    2015-04-03

    The interstitial chemical impurities hydrogen, oxygen, nitrogen, and carbon are important for niobium metal production, and particularly for the optimization of niobium SRF technology. These atoms are present in refined sheets and can be absorbed into niobium during processing treatments, resulting in changes to the residual resistance and the performance of SRF cavities. A first-principles approach is taken to study the properties of carbon in niobium, and the results are compared and contrasted with the properties of the other interstitial impurities. The results indicate that C will likely form precipitates or atmospheres around defects rather than strongly bound complexes withmore » other impurities. Based on the analysis of carbon and hydrogen near niobium lattice vacancies and small vacancy chains and clusters, the formation of extended carbon chains and hydrocarbons is not likely to occur. Association of carbon with hydrogen atoms can, however, occur through the strain fields created by interstitial binding of the impurity atoms. In conclusion, calculated electronic densities of states indicate that interstitial C may have a similar effect as interstitial O on the superconducting transition temperature of Nb.« less

  7. A first principles study of structural, electronic mechanical and magnetic properties of rare earth nitride:TmN

    NASA Astrophysics Data System (ADS)

    Murugan, A.; Rajeswarapalanichamy, R.; Santhosh, M.; Manikandan, M.

    2016-05-01

    The structural, electronic and mechanical properties of rare earth nitride TmN is investigated by the first principles calculations based on density functional theory using the Vienna ab-initio simulation package. At ambient pressure TmN is stable in the ferromagnetic state with NaCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that TmN is metallic at normal pressure. Ferromagnetic to non magnetic phase transition is predicted in TmN at high pressure.

  8. First-principles study of lithium ion migration in lithium transition metal oxides with spinel structure.

    PubMed

    Nakayama, Masanobu; Kaneko, Mayumi; Wakihara, Masataka

    2012-10-28

    The migration of lithium (Li) ions in electrode materials is an important factor affecting the rate performance of rechargeable Li ion batteries. We have examined Li migration in spinels LiMn(2)O(4), LiCo(2)O(4), and LiCo(1/16)Mn(15/16)O(4) by means of first-principles calculations based on density functional theory (DFT). The results showed that the trajectory of the Li jump was straight between the two adjacent Li ions for all of the three spinel compounds. However, there were significant differences in the energy profiles and the Li jump path for LiMn(2)O(4) and LiCo(2)O(4). For LiMn(2)O(4) the highest energy barrier was in the middle of the two tetrahedral sites, or in the octahedral vacancy (16c). For LiCo(2)O(4) the lowest energy was around the octahedral 16c site and the energy barrier was located at the bottleneck sites. The difference in the energy profile for LiCo(2)O(4) stemmed from the charge disproportion of Co(3.5+) to Co(3+)/Co(4+) caused by a Li vacancy forming and jumping, which was not observed for LiMn(2)O(4). Charge disproportion successfully accounted for the faster Li migration mechanism observed in LiCo(1/16)Mn(15/16)O(4). Our computational results demonstrate the importance of the effect of charge distribution on the ion jump. PMID:22986640

  9. Electronic structures of Stone-Wales defective chiral (6,2) silicon carbide nanotubes: First-principles calculations

    NASA Astrophysics Data System (ADS)

    Song, Jiuxu; Liu, Hongxia; Guo, Yingna; Zhu, Kairan

    2015-11-01

    By using first-principle calculations based on density functional theory, the geometries and electronic structures of the Stone-Wales defective chiral (6,2) silicon carbide nanotubes (SiCNTs) are investigated. Independent on their orientations, Stone-Wales defects form two asymmetric pentagons and heptagons coupled in pairs (5-7-7-5) and a defect energy level in the band gap of the SiCNT. By applying transverse electric fields, significant differences in the electronic structures of the defective (6,2) SiCNTs are achieved, which may provide the foundation of identifying the orientation of Stone-Wales defects in chiral SiCNTs.

  10. Controlling Electronic Structures by Irradiation in Single-walled SiC Nanotubes: A First-Principles Molecular Dynamics Study

    SciTech Connect

    Wang, Zhiguo; Gao, Fei; Li , Jingbo; Zu, Xiaotao T.; Weber, William J.

    2009-02-18

    Using first principles molecular dynamics simulations, the displacement threshold energy and defect configurations are determined in SiC nanotubes. The simulation results reveal that a rich variety of defect structures (vacancies, Stone-Wales defects, and antisite defects) are formed with threshold energies of from 11 to 64 eV. The threshold energy shows an anisotropic behavior and exhibits a dramatic decrease with decreasing tube diameter. The electronic structure can be altered by the defects formed by irradiation, which suggests that the electron irradiation may be a way to use defect engineering to tailor electronic properties of SiC nanotubes.

  11. Phase stability, electronic structure and equation of state of cubic TcN from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Song, T.; Ma, Q.; Sun, X. W.; Liu, Z. J.; Fu, Z. J.; Wei, X. P.; Wang, T.; Tian, J. H.

    2016-09-01

    The phase transition, electronic band structure, and equation of state (EOS) of cubic TcN are investigated by first-principles pseudopotential method based on density-functional theory. The calculated enthalpies show that TcN has a transformation between zincblende and rocksalt phases and the pressure determined by the relative enthalpy is 32 GPa. The calculated band structure indicates the metallic feature and it might make cubic TcN a better candidate for hard materials. Particular attention is paid to the predictions of volume, bulk modulus and its pressure derivative which play a central role in the formulation of approximate EOSs using the quasi-harmonic Debye model.

  12. Thermodynamics of solid electrolytes and related oxide ceramics based on the fluorite structure

    SciTech Connect

    Navrotsky, Alexandra

    2010-01-01

    Oxides based on the fluorite structure are important as electrolytes in solid oxide fuel cells, thermal barrier coatings, gate dielectrics, catalysts, and nuclear materials. Though the parent fluorite structure is simple, the substitution of trivalent for tetravalent cations, coupled with the presence of charge-balancing oxygen vacancies, leads to a wealth of short-range and long-range ordered structures and complex thermodynamic properties. The location of vacancies and the nature of clusters affect the energetics of mixing in rare earth doped zirconia, hafnia, ceria, urania, and thoria, with systematic trends in energetics as a function of cation radius. High temperature oxide melt solution calorimetry has provided direct measurement of formation enthalpies of these refractory materials. Surface and interfacial energies have also been measured in yttria stabilized zirconia (YSZ) nanomaterials. Other ionic conductors having perovskite, apatite, and mellilite structures are discussed briefly.

  13. First-principles study on the structural, elastic and electronic properties of Ti2SiN under high pressure

    NASA Astrophysics Data System (ADS)

    Li, Hui; Wang, Zhenjun; Sun, Guodong; Yu, Pengfei; Zhang, Wenxue

    2016-07-01

    The structural, elastic and electronic properties of Ti2SiN under pressure range of 0-50 GPa have been systemically investigated by first-principles calculations. It is found that both Poisson's ratio and shear anisotropy factor of Ti2SiN increase with pressure, and Ti2SiN is elastic anisotropic. The DOS and Mulliken population analysis have been explored, which indicts that Ti2SiN is metallic-covalent-ionic in nature. The present calculations may contribute preliminary results and a better understanding of Ti2SiN for its applications under high pressure environments.

  14. Electronic structures and elastic properties of X3Sb (X = Li, K, Cs) from the first-principles calculations

    NASA Astrophysics Data System (ADS)

    Guo, San-Dong

    2014-03-01

    We investigate the electronic structures of {{\\rm{X}}_{3}}{\\rm{Sb}} (X = Li, K, Cs) by using Tran and Blaha's modified Becke and Johnson exchange potential. Calculated energy gaps are substantially better than previous first-principles results with respect to experimental values. The substantial improvement is achieved because the conduction bands are correctly calculated with the new exchange potential. The approach should be applicable to other similar materials. The elastic properties of {{\\rm{X}}_{3}}{\\rm{Sb}} (X = Li, K, Cs) are also studied in detail with the generalized gradient approximation such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio, sound velocities, and Debye temperature.

  15. First-principles study of phonon effects in x-ray absorption near-edge structure spectroscopy

    NASA Astrophysics Data System (ADS)

    Nemausat, R.; Brouder, Ch; Gervais, Ch; Cabaret, D.

    2016-05-01

    Usually first-principles x-ray absorption near-edge structure (XANES) calculations are performed in the Born-Oppenheimer approximation assuming a static lattice, whereas the nuclear motion undoubtedly impacts XANES spectra notably at the K pre-edge of light elements in oxides. Here, an efficient method based on density-functional theory to account for quantum thermal fluctuations of nuclei is developed and is successfully applied to the K edge of corundum for temperatures up to 930 K. The zero-point motion influence is estimated. Comparison is made with previous theoretical approaches also developed to account for vibrations in XANES.

  16. First-principles study on the adsorption properties of phenylalanine on carbon graphitic structures

    NASA Astrophysics Data System (ADS)

    Kang, Seoung-Hun; Kwon, Dae-Gyeon; Park, Sora; Kwon, Young-Kyun

    2015-12-01

    Using ab-initio density functional theory, we investigate the binding properties of phenylalanine, an amino acid, on graphitic carbon structures, such as graphene, nanotubes, and their modified structures. We focus especially on the effect of the adsorbate on the geometrical and the electronic structures of the absorbents. The phenylalanine molecule is found to bind weakly on pristine graphitic structures with a binding energy of 40-70 meV and not to change the electronic configuration of the graphitic structures, implying that the phenylalanine molecule may not be detected on pristine graphitic structures. On the other hand, the phenylalanine molecule exhibits a substantial increase in its binding energy up to ~2.60 eV on the magnesium-decorated boron-doped graphitic structures. We discover that the Fermi level of the system, which was shifted below the Dirac point of the graphitic structures due to p-doping by boron substitution, can be completely restored to the Dirac point because of the amino acid adsorption. This behavior implies that such modified structures can be utilized to detect phenylalanine molecules.

  17. First-principles study of structural & electronic properties of pyramidal silicon nanowire

    NASA Astrophysics Data System (ADS)

    Jariwala, Pinank; Singh, Deobrat; Sonvane, Y. A.; Gupta, Sanjeev K.; Thakor, P. B.

    2016-05-01

    We have investigated the stable structural and electronic properties of Silicon (Si) nanowires having different cross-sections with 5-7 Si atoms per unit cell. These properties of the studied Si nanowires were significantly changed from those of diamond bulk Si structure. The binding energy increases as increasing atoms number per unit cell in different SiNWs structures. All the nanowires structures are behave like metallic rather than semiconductor in bulk systems. In general, the number of conduction channels increases when the nanowire becomes thicker. The density of charge revealed delocalized metallic bonding for all studied Si nanowires.

  18. First-Principles Mobility Calculations and Atomic-Scale Interface Roughness in Nanoscale Structures

    SciTech Connect

    Evans, Matthew H; Zhang, Xiaoguang; Joannopoulos, J. D.; Pantelides, Sokrates T

    2005-01-01

    Calculations of mobilities have so far been carried out using approximate methods that suppress atomic-scale detail. Such approaches break down in nanoscale structures. Here we report the development of a method to calculate mobilities using atomic-scale models of the structures and density functional theory at various levels of sophistication and accuracy. The method is used to calculate the effect of atomic-scale roughness on electron mobilities in ultrathin double-gate silicon-on-insulator structures. The results elucidate the origin of the significant reduction in mobility observed in ultrathin structures at low electron densities.

  19. Crystal structure and physical properties of Mo{sub 2}B: First-principle calculations

    SciTech Connect

    Zhou, Dan; Cui, Qiliang E-mail: liquan777@jlu.edu.cn; Li, Quan E-mail: liquan777@jlu.edu.cn; Wang, Jingshu

    2014-03-21

    Several decades ago, Mo{sub 2}B was assumed to have an Al{sub 2}Cu-type structure with I4/mcm space group. Using ab initio phonon calculations, we identify the earlier proposed Al{sub 2}Cu-type structure is dynamically unstable at ambient pressure. An energetically more favorable phase with the tetragonal I4/m structure was then predicted by employing frozen-phonon technique. The currently predicted I4/m phase is mechanically and dynamically stable and energetically more favorable than that of the earlier proposed Al{sub 2}Cu-type structure. The electronic structures calculations indicate that Mo{sub 2}B is a metal with several bands crossing the Fermi level. Our analysis indicates that the three-dimensional network of the covalent Mo-B bond is responsible for the ultra-incompressible property of Mo{sub 2}B.

  20. Investigating the structural evolution of thiolate protected gold clusters from first-principles.

    PubMed

    Pei, Yong; Zeng, Xiao Cheng

    2012-07-21

    Unlike bulk materials, the physicochemical properties of nano-sized metal clusters can be strongly dependent on their atomic structure and size. Over the past two decades, major progress has been made in both the synthesis and characterization of a special class of ligated metal nanoclusters, namely, the thiolate-protected gold clusters with size less than 2 nm. Nevertheless, the determination of the precise atomic structure of thiolate-protected gold clusters is still a grand challenge to both experimentalists and theorists. The lack of atomic structures for many thiolate-protected gold clusters has hampered our in-depth understanding of their physicochemical properties and size-dependent structural evolution. Recent breakthroughs in the determination of the atomic structure of two clusters, [Au(25)(SCH(2)CH(2)Ph)(18)](q) (q = -1, 0) and Au(102)(p-MBA)(44), from X-ray crystallography have uncovered many new characteristics regarding the gold-sulfur bonding as well as the atomic packing structure in gold thiolate nanoclusters. Knowledge obtained from the atomic structures of both thiolate-protected gold clusters allows researchers to examine a more general "inherent structure rule" underlying this special class of ligated gold nanoclusters. That is, a highly stable thiolate-protected gold cluster can be viewed as a combination of a highly symmetric Au core and several protecting gold-thiolate "staple motifs", as illustrated by a general structural formula [Au](a+a')[Au(SR)(2)](b)[Au(2)(SR)(3)](c)[Au(3)(SR)(4)](d)[Au(4)(SR)(5)](e) where a, a', b, c, d and e are integers that satisfy certain constraints. In this review article, we highlight recent progress in the theoretical exploration and prediction of the atomic structures of various thiolate-protected gold clusters based on the "divide-and-protect" concept in general and the "inherent structure rule" in particular. As two demonstration examples, we show that the theoretically predicted lowest-energy structures of

  1. Investigating the structural evolution of thiolate protected gold clusters from first-principles

    NASA Astrophysics Data System (ADS)

    Pei, Yong; Zeng, Xiao Cheng

    2012-06-01

    Unlike bulk materials, the physicochemical properties of nano-sized metal clusters can be strongly dependent on their atomic structure and size. Over the past two decades, major progress has been made in both the synthesis and characterization of a special class of ligated metal nanoclusters, namely, the thiolate-protected gold clusters with size less than 2 nm. Nevertheless, the determination of the precise atomic structure of thiolate-protected gold clusters is still a grand challenge to both experimentalists and theorists. The lack of atomic structures for many thiolate-protected gold clusters has hampered our in-depth understanding of their physicochemical properties and size-dependent structural evolution. Recent breakthroughs in the determination of the atomic structure of two clusters, [Au25(SCH2CH2Ph)18]q (q = -1, 0) and Au102(p-MBA)44, from X-ray crystallography have uncovered many new characteristics regarding the gold-sulfur bonding as well as the atomic packing structure in gold thiolate nanoclusters. Knowledge obtained from the atomic structures of both thiolate-protected gold clusters allows researchers to examine a more general ``inherent structure rule'' underlying this special class of ligated gold nanoclusters. That is, a highly stable thiolate-protected gold cluster can be viewed as a combination of a highly symmetric Au core and several protecting gold-thiolate ``staple motifs'', as illustrated by a general structural formula [Au]a+a'[Au(SR)2]b[Au2(SR)3]c[Au3(SR)4]d[Au4(SR)5]e where a, a', b, c, d and e are integers that satisfy certain constraints. In this review article, we highlight recent progress in the theoretical exploration and prediction of the atomic structures of various thiolate-protected gold clusters based on the ``divide-and-protect'' concept in general and the ``inherent structure rule'' in particular. As two demonstration examples, we show that the theoretically predicted lowest-energy structures of Au25(SR)8- and Au38(SR)24 (-R

  2. First-principles calculation of the electronic structure and EELS spectra at the graphene/Ni(111) interface

    NASA Astrophysics Data System (ADS)

    Bertoni, Giovanni; Calmels, Lionel; Altibelli, Anne; Serin, Virginie

    2005-02-01

    A spin-polarized first-principles calculation of the atomic and electronic structure of the graphene/Ni(111) interface is presented. Different structural models have been considered, which differ in the positions of the carbon atoms with respect to the nickel topmost layer. The most probable structure, which has the lowest energy, has been determined. The distance between the floating carbon layer and the nickel surface is found smaller than the distance between graphene sheets in bulk graphite, in accordance with experimental measurements. The electronic structure of the graphene layer is strongly modified by interaction with the substrate and the magnetic moment of the surface nickel atoms is lowered in the presence of the graphene layer. Several interface states have been identified in different parts of the interface two-dimensional Brillouin zone. Their influence on the electron energy loss spectra has been evaluated.

  3. Structure of liquid phosphorus: A liquid-liquid phase transition via constant-pressure first-principles molecular dynamics

    NASA Astrophysics Data System (ADS)

    Morishita, Tetsuya

    2001-12-01

    Constant-pressure first-principles molecular dynamics simulations have been carried out to study structural phase transitions of liquid black phosphorus. By compressing the tetrahedral molecular liquid (a low-pressure phase), a structural phase transition from the molecular to polymeric liquid (a high-pressure phase) was successfully realized just as observed in the recent experiment by Katayama et al. [Nature 170 (2000) 403]. Structural properties in the polymeric liquid were investigated and it is found that the covalent p-state bonds are dominant within the first nearest neighbors of each atom. However, further compression of the polymeric liquid shows that the covalent bonding is weakened as pressure is increased. As a result, liquid phosphorus becomes similar to the simple liquid in which atoms form a close-packed structure at very high pressure.

  4. Interpretation of the optical absorption spectrum of Co3O4 with normal spinel structure from first principles calculations

    NASA Astrophysics Data System (ADS)

    Lima, A. F.

    2014-01-01

    First principles calculations based on density functional theory have been employed to study the electronic, magnetic and optical properties of Co3O4 in a cubic normal spinel structure. Exchange and correlation effects between electrons were treated by a B3PW91 hybrid functional, which produced better results than others scheme, such as GGA+U or PBE0 hybrid functionals or mBJ semilocal potential. The work focuses on clarifying the nature of the optical absorption bands, which have motivated various theoretical and experimental works in the literature. The calculated optical absorption spectrum was compared with available experimental data. On the basis of this calculated electronic and magnetic structure, the optical absorption peaks (theoretical and experimental) could be satisfactorily explained in terms of d3d charge transfer transitions between both CO2+→CO2+ and CO3+→CO3+ ions. The calculations also predicted that the crystal field splittings at both octahedral and tetrahedral sites in the Co3O4 compound are of the same magnitude. First principles calculations were used to predict optical properties of Co3O4. Exchange-correlation electronic effects were treated by a B3PW91 hybrid functional. Calculated optical absorption spectrum was compared with experimental data. Optical absorption peaks could be satisfactorily explained.

  5. Structural and mechanical properties of alkali hydrides investigated by the first-principles calculations and principal component analysis

    NASA Astrophysics Data System (ADS)

    Settouti, Nadera; Aourag, Hafid

    2016-08-01

    The structural and mechanical properties of alkali hydrides (LiH, NaH, KH, RbH, and CsH) were investigated via first-principles calculations which cover the optimized structural parameters. The density functional theory in combination with the generalized gradient approximation (GGA) were used in this study. From the present study, one could note that alkali hydrides are brittle materials and mechanically stable. It was found that stiffness and shear resistance are greater in LiH than in other hydrides. It is more brittle in nature, and comparatively harder than the other materials under study; it also presents a high degree of anisotropy. The results were then investigated and analyzed with principal component analysis (PCA), which is one of the most common techniques in multivariate analysis, was used to explore the correlations among material properties of alkali hydrides and to study their trends. The alkali hydrides obtained by the first-principles calculations were also compared with the alkaline-earth metal hydrides (BeH2, MgH2, CaH2, SrH2, and BaH2) and discussed in this work.

  6. First-Principles Study of Electronic Structure and Thermoelectric Properties of Ge-Doped Tin Clathrates

    NASA Astrophysics Data System (ADS)

    Akai, K.; Kishimoto, K.; Koyanagi, T.; Kono, Y.; Yamamoto, S.

    2014-06-01

    We calculated the electronic structure and thermoelectric properties of the Ge-doped quaternary clathrate Ba-Ga-Sn-Ge. The electronic structure was calculated by using the WIEN2k code, which is based on the full-potential augmented plane-wave method. Using this method, we calculated the total energies for several Ge configurations to determine the positions of Ge atoms in the unit cell. The calculated Ge positions were in good agreement with the experimental results. Based on the resulting Ge positions, the band structure and thermoelectric properties of the Ba-Ga-Sn-Ge clathrates were calculated.

  7. First-principles calculation of atomic forces and structural distortions in strongly correlated materials.

    PubMed

    Leonov, I; Anisimov, V I; Vollhardt, D

    2014-04-11

    We introduce a novel computational approach for the investigation of complex correlated electron materials which makes it possible to evaluate interatomic forces and, thereby, determine atomic displacements and structural transformations induced by electronic correlations. It combines ab initio band structure and dynamical mean-field theory and is implemented with the linear-response formalism regarding atomic displacements. We apply this new technique to explore structural transitions of prototypical correlated systems such as elemental hydrogen, SrVO3, and KCuF3. PMID:24765993

  8. First principle calculations of structural phase transition and electronic properties in AmTe

    SciTech Connect

    Pataiya, Jagdeesh Makode, C.; Aynyas, Mahendra; Singh, A.; Sanyal, S. P.

    2015-06-24

    The tight-binding linear muffin-tin orbital (TB-LMTO) with in the local density approximation is used to calculate total energy, lattice parameters, bulk modulus, density of states and energy band structure of americium telluride at ambient as well as at high pressure. It is found that AmTe is stable in NaCl – type structure under ambient pressure. The phase transition pressure was found to be 15.0 GPa from NaCl-type (B{sub 1}-phase) structure to CsCl-type (B{sub 2}-phase) structure for this compound. From energy band diagram it is observed that AmTe exhibit metallic behaviour. The calculated ground state properties such as lattice parameters and bulk modulus are in general good agreement with the available results.

  9. First principle calculations of structural phase transition and electronic properties in AmTe

    NASA Astrophysics Data System (ADS)

    Pataiya, Jagdeesh; Aynyas, Mahendra; Makode, C.; Singh, A.; Sanyal, S. P.

    2015-06-01

    The tight-binding linear muffin-tin orbital (TB-LMTO) with in the local density approximation is used to calculate total energy, lattice parameters, bulk modulus, density of states and energy band structure of americium telluride at ambient as well as at high pressure. It is found that AmTe is stable in NaCl - type structure under ambient pressure. The phase transition pressure was found to be 15.0 GPa from NaCl-type (B1-phase) structure to CsCl-type (B2-phase) structure for this compound. From energy band diagram it is observed that AmTe exhibit metallic behaviour. The calculated ground state properties such as lattice parameters and bulk modulus are in general good agreement with the available results.

  10. Large-Scale Computations Leading to a First-Principles Approach to Nuclear Structure

    SciTech Connect

    Ormand, W E; Navratil, P

    2003-08-18

    We report on large-scale applications of the ab initio, no-core shell model with the primary goal of achieving an accurate description of nuclear structure from the fundamental inter-nucleon interactions. In particular, we show that realistic two-nucleon interactions are inadequate to describe the low-lying structure of {sup 10}B, and that realistic three-nucleon interactions are essential.

  11. First principle investigation of structural and electronic properties of bulk ZnSe

    SciTech Connect

    Khatta, Swati; Tripathi, S. K. Prakash, Satya

    2015-08-28

    Electronic and structural properties of ZnSe are investigated using plane-wave self-consistent field method within the framework of density functional theory. The pseudopotential method within the local density approximation is used for the exchange-correlation potential. The equilibrium lattice parameter, static bulk modulus and its pressure derivative are calculated. The electronic band structure, partial density of states and density of states are also obtained. The results are compared with available theoretical calculations and experimental results.

  12. Structural stability and magnetism of FeN from first principles

    NASA Astrophysics Data System (ADS)

    Houari, A.; Matar, S. F.; Belkhir, M. A.; Nakhl, M.

    2007-02-01

    In the framework of density-functional theory, the structural and magnetic properties of FeN mononitride have been investigated using the all-electron augmented spherical wave method with a generalized gradient approximation functional for treating the effects of exchange and correlation. Calculation of the energy versus volume in hypothetic rocksalt (RS-), zinc-blende (ZB-), and wurtzite (W)-type structures shows that the RS-type structure is more stable than the others. Spin-polarized calculation results at equilibrium volume indicate that the ground state of RS-FeN is ferromagnetic with a high moment, while ZB-FeN and W-FeN are nonmagnetic. The influence of distortions on the stability is taken into account by considering FeN in two different face-centered-tetragonal structures (fcts): fct rocksalt and fct zinc blende. The magnetovolume effects with respect to Slater-Pauling-Friedel model are discussed. The electronic structures analyzed from site- and spin-projected density of states are reported. A discussion of the structural and magnetic properties of FeN is given with respect to N local environment of Fe.

  13. Bridge Structure for the graphene/Ni(111) system: A first principles study

    SciTech Connect

    Fuentes-Cabrera, Miguel A; Baskes, Mike I.; Melechko, Anatoli Vasilievich; Simpson, Michael L

    2008-01-01

    The structure of graphene on Ni(111) is studied with density functional theory (DFT). Six different structures, i.e., top-fcc, top-hcp, hcp-fcc, bridge-top, bridge-fcc, and bridge-hcp, were investigated. Bridge-top, bridge-fcc, and bridge-hcp are studied here. Top-fcc and hcp-fcc have been considered before, experimentally and theoretically, and regarded as energetically stable structures. The calculations employed the local density approximation (LDA) and the Perdew, Burke, and Ernzerhof (PBE) generalized-gradient approximation to DFT. The results showed that with PBE, none of the structures is stable at the experimentally relevant temperatures; with LDA, only bridge-top and top-fcc are stable. These findings suggest that it will be worthwhile to carry on new experimental studies to revisit the structural determination of the graphene/Ni(111) system, with special emphasis on testing whether bridge-top could exist by itself or coexist with other structures.

  14. High-pressure U3O8 with the fluorite-type structure

    NASA Astrophysics Data System (ADS)

    Zhang, F. X.; Lang, M.; Wang, J. W.; Li, W. X.; Sun, K.; Prakapenka, V.; Ewing, R. C.

    2014-05-01

    A new high-pressure phase of U3O8, which has a fluorite-type structure, forms at pressures greater than ~8.1 GPa that was confirmed by in situ x-ray diffraction (XRD) measurements. The fluorite-type U3O8 is stable at pressures at least up to ~40 GPa and temperatures to 1700 K, and quenchable to ambient conditions. Based on the XRD analysis, there is a huge volume collapse (>20%) for U3O8 during the phase transition and the quenched high-pressure phase is 28% denser than the initial orthorhombic phase at ambient conditions. The high-pressure phase has a very low compressibility comparing with the starting orthorhombic phase.

  15. First-principles study of structural properties of SiO2 bilayers

    NASA Astrophysics Data System (ADS)

    Malashevich, Andrei; Ismail-Beigi, Sohrab; Altman, Eric I.

    Two dimensional (2D) materials draw a tremendous amount of interest because they exhibit unique physical properties due to reduced dimensionality. Recently, SiO2 2D bilayer systems were discovered. The structure of these bilayers is formed by two mirror-image planes of corner-sharing SiO4 tetrahedra and does not have a direct relation to bulk SiO2 systems. SiO2 bilayers may be obtained in crystalline or amorphous forms. In the crystalline form, the bilayers are constructed from six-membered rings of corner-sharing SiO4 tetrahedra. The amorphous form has rings of various sizes typically in the range from four to nine Si atoms in the ring. These structures may be of practical interest as atomically thin membranes and molecular sieves. In our work, we study the effect of strain and doping on the crystalline structure of SiO2 bilayers using density functional theory. We analyze the stability of structures depending on the ring size and establish strain and doping conditions that may render the structures with large ring sizes stable. This work is supported by the National Science Foundation through Grants MRSEC NSF DMR-1119826 and NSF DMR-1506800.

  16. First-principles study of structural, elastic and thermodynamic properties of AuIn2

    NASA Astrophysics Data System (ADS)

    Wu, Hai Ying; Chen, Ya Hong; Deng, Chen Rong; Yin, Peng Fei; Cao, Hong

    2015-12-01

    The structural, elastic and thermodynamic properties of AuIn2 in the CaF2 structure under pressure have been investigated using ab initio plane wave pseudopotential method within the generalized gradient approximation. The calculated structural parameters and equation of state are in excellent agreement with the available experimental and theoretical results. The elastic constants of AuIn2 at ambient condition are calculated, and the bulk modulus obtained from these calculated elastic constants agrees well with the experimental data. The pressure dependence of the elastic constants, bulk modulus, shear modulus and Young’s modulus has also been investigated. The Debye temperature presents a slight increase with pressure. AuIn2 exhibits ductibility and low hardness characteristics, the ductibility increases while the hardness decreases with the increasing of pressure. The pressure effect on the heat capacity and thermal expansion coefficient for AuIn2 is much larger.

  17. Magnetism, structure and chemical order in small CoPd clusters: A first-principles study

    NASA Astrophysics Data System (ADS)

    Mokkath, Junais Habeeb

    2014-01-01

    The structural, electronic and magnetic properties of small ComPdn(N=m+n=8,m=0-N) nanoalloy clusters are studied in the framework of a generalized-gradient approximation to density-functional theory. The optimized cluster structures have a clear tendency to maximize the number of nearest-neighbor CoCo pairs. The magnetic order is found to be ferromagnetic-like (FM) for all the ground-state structures. Antiferromagnetic-like spin arrangements were found in some low-lying isomers. The average magnetic moment per atom μ increases approximately linearly with Co content. A remarkable enhancement of the local Co moments is observed as a result of Pd doping. This is a consequence of the increase in the number of Co d holes, due to CoPd charge transfer, combined with the reduced local coordination. The influence of spin-orbit interactions on the cluster properties is also discussed.

  18. HARES: an efficient method for first-principles electronic structure calculations of complex systems

    NASA Astrophysics Data System (ADS)

    Waghmare, U. V.; Kim, Hanchul; Park, I. J.; Modine, Normand; Maragakis, P.; Kaxiras, Efthimios

    2001-07-01

    We discuss our new implementation of the Real-space Electronic Structure method for studying the atomic and electronic structure of infinite periodic as well as finite systems, based on density functional theory. This improved version which we call HARES (for High-performance-Fortran Adaptive grid Real-space Electronic Structure) aims at making the method widely applicable and efficient, using high performance Fortran on parallel architectures. The scaling of various parts of a HARES calculation is analyzed and compared to that of plane-wave based methods. The new developments that lead to enhanced performance, and their parallel implementation, are presented in detail. We illustrate the application of HARES to the study of elemental crystalline solids, molecules and complex crystalline materials, such as blue bronze and zeolites.

  19. Influence of Sn interaction on the structural evolution of Au clusters: A first principles study

    NASA Astrophysics Data System (ADS)

    Sahoo, Suman Kalyan; Nigam, Sandeep; Sarkar, Pranab; Majumder, Chiranjib

    2012-08-01

    Here we report the structural and electronic properties of AunSn (n = 2-13) clusters by using pseudo-potential and LCAO-MO method. A comparison between the structures of Aun and AunSn clusters reveals that while Aun clusters favor planar isomers up to n = 13, AunSn clusters follow a different trend; 3D structure for n = 3 and 4, quasi planar in the size range n = 5-11, and again 3D isomers from n = 12 onwards. Enhanced contribution of Au p-orbital and significant charge transfer from Sn to the gold atoms is attributed for such interesting growth pattern of AunSn clusters.

  20. Electronic structure of III-V zinc-blende semiconductors from first principles

    NASA Astrophysics Data System (ADS)

    Wang, Yin; Yin, Haitao; Cao, Ronggen; Zahid, Ferdows; Zhu, Yu; Liu, Lei; Wang, Jian; Guo, Hong

    2013-06-01

    For analyzing quantum transport in semiconductor devices, accurate electronic structures are critical for quantitative predictions. Here we report theoretical analysis of electronic structures of all III-V zinc-blende semiconductor compounds. Our calculations are from density functional theory with the semilocal exchange proposed recently [Tran and Blaha, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.102.226401 102, 226401 (2009)], within the linear muffin tin orbital scheme. The calculated band gaps and effective masses are compared to experimental data and good quantitative agreement is obtained. Using the theoretical scheme presented here, quantum transport in nanostructures of III-V compounds can be confidently predicted.

  1. Structural and electronic properties of BxCyNz nanoribbons: A first principles study

    NASA Astrophysics Data System (ADS)

    Gonçalves, R. D.; Azevedo, S.; Machado, M.

    2013-12-01

    We have performed an extensive ab initio study on the energetic stability of hydrogen passivated BxCyNz nanoribbons and at the electronic structure and magnetic properties of BC2N ribbons with different widths and configurations. In particular, it was investigated that BC2N ribbons composed of boron-nitride clusters surrounded by carbon atoms are showing armchair and zigzag edges. It was seen that the zigzag and armchair BC2N ribbons can be small gap semiconductors or metallic according to the ribbons width. Also, magnetic behavior is observed for these structures, for all the considered widths, while the armchair ones do not show any magnetization.

  2. Local spin flip in two- and three-magnetic-center structures: A first-principles approach

    NASA Astrophysics Data System (ADS)

    Lefkidis, G.; Li, C.; Hartenstein, T.; Hübner, W.

    2010-01-01

    We present a fully ab initio theory of ultrafast spin switching in nanostructures using optical control theory and including spin-orbit coupling thus realizing Λ processes. These processes are investigated using high-level quantum chemistry in structures with one, two, and three magnetic centers, where the spin localization and transferability are discussed with respect to their geometry. In particular we study metallic chains with two and three magnetic centers interconnected with Na atoms. We discuss the prerequisites for such scenarios for all structures.

  3. Structure determination of ultra dense magnesium borohydride: A first-principles study

    NASA Astrophysics Data System (ADS)

    Fan, Jing; Duan, Defang; Jin, Xilian; Bao, Kuo; Liu, Bingbing; Cui, Tian

    2013-06-01

    Magnesium borohydride (Mg(BH4)2) is one of the potential hydrogen storage materials. Recently, two experiments [Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev, D. Chernyshov, and H. Hagemann, Angew. Chem., Int. Ed. 50, 11162 (2011);, 10.1002/anie.201100675 L. George, V. Drozd, and S. K. Saxena, J. Phys. Chem. C 113, 486 (2009), 10.1021/jp807842t] found that α-Mg(BH4)2 can irreversibly be transformed to an ultra dense δ-Mg(BH4)2 under high pressure. Its volumetric hydrogen content at ambient pressure (147 g/cm3) exceeds twice of DOE's (U.S. Department of Energy) target (70 g/cm3) and that of α-Mg(BH4)2 (117 g/cm3) by 20%. In this study, the experimentally proposed P42nm structure of δ-phase has been found to be dynamically unstable. A new Fddd structure has been reported as a good candidate of δ-phase instead. Its enthalpy from 0 to 12 GPa is much lower than P42nm structure and the simulated X-ray diffraction spectrum is in satisfied agreement with previous experiments. In addition, the previously proposed P-3m1 structure, which is denser than Fddd, is found to be a candidate of ɛ-phase due to the agreement of Raman shifts.

  4. Interface structure and mechanics between graphene and metal substrates: a first-principles study

    NASA Astrophysics Data System (ADS)

    Xu, Zhiping; Buehler, Markus J.

    2010-12-01

    Graphene is a fascinating material not only for technological applications, but also as a test bed for fundamental insights into condensed matter physics due to its unique two-dimensional structure. One of the most intriguing issues is the understanding of the properties of graphene and various substrate materials. In particular, the interfaces between graphene and metal substrates are of critical importance in applications of graphene in integrated electronics, as thermal materials, and in electromechanical devices. Here we investigate the structure and mechanical interactions at a graphene-metal interface through density functional theory (DFT)-based calculations. We focus on copper (111) and nickel (111) surfaces adhered to a monolayer of graphene, and find that their cohesive energy, strength and electronic structure correlate directly with their atomic geometry. Due to the strong coupling between open d-orbitals, the nickel-graphene interface has a much stronger cohesive energy with graphene than copper. We also find that the interface cohesive energy profile features a well-and-shoulder shape that cannot be captured by simple pair-wise models such as the Lennard-Jones potential. Our results provide a detailed understanding of the interfacial properties of graphene-metal systems, and help to predict the performance of graphene-based nanoelectronics and nanocomposites. The availability of structural and energetic data of graphene-metal interfaces could also be useful for the development of empirical force fields for molecular dynamics simulations.

  5. k.p Parameters with Accuracy Control from Preexistent First-Principles Band Structure Calculations

    NASA Astrophysics Data System (ADS)

    Sipahi, Guilherme; Bastos, Carlos M. O.; Sabino, Fernando P.; Faria Junior, Paulo E.; de Campos, Tiago; da Silva, Juarez L. F.

    The k.p method is a successful approach to obtain band structure, optical and transport properties of semiconductors. It overtakes the ab initio methods in confined systems due to its low computational cost since it is a continuum method that does not require all the atoms' orbital information. From an effective one-electron Hamiltonian, the k.p matrix representation can be calculated using perturbation theory and the parameters identified by symmetry arguments. The parameters determination, however, needs a complementary approach. In this paper, we developed a general method to extract the k.p parameters from preexistent band structures of bulk materials that is not limited by the crystal symmetry or by the model. To demonstrate our approach, we applied it to zinc blende GaAs band structure calculated by hybrid density functional theory within the Heyd-Scuseria-Ernzerhof functional (DFT-HSE), for the usual 8 ×8 k.p Hamiltonian. Our parameters reproduced the DFT-HSE band structure with great accuracy up to 20% of the first Brillouin zone (FBZ). Furthermore, for fitting regions ranging from 7-20% of FBZ, the parameters lie inside the range of values reported by the most reliable studies in the literature. The authors acknowledge financial support from the Brazilian agencies CNPq (Grant #246549/2012-2) and FAPESP (Grants #2011/19333-4, #2012/05618-0 and #2013/23393-8).

  6. First-principles simulations of extended phosphorus oxynitride structures in LiPON glasses

    NASA Astrophysics Data System (ADS)

    Du, Yaojun; Holzwarth, N. A. W.

    2009-03-01

    The thin film electrolyte LiPON, having the composition of Li3+xPO4-yNz with x=3z-2y, was developed at Oak Ridge National Lab in the 1990's for use in solid state batteries and related applications. In an effort to understand and to optimize properties of this electrolyte material, we expanded previous studies of isolated defects in crystalline Li3PO4 to focus on more complicated phosphate structures based on combinations of tetrahedral P-O bonds and bridging P-O-P bonds. For example, crystalline LiPO3 and P2O5 are composed of phosphate structures with linear and branched chains, respectively. Both these and related structures derived from substituting O with N and adjusting mobile Li ion concentrations approximate components found in LiPON films.^2 In the simulated structures, we find that N is energetically more stable at bridging bond sites than at tetrahedral sites by 2-3 eV and that the Li ion migration energies are 0.5-0.6 eV, similar to values measured in LiPON films.

  7. First-principles study on the structural and electronic properties of metallic HfH2 under pressure

    PubMed Central

    Liu, Yunxian; Huang, Xiaoli; Duan, Defang; Tian, Fubo; Liu, Hanyu; Li, Da; Zhao, Zhonglong; Sha, Xiaojing; Yu, Hongyu; Zhang, Huadi; Liu, Bingbing; Cui, Tian

    2015-01-01

    The crystal structures and properties of hafnium hydride under pressure are explored using the first-principles calculations based on density function theory. The material undergoes pressure-induced structural phase transition I4/mmm→Cmma→P21/m at 180 and 250 GPa, respectively, and all of these structures are metallic. The superconducting critical temperature Tc values of I4/mmm, Cmma, and P21/m are 47–193 mK, 5.99–8.16 K and 10.62–12.8 K at 1 atm, 180 and 260 GPa, respectively. Furthermore, the bonding nature of HfH2 is investigated with the help of the electron localization function, the difference charge density and Bader charge analyses, which show that HfH2 is classified as a ionic crystal with the charges transferring from Hf atom to H. PMID:26096298

  8. Nitrogen defects in wide band gap oxides: defect equilibria and electronic structure from first principles calculations.

    PubMed

    Polfus, Jonathan M; Bjørheim, Tor S; Norby, Truls; Haugsrud, Reidar

    2012-09-01

    The nitrogen related defect chemistry and electronic structure of wide band gap oxides are investigated by density functional theory defect calculations of N(O)(q), NH(O)(×), and (NH2)(O)(·) as well as V(O)(··) and OH(O)(·) in MgO, CaO, SrO, Al(2)O(3), In(2)O(3), Sc(2)O(3), Y(2)O(3), La(2)O(3), TiO(2), SnO(2), ZrO(2), BaZrO(3), and SrZrO(3). The N(O)(q) acceptor level is found to be deep and the binding energy of NH(O)(×) with respect to N(O)' and (OH(O)(·) is found to be significantly negative, i.e. binding, in all of the investigated oxides. The defect structure of the oxides was found to be remarkably similar under reducing and nitriding conditions (1 bar N(2), 1 bar H(2) and 1 × 10(-7) bar H(2)O): NH(O)(×) predominates at low temperatures and [N(O)'] = 2[V(O)(··) predominates at higher temperatures (>900 K for most of the oxides). Furthermore, we evaluate how the defect structure is affected by non-equilibrium conditions such as doping and quenching. In terms of electronic structure, N(O)' is found to introduce isolated N-2p states within the band gap, while the N-2p states of NH(O)(×) are shifted towards, or overlap with the VBM. Finally, we assess the effect of nitrogen incorporation on the proton conducting properties of oxides and comment on their corrosion resistance in nitriding atmospheres in light of the calculated defect structures. PMID:22828729

  9. Structure-property relationships of curved aromatic materials from first principles.

    PubMed

    Zoppi, Laura; Martin-Samos, Layla; Baldridge, Kim K

    2014-11-18

    CONSPECTUS: Considerable effort in the past decade has been extended toward achieving computationally affordable theoretical methods for accurate prediction of the structure and properties of materials. Theoretical predictions of solids began decades ago, but only recently have solid-state quantum techniques become sufficiently reliable to be routinely chosen for investigation of solids as quantum chemistry techniques are for isolated molecules. Of great interest are ab initio predictive theories for solids that can provide atomic scale insights into properties of bulk materials, interfaces, and nanostructures. Adaption of the quantum chemical framework is challenging in that no single theory exists that provides prediction of all observables for every material type. However, through a combination of interdisciplinary efforts, a richly textured and substantive portfolio of methods is developing, which promise quantitative predictions of materials and device properties as well as associated performance analysis. Particularly relevant for device applications are organic semiconductors (OSC), with electrical conductivity between that of insulators and that of metals. Semiconducting small molecules, such as aromatic hydrocarbons, tend to have high polarizabilities, small band-gaps, and delocalized π electrons that support mobile charge carriers. Most importantly, the special nature of optical excitations in the form of a bound electron-hole pairs (excitons) holds significant promise for use in devices, such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), and molecular nanojunctions. Added morphological features, such as curvature in aromatic hydrocarbon structure, can further confine the electronic states in one or more directions leading to additional physical phenomena in materials. Such structures offer exploration of a wealth of phenomenology as a function of their environment, particularly due to the ability to tune their electronic

  10. First-principles study of the electronic structure of organic semiconductors

    NASA Astrophysics Data System (ADS)

    Sharifzadeh, Sahar; Biller, Ariel J.; Kronik, Leeor; Neaton, Jeffrey B.

    2010-03-01

    Organic semiconductors are promising materials for next generation organic photovoltaics, with the characterization of their spectroscopic properties vital to improving the potential of such technologies. Here, we use density functional theory and many-body perturbation theory within the GW approximation to explore quantitatively the electronic structure of prototypical organic semiconductor crystals and compare directly with valence-band photoemission data. For pentacene and PTCDA, computed gas-phase ionization energies and electron affinities are compared with calculated crystal-phase quasiparticle band structures, and relationships between shifts in orbital energy with change of phase and static polarization of the bulk are discussed and compared with experiment. We acknowledge DOE, NSF, BASF, and ISF for financial support, and NERSC for computational resources.

  11. Crystal structure analysis and first principle investigation of F doping in LiFePO4

    NASA Astrophysics Data System (ADS)

    Milović, Miloš; Jugović, Dragana; Cvjetićanin, Nikola; Uskoković, Dragan; Milošević, Aleksandar S.; Popović, Zoran S.; Vukajlović, Filip R.

    2013-11-01

    This work presents the synthesis of F-doped LiFePO4/C composite by the specific modification of the recently suggested synthesis procedure based on an aqueous precipitation of precursor material in molten stearic acid, followed by a high temperature treatment. Besides the lattice parameters and the primitive cell volume reductions, compared to the undoped sample synthesized under the same conditions, the Rietveld refinement also shows that fluorine ions preferably occupy specific oxygen sites. Particularly, the best refinement is accomplished when fluorine ions occupy O(2) sites exclusively. By means of up-to-date electronic structure and total energy calculations this experimental finding is theoretically confirmed. Such fluorine doping also produces closing of the gap in the electronic structure and consequently better conductivity properties of the doped compound. In addition, the morphological and electrochemical performances of the synthesized powder are fully characterized.

  12. First principles studies of structure stability and lithium intercalation of ZnCo2 O4

    NASA Astrophysics Data System (ADS)

    Zhang, Yanning; Liu, Weiwei; Beijing Computational Science Research Center Team

    Among the metal oxides, which are the most widely investigated alternative anodes for use in lithium ion batteries (LIBs), binary and ternary transition metal oxides have received special attention due to their high capacity values. ZnCo2O4 is a promising candidate as anode for LIB, and one can expect a total capacity corresponding to 7.0 - 8.33 mol of recyclable Li per mole of ZnCo2O4. Here we studied the structural stability, electronic properties, lithium intercalation and diffusion barrier of ZnCo2O4 through density functional calculations. The calculated structural and energetic parameters are comparable with experiments. Our theoretical studies provide insights in understanding the mechanism of lithium ion displacement reactions in this ternary metal oxide.

  13. Structure and density of basaltic melts at mantle conditions from first-principles simulations

    PubMed Central

    Bajgain, Suraj; Ghosh, Dipta B.; Karki, Bijaya B.

    2015-01-01

    The origin and stability of deep-mantle melts, and the magmatic processes at different times of Earth's history are controlled by the physical properties of constituent silicate liquids. Here we report density functional theory-based simulations of model basalt, hydrous model basalt and near-MORB to assess the effects of iron and water on the melt structure and density, respectively. Our results suggest that as pressure increases, all types of coordination between major cations and anions strongly increase, and the water speciation changes from isolated species to extended forms. These structural changes are responsible for rapid initial melt densification on compression thereby making these basaltic melts possibly buoyantly stable at one or more depths. Our finding that the melt-water system is ideal (nearly zero volume of mixing) and miscible (negative enthalpy of mixing) over most of the mantle conditions strengthens the idea of potential water enrichment of deep-mantle melts and early magma ocean. PMID:26450568

  14. Thermodynamic stability and structures of iron chloride surfaces: A first-principles investigation

    SciTech Connect

    Saraireh, Sherin A.; Altarawneh, Mohammednoor

    2014-08-07

    In this study, we report a comprehensive density functional theory investigation of the structure and thermodynamic stability of FeCl{sub 2} and FeCl{sub 3} surfaces. Calculated lattice constants and heats of formation for bulk FeCl{sub 2} and FeCl{sub 3} were found to be in relatively good agreement with experimental measurements. We provide structural parameters for 15 distinct FeCl{sub 2} and FeCl{sub 3} surfaces along the three low-index orientations. The optimized geometries for all surfaces are compared with analogous bulk values. Ab initio atomistic thermodynamic calculations have been carried out to assess the relative thermodynamic stability of FeCl{sub 2} and FeCl{sub 3} surfaces under practical operating conditions of temperatures and pressures. The FeCl{sub 2} (100-Cl) surface is found to afford the most stable configuration at all experimentally accessible gas phase conditions.

  15. First-Principles Investigation of Structural, Thermal and Transport Properties of Anatase TiO2

    NASA Astrophysics Data System (ADS)

    Naffouti, Wafa; Ben Nasr, Tarek; Meradji, Hocine; Kamoun-Turki, Najoua

    2016-06-01

    A theoretical calculation of the structural, thermal and transport properties of anatase titanium dioxide (TiO2) was investigated with the help of density functional theory and Boltzmann theory. The fully optimized structure was obtained by minimizing the total energy. The variations of the volume (V), bulk modulus (B), Debye temperature (Θ), heat capacities at constant volume (C v ) and constant pressure (C p ), entropy (S), Grüneisen parameter (γ) and thermal expansion coefficient (α) as a function of the pressure (P) and temperature (T) were all obtained and analyzed in detail. Boltzmann theory calculations have been used to evaluate important transport properties such as Seebeck coefficient (S), electrical conductivity (σ), electronic thermal conductivity (K el ) and power factor (S 2 σ) with respect to scattering time (τ) as a function of chemical potential (μ).

  16. Structure and density of basaltic melts at mantle conditions from first-principles simulations.

    PubMed

    Bajgain, Suraj; Ghosh, Dipta B; Karki, Bijaya B

    2015-01-01

    The origin and stability of deep-mantle melts, and the magmatic processes at different times of Earth's history are controlled by the physical properties of constituent silicate liquids. Here we report density functional theory-based simulations of model basalt, hydrous model basalt and near-MORB to assess the effects of iron and water on the melt structure and density, respectively. Our results suggest that as pressure increases, all types of coordination between major cations and anions strongly increase, and the water speciation changes from isolated species to extended forms. These structural changes are responsible for rapid initial melt densification on compression thereby making these basaltic melts possibly buoyantly stable at one or more depths. Our finding that the melt-water system is ideal (nearly zero volume of mixing) and miscible (negative enthalpy of mixing) over most of the mantle conditions strengthens the idea of potential water enrichment of deep-mantle melts and early magma ocean. PMID:26450568

  17. First principles calculation of the structural, electronic, and magnetic properties of Au-Pd atomic chains

    SciTech Connect

    Dave, Mudra R.; Sharma, A. C.

    2015-06-24

    The structural, electronic and magnetic properties of free standing Au-Pd bimetallic atomic chain is studied using ab-initio method. It is found that electronic and magnetic properties of chains depend on position of atoms and number of atoms. Spin polarization factor for different atomic configuration of atomic chain is calculated predicting a half metallic behavior. It suggests a total spin polarised transport in these chains.

  18. Structure and dynamics of the water films confined between edges of pyrophyllite: A first principle study

    NASA Astrophysics Data System (ADS)

    Churakov, Sergey V.

    2007-03-01

    Edge sites of clay minerals play a key role for pH dependent sorption of ions from solutions of electrolytes. Pyrophyllite, Al 2[Si 4O 10](OH) 2, is an important structural prototype for a variety of 2:1 dioctahedral phyllosilicates but in contrast to the other clays has no permanent structural charge. The structure of thin water films confined between most common edges of 1Tc pyrophyllite: (0 1 0), (1 1 0) and (1 0 0), was analyzed by means of ab initio molecular dynamic simulations. The system setup allowed for a full flexibility of the interfaces and a proton exchange between the edges of pyrophyllite and water molecules in solution. The structure of hydrated surfaces is compared with the recent predictions of static geometry optimizations for edge-vacuum interfaces. All surfaces studied reveal a strong hydrophilic character of edge similar to the hydrated silica surface and the facets of simple layered hydroxides. Spontaneous proton transfer between different surface sites were observed in molecular dynamics simulations of the (0 1 0) interface. The proton bound to the tbnd Si sbnd OH site was found to exchange with the tbnd Al sbnd OH group by the mechanism tbnd Si sbnd OH +tbnd Al sbnd OH ↔tbnd Si sbnd O+tbnd Al sbnd OH 2+. The direction of the proton transfer agrees with the scale of relative proton affinities for surface sites obtained from the static calculations. Alternatively, the proton attached to the tbnd Al sbnd OH 2 site exchanges with the tbnd Al sbnd OH group. In both reactions, the protons are transferred through the chains of hydrogen bonds formed between water molecules in the solution and the surface sites. The observed mechanisms might be one of the basic schemes for the surface proton diffusion in compacted clays. Kinetics of the proton transfer at edge sites is limited by the rate of rearrangements of the water molecules near interface.

  19. First-principles study of the structure of water layers on flat and stepped Pb electrodes

    PubMed Central

    Lin, Xiaohang; Evers, Ferdinand

    2016-01-01

    Summary On the basis of perodic density functional theory (DFT) calculations, we have addressed the geometric structures and electronic properties of water layers on flat and stepped Pb surfaces. In contrast to late d-band metals, on Pb(111) the energy minimum structure does not correspond to an ice-like hexagonal arrangement at a coverage of 2/3, but rather to a distorted structure at a coverage of 1 due to the larger lattice constant of Pb. At stepped Pb surfaces, the water layers are pinned at the step edge and form a complex network consisting of rectangles, pentagons and hexagons. The thermal stability of the water layers has been studied by using ab initio molecular dynamics simulations (AIMD) at a temperature of 140 K. Whereas the water layer on Pb(111) is already unstable at this temperature, the water layers on Pb(100), Pb(311), Pb(511) and Pb(711) exhibit a higher stability because of stronger water–water interactions. The vibrational spectra of the water layers at the stepped surfaces show a characteristic splitting into three modes in the O–H stretch region. PMID:27335744

  20. First principles study of the structural, electronic, and transport properties of triarylamine-based nanowires

    SciTech Connect

    Akande, Akinlolu Bhattacharya, Sandip; Cathcart, Thomas; Sanvito, Stefano

    2014-02-21

    We investigate with state of the art density functional theory the structural, electronic, and transport properties of a class of recently synthesized nanostructures based on triarylamine derivatives. First, we consider the single molecule precursors in the gas phase and calculate their static properties, namely (i) the geometrical structure of the neutral and cationic ions, (ii) the electronic structure of the frontier molecular orbitals, and (iii) the ionization potential, hole extraction potential, and internal reorganization energy. This initial study does not evidence any direct correlation between the properties of the individual molecules and their tendency to self-assembly. Subsequently, we investigate the charge transport characteristics of the triarylamine derivatives nanowires, by using Marcus theory. For one derivative we further construct an effective Hamiltonian including intermolecular vibrations and evaluate the mobility from the Kubo formula implemented with Monte Carlo sampling. These two methods, valid respectively in the sequential hopping and polaronic band limit, give us values for the room-temperature mobility in the range 0.1–12 cm{sup 2}/Vs. Such estimate confirms the superior transport properties of triarylamine-based nanowires, and make them an attracting materials platform for organic electronics.

  1. Electronic Structures of Silicene Nanoribbons: Two-Edge-Chemistry Modification and First-Principles Study.

    PubMed

    Yao, Yin; Liu, Anping; Bai, Jianhui; Zhang, Xuanmei; Wang, Rui

    2016-12-01

    In this paper, we investigate the structural and electronic properties of zigzag silicene nanoribbons (ZSiNRs) with edge-chemistry modified by H, F, OH, and O, using the ab initio density functional theory method and local spin-density approximation. Three kinds of spin polarized configurations are considered: nonspin polarization (NM), ferromagnetic spin coupling for all electrons (FM), ferromagnetic ordering along each edge, and antiparallel spin orientation between the two edges (AFM). The H, F, and OH groups modified 8-ZSiNRs have the AFM ground state. The directly edge oxidized (O1) ZSiNRs yield the same energy and band structure for NM, FM, and AFM configurations, owning to the same s p (2) hybridization. And replacing the Si atoms on the two edges with O atoms (O2) yields FM ground state. The edge-chemistry-modified ZSiNRs all exhibit metallic band structures. And the modifications introduce special edge state strongly localized at the Si atoms in the edge, except for the O1 form. The modification of the zigzag edges of silicene nanoribbons is a key issue to apply the silicene into the field effect transistors (FETs) and gives more necessity to better understand the experimental findings. PMID:27550051

  2. First-Principles Study of Electronic Structure of Type I Hybrid Carbon-Silicon Clathrates

    NASA Astrophysics Data System (ADS)

    Chan, Kwai S.; Peng, Xihong

    2016-05-01

    A new class of type I hybrid carbon-silicon clathrates has been designed using computational methods by substituting some of the Si atoms in the silicon clathrate framework with carbon atoms. In this work, the electronic structure of hybrid carbon-silicon clathrates with and without alkaline or alkaline-earth metal guest atoms has been computed within the density functional theory framework. The theoretical calculations indicate that a small number of carbon substitutions in the Si46 framework slightly reduces the density of states (DOS) near the band edge and narrows the bandgap of carbon-silicon clathrates. Weak hybridization of the conduction band occurs when alkaline metal (Li, Na, K) atoms are inserted into the structure, while strong hybridization of the conduction band occurs when alkaline-earth metal (Mg, Ca, Ba) atoms are inserted into the hybrid structure. Empty C y Si46-y clathrates within the composition range of 2 ≤ y ≤ 15 can be tuned to exhibit indirect bandgaps of 1.5 eV or less, and may be considered as potential electronic materials.

  3. First-Principles Study of Electronic Structure of Type I Hybrid Carbon-Silicon Clathrates

    NASA Astrophysics Data System (ADS)

    Chan, Kwai S.; Peng, Xihong

    2016-08-01

    A new class of type I hybrid carbon-silicon clathrates has been designed using computational methods by substituting some of the Si atoms in the silicon clathrate framework with carbon atoms. In this work, the electronic structure of hybrid carbon-silicon clathrates with and without alkaline or alkaline-earth metal guest atoms has been computed within the density functional theory framework. The theoretical calculations indicate that a small number of carbon substitutions in the Si46 framework slightly reduces the density of states (DOS) near the band edge and narrows the bandgap of carbon-silicon clathrates. Weak hybridization of the conduction band occurs when alkaline metal (Li, Na, K) atoms are inserted into the structure, while strong hybridization of the conduction band occurs when alkaline-earth metal (Mg, Ca, Ba) atoms are inserted into the hybrid structure. Empty C y Si46- y clathrates within the composition range of 2 ≤ y ≤ 15 can be tuned to exhibit indirect bandgaps of 1.5 eV or less, and may be considered as potential electronic materials.

  4. First-principles study of formation of Se submonolayer structures on Ru surfaces

    NASA Astrophysics Data System (ADS)

    Stolbov, Sergey

    2010-10-01

    The Ru nanoparticles with Se submonolayer coverage (Se/Ru) demonstrate high electrocatalytic activity toward oxygen reduction reaction (ORR) on cathodes of proton exchange membrane fuel cells. To understand the mechanisms of formation of Se structures on Ru surfaces, the geometric and electronic structures and energetics have been calculated in the present work for various distributions of Se atoms on the Ru(0001) surface and in the vicinity of the edge between the (0001) and (1101) facets. The calculations were performed within the density-functional theory with plane-wave expansion for wave functions and the projector augmented wave potentials. It has been found that due to electronic charge transfer from Ru to Se upon selenium adsorption, Se atoms become negatively charged and repel each other. This repulsion makes compact Se islands on Ru(0001) unstable. Se atoms prefer to separate from each other by the distance of ˜5.47Å or larger, which is possible for all Se adsorbates if coverage is not exceeding 1/3 ML. Further increase in Se coverage weakens Se-Ru bonding. Three-dimensional Se structure such as 4- and 11-atom pyramids are found to decompose spontaneously with scattering of Se atoms over the Ru(0001) surface. The Se adsorbates are also found to repel in the vicinity of the edge between the Ru facets, and a small increase in Se bonding to undercoordinated Ru atom does not change the trend of Se adsorbates to separate from each other. The obtained most stable configurations of Se on Ru with 1/3 ML coverage or less may also be optimal for ORR because they provide Ru sites available for O and OH adsorption.

  5. First-principles study of crystal and electronic structure of rare-earth cobaltites

    NASA Astrophysics Data System (ADS)

    Topsakal, M.; Leighton, C.; Wentzcovitch, R. M.

    2016-06-01

    Using density functional theory plus self-consistent Hubbard U (DFT + Usc) calculations, we have investigated the structural and electronic properties of the rare-earth cobaltites RCoO3 (R = Pr - Lu). Our calculations show the evolution of crystal and electronic structure of the insulating low-spin RCoO3 with increasing rare-earth atomic number (decreasing ionic radius), including the invariance of the Co-O bond distance (dCo-O), the decrease of the Co-O-Co bond angle (Θ), and the increase of the crystal field splitting (ΔCF) and band gap energy (Eg). Agreement with experiment for the latter improves considerably with the use of DFT + Usc and all trends are in good agreement with the experimental data. These trends enable a direct test of prior rationalizations of the trend in spin-gap associated with the spin crossover in this series, which is found to expose significant issues with simple band based arguments. We also examine the effect of placing the rare-earth f-electrons in the core region of the pseudopotential. The effect on lattice parameters and band structure is found to be small, but distinct for the special case of PrCoO3 where some f-states populate the middle of the gap, consistent with the recent reports of unique behavior in Pr-containing cobaltites. Overall, this study establishes a foundation for future predictive studies of thermally induced spin excitations in rare-earth cobaltites and similar systems.

  6. First principles electronic band structure and phonon dispersion curves for zinc blend beryllium chalcogenide

    SciTech Connect

    Dabhi, Shweta Mankad, Venu Jha, Prafulla K.

    2014-04-24

    A detailed theoretical study of structural, electronic and Vibrational properties of BeX compound is presented by performing ab-initio calculations based on density-functional theory using the Espresso package. The calculated value of lattice constant and bulk modulus are compared with the available experimental and other theoretical data and agree reasonably well. BeX (X = S,Se,Te) compounds in the ZB phase are indirect wide band gap semiconductors with an ionic contribution. The phonon dispersion curves are represented which shows that these compounds are dynamically stable in ZB phase.

  7. Surface structure and hole localization in bismuth vanadate: A first principles study

    NASA Astrophysics Data System (ADS)

    Kweon, Kyoung E.; Hwang, Gyeong S.

    2013-09-01

    The monoclinic and tetragonal phases of bismuth vanadate (BiVO4) have been found to exhibit significantly different photocatalytic activities for water splitting. To assess a possible surface effect on the phase-dependent behavior, we calculate and compare the geometries and electronic structures of the monoclinic and tetragonal BiVO4 (001) surfaces using hybrid density functional theory. The relaxed atomic configurations of these two surfaces are found to be nearly identical, while an excess hole shows a relatively stronger tendency to localize at the surface than the bulk in both phases. Possible factors for the phase-dependent photocatalytic activity of BiVO4 are discussed.

  8. First-principles investigation of structural, mechanical, electronic, and bonding properties of NaZnSb

    NASA Astrophysics Data System (ADS)

    Gu, Jian-Bing; Wang, Chen-Ju; Zhang, Lin; Cheng, Yan; Yang, Xiang-Dong

    2015-08-01

    The structural, mechanical, electronic, and bonding properties and phase transition of NaZnSb are explored using the generalized gradient approximation based on ab initio plane-wave pseudopotential density functional theory.With the help of the quasi-harmonic Debye model, we probe the Grüneisen parameter, thermal expansivity, heat capacity, Debye temperature, and entropy of NaZnSb in the tetragonal phase. The results indicate that the lattice constants and the bulk modulus and its first pressure derivative agree well with the available theoretical and experimental data. NaZnSb in its ground state structure exhibits a distinct energy gap of about 0.41 eV, which increases with increasing pressure. Our conclusions are consistent with the theoretical predictions obtained by the ABINIT package, but are different from those obtained through the tight-binding linear muffin-tin orbital method. As a result, further experimental and theoretical researches need to be carried out. For the purpose of providing a comparative and complementary study for future research, we first investigate the thermodynamic properties of NaZnSb.

  9. Structural and configurational properties of nanoconfined monolayer ice from first principles

    NASA Astrophysics Data System (ADS)

    Corsetti, Fabiano; Matthews, Paul; Artacho, Emilio

    2016-01-01

    Understanding the structural tendencies of nanoconfined water is of great interest for nanoscience and biology, where nano/micro-sized objects may be separated by very few layers of water. Here we investigate the properties of ice confined to a quasi-2D monolayer by a featureless, chemically neutral potential, in order to characterize its intrinsic behaviour. We use density-functional theory simulations with a non-local van der Waals density functional. An ab initio random structure search reveals all the energetically competitive monolayer configurations to belong to only two of the previously-identified families, characterized by a square or honeycomb hydrogen-bonding network, respectively. We discuss the modified ice rules needed for each network, and propose a simple point dipole 2D lattice model that successfully explains the energetics of the square configurations. All identified stable phases for both networks are found to be non-polar (but with a topologically non-trivial texture for the square) and, hence, non-ferroelectric, in contrast to previous predictions from a five-site empirical force-field model. Our results are in good agreement with very recently reported experimental observations.

  10. First principles study of the structural, electronic and optical properties of crystalline o-phenanthroline

    NASA Astrophysics Data System (ADS)

    Nejatipour, Hajar; Dadsetani, Mehrdad

    2016-05-01

    In a comprehensive study, structural properties, electronic structure and optical response of crystalline o-phenanthroline were investigated. Our results show that in generalized gradient approximation (GGA) approximation, o-phenanthroline is a direct bandgap semiconductor of 2.60 eV. In the framework of many-body approach, by solving the Bethe-Salpeter equation (BSE), dielectric properties of crystalline o-phenanthroline were studied and compared with phenanthrene. Highly anisotropic components of the imaginary part of the macroscopic dielectric function in o-phenanthroline show four main excitonic features in the bandgap region. In comparison to phenanthrene, these excitons occur at lower energies. Due to smaller bond lengths originated from the polarity nature of bonds in presence of nitrogen atoms, denser packing, and therefore, a weaker screening effect, exciton binding energies in o-phenanthroline were found to be larger than those in phenanthrene. Our results showed that in comparison to the independent-particle picture, excitonic effects highly redistribute the oscillator strength.

  11. Structure and Properties of the Fe/Y2O3 Interface from First Principles Calculations

    SciTech Connect

    Choudhury, Samrat; Stanek, Christopher R.; Uberuaga, Blas P.

    2012-07-31

    Fundamentals of radiation damage are: (1) Formation of Frenkel pair (interstitial-vacancy pair) defects in the lattice; (2) Concentration of Frenkel pair defects >>> thermal equilibrium thermodynamic concentration; and (3) The radiation damage response of a material is determined by the fate of these excess Frenkel pair defects in the lattice. The objective is to understand the electronic and atomic structure of Fe/Y{sub 2}O{sub 3} interface and segregation behavior of the alloying elements at the interface. The significance of the results of this report are: (1) Provides a science based approach to design new radiation resistant materials. Obtained two controlling parameters - Dislocation density (composition, orientation relationship) and Oxygen partial pressure; (2) Applicable to any other metal/oxide interfaces (both functional and structural properties at the interface) - (a) Nano Catalysts: Oxide-supported metal catalysts Ni/ZrO{sub 2}, (b) Thermal barrier coatings (Ni/Al{sub 2}O{sub 3}), (c) Corrosion of metals and alloys.

  12. First-principles study of structural, electronic and optical properties of ZnF2

    NASA Astrophysics Data System (ADS)

    Wu, Jian-Bang; Cheng, Xin-Lu; Zhang, Hong; Xiong, Zheng-Wei

    2014-07-01

    The structural, electronic, and optical properties of rutile—, CaCl2-, and PdF2—ZnF2 are calculated by the plane-wave pseudopotential method within the density functional theory. The calculated equilibrium lattice constants are in reasonable agreement with the available experimental and other calculated results. The band structures show that the rutile—, CaCl2-, and PdF2—ZnF2 are all direct band insulator. The band gaps are 3.63, 3.62, and 3.36 eV, respectively. The contribution of the different bands was analyzed by the density of states. The Mulliken population analysis is performed. A mixture of covalent and weak ionic chemical bonding exists in ZnF2. Furthermore, in order to understand the optical properties of ZnF2, the dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, and optical reflectivity are also performed in the energy range from 0 to 30 eV. It is found that the main absorption parts locate in the UV region for ZnF2. This is the first quantitative theoretical prediction of the electronic and optical properties of ZnF2 compound, and it still awaits experimental confirmation.

  13. Structural and configurational properties of nanoconfined monolayer ice from first principles.

    PubMed

    Corsetti, Fabiano; Matthews, Paul; Artacho, Emilio

    2016-01-01

    Understanding the structural tendencies of nanoconfined water is of great interest for nanoscience and biology, where nano/micro-sized objects may be separated by very few layers of water. Here we investigate the properties of ice confined to a quasi-2D monolayer by a featureless, chemically neutral potential, in order to characterize its intrinsic behaviour. We use density-functional theory simulations with a non-local van der Waals density functional. An ab initio random structure search reveals all the energetically competitive monolayer configurations to belong to only two of the previously-identified families, characterized by a square or honeycomb hydrogen-bonding network, respectively. We discuss the modified ice rules needed for each network, and propose a simple point dipole 2D lattice model that successfully explains the energetics of the square configurations. All identified stable phases for both networks are found to be non-polar (but with a topologically non-trivial texture for the square) and, hence, non-ferroelectric, in contrast to previous predictions from a five-site empirical force-field model. Our results are in good agreement with very recently reported experimental observations. PMID:26728125

  14. Structural and excited-state properties of oligoacene crystals from first principles

    NASA Astrophysics Data System (ADS)

    Rangel, Tonatiuh; Berland, Kristian; Sharifzadeh, Sahar; Brown-Altvater, Florian; Lee, Kyuho; Hyldgaard, Per; Kronik, Leeor; Neaton, Jeffrey B.

    2016-03-01

    Molecular crystals are a prototypical class of van der Waals (vdW) bound organic materials with excited-state properties relevant for optoelectronics applications. Predicting the structure and excited-state properties of molecular crystals presents a challenge for electronic structure theory, as standard approximations to density functional theory (DFT) do not capture long-range vdW dispersion interactions and do not yield excited-state properties. In this work, we use a combination of DFT including vdW forces, using both nonlocal correlation functionals and pairwise correction methods, together with many-body perturbation theory (MBPT) to study the geometry and excited states, respectively, of the entire series of oligoacene crystals, from benzene to hexacene. We find that vdW methods can predict lattice constants within 1% of the experimental measurements, on par with the previously reported accuracy of pairwise approximations for the same systems. We further find that excitation energies are sensitive to geometry, but if optimized geometries are used MBPT can yield excited-state properties within a few tenths of an eV from experiment. We elucidate trends in MBPT-computed charged and neutral excitation energies across the acene series and discuss the role of common approximations used in MBPT.

  15. The ultraviolet spectrum of OCS from first principles: Electronic transitions, vibrational structure and temperature dependence

    NASA Astrophysics Data System (ADS)

    Schmidt, J. A.; Johnson, M. S.; McBane, G. C.; Schinke, R.

    2012-08-01

    Global three dimensional potential energy surfaces and transition dipole moment functions are calculated for the lowest singlet and triplet states of carbonyl sulfide at the multireference configuration interaction level of theory. The first ultraviolet absorption band is then studied by means of quantum mechanical wave packet propagation. Excitation of the repulsive 2 1A' state gives the main contribution to the cross section. Excitation of the repulsive 1 1A″ state is about a factor of 20 weaker at the absorption peak (Eph ≈ 45 000 cm-1) but becomes comparable to the 2 1A' state absorption with decreasing energy (35 000 cm-1) and eventually exceeds it. Direct excitation of the repulsive triplet states is negligible except at photon energies Eph < 38 000 cm-1. The main structure observed in the cross section is caused by excitation of the bound 2 3A″ state, which is nearly degenerate with the 2 1A' state in the Franck-Condon region. The structure observed in the low energy tail of the spectrum is caused by excitation of quasi-bound bending vibrational states of the 2 1A' and 1 1A″ electronic states. The absorption cross sections agree well with experimental data and the temperature dependence of the cross section is well reproduced.

  16. Structural and configurational properties of nanoconfined monolayer ice from first principles

    PubMed Central

    Corsetti, Fabiano; Matthews, Paul; Artacho, Emilio

    2016-01-01

    Understanding the structural tendencies of nanoconfined water is of great interest for nanoscience and biology, where nano/micro-sized objects may be separated by very few layers of water. Here we investigate the properties of ice confined to a quasi-2D monolayer by a featureless, chemically neutral potential, in order to characterize its intrinsic behaviour. We use density-functional theory simulations with a non-local van der Waals density functional. An ab initio random structure search reveals all the energetically competitive monolayer configurations to belong to only two of the previously-identified families, characterized by a square or honeycomb hydrogen-bonding network, respectively. We discuss the modified ice rules needed for each network, and propose a simple point dipole 2D lattice model that successfully explains the energetics of the square configurations. All identified stable phases for both networks are found to be non-polar (but with a topologically non-trivial texture for the square) and, hence, non-ferroelectric, in contrast to previous predictions from a five-site empirical force-field model. Our results are in good agreement with very recently reported experimental observations. PMID:26728125

  17. First principles simulations of liquid semiconductors: Electronic, structural and dynamic properties

    NASA Astrophysics Data System (ADS)

    Godlevsky, Vitaliy

    We develop ab initio molecular dynamics simulation technique to examine liquid semiconductors. Our methods use quantum interatomic forces, computed within the pseudopotential-density functional method (PDFM). In our work, we study typical representatives of IV, III-V and II-VI materials: Si, Ge, GaAs and CdTe. We show that, upon melting, IV and III-V semiconductors experience semiconductor → metal transition, while more ionic II-VI compounds remain semiconductors in the melt. Metallic type conductivity of liquid IV and III-V materials results from the structural changes of the systems in the melt. In our simulations, "open" zinc-blende (diamond for Si and Ge) structures transform into a more close-packed configuration during solid → liquid transition. Their coordination number, equal to 4 in the crystalline phase, changes to ˜6 in the liquid. We demonstrate that this leads to the breaking of covalent bonds and delocalization of electrons. According to our results, the density of states function of liquid IV and III-V semiconductors has a well defined "free electron" character. For these materials, the electrical conductivity jumps by one to two orders of magnitude during melting. This is opposite to the behavior of the majority of II-VI compounds. In our work, we examine CdTe, typical II-VI semiconductor. Although the dc conductivity of CdTe increases by a factor of 40 as it melts, this material remains a semiconductor in the liquid: its electrical conductivity increases with the temperature. At variance with IV and III-V semiconductors, liquid CdTe retains its tetrahedral environment with the coordination number of ˜4. We discover that a significant number of anion-cation bonds are conserved in liquid CdTe as opposed to IV and III-V materials. This is in agreement with the small entropy change observed in the melting process of CdTe. In our simulations, we find that further heating of molten CdTe results in significant structural changes with a

  18. First-principles investigations of electronic structures of pristine and doped anatase titanium dioxide

    NASA Astrophysics Data System (ADS)

    Wang, Yushan

    2007-12-01

    The formation and development of quantum theory in the first half of the 20th century has led to a revolution in our understanding of pure and applied physics. Quantum theory has nowadays demonstrated a surprisingly accurate and predictive power in modern science and engineering. In this study, an important branch of quantum theory, density functional theory (DFT), is applied to studies of TiO2 and doped TiO2, which have shown considerable applications in industry. The first chapter is an introduction to the theoretical background of DFT, in which a large quantity of efforts are focused on the analysis of exchange-correlation energy and how to approximate it by using local density approximation (LDA), generalized gradient approximation (GGA), and LDA+U, where the U is the Hubbard coefficient. This is followed in the second chapter by a discussion of practical implementations of the DFT-based calculations. We primarily introduce linearized augmented plane wave (LAPW) and augmented plane wave plus local orbital (APW+LO) methods, both of which are applied in our calculations. In chapter 3, we briefly introduce some fundamental properties of TiO2 and its applications in industry. Chapters 4 through 8 are divided into two categories. Chapters 4 through 6 are mainly concerned with insights into the mechanism of optical excitation in anatase TiO2. Chapters 7 and 8 are concerned with TiO2-based dilute magnetic semiconductors (DMS). Chapter 4 presents detailed calculations on pristine TiO2, including the structural optimization, density of states (DOS), band structure, and optical properties. Our calculations involve both bulk and slab TiO 2, presenting reasonable results without considering inherent drawbacks of the calculation methods involved. Calculations on slab TiO2 provide insight to account for the particular property of TiO2 in nanoscale particles where a significant fraction of atoms are on the surface. In chapter 5, we investigate effects of the non-metal dopants

  19. Structure and Formation of Synthetic Hemozoin: Insights from First Principles Calculations

    NASA Astrophysics Data System (ADS)

    Marom, Noa; Tkatchenko, Alexandre; Kapishnikov, Sergey; Kronik, Leeor; Leiserowitz, Leslie

    2011-03-01

    Malaria has reemerged due to parasite resistance to synthetic drugs that act by inhibiting crystallization of the malaria pigment, hemozoin (HZ). Understanding the process of HZ nucleation is therefore vital. The crystal structure of synthetic HZ, β -hematin (β H), has recently been determined via x-ray diffraction. We employ van der Waals (vdW) corrected density functional theory to study the β H crystal and its repeat unit, a heme dimer. We find that vdW interactions play a major role in the binding of the heme dimer and the β H crystal. Accounting for the β H periodicity is a must for obtaining the correct geometry of the heme dimer, due to vdW interactions with adjacent dimers. The different isomers of the heme dimer are close in energy, consistent with the observed pseudo-polymorphism. We use these findings to comment on β H crystallization mechanisms.

  20. Electronic Structure of ABC-stacked Multilayer Graphene and Trigonal Warping:A First Principles Calculation

    NASA Astrophysics Data System (ADS)

    Yelgel, Celal

    2016-04-01

    We present an extensive density functional theory (DFT) based investigation of the electronic structures of ABC–stacked N–layer graphene. It is found that for such systems the dispersion relations of the highest valence and the lowest conduction bands near the K point in the Brillouin zone are characterised by a mixture of cubic, parabolic, and linear behaviours. When the number of graphene layers is increased to more than three, the separation between the valence and conduction bands decreases up until they touch each other. For five and six layer samples these bands show flat behaviour close to the K point. We note that all states in the vicinity of the Fermi energy are surface states originated from the top and/or bottom surface of all the systems considered. For the trilayer system, N = 3, pronounced trigonal warping of the bands slightly above the Fermi level is directly obtained from DFT calculations.

  1. Effect of tensile strain on the electronic structure of Ge: A first-principles calculation

    SciTech Connect

    Liu, Li; Zhang, Miao; Di, Zengfeng E-mail: shijin.zhao@shu.edu.cn; Hu, Lijuan; Zhao, Shi-Jin E-mail: shijin.zhao@shu.edu.cn

    2014-09-21

    Taking the change of L-point conduction band valley degeneracy under strain into consideration, we investigate the effect of biaxially tensile strain (parallel to the (001), (110), and (111) planes) and uniaxially tensile strain (along the [001], [110], and [111] directions) on the electronic structure of Ge using density functional theory calculations. Our calculation shows that biaxial tension parallel to (001) is the most efficient way to transform Ge into a direct bandgap material among all tensile strains considered. [111]-tension is the best choice among all uniaxial approaches for an indirect- to direct-bandgap transition of Ge. The calculation results, which are further elaborated by bond-orbital approximation, provide a useful guidance on the optical applications of Ge through strain engineering.

  2. First-principles investigations on the electronic structures of U3Si2

    NASA Astrophysics Data System (ADS)

    Wang, Tong; Qiu, Nianxiang; Wen, Xiaodong; Tian, Yonghui; He, Jian; Luo, Kan; Zha, Xianhu; Zhou, Yuhong; Huang, Qing; Lang, Jiajian; Du, Shiyu

    2016-02-01

    U3Si2 has been widely utilized as a high-power uranium fuel for research reactors due to its high density of uranium. However, theoretical investigations on this material are still scarce up to now. For this reason, the computational study via density functional theory (DFT) is performed on the U3Si2 compound in this work. The properties of U3Si2, such as stable crystalline structures, density of states, charge distributions, formation energy of defects, as well as the mechanical properties are explored. The calculation results show that the U3Si2 material is metallic and brittle, which is in good agreement with the previous experimental observations. The formation energy of uranium vacancy defect is predicted to be the lowest, similar with that of UN. The theoretical investigation of this work is expected to provide new insight of uranium silicide fuels.

  3. Surface structure and hole localization in bismuth vanadate: A first principles study

    SciTech Connect

    Kweon, Kyoung E.; Hwang, Gyeong S.

    2013-09-23

    The monoclinic and tetragonal phases of bismuth vanadate (BiVO{sub 4}) have been found to exhibit significantly different photocatalytic activities for water splitting. To assess a possible surface effect on the phase-dependent behavior, we calculate and compare the geometries and electronic structures of the monoclinic and tetragonal BiVO{sub 4} (001) surfaces using hybrid density functional theory. The relaxed atomic configurations of these two surfaces are found to be nearly identical, while an excess hole shows a relatively stronger tendency to localize at the surface than the bulk in both phases. Possible factors for the phase-dependent photocatalytic activity of BiVO{sub 4} are discussed.

  4. Structural, electronic, mechanical, and dynamical properties of graphene oxides: A first principles study

    SciTech Connect

    Dabhi, Shweta D.; Gupta, Sanjay D.; Jha, Prafulla K.

    2014-05-28

    We report the results of a theoretical study on the structural, electronic, mechanical, and vibrational properties of some graphene oxide models (GDO, a-GMO, z-GMO, ep-GMO and mix-GMO) at ambient pressure. The calculations are based on the ab-initio plane-wave pseudo potential density functional theory, within the generalized gradient approximations for the exchange and correlation functional. The calculated values of lattice parameters, bulk modulus, and its first order pressure derivative are in good agreement with other reports. A linear response approach to the density functional theory is used to derive the phonon frequencies. We discuss the contribution of the phonons in the dynamical stability of graphene oxides and detailed analysis of zone centre phonon modes in all the above mentioned models. Our study demonstrates a wide range of energy gap available in the considered models of graphene oxide and hence the possibility of their use in nanodevices.

  5. Structural, electronic, mechanical, and dynamical properties of graphene oxides: A first principles study

    NASA Astrophysics Data System (ADS)

    Dabhi, Shweta D.; Gupta, Sanjay D.; Jha, Prafulla K.

    2014-05-01

    We report the results of a theoretical study on the structural, electronic, mechanical, and vibrational properties of some graphene oxide models (GDO, a-GMO, z-GMO, ep-GMO and mix-GMO) at ambient pressure. The calculations are based on the ab-initio plane-wave pseudo potential density functional theory, within the generalized gradient approximations for the exchange and correlation functional. The calculated values of lattice parameters, bulk modulus, and its first order pressure derivative are in good agreement with other reports. A linear response approach to the density functional theory is used to derive the phonon frequencies. We discuss the contribution of the phonons in the dynamical stability of graphene oxides and detailed analysis of zone centre phonon modes in all the above mentioned models. Our study demonstrates a wide range of energy gap available in the considered models of graphene oxide and hence the possibility of their use in nanodevices.

  6. First principle electronic, structural, elastic, and optical properties of strontium titanate

    NASA Astrophysics Data System (ADS)

    Ekuma, Chinedu E.; Jarrell, Mark; Moreno, Juana; Bagayoko, Diola

    2012-03-01

    We report self-consistent ab-initio electronic, structural, elastic, and optical properties of cubic SrTiO3 perovskite. Our non-relativistic calculations employed a generalized gradient approximation (GGA) potential and the linear combination of atomic orbitals (LCAO) formalism. The distinctive feature of our computations stem from solving self-consistently the system of equations describing the GGA, using the Bagayoko-Zhao-Williams (BZW) method. Our results are in agreement with experimental ones where the later are available. In particular, our theoretical, indirect band gap of 3.24 eV, at the experimental lattice constant of 3.91 Å, is in excellent agreement with experiment. Our predicted, equilibrium lattice constant is 3.92 Å, with a corresponding indirect band gap of 3.21 eV and bulk modulus of 183 GPa.

  7. Re-examining the electronic structure of germanium: A first-principle study

    NASA Astrophysics Data System (ADS)

    Ekuma, C. E.; Jarrell, M.; Moreno, J.; Bagayoko, D.

    2013-11-01

    We report results from an efficient, ab initio method for self-consistent calculations of electronic and structural properties of Ge. Our non-relativistic calculations employed a GGA potential and LCAO formalism. The distinctive feature of our computations stem from the use of Bagayoko-Zhao-Williams-Ekuma-Franklin method. Our results are in agreement with experimental ones where the latter are available. In particular, our theoretical, indirect band gap (EgΓ-L) of 0.65 eV, at the experimental lattice constant of 5.66 Å, is in excellent agreement with experiment. Our predicted, equilibrium lattice constant is 5.63 Å, with corresponding EgΓ-L of 0.65 eV and a bulk modulus of 80 GPa.

  8. First-principles Wannier function analysis of the electronic structure of PdTe: weaker magnetism and superconductivity.

    PubMed

    Ekuma, Chinedu E; Lin, Chia-Hui; Moreno, Juana; Ku, Wei; Jarrell, Mark

    2013-10-01

    We report a first-principles Wannier function study of the electronic structure of PdTe. Its electronic structure is found to be a broad three-dimensional Fermi surface with highly reduced correlation effects. In addition, the higher filling of the Pd d-shell, its stronger covalency resulting from the closer energy of the Pd d and Te p shells, and the larger crystal field effects of the Pd ion due to its near octahedral coordination, all serve to weaken significantly electronic correlations in the particle-hole (spin, charge, and orbital) channel. In comparison to the Fe chalcogenides, e.g. FeSe, we highlight the essential features (quasi-two-dimensionality, proximity to half-filling, weaker covalency, and higher orbital degeneracy) of Fe-based high-temperature superconductors. PMID:24025790

  9. First-Principles Calculation of the Electronic Structure and Magnetism at the GRAPHENE/Ni(111) Interface

    NASA Astrophysics Data System (ADS)

    Chen, L.; Ouyang, Y.; Pan, H. Z.; Sun, Y. Y.; Wang, Y. L.

    A spin-polarized first-principles calculation of the atomic and electronic structure of the graphene/Ni(111) interface is studied. The electronic structure of the graphene layer is strongly modified by interaction with the substrate and a behavior where magnetic moments are localized at the edges of nanoscale holes of isolated graphene does not happen in the defect-graphene/Ni(111) system. The magnetic moment of the surface nickel atoms is lowered in the presence of the graphene layer and nanoscale holes of graphene, which control the strength of the hybridization between electronic states of graphene and Ni substrate. Our findings show that an electron spin in the graphene/Ni(111) interface can be manipulated in a controlled way and have important implications for graphene-based spintronic devices.

  10. On the influence of tetrahedral covalent-hybridization on electronic band structure of topological insulators from first principles

    SciTech Connect

    Zhang, X. M.; Xu, G. Z.; Liu, E. K.; Wang, W. H. Wu, G. H.; Liu, Z. Y.

    2015-01-28

    Based on first-principles calculations, we investigate the influence of tetrahedral covalent-hybridization between main-group and transition-metal atoms on the topological band structures of binary HgTe and ternary half-Heusler compounds, respectively. Results show that, for the binary HgTe, when its zinc-blend structure is artificially changed to rock-salt one, the tetrahedral covalent-hybridization will be removed and correspondingly the topologically insulating band character lost. While for the ternary half-Heusler system, the strength of covalent-hybridization can be tuned by varying both chemical compositions and atomic arrangements, and the competition between tetrahedral and octahedral covalent-hybridization has been discussed in details. As a result, we found that a proper strength of tetrahedral covalent-hybridization is probably in favor to realizing the topologically insulating state with band inversion occurring at the Γ point of the Brillouin zone.

  11. Electronic structure and thermoelectric properties of (PbSe)m/(SnSe)n superlattice: A first principles study

    NASA Astrophysics Data System (ADS)

    Do, Duc Cuong; Rhim, S. H.; Hong, Soon Cheol

    2015-03-01

    Figure of merit (ZT) of thermoelectric materials can be enhanced by lowering thermal conductivity or/and increasing electrical conductivity. The extremely high ZT of layered structure SnSe opened up a new direction in study of thermoelectricity due to its low thermal conductivity, which, however, is limited to high temperature. Here, we performed first principles density functional calculations to explore room-temperature thermoelectricity. We consider (PbSe)m/(SnSe)n superlattices with different period, whose quantum well structure is expected to increase electrical conductivity by modulation of charge doping at interface. Calculations of Seebeck coefficients for the superlattices are presented. Supported by the Ministry of Trade, Industry & Energy, Korea (20132020000110) and Priority Research Centers Program (2009-0093818) through National Research Foundation of Korea.

  12. On the dynamical stability of ferromagnetic Ru and Os in the bct structure: a first-principles study

    NASA Astrophysics Data System (ADS)

    Cifuentes-Quintal, M. E.; de Coss, R.

    2015-08-01

    Recent theoretical studies have predicted magnetic states for Ru and Os in the body-centred tetragonal structure (bct) with ?. In this study, we present first principles calculations of the phonon dispersion for ferromagnetic Ru- and Os-bct along the epitaxial and uniaxial Bain paths, to evaluate their dynamical stability. The phonon dispersions were computed using the density functional perturbation theory, including the gradient corrections to the exchange-correlation functional within the plane-waves ultrasoft-pseudopotential approximation. The phonon dispersion for the local minimum in the Bain path with ? as well as the uniaxial and epitaxial strained structures are analysed. We find imaginary frequencies along different directions of the Brillouin zone, which indicates that both systems are dynamically unstable. Consequently, ferromagnetic Ru and Os in the bct with ? are not truly metastable phases.

  13. First principles study of electronic structures of defects in zirconium germanium phosphate and defect chalcopyrites

    NASA Astrophysics Data System (ADS)

    Jiang, Xiaoshu

    2005-11-01

    This thesis mainly focuses on a study of the point defects in ZnGeP 2. Density functional theory (DFT) is used in the local density approximation (LDA) in conjunction with the full-potential linearized muffin-tin orbital (FP-LMTO) method, modeling defects with the supercell approach. Under prevalent Zn-poor conditions, the GeZn double donor and VZn shallow acceptor are found to have the lowest formation energies, which explains the compensated p-type nature of the material. Good agreement is obtained with the energy levels deduced from optical quenching and activation of the EPR signals, if a direct transfer of electrons from V2-Zn to Ge2+Zn is assumed to occur rather than a process via the conduction band. The VGe is found to have high energy of formation under any chemical potential conditions and is found to be unstable towards formation of VZn and GeZn. Structural relaxation of all defects is performed but no symmetry breaking distortions are found. The defect wavefunctions of the unpaired electron in the V-Zn is found to be spread equally over the four neighboring P atoms, in disagreement with electron nuclear double resonance (ENDOR) data which indicate primary localization on a pair of P atoms. Several possible origins for this discrepancy are examined. Alternative assignments of the AL1 EPR signal to ZnGe, or complexes such as Zni-VZn, V-Zn -G2+Zn-V- Zn are discarded although the latter complex is found to be favorable in energy. The possibility of a failure of the LDA due to its incompletely cancelled self-interaction is examined using Hartree-Fock cluster calculations. A distortion is found to occur in Hartree-Fock but not in LDA. However, it is different from the experimental one. Finally, a dangling bond and group theory model is proposed for a Jahn-Teller distortion which can explain the localization observed by ENDOR. In the final chapter, the electronic band structures of the ordered vacancy defect chalcopyrites with formula II-III2-VI4 for II=Zn, Cd

  14. Unexpected electronic structure of the alloyed and doped arsenene sheets: First-Principles calculations

    NASA Astrophysics Data System (ADS)

    Liu, Ming-Yang; Huang, Yang; Chen, Qing-Yuan; Cao, Chao; He, Yao

    2016-07-01

    We study the equilibrium geometry and electronic structure of alloyed and doped arsenene sheets based on the density functional theory calculations. AsN, AsP and SbAs alloys possess indirect band gap and BiAs is direct band gap. Although AsP, SbAs and BiAs alloyed arsenene sheets maintain the semiconducting character of pure arsenene, they have indirect-direct and semiconducting-metallic transitions by applying biaxial strain. We find that B- and N-doped arsenene render p-type semiconducting character, while C- and O-doped arsenene are metallic character. Especially, the C-doped arsenene is spin-polarization asymmetric and can be tuned into the bipolar spin-gapless semiconductor by the external electric field. Moreover, the doping concentration can effectively affect the magnetism of the C-doped system. Finally, we briefly study the chemical molecule adsorbed arsenene. Our results may be valuable for alloyed and doped arsenene sheets applications in mechanical sensors and spintronic devices in the future.

  15. The electronic band structures of gadolinium chalcogenides: a first-principles prediction for neutron detecting

    NASA Astrophysics Data System (ADS)

    Li, Kexue; Liu, Lei; Yu, Peter Y.; Chen, Xiaobo; Shen, D. Z.

    2016-05-01

    By converting the energy of nuclear radiation to excited electrons and holes, semiconductor detectors have provided a highly efficient way for detecting them, such as photons or charged particles. However, for detecting the radiated neutrons, those conventional semiconductors hardly behave well, as few of them possess enough capability for capturing these neutral particles. While the element Gd has the highest nuclear cross section, here for searching proper neutron-detecting semiconductors, we investigate theoretically the Gd chalcogenides whose electronic band structures have never been characterized clearly. Among them, we identify that γ-phase Gd2Se3 should be the best candidate for neutron detecting since it possesses not only the right bandgap of 1.76 eV for devices working under room temperature but also the desired indirect gap nature for charge carriers surviving longer. We propose further that semiconductor neutron detectors with single-neutron sensitivity can be realized with such a Gd-chalcogenide on the condition that their crystals can be grown with good quality.

  16. The electronic band structures of gadolinium chalcogenides: a first-principles prediction for neutron detecting.

    PubMed

    Li, Kexue; Liu, Lei; Yu, Peter Y; Chen, Xiaobo; Shen, D Z

    2016-05-11

    By converting the energy of nuclear radiation to excited electrons and holes, semiconductor detectors have provided a highly efficient way for detecting them, such as photons or charged particles. However, for detecting the radiated neutrons, those conventional semiconductors hardly behave well, as few of them possess enough capability for capturing these neutral particles. While the element Gd has the highest nuclear cross section, here for searching proper neutron-detecting semiconductors, we investigate theoretically the Gd chalcogenides whose electronic band structures have never been characterized clearly. Among them, we identify that γ-phase Gd2Se3 should be the best candidate for neutron detecting since it possesses not only the right bandgap of 1.76 eV for devices working under room temperature but also the desired indirect gap nature for charge carriers surviving longer. We propose further that semiconductor neutron detectors with single-neutron sensitivity can be realized with such a Gd-chalcogenide on the condition that their crystals can be grown with good quality. PMID:27049355

  17. Unexpected electronic structure of the alloyed and doped arsenene sheets: First-Principles calculations

    PubMed Central

    Liu, Ming-Yang; Huang, Yang; Chen, Qing-Yuan; Cao, Chao; He, Yao

    2016-01-01

    We study the equilibrium geometry and electronic structure of alloyed and doped arsenene sheets based on the density functional theory calculations. AsN, AsP and SbAs alloys possess indirect band gap and BiAs is direct band gap. Although AsP, SbAs and BiAs alloyed arsenene sheets maintain the semiconducting character of pure arsenene, they have indirect-direct and semiconducting-metallic transitions by applying biaxial strain. We find that B- and N-doped arsenene render p-type semiconducting character, while C- and O-doped arsenene are metallic character. Especially, the C-doped arsenene is spin-polarization asymmetric and can be tuned into the bipolar spin-gapless semiconductor by the external electric field. Moreover, the doping concentration can effectively affect the magnetism of the C-doped system. Finally, we briefly study the chemical molecule adsorbed arsenene. Our results may be valuable for alloyed and doped arsenene sheets applications in mechanical sensors and spintronic devices in the future. PMID:27373712

  18. Unexpected electronic structure of the alloyed and doped arsenene sheets: First-Principles calculations.

    PubMed

    Liu, Ming-Yang; Huang, Yang; Chen, Qing-Yuan; Cao, Chao; He, Yao

    2016-01-01

    We study the equilibrium geometry and electronic structure of alloyed and doped arsenene sheets based on the density functional theory calculations. AsN, AsP and SbAs alloys possess indirect band gap and BiAs is direct band gap. Although AsP, SbAs and BiAs alloyed arsenene sheets maintain the semiconducting character of pure arsenene, they have indirect-direct and semiconducting-metallic transitions by applying biaxial strain. We find that B- and N-doped arsenene render p-type semiconducting character, while C- and O-doped arsenene are metallic character. Especially, the C-doped arsenene is spin-polarization asymmetric and can be tuned into the bipolar spin-gapless semiconductor by the external electric field. Moreover, the doping concentration can effectively affect the magnetism of the C-doped system. Finally, we briefly study the chemical molecule adsorbed arsenene. Our results may be valuable for alloyed and doped arsenene sheets applications in mechanical sensors and spintronic devices in the future. PMID:27373712

  19. First-principles electronic structure and relative stability of pyrite and marcasite: Implications for photovoltaic performance

    NASA Astrophysics Data System (ADS)

    Sun, Ruoshi; Chan, M. K. Y.; Ceder, G.

    2011-06-01

    Despite the many advantages (e.g., suitable band gap, exceptional optical absorptivity, earth abundance) of pyrite as a photovoltaic material, its low open-circuit voltage (OCV) has remained the biggest challenge preventing its use in practical devices. Two of the most widely accepted reasons for the cause of the low OCV are (i) Fermi level pinning due to intrinsic surface states that appear as gap states, and (ii) the presence of the metastable polymorph, marcasite. In this paper, we investigate these claims, via density-functional theory, by examining the electronic structure, bulk, surface, and interfacial energies of pyrite and marcasite. Regardless of whether the Hubbard U correction is applied, the intrinsic {100} surface states are found to be of dz2 character, as expected from ligand field theory. However, they are not gap states but rather located at the conduction-band edge. Thus, ligand field splitting at the symmetry-broken surface cannot be the sole cause of the low OCV. We also investigate epitaxial growth of marcasite on pyrite. Based on the surface, interfacial, and strain energies of pyrite and marcasite, we find from our model that only one layer of epitaxial growth of marcasite is thermodynamically favorable. Within all methods used (LDA, GGA-PBE, GGA-PBE+U, GGA-AM05, GGA-AM05+U, HSE06, and Δ-sol), the marcasite band gap is not less than the pyrite band gap, and is even larger than the experimental marcasite gap. Moreover, gap states are not observed at the pyrite-marcasite interface. We conclude that intrinsic surface states or the presence of marcasite are unlikely to undermine the photovoltaic performance of pyrite.

  20. Linear optical properties and electronic structures of poly(3-hexylthiophene) and poly(3-hexylselenophene) crystals from first principles

    NASA Astrophysics Data System (ADS)

    Tsumuraya, Takao; Song, Jung-Hwan; Freeman, Arthur J.

    2012-08-01

    Linear optical properties of regio-regular-poly(3-hexythiophene) (rr-P3HT) and regio-regular-poly(3-hexyselenophene) (rr-P3HS) are investigated in relation to their anisotropic crystal structure by means of first-principles density functional calculations. The optical spectra are evaluated by calculating its dielectric functions, focusing on the frequency dependence of the imaginary part. The optical transition along the π conjugation-connecting backbone direction is found to be the most significant at the band edges. A group-theoretical analysis of the matrix elements is given to explain the interband transitions. The optical spectra, electronic structures, and structural stabilities are calculated using the all-electron full-potential linearized augmented plane wave (FLAPW) method within the local-density approximation. We proposed several possible crystal structures of rr-P3HT and performed structural optimizations to determine a stable structure. Comparing the total energy differences among these relaxed structures, a base-centered monoclinic structure belonging to the space group A2 is found to be the most stable structure. In the electronic structure, C and S orbitals belonging to polythiophene backbones are the biggest contributors at the valence band maximum and conduction band minimum, but there is almost no contribution from the hexyl side chains. Last, the differences in electronic and optical properties between rr-P3HT and rr-P3HS are discussed.

  1. Electronic Structure of Organic/Inorganic Interfaces: Insights from First Principles Calculations

    NASA Astrophysics Data System (ADS)

    Segev, Lior

    Electronic devices based on molecules draw a lot of attention in both scientific and industrial activities. Molecules in electronic devices can serve as the heart of the device, featuring versatile physical properties i.e. electronical, optical, magnetic, etc. Molecules can also function as an assist mechanism in which the electronic properties of the underlying material are modified in a predictable fashion according to the molecular monolayer properties. But, the route to applications in both these directions lies in answering fundamental questions related to band offsets between two materials, full electronic structure determination of molecule and substrates, work function modifications, etc. To tackle these questions, we chose to study the interface formed by an alkyl monolayer adsorbed on a Si substrate by utilizing two ab initio methods. First, the density functional theory (DFT) utilizing the local density or the B3LYP approximations for the exchange-correlation potential and, second, the many-body perturbation theory based on the GW approximation. We adapted a "divide and conquer" approach to our system by simulating the infinite counterpart, polyethylene, of our finite alkyl chain to test how the band gap of the two molecules changes when moving from an infinite 1D molecule to a finite length molecule. We find excellent agreement between our GW simulation results for polyethylene and experimental results for the bandstructure, ionization potential and band gap values. From DFT simulations, we analyze the ultra-violet photoelectron spectra (UPS) of odd and even number of carbons alkyl chains and identify the origin of their differences in spectral signature. GW simulations of the full alkyl monolayer/Si(111) system reveal that the projected density of states (DOS) of the upper alkyl chain have an excellent agreement to experimental UPS and inverse-photoemission spectra results. Based on this correspondence, we find the band alignment between the alkyl

  2. Structural, electronic and magnetic properties of (N, C)-codoped ZnO nanotube: First principles study

    NASA Astrophysics Data System (ADS)

    Esmailian, Amirhosein; Shahrokhi, Masoud; Kanjouri, Faramarz

    2015-04-01

    We have studied the electronic structure and magnetic properties of Nitrogen and Carbon codoped ZnO (5,0) single-walled zigzag nanotube using first-principle calculations based on the density functional theory. We performed our calculations for N- and C- codoping ZnO nanotube in two different configurations. For the first configuration in which the two impurity atoms (N or C) are on first nearest-neighbor sites in the plane of codoping, our calculation predicts that the N- and C-codoped ZnO nanotubes are antiferromagnetic material with no net magnetization. On the other hand, it is found that for the configuration in which the two impurity atoms are next nearest-neighbors, a spin polarization results in a magnetic moment in the N- and C-codoped ZnO nanotubes.

  3. Strong interplay between structure and electronic properties in CuIn(S,Se){2}: a first-principles study.

    PubMed

    Vidal, Julien; Botti, Silvana; Olsson, Pär; Guillemoles, Jean-François; Reining, Lucia

    2010-02-01

    We present a first-principles study of the electronic properties of CuIn(S,Se){2} (CIS) using state-of-the-art self-consistent GW and hybrid functionals. The calculated band gap depends strongly on the anion displacement u, an internal structural parameter that measures lattice distortion. This contrasts with the observed stability of the band gap of CIS solar panels under operating conditions, where a relatively large dispersion of values for u occurs. We solve this apparent paradox considering the coupled effect on the band gap of copper vacancies and lattice distortions. The correct treatment of d electrons in these materials requires going beyond density functional theory, and GW self-consistency is critical to evaluate the quasiparticle gap and the valence band maximum. PMID:20366776

  4. Structural, electronic and magnetic properties of the (Co, Ni) codoped ZnS: A first-principles study

    NASA Astrophysics Data System (ADS)

    Yin, Zhu-Hua; Zhang, Jian-Min

    2016-08-01

    Using spin-polarized first-principles calculation, we investigate the structural, electronic and magnetic properties of the Zn31Co1S32, Zn31Ni1S32 and Zn30Co1Ni1S32 systems. The results show that the Zn31Co1S32 system is a magnetic semiconductor, while the Zn31Ni1S32 system exhibits a magnetic half-metallic (HM) character. The Zn30Co1Ni1S32 system exhibits a HM ferrimagnetic character explained by the superexchange mechanism. The Co and Ni atoms favorably occupy nearest neighbor positions of the metal sublattice with antiparallel arrangement of their magnetic moments. Furthermore, it is observed that the magnetic moment of Co/Ni atom reduces from an isolated atom magnetic moment due to p-d hybridization which yields small parallel magnetic moments on the nearest S atoms.

  5. First principles DFT investigation of yttrium-decorated boron-nitride nanotube: Electronic structure and hydrogen storage

    NASA Astrophysics Data System (ADS)

    Jain, Richa Naja; Chakraborty, Brahmananda; Ramaniah, Lavanya M.

    2015-06-01

    The electronic structure and hydrogen storage capability of Yttrium-doped BNNTs has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom prefers the hollow site in the center of the hexagonal ring with a binding energy of 0.8048eV. Decorating by Y makes the system half-metallic and magnetic with a magnetic moment of 1.0µB. Y decorated Boron-Nitride (8,0) nanotube can adsorb up to five hydrogen molecules whose average binding energy is computed as 0.5044eV. All the hydrogen molecules are adsorbed with an average desorption temperature of 644.708 K. Taking that the Y atoms can be placed only in alternate hexagons, the implied wt% comes out to be 5.31%, a relatively acceptable value for hydrogen storage materials. Thus, this system can serve as potential hydrogen storage medium.

  6. Phase relationships and structures of inorganic crystals by a combination of the cluster expansion method and first principles calculations.

    PubMed

    Tanaka, Isao; Seko, Atsuto; Togo, Atsushi; Koyama, Yukinori; Oba, Fumiyasu

    2010-09-29

    Properties of crystalline solutions are generally dependent not only on their chemical composition but also on the configurations of solute atoms and/or point defects. Quantitative knowledge of the configuration-dependent properties is therefore essential for materials design. The cluster expansion (CE) method has been widely used to describe the configurational properties. Increases in computational power and advances in numerical techniques enable us to perform a large set of systematic first principles calculations based on density functional theory (DFT) to be combined with CE calculations. In this paper, our procedure of CE with optimal selections of clusters and DFT structures is described. Two examples of such calculations are then shown. One is the cation arrangement in a series of spinel oxides. The other is arrangement of the oxygen vacancy in a series of tin sub-dioxides. PMID:21386541

  7. Phase relationships and structures of inorganic crystals by a combination of the cluster expansion method and first principles calculations

    NASA Astrophysics Data System (ADS)

    Tanaka, Isao; Seko, Atsuto; Togo, Atsushi; Koyama, Yukinori; Oba, Fumiyasu

    2010-09-01

    Properties of crystalline solutions are generally dependent not only on their chemical composition but also on the configurations of solute atoms and/or point defects. Quantitative knowledge of the configuration-dependent properties is therefore essential for materials design. The cluster expansion (CE) method has been widely used to describe the configurational properties. Increases in computational power and advances in numerical techniques enable us to perform a large set of systematic first principles calculations based on density functional theory (DFT) to be combined with CE calculations. In this paper, our procedure of CE with optimal selections of clusters and DFT structures is described. Two examples of such calculations are then shown. One is the cation arrangement in a series of spinel oxides. The other is arrangement of the oxygen vacancy in a series of tin sub-dioxides.

  8. Structural, energetic and thermodynamic analyses of Ca(BH4)2·2NH3 from first principles calculations

    NASA Astrophysics Data System (ADS)

    Yuan, Peng-Fei; Wang, Fei; Sun, Qiang; Jia, Yu; Guo, Zheng-Xiao

    2012-01-01

    Ca(BH4)2·2NH3 is a relatively new compound with potential application in hydrogen storage. Here the fundamental properties of the compound, such as electronic structure, energetic and thermodynamic properties, were comprehensively studied using first-principles calculations. Results from electronic density of states (DOS) and electron localization function (ELF) indicate the covalent bond nature of the N-H bond and the B-H bond. Charge density analyses show weak ionic interactions between the Ca atom and the NH3 complexes or the (BH4)- complexes. The calculated vibration frequencies of B-H and N-H are in good agreement with other theoretical and experimental results. Furthermore, we calculated the reaction enthalpy and reaction Gibbs free energy at a range of temperature 0-700 K. Our results are in good agreement with experimental results in literature. Possible reaction mechanism of the decomposition reaction is proposed.

  9. Structural and electronic properties of Li-ion battery cathode material MoF3 from first-principles

    NASA Astrophysics Data System (ADS)

    Li, A. Y.; Wu, S. Q.; Yang, Y.; Zhu, Z. Z.

    2015-07-01

    The transition metal fluorides have been extensively investigated recently as the electrode materials with high working voltage and large capacity. The structural, electronic and magnetic properties of MoF3 are studied by the first-principles calculations within both the generalized gradient approximation (GGA) and GGA+U frameworks. Our results show that the antiferromagnetic configuration of MoF3 is more stable than the ferromagnetic one, which is consistent with experimental results. The analysis of the electronic density of states shows that MoF3 is a Mott-Hubbard insulator with a d-d type band gap, which is similar to the case of FeF3. Moreover, small spin polarizations were found on the sites of fluorine ions, which accords with a fluorine-mediated superexchange mechanism for the Mo-Mo magnetic interaction.

  10. First-principles study on the physical properties of a layered ZnO with hexagonal α-BN structure

    NASA Astrophysics Data System (ADS)

    Su, Y. L.; Zhang, Q. Y.; Zhao, J. J.

    2016-05-01

    A layered ZnO with α-BN structure has been studied using first-principles calculations based on density functional theory. The physical properties of the layered ZnO are calculated in terms of dielectric function, infrared reflectance, elastic coefficients, modulus, hardness, and heat capacity. The layered ZnO exhibits a good infrared reflectance with a broad reststrahlen band covering the infrared band below 600 cm-1. The layered ZnO is predicted to be a material behaving in a brittle manner, with a microhardness ~3.6 times higher than that of the wurtzite ZnO. The temperature-dependent thermodynamic functions suggest that the layered ZnO has the thermal properties similar to those of wurtzite ZnO, but having a little higher Debye temperature above room temperature.

  11. First principles DFT investigation of yttrium-decorated boron-nitride nanotube: Electronic structure and hydrogen storage

    SciTech Connect

    Jain, Richa Naja; Chakraborty, Brahmananda; Ramaniah, Lavanya M.

    2015-06-24

    The electronic structure and hydrogen storage capability of Yttrium-doped BNNTs has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom prefers the hollow site in the center of the hexagonal ring with a binding energy of 0.8048eV. Decorating by Y makes the system half-metallic and magnetic with a magnetic moment of 1.0µ{sub B}. Y decorated Boron-Nitride (8,0) nanotube can adsorb up to five hydrogen molecules whose average binding energy is computed as 0.5044eV. All the hydrogen molecules are adsorbed with an average desorption temperature of 644.708 K. Taking that the Y atoms can be placed only in alternate hexagons, the implied wt% comes out to be 5.31%, a relatively acceptable value for hydrogen storage materials. Thus, this system can serve as potential hydrogen storage medium.

  12. Electronic structure of quasi-one-dimensional superconductor K2Cr3As3 from first-principles calculations

    PubMed Central

    Jiang, Hao; Cao, Guanghan; Cao, Chao

    2015-01-01

    The electronic structure of quasi-one-dimensional superconductor K2Cr3As3 is studied through systematic first-principles calculations. The ground state of K2Cr3As3 is paramagnetic. Close to the Fermi level, the , dxy, and orbitals dominate the electronic states, and three bands cross EF to form one 3D Fermi surface sheet and two quasi-1D sheets. The electronic DOS at EF is less than 1/3 of the experimental value, indicating a large electron renormalization factor around EF. Despite of the relatively small atomic numbers, the antisymmetric spin-orbit coupling splitting is sizable (≈60 meV) on the 3D Fermi surface sheet as well as on one of the quasi-1D sheets. Finally, the imaginary part of bare electron susceptibility shows large peaks at Γ, suggesting the presence of large ferromagnetic spin fluctuation in the compound. PMID:26525099

  13. On structural and lattice dynamic stability of LaF{sub 3} under high pressure: A first principle study

    SciTech Connect

    Sahoo, B. D. Joshi, K. D.; Gupta, Satish C.

    2015-06-24

    Structural and lattice dynamical stability of the LaF3 has been analyzed as a function of hydrostatic compression through first principle electronic band structure calculations. The comparison of enthalpies of various plausible structures calculated at various pressures suggests a phase transition from ambient condition tysonite structure (space group P-3c1) to a primitive orthorhombic structure (space group Pmmn) at a pressure of ∼19.5 GPa, in line with the experimental value of 16 GPa. Further, it is predicted that this phase will remain stable up to 100 GPa (the maximum pressure up to which calculations have been performed in the present work). The theoretically determined equation of state displays a good agreement with experimental data. Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state and compared with the available experimental data. Our lattice dynamic calculations correctly demonstrate that at zero pressure the tysonite structure is lattice dynamically stable whereas the Pmmn structure is unstable lattice dynamically. Further, at transition pressure the theoretically calculated phonon spectra clearly show that the Pmmn phase emerges as lattice dynamically stable phase whereas the tysonite structure becomes unstable dynamically, supporting our static lattice calculations.

  14. Crystal structure prediction of Fe3Se4 using the evolutionary algorithm coupled with first principles DFT simulations

    NASA Astrophysics Data System (ADS)

    Al-Aqtash, Nabil; Sabirianov, Renat

    2014-03-01

    The evolutionary algorithm coupled with the first-principles Density Functional Theory (DFT) method is used to identify the global energy minimum structure of Fe3Se4. The structure is processed by free-energy based evolutionary crystal structure optimization algorithms, as implemented USPEX and XtalOpt codes, which predict structure of the system solely based on the chemical formula without prior experimental information. This is very challenging task for verifying the validity of this approach on Fe3Se4 structure. Fe3Se4 has highly anisotropic structure, and its structure demonstrates ordering of vacancies that makes the system ``open'', i.e. breaking traditional coordination rules. By using USPEX and XtalOpt we identify the global minimum of Fe3Se4 structure. The randomly generated initial population had 20 structures. The enthalpy (tolerance of 0.002 eV), and space group were used for niching. The enthalpy of the lowest energy structure, out of 700 generated structures that were generated, is (-81.126 eV). Bulk Fe3Se4 has a monoclinic structure with a space group of I2/m and a = 6.208Å, b = 3.541Å, and c = 11.281Å. The crystal structure and the lattice parameters of Fe3Se4 optimized from our calculations are similar to the experimental existing structure parameters. Fe3Se4 exhibits large magnetocrystalline anisotropy of 6x106 erg/cm3 and coercivity up to 40kOe due to its unusual properties.

  15. First-principles study of the structural, defect, and mechanical properties of B2FeCo alloys

    NASA Astrophysics Data System (ADS)

    Fu, C. L.; Krčmar, Maja

    2006-11-01

    B2FeCo has the highest saturation magnetization of any material, but has zero room temperature ductility in the ordered state that somewhat increases in the disordered state. Brittleness of FeCo has long been a puzzle given its high-symmetry B2 structure, 1/2⟨111⟩{110} slip, and low ordering temperature—all features of intrinsically ductile intermetallics. Employing first-principles calculations and statistical mechanics, we study the structural stability, point defects and order-disorder transition of B2FeCo , and suggest a mechanism potentially leading to its intrinsic brittleness. We find that B2FeCo is marginally stable, weakly ordered with a high density of antisite defects, and low anti-phase boundary energies for ⟨111⟩ slip on {110} and {112} planes. Most importantly, this system is very sensitive to the change in local atomic environment: structural instability and transformation into low-symmetry L10 structure or sheared L10 structure can be caused by reduced dimensionality or applied shear stress, respectively. We suggest that the internal stress (e.g., near the dislocation cores) may be closely connected with the B2FeCo intrinsic brittleness, since it is likely to induce local B2→L10 structural transformations.

  16. First-principles study of MoHn (n =1, 2 and 3) crystal structures under high pressure

    NASA Astrophysics Data System (ADS)

    Feng, Xiaolei; Zhang, Jurong; Liu, Hanyu; Wang, Hui

    Hydrogen-rich materials have attracted attention recently, owing to their fascinating chemical bonding and potential high superconducting critical temperatures temperature. Inspired by the recent identification of polyhydrides of d metals and molybdenum hydride molecules with a high H content, we explored the crystal structures of MoHn (n = 1, 2, and 3) under high pressures using particle swarm optimization combined with first-principles electronic structure calculations. Several novel structures of MoH2 and MoH3 are predicted at high pressures. MoH is calculated to be stable at ambient pressure; at P >2.3 GPa the hexagonal phase of MoH2 becomes stable, and at 24 GPa it transforms into an orthorhombic structure, which remains stable up to 100 GPa. All three stable structures show metallic behavior under pressure. The calculated electronic properties suggest that the d-orbitals of the Mo atoms provide the dominant contribution to the density of states at the Fermi level, which is different from the density of states previously predicted for H-rich materials. The present results offer insights in understanding of chemical and physical properties in hydrogen-rich materials, especially in extreme environments.

  17. First-principles study on structure stabilities of α-S and Na-S battery systems

    NASA Astrophysics Data System (ADS)

    Momida, Hiroyoshi; Oguchi, Tamio

    2014-03-01

    To understand microscopic mechanisms of charge and discharge reactions in Na-S batteries, there has been increasing needs to study fundamental atomic and electronic structures of elemental S as well as that of Na-S phases. The most stable form of S is known to be an orthorhombic α-S crystal at ambient temperature and pressure, and α-S consists of puckered S8 rings which crystallize in space group Fddd . In this study, the crystal structure of α-S is examined by using first-principles calculations with and without the van der Waals interaction corrections of Grimme's method, and results clearly show that the van der Waals interactions between the S8 rings have crucial roles on cohesion of α-S. We also study structure stabilities of Na2S, NaS, NaS2, and Na2S5 phases with reported crystal structures. Using calculated total energies of the crystal structure models, we estimate discharge voltages assuming discharge reactions from 2Na+ xS -->Na2Sx, and discharge reactions in Na/S battery systems are discussed by comparing with experimental results. This work was partially supported by Elements Strategy Initiative for Catalysts and Batteries (ESICB) of Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

  18. First-principles calculations of the electronic structure, phase transition and properties of ZrSiO4 polymorphs

    SciTech Connect

    Du, Jincheng; Devanathan, Ramaswami; Corrales, Louis R.; Weber, William J.

    2012-05-01

    First-principles periodic density functional theory (DFT) calculations have been performed to understand the electronic structure, chemical bonding, phase transition, and physical properties of the mineral zircon (in the chemical composition of ZrSiO4) and its high pressure phase reidite. Temperature effect on phase transition and thermal–mechanical properties such as heat capacity and bulk modulus have been studied by combining the equation of states obtained from DFT calculations with the quasi-harmonic Debye model to take into account the entropy contribution to free energy. Local density approximation (LDA) and generalized gradient approximation (GGA) DFT functionals have been systematically compared in predicting the structure and property of this material. It is found that the LDA functional provides a better description of the equilibrium structure and bulk modulus, while GGA predicts a transition pressure closer to experimental values. Both functionals correctly predict the relative stability of the two phases, with GGA giving slightly larger energy differences. The calculated band structures show that both zircon and reidite have indirect bandgaps and the reidite phase has a narrower bandgap than the zircon phase. The electronic density of states and atomic charges analyses show that bonding in the high-pressure reidite phase has a stronger covalent character.

  19. First Principle Calculations of the Electronic Structure, Phase Transition and Properties of ZrSiO4 Polymorphs

    SciTech Connect

    Du, Jincheng; Devanathan, Ram; Corrales, L Rene; Weber, William J

    2012-01-01

    First principle periodic density functional theory (DFT) calculations have been performed to understand the electronic structure, chemical bonding, phase transition, and physical properties of the zircon (in the chemical composition of ZrSiO4) and its high pressure phase reidite. Temperature effect on phase transition and thermal-mechanical properties such as heat capacity and bulk modulus have been studied by combining the equation of states obtained from DFT calculations with the quasi-harmonic Debye model to take into account the entropy contribution to free energy. Local density approximation (LDA) and generalized gradient approximation (GGA) DFT functionals have been systematically compared in predicting the structure and property of this material. It is found that the LDA functional provides a better description of the equilibrium structure and bulk modulus, while GGA predicts a transition pressure closer to experimental values. Both functionals correctly predict the relative stability of the two phases, with GGA giving slightly larger energy differences. The calculated band structures show that both zircon and reidite have indirect bandgaps and the reidite phase has a narrower bandgap than the zircon phase. The atomic charges determined using the Bader method show that bonding in reidite has a stronger covalent character.

  20. Structural and magnetic properties of ternary Fe1– xMnxPt nanoalloys from first principles

    PubMed Central

    Entel, Peter

    2011-01-01

    Summary Background: Structural and magnetic properties of binary Mn–Pt and ternary Fe1– xMnxPt nanoparticles in the size range of up to 2.5 nm (561 atoms) have been explored systematically by means of large scale first principles calculations in the framework of density functional theory. For each composition several magnetic and structural configurations have been compared. Results: The concentration dependence of magnetization and structural properties of the ternary systems are in good agreement with previous bulk and thin film measurements. At an intermediate Mn-content around x = 0.25 a crossover between several phases with magnetic and structural properties is encountered, which may be interesting for exploitation in functional devices. Conclusion: Addition of Mn effectively increases the stability of single crystalline L10 particles over multiply twinned morphologies. This, however, compromises the stability of the ferromagnetic phase due to an increased number of antiferromagnetic interactions. The consequence is that only small additions of Mn can be tolerated for data recording applications. PMID:21977428

  1. First-principles Study on the Vibration Modes and Electronic Structure of Alkali and Alkaline-earth Amides and Alanates

    NASA Astrophysics Data System (ADS)

    Tsumuraya, Takao; Shishidou, Tatsuya; Oguchi, Tamio

    2009-03-01

    Light alkaline and alkaline-earth metal hydrides such as amides M(NH2)n and alanates M(AlH4)n (M=K, Na, Li, Ca, and Mg) have attracted a growing interest as reversible hydrogen storage materials recently because of their innately high hydrogen contents. [1, 2] We study the electronic structure of the amides and alanates with different cations, focusing on the role of cation states from first-principles calculations based on the all-electron FLAPW method. Calculated breathing stretch vibration modes for these compounds are compared with measured infrared and Raman spectra. In the amides, we find a significant tendency such that the breathing stretch vibration frequencies and the structural parameters of NH2 vary in accordance with the ionization energy of cation, which may be explained by the strength in hybridization between cation orbitals and molecular orbitals of (NH2)^-. We elucidate the microscopic mechanism of correlations between the breathing stretch vibration frequencies of N-H and structural parameters by analyzing the calculated electronic structure from a view point of the molecular-orbitals. A similar tendency in the alanates is also discussed. [1] P. Chen, Z. Xiong, J. Luo, J. Lin and K.L. Tan, Nature 420, 302 (2002). [2] B. Bogdanovi and M. Schwickardi, J. Alloys Compd. 253-254, 1 (1997).

  2. First-principles determination of LaMnxM1-xO3 surface structures under catalytic conditions

    NASA Astrophysics Data System (ADS)

    Rong, Xi; Kolpak, Alexie; Kolpak Group Team

    2013-03-01

    The design of efficient and cost-effective catalysts for the oxygen evolution reaction (OER) is crucial for the development of electrochemical conversion technologies. One of the most important factors determining the activity is the surface/interface structures of catalysts. However, little is known about the atomic and electronic structures and thermodynamic properties of realistic interface reconstructions, which are caused by different environments during fabrication, measurement, and eventual device operation. In this work, we apply first-principles density functional theory computations in combination with kinetic modeling to investigate the environment-dependent chemical and physical properties of perovskite oxide heterostrucutre catalysts, particularly LaMnxM1-xO3. We develop a methodology for accurate identification of constraints on the interface structure phase space and rapid computation of this identification as a function of temperature, pressure, and other chemical environments. Our work could lead to accurate and rapid prediction of surface/interface structures and properties under different environmental conditions, and contribute to the design of new high-activity OER catalysts.

  3. First-principles molecular dynamics study of glassy GeS2: Atomic structure and bonding properties

    NASA Astrophysics Data System (ADS)

    Celino, M.; Le Roux, S.; Ori, G.; Coasne, B.; Bouzid, A.; Boero, M.; Massobrio, C.

    2013-11-01

    The structure of glassy GeS2 is studied in the framework of density functional theory, by using a fully self-consistent first-principles molecular dynamics (FPMD) scheme. A comparative analysis is performed with previous molecular dynamics data obtained within the Harris functional (HFMD) total energy approach. The calculated total neutron structure factor exhibits an unprecedented agreement with the experimental counterpart. In particular, the height of the first sharp diffraction peak (FSDP) improves considerably upon the HFMD results. Both the Ge and the S subnetworks are affected by a consistent number of miscoordinations, coexisting with the main tetrahedral structural motif. Glassy GeS2 features a short-range order quite similar to the one found in glassy GeSe2, a notable exception being the larger number of edge-sharing connections. An electronic structure localization analysis, based on the Wannier functions formalism, provides evidence of a more enhanced ionic character in glassy GeS2 when compared to glassy GeSe2.

  4. Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7

    PubMed Central

    Aidhy, Dilpuneet S.; Sachan, Ritesh; Zarkadoula, Eva; Pakarinen, Olli; Chisholm, Matthew F.; Zhang, Yanwen; Weber, William J.

    2015-01-01

    The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd2Ti2O7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. In view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties. PMID:26555848

  5. Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7

    DOE PAGESBeta

    Aidhy, Dilpuneet S.; Sachan, Ritesh; Zarkadoula, Eva; Pakarinen, Olli; Chisholm, Matthew F.; Zhang, Yanwen; Weber, William J.

    2015-11-10

    The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd2Ti2O7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallizationmore » during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. Lastly, in view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties.« less

  6. First-Principles Study on Structural and Chemical Asymmetry of a Biomimetic Water-Splitting Dimanganese Complex.

    PubMed

    Zhou, Ting; Lin, Xiangsong; Zheng, Xiao

    2013-02-12

    Density-functional theory calculations are carried out for a biomimetic dimanganese complex, [H2O(terpy)Mn(III)(μ-O)2Mn(IV)(terpy)OH2](3+)(1, terpy = 2,2':6',2″-terpyridine), which is a structural model for the oxygen evolving center of photosystem II. Theoretical investigations aim at elucidating the asymmetry features in the geometric and electronic structures of complex 1, as well as their influences on the chemical functions of the two manganese centers, in the presence of water solvent. With the insight gained from the first-principles calculations, we study the oxidation state of complex 1 in the acetate buffer solution. Both the thermodynamic and kinetic aspects are explored in detail, and the structural and chemical asymmetry of the two manganese centers is fully considered. It is found that the larger steric repulsion associated with the Mn(IV) center plays a decisive role, which leads to the predominant acetate coordination at the Mn(III) ion. This thus resolves the existing controversy on the preferential acetate binding to complex 1. PMID:26588750

  7. First-Principles Study on Structural and Thermoelectric Properties of Al- and Sb-Doped Mg2Si

    NASA Astrophysics Data System (ADS)

    Hirayama, Naomi; Iida, Tsutomu; Funashima, Hiroki; Morioka, Shunsuke; Sakamoto, Mariko; Nishio, Keishi; Kogo, Yasuo; Takanashi, Yoshifumi; Hamada, Noriaki

    2015-06-01

    We theoretically investigate the structural and thermoelectric properties of magnesium silicide (Mg2Si) incorporating Al or Sb atoms as impurities using first-principles calculations. We optimized the structural properties through variable-cell relaxation using a pseudopotential method based on density functional theory. The result indicates that the lattice constant can be affected by the insertion of impurity atoms into the system, mainly because the ionic radii of these impurities differ from those of the matrix constituents Mg and Si. We then estimate, on the basis of the optimized structures, the site preferences of the impurity atoms using a formation energy calculation. The result shows a nontrivial concentration-dependence of the site occupation, such that Al tends to go into the Si, Mg, and interstitial sites with comparable formation energies at low doping levels (<2 at.%); it can start to substitute for the Mg sites preferentially at higher doping levels (<4 at.%). Sb, on the other hand, shows a strong preference for the Si sites at all impurity concentrations. Furthermore, we obtain the temperature-dependence of the thermoelectromotive force (Seebeck coefficient) of the Al- and Sb-doped Mg2Si using the full-potential linearized augmented-plane-wave method and the Boltzmann transport equation.

  8. First-principle investigations of structure, elastic and bond hardness of FexB (x=1, 2, 3) under pressure

    NASA Astrophysics Data System (ADS)

    Gueddouh, Ahmed; Bentria, Bachir; Lefkaier, I. K.

    2016-05-01

    First-principles calculations are performed to study pressure effects on structure, magnetic and mechanical properties of FexB (x=1, 2, 3) compounds using density functional theory (DFT) within GGA approximation. The three structures are studied in the pressure range from 0 to 90 GPa, in order to predict the critical transition pressure from magnetic to nonmagnetic states (NM) and a possibility of superconductivity in this state was predicted. The density of states of FexB ferromagnetic compounds are significantly modified with increasing pressure and at particular critical pressures, our compounds undergo an abrupt loss of ferromagnetic character that cause mirror in upper and lowers half panels on both spin channels. Furthermore, the relationship between crystal structure and material hardness of FexB is also investigated by calculating hardness of Fe-B and B-B bonds using Mulikan population analysis and semi empirical hardness theory. This model proved effective in hardness prediction of metal borides and agrees well with the experimental values. By the elastic stability criteria, it is predicted that FexB are stable up to the selected pressures.

  9. First-principles calculations for the structural and electronic properties of GaAs1-xPx nanowires

    NASA Astrophysics Data System (ADS)

    Mohammad, Rezek; Katırcıoğlu, Şenay

    2016-09-01

    Structural stability and electronic properties of GaAs1-xPx (0.0≤x≤1.0) nanowires (NWs) in zinc-blende (ZB) (˜5≤ diameter ≤˜21Å) and wurtzite (WZ) (˜5≤diameter≤˜29Å) phases are investigated by first-principles calculations based on density functional theory (DFT). GaAs (x=0.0) and GaP (x=1.0) compound NWs in WZ phase are found energetically more stable than in ZB structural ones. In the case of GaAs1-xPx alloy NWs, the energetically favorable phase is found size and composition dependent. All the presented NWs have semiconductor characteristics. The quantum size effect is clearly demonstrated for all GaAs1-xPx (0.0≤x≤1.0) NWs. The band gaps of ZB and WZ structural GaAs compound NWs with ˜10≤ diameter ≤˜21Å and ˜5≤diameter≤˜29Å, respectively are enlarged by the addition of concentrations of phosphorus for obtaining GaAs1-xPx NWs proportional to the x values around 0.25, 0.50 and 0.75.

  10. First-principles study of the Pd–Si system and Pd(001)/SiC(001) hetero-structure

    SciTech Connect

    Turchi, P.E.A.; Ivashchenko, V.I.

    2014-11-01

    First-principles molecular dynamics simulations of the Pd(001)/3C–SiC(001) nano-layered structure were carried out at different temperatures ranging from 300 to 2100 K. Various PdSi (Pnma, Fm3m, P6m2, Pm3m), Pd2Si (P6⁻2m, P63/mmc, P3m1, P3⁻1m) and Pd3Si (Pnma, P6322, Pm3m, I4/mmm) structures under pressure were studied to identify the structure of the Pd/Si and Pd/C interfaces in the Pd/SiC systems at high temperatures. It was found that a large atomic mixing at the Pd/Si interface occurred at 1500–1800 K, whereas the Pd/C interface remained sharp even at the highest temperature of 2100 K. At the Pd/C interface, voids and a graphite-like clustering were detected. Palladium and silicon atoms interact at the Pd/Si interface to mostly form C22-Pd2Si and D011-Pd3Si fragments, in agreement with experiment.

  11. First-principles investigation of the structural characteristics of LiMO2 cathode materials for lithium secondary batteries

    NASA Astrophysics Data System (ADS)

    Kim, Yongseon

    2015-11-01

    The structural features related to the defects of LiMO2 (M = Ni, Co, Mn) cathode materials for lithium secondary batteries were investigated by a simulation of phase diagrams based on first-principle calculations. Crystal models with various types of point defects were designed and dealt with as independent phases, which enabled an examination of the thermodynamic stability of the defects. A perfect phase without defects appeared to be the most stable for LiCoO2, whereas the formation of Li vacancies, O vacancies, and antisites between Li and Ni was thermodynamically unavoidable for LiNiO2. The introduction of both Co and Mn in LiNiO2 was effective in reducing the formation of point defects, but increasing the relative amount of Mn was undesirable because the antisite defect remained stable with Mn doping. The simulation showed good agreement with the experimental data and previous reports. Therefore, the method and the results of this study are expected to be useful for examining the synthesis, structure and related properties of layer-structured cathode materials.

  12. Attempts at a determination of the fine-structure constant from first principles: a brief historical overview

    NASA Astrophysics Data System (ADS)

    Jentschura, U. D.; Nándori, I.

    2014-12-01

    It has been a notably elusive task to find a remotely sensical ansatz for a calculation of Sommerfeld's electrodynamic fine-structure constant αQED ≈ 1 / 137.036 based on first principles. However, this has not prevented a number of researchers to invest considerable effort into the problem, despite the formidable challenges, and a number of attempts have been recorded in the literature. Here, we review a possible approach based on the quantum electrodynamic (QED) β function, and on algebraic identities relating αQED to invariant properties of "internal" symmetry groups, as well as attempts to relate the strength of the electromagnetic interaction to the natural cutoff scale for other gauge theories. Conjectures based on both classical as well as quantum-field theoretical considerations are discussed. We point out apparent strengths and weaknesses of the most prominent attempts that were recorded in the literature. This includes possible connections to scaling properties of the Einstein-Maxwell Lagrangian which describes gravitational and electromagnetic interactions on curved space-times. Alternative approaches inspired by string theory are also discussed. A conceivable variation of the fine-structure constant with time would suggest a connection of αQED to global structures of the Universe, which in turn are largely determined by gravitational interactions.

  13. First-principle electronic structure calculations for magnetic moment in iron-based superconductors: An LSDA + negative U study

    NASA Astrophysics Data System (ADS)

    Nakamura, H.; Hayashi, N.; Nakai, N.; Okumura, M.; Machida, M.

    2009-10-01

    In order to resolve a discrepancy of the magnetic moment on Fe between the experimental and calculation results, we perform first-principle electronic structure calculations for iron-based superconductors LaFeAsO1-x and LiFeAs also show similar SDW. So far, the first-principle calculations on LaFeAsO actually predicted the SDW state as a ground state. However, the predicted magnetic moment (∼2 μB) per an Fe atom is much larger than the observed one (∼0.35 μB) in experiments [2,4]. The authors suggested that the discrepancy can be resolved by expanding U into a negative U range within LSDA + U framework. In this paper, we revisit the discrepancy and clarify why the negative correction is essential in these compounds. See Ref. [5] for the details of calculation data by LSDA + negative U. In the first-principle calculation on compounds including transition metals, the total energy is frequently corrected by “LSDA + U” approach. The parameter U is theoretically re-expressed as U(≡U-J), where U is the on-site Coulomb repulsion (Hubbard U) and J is the atomic-orbital intra-exchange energy (Hund’s coupling parameter) [6]. The parameter U employed in the electronic structure calculations is usually positive. The positivity promotes the localized character of d-electrons and enhances the magnetic moment in the cases of magnetically ordered compounds. Normally, this positive correction successfully works. In choosing the parameter, one can principally extend the parameter U range to a negative region. The negative case [7] is not popular, but it can occur in the following two cases [8]: (i) the Hubbard U becomes negative and (ii) the intra-exchange J is effectively larger than the Hubbard U. The case (i) has been suggested by many authors based on various theoretical considerations. Here, we note that U should be estimated once screening effects on the long-range Coulomb interaction are taken into account. In fact, small U has been reported [9]. Thus, when the

  14. First-principles study of the electronic structure of nonmetal-doped anatase TiO2

    NASA Astrophysics Data System (ADS)

    Wang, Yuan; Ma, Jing; Zhou, Jian-Ping; Chen, Xiao-Ming; Wang, Jing-Zhou

    2016-02-01

    In this paper, we present a detailed study of the structure, defect formation energy, and electronic and magnetic properties of nonmetal-doped TiO2 by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA). The formation energy reduces with increasing electronegativity of the dopant. After doping with nonmetal elements, some band gaps of the doped-TiO2 become narrow, and others become wide, in which impurity states appear in the band gap. The relative positions of the impurity states are much different, mainly caused by the different electronegativities of the nonmetal elements F, O, B, C and N. When H is added to achieve a charge balance, the impurity states approach the valence band maximum, because the electronegativity difference among the nonmetal elements is decreased. Therefore, nonmetal and H codoping is an effective way to improve the visible-light catalytic activity of anatase TiO2. In addition, N-doping and C-doping can cause spin polarization of the TiO2 electronic structure and form 1.0 μ B and 2.0 μ B magnetic moment, respectively.

  15. The electronic structure, mechanical flexibility and carrier mobility of black arsenic-phosphorus monolayers: a first principles study.

    PubMed

    Sun, Jie; Lin, Na; Ren, Hao; Tang, Cheng; Yang, Letao; Zhao, Xian

    2016-04-14

    New artificial layered semiconductors - black arsenic-phosphorus (b-AsP) - which have tunable band gaps owing to good tunability of the chemical compositions have been synthesized in a recent experiment. In the present work, first principles calculations are performed to systematically study the structure, and mechanical, electrical, and transport properties of b-AsP monolayers. The mechanical analysis demonstrates that the exfoliation of single-layer b-AsP systems from the bulk form is more difficult compared with that of pure black phosphorus (BP). In addition, the breaking strain of the b-AsP monolayer is comparable with other widely studied two dimensional materials, indicating their excellent mechanical flexibility and good potential for flexible device applications. Besides, the electronic structures of b-AsP system monolayers are not sensitive to their specific compositions, which however, can be flexibly modulated by the strain effect. The predicted carrier mobilities of b-AsP systems are directionally anisotropic, similar to pure BP. However, the degradation of their carrier mobilities may become a practical limitation in real electronic device applications. PMID:27003857

  16. Competing collinear magnetic structures in superconducting FeSe by first-principles quantum Monte Carlo calculations

    NASA Astrophysics Data System (ADS)

    Busemeyer, Brian; Dagrada, Mario; Sorella, Sandro; Casula, Michele; Wagner, Lucas K.

    2016-07-01

    Resolving the interplay between magnetic interactions and structural properties in strongly correlated materials through a quantitatively accurate approach has been a major challenge in condensed-matter physics. Here we apply highly accurate first-principles quantum Monte Carlo (QMC) techniques to obtain structural and magnetic properties of the iron selenide (FeSe) superconductor under pressure. Where comparable, the computed properties are very close to the experimental values. Of potential ordered magnetic configurations, collinear spin configurations are the most energetically favorable over the explored pressure range. They become nearly degenerate in energy with bicollinear spin orderings at around 7 GPa, when the experimental critical temperature Tc is the highest. On the other hand, ferromagnetic, checkerboard, and staggered dimer configurations become relatively higher in energy as the pressure increases. The behavior under pressure is explained by an analysis of the local charge compressibility and the orbital occupation as described by the QMC many-body wave function, which reveals how spin, charge, and orbital degrees of freedom are strongly coupled in this compound. This remarkable pressure evolution suggests that stripelike magnetic fluctuations may be responsible for the enhanced Tc in FeSe and that higher Tc is associated with nearness to a crossover between collinear and bicollinear ordering.

  17. Structure and dynamics of bulk liquid iron at pressures up to 58 GPa. A first-principles study

    NASA Astrophysics Data System (ADS)

    Gonzalez, David; Marques, Miriam; Gonzalez, Luis Enrique

    The static and dynamic properties of bulk liquid Fe at a several high pressure states, have been studied by using first-principles molecular dynamics simulations based on the density functional theory and the projector augmented wave technique. Results are reported for four thermodynamic states at pressures of 27, 42, 50 and 58 GPa for which x-ray scattering data are available. The calculated static structure shows very good agreement with the available experimental data, including an asymmetric second peak which becomes more marked with increasing pressure. The dynamical structure reveals the existence of propagating density fluctuations and the associated dispersion relation has also been determined. The relaxation mechanisms for the density fluctuations have been analyzed in terms of a model with two decay channels (fast and slow, respectively). We found that the thermal relaxation proceeds along the slow decaying channel whereas the fast one is that of the viscoelastic relaxation. Finally, results are also reported for some transport coefficients. We acknowledge financial support from Spanish MCI (FIS2014-59279-P) and JCyL (CIP13/03 and VA104A11-2).

  18. The structure of water at a Pt(111) electrode and the potential of zero charge studied from first principles.

    PubMed

    Sakong, Sung; Forster-Tonigold, Katrin; Groß, Axel

    2016-05-21

    The structure of a liquid water layer on Pt(111) has been studied by ab initio molecular dynamics simulations based on periodic density functional theory calculations. First the reliability of the chosen exchange-correlation function has been validated by considering water clusters, bulk ice structures, and bulk liquid water, confirming that the dispersion corrected RPBE-D3/zero functional is a suitable choice. The simulations at room temperature yield that a water layer that is six layers thick is sufficient to yield liquid water properties in the interior of the water film. Performing a statistical average along the trajectory, a mean work function of 5.01 V is derived, giving a potential of zero charge of Pt(111) of 0.57 V vs. standard hydrogen electrode, in good agreement with experiments. Therefore we propose the RPBE-D3/zero functional as the appropriate choice for first-principles calculations addressing electrochemical aqueous electrolyte/metal electrode interfaces. PMID:27208959

  19. Anisotropy and temperature dependence of structural, thermodynamic, and elastic properties of crystalline cellulose Iβ: a first-principles investigation

    NASA Astrophysics Data System (ADS)

    Dri, Fernando L.; Shang, ShunLi; Hector, Louis G., Jr.; Saxe, Paul; Liu, Zi-Kui; Moon, Robert J.; Zavattieri, Pablo D.

    2014-12-01

    Anisotropy and temperature dependence of structural, thermodynamic and elastic properties of crystalline cellulose Iβ were computed with first-principles density functional theory (DFT) and a semi-empirical correction for van der Waals interactions. Specifically, we report the computed temperature variation (up to 500 K) of the monoclinic cellulose Iβ lattice parameters, constant pressure heat capacity, Cp, entropy, S, enthalpy, H, the linear thermal expansion components, ξi, and components of the isentropic and isothermal (single crystal) elastic stiffness matrices, CijS (T) and CijT (T) , respectively. Thermodynamic quantities from phonon calculations computed with DFT and the supercell method provided necessary inputs to compute the temperature dependence of cellulose Iβ properties via the quasi-harmonic approach. The notable exceptions were the thermal conductivity components, λi (the prediction of which has proven to be problematic for insulators using DFT) for which the reverse, non-equilibrium molecular dynamics approach with a force field was applied. The extent to which anisotropy of Young's modulus and Poisson's ratio is temperature-dependent was explored in terms of the variations of each with respect to crystallographic directions and preferred planes containing specific bonding characteristics (as revealed quantitatively from phonon force constants for each atomic pair, and qualitatively from charge density difference contours). Comparisons of the predicted quantities with available experimental data revealed reasonable agreement up to 500 K. Computed properties were interpreted in terms of the cellulose Iβ structure and bonding interactions.

  20. First-Principles Study on Electronic Structure and Thermodynamic Stability of Sr(Ti,Ru)O3

    NASA Astrophysics Data System (ADS)

    Shimizu, Tatsuo; Kawakubo, Takashi

    2001-02-01

    Not only the electronic structure but also the thermodynamic stability of Sr(Ti,Ru)O3 is investigated using the first-principles method. For the electronic structure with small Ru concentration, the Ru t2g states appear below the middle of the band gap of SrTiO3. In this case, the wave functions of the Ru t2g states are localized at the Ru site. As a Ru concentration increases to more than 50 mol%, the inside-gap states gradually become delocalized. Therefore, a conductive Sr(Ti,Ru)O3 thin film is expected. On the other hand, thermodynamic stability of a Sr(Ti,Ru)O3 thin film is enhanced drastically relative to that of SrRuO3, as Ru concentration decreases. Sr(Ti,Ru)O3 with Ru concentration of more than 60 mol% may oxidize the neighboring (Ti,Al)N layer, which is often used as a barrier metal in memory-cell capacitors. In short, Sr(Ti,Ru)O3 with Ru concentration of about 50 mol% is proposed as a promising candidate material for new electrodes.

  1. First-Principles Electronic Structure Calculations of N2H4 Adsorbed on Single-Wall Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Yu, M.; Tian, W. Q.; Jayanthi, C. S.; Wu, S. Y.

    2008-03-01

    Recent experiments conducted by Desai et al. [1] reveal that single-wall carbon nanotube (SWCNT) networks exposed to N2H4 vapor at various pressures exhibit considerable drop in resistance with respect to the pristine sample. Experimental findings reveal: (i) n-type behavior for the adsorption of N2H4/SWCNT, and (ii) the binding of N2H4 on SWCNT as chemisorption. In the present work, we have performed first-principles electronic structure calculations [2] for the N2H4 adsorbed on the (14, 0) SWCNT, where several orientations for the N2H4 molecule were considered. Calculations for the combined system were performed using 3 unit cells with the DFT/GGA and ultra soft pseudo-potentials. Our calculations reveal: (i) the binding of N2H4 on SWCNT as physisorption, and (ii) the electronic structure of SWCNT to be practically unaltered by the adsorption of N2H4, suggesting that there will not be a dramatic drop in resistance for N2H4/SWCNT. This is in disagreement with the experimental findings. To further understand the experimental observations, we will discuss mechanisms that may alter the binding nature of N2H4 on SWCNT. [1] S. Desai, G. Sumanasekera, et al. (APS, March 2008). [2] G. Kresse and J. Furthmuller, Phys. Rev. B 54, 11169 (1996).

  2. First principles prediction of the electronic structure and carrier mobilities of biaxially strained molybdenum trioxide (MoO3)

    NASA Astrophysics Data System (ADS)

    Dandogbessi, Bruno S.; Akin-Ojo, Omololu

    2016-08-01

    The electronic band structures of unstrained and biaxially strained MoO3 were determined by first-principles density functional theory calculations. From the band structures, the effects of strain on the charge carrier mobilities were investigated. These mobilities were calculated based on deformation potential theory. First, we found that the electron effective masses of unstrained bulk pristine MoO3 are about three times smaller than the corresponding hole effective masses, and, second, the electron mobility is about ten times the hole mobility, making the compound an electron transport material. Our results also show that, when compressed biaxially, as the strain increases from 0% to 1.5%, the electron (hole) mobility increases by 0% to 53% (0% to 17%). On the other hand, the application of a biaxial tensile strain decreases the electron (hole) mobility by 65% to 0% (90% to 0%), as the tensile strain increases from 0% to 1.5 % . These changes are caused mainly by the fact that the carrier effective masses reduce (increase) upon application of compressive (tensile) strain. Only the acoustic-phonon limited carrier mobilities were computed; hence, the actual mobilities cannot be less than the values obtained in this work.

  3. First principles study of pressure induced structural phase transition in hydrogen storage material—MgH2

    NASA Astrophysics Data System (ADS)

    Kanagaprabha, S.; Asvinimeenaatci, A. T.; Rajeswarapalanichamy, R.; Iyakutti, K.

    2012-01-01

    First principles calculation were performed using Vienna ab-initio simulation package within the frame work of density functional theory (DFT) to understand the electronic properties of magnesium hydride. At normal pressure, the most stable structure of MgH 2 is rutile type with a wide band gap of 3.52 eV, which agrees well with the available data. A pressure induced semi-conductor to metallic transition at a pressure of 92.54 GPa is predicted. Our results indicate a sequence of pressure induced structural phase transition in MgH 2. The obtained sequence of phase transition was α→γ→β→δ→ε at a pressure of 0.37 GPa, 3.89 GPa,7.23 GPa and 11.26 GPa, respectively. Thus our results indicate that MgH 2 is one of the best hydrogen storage material and the maximum storage capacity achieved was 7.7%.

  4. Lattice structures and electronic properties of MO/MoSe2 interface from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Zhang, Yu; Tang, Fu-Ling; Xue, Hong-Tao; Lu, Wen-Jiang; Liu, Jiang-Fei; Huang, Min

    2015-02-01

    Using first-principles plane-wave calculations within density functional theory, we theoretically studied the atomic structure, bonding energy and electronic properties of the perfect Mo (110)/MoSe2 (100) interface with a lattice mismatch less than 4.2%. Compared with the perfect structure, the interface is somewhat relaxed, and its atomic positions and bond lengths change slightly. The calculated interface bonding energy is about -1.2 J/m2, indicating that this interface is very stable. The MoSe2 layer on the interface has some interface states near the Fermi level, the interface states are mainly caused by Mo 4d orbitals, while the Se atom almost have no contribution. On the interface, Mo-5s and Se-4p orbitals hybridize at about -6.5 to -5.0 eV, and Mo-4d and Se-4p orbitals hybridize at about -5.0 to -1.0 eV. These hybridizations greatly improve the bonding ability of Mo and Se atom in the interface. By Bader charge analysis, we find electron redistribution near the interface which promotes the bonding of the Mo and MoSe2 layer.

  5. Electronic structures and optical properties of Ca5(BO3)3F: a systematical first-principles study.

    PubMed

    Hu, Chun-Li; Xu, Xiang; Sun, Chuan-Fu; Mao, Jiang-Gao

    2011-10-01

    A first-principles study of the electronic structure, the linear optical properties and second-order NLO properties of calcium fluoroborate (Ca(5)(BO(3))(3)F, or CBF) crystal has been performed within density functional theory and the independent-particle approximation. The results indicate that the calculated birefringence Δn and the second-order susceptibilities are very coincident with the experimental measured values, and the χ((2)) curves show stronger anisotropy than the linear optical properties. Further analysis based on the spectral and spatial decomposition of χ((2)) reveals that the main sources of the SHG response of CBF are from the planar BO(3) groups (74%-77%) and Ca(2+) cations (23%-26%) and can be attributed to the interband electronic transition from the nonbonding O 2p states to the B 2p and Ca 4s4p states. The packing arrangement of BO(3) is the principal contributor to the significant differences among SHG tensors in CBF. Meanwhile, for a certain crystal CBF, the SHG tensors' trend can be the trend of the optical transition matrix elements, which are high when the corresponding subscript directions have more parallel BO(3) triangular planes in the structure. PMID:21918290

  6. First principle investigation of electronic structure, chemical bonding and optical properties of tetrabarium gallium trinitride oxide single crystal

    SciTech Connect

    Khan, Saleem Ayaz Azam, Sikander

    2015-10-15

    The electronic band structure, valence electron charge density and optical susceptibilities of tetrabarium gallium trinitride (TGT) were calculated via first principle study. The electronic band structure calculation describes TGT as semiconductor having direct band gap of 1.38 eV. The valence electronic charge density contour verified the non-polar covalent nature of the bond. The absorption edge and first peak of dielectric tensor components showed electrons transition from N-p state to Ba-d state. The calculated uniaxial anisotropy (0.4842) and birefringence (−0.0061) of present paper is prearranged as follow the spectral components of the dielectric tensor. The first peak in energy loss function (ELOS) shows the energy loss of fast traveling electrons in the material. The first sharp peak produced in ELOS around 10.5 eV show plasmon loss having plasma frequencies 0.1536, 0.004 and 0.066 of dielectric tensor components. This plasmon loss also cause decrease in reflectivity spectra.

  7. First-principles calculation of the electronic structure, chemical bonding, and thermodynamic properties of β-US2

    NASA Astrophysics Data System (ADS)

    Li, Shi-Chang; Zheng, Yuan-Lei; Ma, Sheng-Gui; Gao, Tao; Ao, Bing-Yun

    2015-12-01

    The electronic structure, magnetic states, chemical bonding, and thermodynamic properties of β-US2 are investigated by using first-principles calculation through the density functional theory (DFT) +U approach. The obtained band structure exhibits a direct band gap semiconductor at Γ point with a band gap of 0.9 eV for β-US2, which is in good agreement with the recent experimental data. The charge-density differences, the Bader charge analysis, and the Born effective charges suggest that the U-S bonds of the β-US2 have a mixture of covalent and ionic characters, but the ionic character is stronger than covalent character. The Raman-active, infrared-active, and silent modes at the Γ point are further assigned and discussed. The obtained optical-mode frequencies indicate that the three apparent LO-TO (longitudinal optical-transverse optical) splittings occur in B1u, B2u, and B3u modes, respectively. Furthermore, the Helmholtz free energy ΔF, the specific heat ΔE, vibrational entropy S, and constant volume CV are studied over a range from 0 K˜100 K. We expect that our work can provide some valuable information for further experimental investigation of the dielectric properties and the infrared reflectivity spectrum of uranium chalcogenide. Project supported by the National Natural Science Foundation of China (Grant Nos. 21371160 and 21401173).

  8. Interfacial properties and electron structure of Al/B4C interface: A first-principles study

    NASA Astrophysics Data System (ADS)

    Xian, Yajiang; Qiu, Ruizhi; Wang, Xin; Zhang, Pengcheng

    2016-09-01

    This research aims at investigating the structural, mechanical and electronic properties of the Al (111)/B4C (0001) interface by first-principles calculations. This model geometry Al (111)/B4C (0001) is chosen because the close-packed planes of Al and B4C have the (111) and (0001) orientation, respectively, and the lattice mismatch is only ∼2.1%. Among four B4C (0001) surfaces with different terminations, our calculation of surface free energies predicted that C-terminated B4C (0001) surface is the most stable one. Relaxed atomic geometries, the work of adhesion and interfacial free energies were calculated for three C-terminated B4C (0001)/Al (111) interfaces with different stacking sequences (top-site, hollow-site, and bridge-site). Results reveal that the relaxed top-site (hollow-site-like) Al/B4C interface has the best adhesion force and also be the most stable. The interfacial electron structure including charge density difference, Bader charge and density of states (DOS) is analyzed to determine the nature of metal/carbide bonding and we find the formation of Alsbnd C bond and possibly the formation of Al4C3 in the interface.

  9. Understanding the interplay of lattice and magnetic degrees of freedom in structurally complex insulators from first principles

    NASA Astrophysics Data System (ADS)

    Fennie, Craig J.

    Two themes recurring throughout nature are spontaneous symmetry breaking and the emergence of new phenomena through collective behavior of seemingly simpler, well-understood parts. These themes are especially prevalent in condensed matter systems where the interplay between diverse microscopic degrees of freedom, such as spin, charge, and lattice excitations, produces new and unusual macroscopic properties. For example, ferromagnets spontaneously break time-reversal symmetry, while ferroelectrics break space inversion symmetry. Materials displaying either spontaneous spin polarization or electrical polarization are ubiquitous and many possess simple crystalline structures, yet materials breaking both time and space inversion, appropriately called multiferroic, are quite rare and prevalently found in structurally and chemically complex materials. The study of the emergence of new phenomena in multiferroic materials holds much promise to further our fundamental understanding of how spin and lattice degrees of freedom interact with one another. Much effort has already been invested in identifying new multiferroic materials; attention has now turned to the question of how to produce a strong coupling between the two distinct order parameters. In this thesis we investigate the interplay of lattice and spin degrees of freedom in complex solids using first-principles density-functional methods. Specifically, we study (1) lattice-lattice coupling in ferroelectrics and (2) spin-lattice coupling in both simple and geometrically frustrated magnets. Building upon these two studies we developed a unique approach in which the interplay of phonons and spins leads to a strong coupling between the ferroelectric and ferromagnetic order parameters in multiferroics.

  10. Structural phase transitions and fundamental band gaps of MgxZn1 xO alloys from first principles

    SciTech Connect

    Maznichenko, I. V.; Ernst, Arthur; Bouhassoune, M.; Henk, J.; Daene, Markus W; Lueders, Martin; Bruno, Patrick; Wolfam, Hergert; Mertig, I.; Szotek, Zdzislawa; Temmerman, Walter M

    2009-01-01

    The structural phase transitions and the fundamental band gaps of MgxZn1 xO alloys are investigated by detailed first-principles calculations in the entire range of Mg concentrations x, applying a multiple-scattering theoretical approach (Korringa-Kohn-Rostoker method). Disordered alloys are treated within the coherent-potential approximation. The calculations for various crystal phases have given rise to a phase diagram in good agreement with experiments and other theoretical approaches. The phase transition from the wurtzite to the rock-salt structure is predicted at the Mg concentration of x=0.33, which is close to the experimental value of 0.33 0.40. The size of the fundamental band gap, typically underestimated by the local-density approximation, is considerably improved by the self-interaction correction. The increase in the gap upon alloying ZnO with Mg corroborates experimental trends. Our findings are relevant for applications in optical, electrical, and, in particular, in magnetoelectric devices.

  11. Interfacial bonding and electronic structure of GaN/GaAs interface: A first-principles study

    SciTech Connect

    Cao, Ruyue; Zhang, Zhaofu; Wang, Changhong; Li, Haobo; Dong, Hong; Liu, Hui; Wang, Weichao; Xie, Xinjian

    2015-04-07

    Understanding of GaN interfacing with GaAs is crucial for GaN to be an effective interfacial layer between high-k oxides and III-V materials with the application in high-mobility metal-oxide-semiconductor field effect transistor (MOSFET) devices. Utilizing first principles calculations, here, we investigate the structural and electronic properties of the GaN/GaAs interface with respect to the interfacial nitrogen contents. The decrease of interfacial N contents leads to more Ga dangling bonds and As-As dimers. At the N-rich limit, the interface with N concentration of 87.5% shows the most stability. Furthermore, a strong band offsets dependence on the interfacial N concentration is also observed. The valance band offset of N7 with hybrid functional calculation is 0.51 eV. The electronic structure analysis shows that significant interface states exist in all the GaN/GaAs models with various N contents, which originate from the interfacial dangling bonds and some unsaturated Ga and N atoms. These large amounts of gap states result in Fermi level pinning and essentially degrade the device performance.

  12. Interfacial bonding and electronic structure of GaN/GaAs interface: A first-principles study

    NASA Astrophysics Data System (ADS)

    Cao, Ruyue; Zhang, Zhaofu; Wang, Changhong; Li, Haobo; Xie, Xinjian; Dong, Hong; Liu, Hui; Wang, Weichao

    2015-04-01

    Understanding of GaN interfacing with GaAs is crucial for GaN to be an effective interfacial layer between high-k oxides and III-V materials with the application in high-mobility metal-oxide-semiconductor field effect transistor (MOSFET) devices. Utilizing first principles calculations, here, we investigate the structural and electronic properties of the GaN/GaAs interface with respect to the interfacial nitrogen contents. The decrease of interfacial N contents leads to more Ga dangling bonds and As-As dimers. At the N-rich limit, the interface with N concentration of 87.5% shows the most stability. Furthermore, a strong band offsets dependence on the interfacial N concentration is also observed. The valance band offset of N7 with hybrid functional calculation is 0.51 eV. The electronic structure analysis shows that significant interface states exist in all the GaN/GaAs models with various N contents, which originate from the interfacial dangling bonds and some unsaturated Ga and N atoms. These large amounts of gap states result in Fermi level pinning and essentially degrade the device performance.

  13. The ionic structure of liquid sodium obtained by numerical simulation from 'first principles' and ab initio 'norm-conserving' pseudopotentials

    NASA Astrophysics Data System (ADS)

    Harchaoui, N.; Hellal, S.; Grosdidier, B.; Gasser, J. G.

    2008-02-01

    The physical properties of disordered matter depend on the 'atomic structure' i.e. the arrangement of the atoms. This arrangement is described by the structure factor S (q) in reciprocal space and by the pair correlation function g(r) in real space. The structure factor is obtained experimentally while the numerical simulation enables us to obtain the pair correlation function. Liquid sodium is one of the elements the most studied and one can wonder about new scientific contribution appropriateness. The majority of theoretical calculations are compared with the experiment of Waseda. However two other posterior measurements have been published and give different results, in particular with regard to the height of the first peak of the structure factor. Three models of pseudopotential are considered to describe the electron-ion interaction. The first is a local pseudopotential with the alternative known as 'individual' of the model suggested by Fiolhais et al. The second model considered is that of Bachelet et al. This one, ab-initio and 'norm conserving', is non local. The last model is that proposed by Shaw known as 'first principles' and 'energy dependent'. Various static dielectric functions characteristic of the effects of exchange and correlation have been used and developed by Hellal et al. We calculated the form factors (pseudopotential in reciprocal space) and deduce the normalized energy-wave-number characteristic FN (q), the interatomic pair potential Veff (r), then the pair correlation function g(r) by molecular dynamics. The structure factor S(q) is obtained by Fourier transform and is compared with the experiment. Our calculations with the Bachelet and Shaw pseudopotentials are close to the last experiments of Greenfield et al. and of Huijben et al. Our results are discussed.

  14. Atomic partial charges on CH3NH3PbI3 from first-principles electronic structure calculations

    NASA Astrophysics Data System (ADS)

    Madjet, Mohamed E.; El-Mellouhi, Fedwa; Carignano, Marcelo A.; Berdiyorov, Golibjon R.

    2016-04-01

    We calculated the partial charges in methylammonium (MA) lead-iodide perovskite CH3NH3PbI3 in its different crystalline phases using different first-principles electronic charge partitioning approaches, including the Bader, ChelpG, and density-derived electrostatic and chemical (DDEC) schemes. Among the three charge partitioning methods, the DDEC approach provides chemically intuitive and reliable atomic charges for this material, which consists of a mixture of transition metals, halide ions, and organic molecules. The DDEC charges are also found to be robust against the use of hybrid functionals and/or upon inclusion of spin-orbit coupling or dispersive interactions. We calculated explicitly the atomic charges with a special focus on the dipole moment of the MA molecules within the perovskite structure. The value of the dipole moment of the MA is reduced with respect to the isolated molecule due to charge redistribution involving the inorganic cage. DDEC charges and dipole moment of the organic part remain nearly unchanged upon its rotation within the octahedral cavities. Our findings will be of both fundamental and practical importance, as the accurate and consistent determination of the atomic charges is important in order to understand the average equilibrium distribution of the electrons and to help in the development of force fields for larger scale atomistic simulations to describe static, dynamic, and thermodynamic properties of the material.

  15. SEMICONDUCTOR PHYSICS: First-principles of wurtzite ZnO (0001) and (000bar 1) surface structures

    NASA Astrophysics Data System (ADS)

    Yufei, Zhang; Zhiyou, Guo; Xiaoqi, Gao; Dongxing, Cao; Yunxiao, Dai; Hongtao, Zhao

    2010-08-01

    The surface structures of wurtzite ZnO (0001) and (000bar 1) surfaces are investigated by using a first-principles calculation of plane wave ultra-soft pseudo-potential technology based on density functional theory (DFT). The calculated results reveal that the surface energy of ZnO-Zn is bigger than that of ZnO-O, and the ZnO-Zn surface is more unstable and active. These two surfaces are apt to relax inward, but the contractions of the ZnO-Zn surface are smaller than the ZnO-O surface. Due to the dispersed Zn4s states and the states of stronger hybridization between the Zn and O atoms, the ZnO-Zn surface shows n-type conduction, while the O2p dangling-bond bands in the upper part of the valence cause the ZnO-O surface to have p-type conduction. The above results are broadly consistent with the experimental results.

  16. First principles calculations of structural, electronic, thermodynamic and optical properties of BAs1 - xPx alloy

    NASA Astrophysics Data System (ADS)

    Drablia, S.; Meradji, H.; Ghemid, S.; Labidi, S.; Bouhafs, B.

    2009-04-01

    First principles calculations have been used to investigate the structural, electronic, thermodynamic and optical properties of boron ternary alloy BAs1 - x Px, using a hybrid full-potential (linear) augmented plane wave plus the local orbitals (APW + lo) method within the density-functional theory (DFT). The Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) as well as the Engel-Vosko (EV)-GGA are used to calculate the band gap. We investigated the effect of composition on lattice constant, bulk modulus and band gap. Deviations of the lattice constant from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed for the alloy. Using the approach of Zunger and co-workers, the microscopic origins of the gap bowing are explained. The thermodynamic stability of the alloy is investigated by calculating the excess enthalpy of mixing ΔHm as well as the phase diagram. The calculated phase diagram showed a broad miscibility gap for the alloy of interest with a high critical temperature. For optical properties, the compositional dependence of the refractive index and the dielectric constant is studied.

  17. First-principle study of the electronic structure and optical property of Nb-doped anatase TiO2

    NASA Astrophysics Data System (ADS)

    Hou, Q. Y.; Liu, Q. L.; Zhao, C. W.; Zhang, Y.

    2014-04-01

    The absorption edge shifted to long wavelength direction and short wavelength direction of two opposite experimental conclusions have been reported, when the band-gap and absorption spectra of Nb-doped anatase TiO2 were studied. In order to solve this contradiction, the electronic structure and the optical property of Nb heavy doped anatase TiO2 have been studied by the first-principles plane-wave ultrasoft pseudopotential method based on the density functional theory with +U method modification. The calculated results indicate that the higher the Nb-doping is, the higher the total energy is, the worse the stability is, the higher the formation energy is, the more difficult the doping is, the wider the optical band-gap is, the more obvious the absorption edge shifting to short wavelength direction is, the lower the absorptivity and the reflectivity is, which is in agreement with the experimental results. The reasonable interpretation of the contradiction has been reported in this paper, too.

  18. Electronic structures and optical properties of Nb-doped SrTiO3 from first principles

    NASA Astrophysics Data System (ADS)

    Shujuan, Jiao; Jinliang, Yan; Guipeng, Sun; Yinnü, Zhao

    2016-07-01

    The n-type Nb-doped SrTiO3 with different doping concentrations were studied by first principles calculations. The effects of Nb concentration on the formation enthalpy, electronic structure and optical property were investigated. Results show that Nb preferentially enters the Ti site in SrTiO3, which is in good agreement with the experimental observation. The Fermi level of Nb-doped SrTiO3 moves into the bottom of the conduction band, and the system becomes an n-type semiconductor. The effect of Nb-doping concentration on the conductivity was discussed from the microscopic point of view. Furthermore, the 1.11 at% Nb-doped SrTiO3 shows strong absorption in the visible light and becomes a very useful material for photo-catalytic activity. The 1.67 at% and 2.5 at% Nb-doped models will be potential transparent conductive materials. Project supported by the National Natural Science Foundation of China (No. 10974077) and the Innovation Project of Shandong Graduate Education, China (No. SDYY13093).

  19. Structural and electronic properties of free standing one-sided and two-sided hydrogenated silicene: A first principle study

    SciTech Connect

    Mohan, Brij Kumar, Ashok Ahluwalia, P. K.

    2014-04-24

    We performed first-principle study of the structural and electronic properties of two-dimensional hydrogenated silicene for two configurations; one is hydrogenation along one side of silicene sheet and second is hydrogenation in both sides of silicene sheet. The one-side hydrogenated silicene is found stable at planar geometry while increased buckling of 0.725 Å is found for both-side hydrogenated silicene. The result shows that the hydrogenation occupy the extended π-bonding network of silicene, and thus it exhibits semi-conducting behaviour with a band gap of 1.77 eV and 2.19 eV for one-side hydrogenated silicene and both-side hydrogenated silicene respectively. However, both-side hydrogenated silicene of binding energy 4.56 eV is more stable than one-side hydrogenated silicene of binding energy 4.30 eV, but experimentally silicene is synthesized on substrates which interacts one side of silicene layer and only other side is available for H-atoms. Therefore, practically one-side hydrogenation is also important.

  20. First Principles Electronic Structure of Mn doped GaAs, GaP, and GaN Semiconductors

    SciTech Connect

    Schulthess, Thomas C; Temmerman, Walter M; Szotek, Zdzislawa; Svane, Axel; Petit, Leon

    2007-01-01

    We present first-principles electronic structure calculations of Mn doped III-V semiconductors based on the local spin-density approximation (LSDA) as well as the self-interaction corrected local spin density method (SIC-LSD). We find that it is crucial to use a self-interaction free approach to properly describe the electronic ground state. The SIC-LSD calculations predict the proper electronic ground state configuration for Mn in GaAs, GaP, and GaN. Excellent quantitative agreement with experiment is found for magnetic moment and p-d exchange in (GaMn)As. These results allow us to validate commonly used models for magnetic semiconductors. Furthermore, we discuss the delicate problem of extracting binding energies of localized levels from density functional theory calculations. We propose three approaches to take into account final state effects to estimate the binding energies of the Mn-d levels in GaAs. We find good agreement between computed values and estimates from photoemisison experiments.

  1. The structure, stability, and electronic properties of ultra-thin BC2N nanotubes: a first-principles study.

    PubMed

    Wang, Yue; Zhang, Juan; Huang, Gang; Yao, Xinhua; Shao, Qingyi

    2014-12-01

    Rapid developments of the silicon electronics industry have close to the physical limits and nanotube materials are the ideal materials to replace silicon for the preparation of next generation electronic devices. Boron-carbon-nitrogen nanotubes (BCNNT) can be formed by joining carbon nanotube (CNT) and boron nitride nanotube (BNNT) segments, and BC2N nanotubes have been widely and deeply studied. Here, we employed first-principles calculations based on density function theory (DFT) to study the structure, stability, and electronic properties of ultra thin (4 Å diameter) BC2N nanotubes. Our results showed that the cross sections of BC2N nanotubes can transform from round to oval when CNT and BNNT segments are parallel to the tube axis. It results when the curvature of BNNT segments become larger than CNT segments. Further, we found the stability of BC2N nanotubes is sensitive to the number of B-N bonds, and the phase segregation of BNNT and CNT segments is energetically favored. We also obtained that all (3,3) BC2N nanotubes are semiconductor, whereas (5,0) BC2N nanotubes are conductor when CNT and BNNT segments are perpendicular to the tube axis; and semiconductor when CNT and BNNT segments are parallel to the tube axis. These electronic properties are abnormal when compared to the relative big ones. PMID:25451142

  2. First-principles study of high-pressure stability, structure, and elasticity of FeS2 polymorphs

    NASA Astrophysics Data System (ADS)

    Liu, Shanqi; Li, Yongbing; Yang, Junli; Tian, Huiquan; Zhu, Bojing; Shi, Yaolin

    2014-03-01

    The pressure-dependent elastic properties of the Fe-S system are important to understand the dynamic properties of the Earth's interior. We have therefore undertaken a first-principles study of the structural and elastic properties of FeS2 polymorphs under high pressure using a method based on plane-wave pseudopotential density function theory. The lattice constants, elastic constants, zero-pressure bulk modulus, and its pressure derivative of pyrite are in good agreement with the previous experiments and theoretical approaches; the lattice constants of marcasite are also consistent with the available experimental data. Calculations of the elastic constants of pyrite and marcasite have been determined from 0 to 200 GPa. Based on the relationship between the calculated elastic constants and the pressure, which can provide the stability of mineral, it would appear that pyrite is stable, whereas marcasite is unstable when the pressure rises above 130 GPa. Static lattice energy calculations predict the marcasite-to-pyrite phase transition to occur at 5.4 GPa at 0 K.

  3. Electronic structure of quasi-one-dimensional superconductor K2Cr3As3 from first-principles calculations.

    PubMed

    Jiang, Hao; Cao, Guanghan; Cao, Chao

    2015-01-01

    The electronic structure of quasi-one-dimensional superconductor K2Cr3As3 is studied through systematic first-principles calculations. The ground state of K2Cr3As3 is paramagnetic. Close to the Fermi level, the Cr-3dz(2), dxy, and d(x(2)-y(2)) orbitals dominate the electronic states, and three bands cross EF to form one 3D Fermi surface sheet and two quasi-1D sheets. The electronic DOS at EF is less than 1/3 of the experimental value, indicating a large electron renormalization factor around EF. Despite of the relatively small atomic numbers, the antisymmetric spin-orbit coupling splitting is sizable (≈60 meV) on the 3D Fermi surface sheet as well as on one of the quasi-1D sheets. Finally, the imaginary part of bare electron susceptibility shows large peaks at Γ, suggesting the presence of large ferromagnetic spin fluctuation in the compound. PMID:26525099

  4. Structural and electronic properties of Li-ion battery cathode material MoF{sub 3} from first-principles

    SciTech Connect

    Li, A.Y.; Wu, S.Q.; Yang, Y.; Zhu, Z.Z.

    2015-07-15

    The transition metal fluorides have been extensively investigated recently as the electrode materials with high working voltage and large capacity. The structural, electronic and magnetic properties of MoF{sub 3} are studied by the first-principles calculations within both the generalized gradient approximation (GGA) and GGA+U frameworks. Our results show that the antiferromagnetic configuration of MoF{sub 3} is more stable than the ferromagnetic one, which is consistent with experimental results. The analysis of the electronic density of states shows that MoF{sub 3} is a Mott–Hubbard insulator with a d–d type band gap, which is similar to the case of FeF{sub 3}. Moreover, small spin polarizations were found on the sites of fluorine ions, which accords with a fluorine-mediated superexchange mechanism for the Mo–Mo magnetic interaction. - Graphical abstract: Deformation charge density and spin-density for MoF{sub 3} in the AF configuration. - Highlights: • The ground state of MoF{sub 3} is shown to be antiferromagnetic, in consistent with experiments. • The electronic states show that MoF{sub 3} is a Mott–Hubbard insulator with a d–d type band gap. • A fluorine-mediated super-exchange mechanism for the Mo–Mo magnetic interaction is shown.

  5. First-principles electronic structure study of the monoclinic crystal bismuth triborate BiB3O6.

    PubMed

    Yang, Jun; Dolg, Michael

    2006-10-01

    Monoclinic BiB(3)O(6) is an excellent nonlinear optical material with many advantages compared to other borate crystals. The origins of the optical effects and the chemical stability of BiB(3)O(6) are studied with gradient-corrected hybrid B3PW density functional theory within the Gaussian-orbital-based CO-LCAO scheme. Including spin-orbit coupling, the B3PW hybrid functional provides an estimate of the indirect band gap of 4.29-4.99 eV closer to the experimental value of 4.3 eV than HF, LDA, or GGA. The crystal orbital overlap population to give a detailed first-principles analysis of chemical bonding and the density of optical absorptions by convoluting the occupied density of states and the virtual density of states have been calculated. Obvious Bi-O covalent bonds have been found with different energy ranges for 6s-2p and 6p-2p interactions. The reason that [BiO(4)](5-) units are mainly responsible for the optics of BiB(3)O(6) in the long-wavelength region is due to the electronic transfer from occupied O 2p to empty Bi 6p orbitals favored by the Bi-O covalent bonds. The relativistic and correlation effects lead to fundamental differences of the band structure, chemical bonds, and optical effects for BiB(3)O(6) compared with nonrelativistic and uncorrelated calculations. PMID:17004777

  6. First-Principles Study of Electronic Structure and Hydrogen Adsorption of 3d Transition Metal Exposed Paddle Wheel Frameworks

    SciTech Connect

    Bak, J. H.; Le, V. D.; Kang, J.; Wei, S. H.; Kim, Y. H.

    2012-04-05

    Open-site paddle wheels, comprised of two transition metals bridged with four carboxylate ions, have been widely used for constructing metal-organic frameworks with large surface area and high binding energy sites. Using first-principles density functional theory calculations, we have investigated atomic and electronic structures of various 3d transition metal paddle wheels before and after metal exposure and their hydrogen adsorption properties at open metal sites. Notably, the hydrogen adsorption is impeded by covalent metal-metal bonds in early transition metal paddle wheels from Sc to Cr and by the strong ferromagnetic coupling of diatomic Mn and Fe in the paddle wheel configurations. A significantly enhanced H{sub 2} adsorption is predicted in the nonmagnetic Co{sub 2} and Zn{sub 2} paddle wheel with the binding energy of {approx}0.2 eV per H{sub 2}. We also propose the use of two-dimensional Co{sub 2} and Zn{sub 2} paddle wheel frameworks that could have strongly adsorbed dihydrogen up to 1.35 wt % for noncryogenic hydrogen storage applications.

  7. Electronic band structures of Ge1-xSnx semiconductors: A first-principles density functional theory study

    NASA Astrophysics Data System (ADS)

    Lee, Ming-Hsien; Liu, Po-Liang; Hong, Yung-An; Chou, Yen-Ting; Hong, Jia-Yang; Siao, Yu-Jin

    2013-02-01

    We conduct first-principles total-energy density functional calculations to study the band structures in Ge1-xSnx infrared semiconductor alloys. The norm-conserving optimized pseudopotentials of Ge and Sn have been constructed for electronic structure calculations. The composition-bandgap relationships in Ge1-xSnx lattices are evaluated by a detailed comparison of structural models and their electronic band structures. The critical Sn composition related to the transition from indirect- to direct-gap in Ge1-xSnx alloys is estimated to be as low as x ˜ 0.016 determined from the parametric fit. Our results show that the crossover Sn concentration occurs at a lower critical Sn concentration than the values predicted from the absorption measurements. However, early results indicate that the reliability of the critical Sn concentration from such measurements is hard to establish, since the indirect gap absorption is much weaker than the direct gap absorption. We find that the direct band gap decreases exponentially with the Sn composition over the range 0 0.375, in very good agreement with the theoretical observed behavior [D. W. Jenkins and J. D. Dow, Phys. Rev. B 36, 7994, 1987]. For homonuclear and heteronuclear complexes of Ge1-xSnx alloys, the indirect band gap at L-pointis is found to decrease homonuclear Ge-Ge bonds or increase homonuclear Sn-Sn bonds as a result of the reduced L valley. All findings agree with previously reported experimental and theoretical results. The analysis suggests that the top of valence band exhibits the localization of bond charge and the bottom of the conduction band is composed of the Ge 4s4p and/or Sn 5s5p atomic orbits.

  8. The effect of moisture on the structures and properties of lead halide perovskites: a first-principles theoretical investigation.

    PubMed

    Zhang, Lei; Ju, Ming-Gang; Liang, WanZhen

    2016-08-17

    With efficiencies exceeding 20% and low production costs, lead halide perovskite solar cells (PSCs) have become potential candidates for future commercial applications. However, there are serious concerns about their long-term stability and environmental friendliness, heavily related to their commercial viability. Herein, we present a theoretical investigation based on the ab initio molecular dynamics (AIMD) simulations and the first-principles density functional theory (DFT) calculations to investigate the effects of sunlight and moisture on the structures and properties of MAPbI3 perovskites. AIMD simulations have been performed to simulate the impact of a few water molecules on the structures of MAPbI3 surfaces terminated in three different ways. The evolution of geometric and electronic structures as well as the absorption spectra has been shown. It is found that the PbI2-terminated surface is the most stable while both the MAI-terminated and PbI2-defective surfaces undergo structural reconstruction, leading to the formation of hydrated compounds in a humid environment. The moisture-induced weakening of photoabsorption is closely related to the formation of hydrated species, and the hydrated crystals MAPbI3·H2O and MA4PbI6·2H2O scarcely absorb the visible light. The electronic excitation in the bare and water-absorbed MAPbI3 nanoparticles tends to weaken Pb-I bonds, especially those around water molecules, and the maximal decrease of photoexcitation-induced bond order can reach up to 20% in the excited state in which the water molecules are involved in the electronic excitation, indicating the accelerated decomposition of perovskites in the presence of sunlight and moisture. This work is valuable for understanding the mechanism of chemical or photochemical instability of MAPbI3 perovskites in the presence of moisture. PMID:27499005

  9. First principles molecular dynamics of molten NaI: Structure, self-diffusion, polarization effects, and charge transfer

    NASA Astrophysics Data System (ADS)

    Galamba, N.; Costa Cabral, B. J.

    2007-09-01

    The structure and self-diffusion of NaI and NaCl at temperatures close to their melting points are studied by first principles Hellmann-Feynman molecular dynamics (HFMD). The results are compared with classical MD using rigid-ion (RI) and shell-model (ShM) interionic potentials. HFMD for NaCl was reported before at a higher temperature [N. Galamba and B. J. Costa Cabral, J. Chem. Phys. 126, 124502 (2007)]. The main differences between the structures predicted by HFMD and RI MD for NaI concern the cation-cation and the anion-cation pair correlation functions. A ShM which allows only for the polarization of I- reproduces the main features of the HFMD structure of NaI. The inclusion of polarization effects for both ionic species leads to a more structured ionic liquid, although a good agreement with HFMD is also observed. HFMD Green-Kubo self-diffusion coefficients are larger than those obtained from RI and ShM simulations. A qualitative study of charge transfer in molten NaI and NaCl was also carried out with the Hirshfeld charge partitioning method. Charge transfer in molten NaI is comparable to that in NaCl, and results for NaCl at two temperatures support the view that the magnitude of charge transfer is weakly state dependent for ionic systems. Finally, Hirshfeld charge distributions indicate that differences between RI and HFMD results are mainly related to polarization effects, while the influence of charge transfer fluctuations is minimal for these systems.

  10. First-principles study of homologous series of layered Bi-Sb-Te-Se and Sn-O structures

    NASA Astrophysics Data System (ADS)

    Govaerts, Kirsten

    In the first part of the thesis, we present a systematic study of the stable layered structures at T = 0 K for the Bi-Sb-Te-Se system by means of a combination of the Cluster Expansion (CE) method and first-principles electronic structure calculations. In order to account for the existence of long-periodic layered structures and the strong structural relaxations we have developed a one-dimensional CE with occupation variables explicitly accounting for the fact that Bi or Sb atoms are part of an even or odd number of layers. For the binary systems A1-xQx (A = Sb, Bi; Q = Te, Se) the resulting (meta)stable structures are the homologous series (A2) n(A2Q3)m built up from successive bilayers A 2 and quintuple units A2Q3. The Bi1-xSb x system is found to be an almost ideal solution. The CE for the ternary Bi-Sb-Te system not only reproduces the binary stable structures but also finds stable ternary layered compounds with an arbitrary stacking of Sb 2Te3, Bi2Te3 and Te-Bi-Te-Sb-Te quintuple units, optionally separated by mixed Bi/Sb bilayers. We also investigate the electronic properties of the newly found ground state structures, and in particular the effect of Bi bilayers on the electronic structure of the topological insulator Bi2Se3. Due to the charge transfer from the Bi bilayers to the quintuple layers, the top- and bottom-surface Dirac cones shift down in energy. Also the Rashba-split conduction band states shift down, resulting in a new Dirac cone. The bands of the additional Bi bilayer are just ordinary Rashba-split states originating from the dipole built up by the charge transfer. These results offer new insight in experimental results, where cones are not always correctly identified. In a second part of the thesis, we investigate the Sn-O system. First we show that a combination of current van der Waals-corrected functionals and many-body calculations within the GW approximation provide accurate values for both structural and electronic properties of Sn

  11. First-principles prediction of disordering tendencies in pyrochlore oxides

    NASA Astrophysics Data System (ADS)

    Jiang, Chao; Stanek, C. R.; Sickafus, K. E.; Uberuaga, B. P.

    2009-03-01

    Using first-principles calculations, we systematically predict the order-disorder energetics of series of zirconate (A2Zr2O7) , hafnate (A2Hf2O7) , titanate (A2Ti2O7) , and stannate (A2Sn2O7) pyrochlores. The disordered defect-fluorite structure is modeled using an 88-atom two-sublattice special quasirandom structure (SQS) that closely reproduces the most relevant near-neighbor intrasublattice and intersublattice pair-correlation functions of the random mixture. The order-disorder transition temperatures of these pyrochlores estimated from our SQS calculations show overall good agreement with existing experiments. We confirm previous studies suggesting that the bonding in pyrochlores is not purely ionic and thus electronic effects also play a role in determining their disordering tendencies. Our results have important consequences for numerous applications, including nuclear waste forms and fast ion conductors.

  12. Tunable electronic structures of germanium monochalcogenide nanosheets via light non-metallic atom functionalization: a first-principles study.

    PubMed

    Ding, Yi; Wang, Yanli

    2016-08-17

    Germanium monochalcogenides, i.e. GeS and GeSe sheets, are isoelectronic analogues of phosphorene, which have been synthesized in recent experiments (P. Ramasamy et al., J. Mater. Chem. C, 2016, 4, 479). Utilizing first-principles calculations, we have investigated their tunable electronic and magnetic properties via light non-metallic atom (B, C, N, O, Si, P, S) functionalization. We find that on these GeS and GeSe sheets O and S adatoms prefer to locate at the top site above the Ge atom, while the other ones like to occupy the anion site, which push the original S/Se atom to the hollow site instead. O and S adatoms slightly affect the semiconducting behaviour of the doped systems, while B, C, N, Si, P ones will drastically modify their band structures and induce versatile spintronic properties. Through the supercell calculations, B and C adatoms are found to induce a bipolar semiconducting behaviour in the decorated systems, while the N/P adatom will cause a spin-gapless-semiconducting/nearly-half-metallic feature in them. The B/C/N/Si/P-substituted GeS/GeSe sheet can be formed by removing the hollow-site S/Se atom from the adatom-decorated structures, which exhibit an opposite semiconducting/metallic behaviour to their phosphorene counterparts. A general odd-even rule is proposed for this phenomenon, which shows that an odd (even) number of valence electron difference between the substitution and host atoms would cause a metallic (semiconducting) feature in the substituted systems. Our study demonstrates that atom functionalization is an efficient way to tailor the properties of GeS and GeSe nanosheets, which have adaptable electronic properties for potential applications in nanoelectronics and spintronics. PMID:27491896

  13. Structural, thermodynamic and optical properties of MgF{sub 2} studied from first-principles theory

    SciTech Connect

    Ramesh Babu, K.; Bheema Lingam, Ch.; Auluck, S.; Tewari, Surya P.; Vaitheeswaran, G.

    2011-02-15

    A detailed theoretical study of structural, electronic, elastic, thermodynamic and optical properties of rutile type MgF{sub 2} has been carried out by means of first-principles Density Functional Theory (DFT) calculations using plane wave pseudo-potentials within the local density approximation and generalized-gradient approximation for the exchange and correlation functionals. The calculated ground state properties and elastic constants agree quite well with experimental values. From the calculated elastic constants we conclude that MgF{sub 2} is relatively hard when compared to other alkaline-earth fluorides and ductile in nature. The thermodynamic properties such as heat capacity, entropy, free energy, phonon density of states and Debye temperatures are calculated at various temperatures from the lattice dynamical data obtained through the quasi-harmonic Debye model. From free energy and entropy it is found that the system is thermodynamically stable up to 1200 K. The imaginary part of the calculated dielectric function {epsilon}{sub 2}({omega}) could reproduce the six prominent peaks which are observed in experiment. From the calculated {epsilon}({omega}), other optical properties such as refractive index, reflectivity and electron energy-loss spectrum are obtained up to the photon energy range of 30 eV. -- Graphical abstract: The calculated imaginary part {epsilon}{sub 2}({omega}) of the complex dielectric function {epsilon}({omega}) of MgF{sub 2} as a function of photon energy is shown. The calculated {epsilon}{sub 2}({omega}) could reproduce the major peaks observed in experiment. All the peaks observed are corresponds to interband transitions from 'p' states of Fluorine in valence band to the 's' states of Mg in conduction band. Display Omitted Research highlights: > Structural and bonding properties. > Optical properties. > Single and polycrystalline elastic properties. > Thermodynamic properties.

  14. First-principles study of structural, electronic, vibrational, dielectric and elastic properties of tetragonal Ba₂YTaO₆

    SciTech Connect

    Ganeshraj, C.; Santhosh, P. N.

    2014-10-14

    We report first-principles study of structural, electronic, vibrational, dielectric, and elastic properties of Ba₂YTaO₆, a pinning material in high temperature superconductors (HTS), by using density functional theory. By using different exchange-correlation potentials, the accuracy of the calculated lattice constants of Ba₂YTaO₆ has been achieved with GGA-RPBE, since many important physical quantities crucially depend on change in volume. We have calculated the electronic band structure dispersion, total and partial density of states to study the band gap origin and found that Ba₂YTaO₆ is an insulator with a direct band gap of 3.50 eV. From Mulliken population and charge density studies, we conclude that Ba₂YTaO₆ have a mixed ionic-covalent character. Moreover, the vibrational properties, born effective charges, and the dielectric permittivity tensor have been calculated using linear response method. Vibrational spectrum determined through our calculations agrees well with the observed Raman spectrum, and allows assignment of symmetry labels to modes. We perform a detailed analysis of the contribution of the various infrared-active modes to the static dielectric constant to explain its anisotropy, while electronic dielectric tensor of Ba₂YTaO₆ is nearly isotropic, and found that static dielectric constant is in good agreement with experimental value. The six independent elastic constants were calculated and found that tetragonal Ba₂YTaO₆ is mechanically stable. Other elastic properties, including bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and elastic anisotropy ratios are also investigated and found that Poisson's ratio and Young's modulus of Ba₂YTaO₆ are similar to that of other pinning materials in HTS.

  15. First-principles study of configurational disorder in B4C using a superatom-special quasirandom structure method

    NASA Astrophysics Data System (ADS)

    Ektarawong, A.; Simak, S. I.; Hultman, L.; Birch, J.; Alling, B.

    2014-07-01

    Configurationally disordered crystalline boron carbide, with the composition B4C, is studied using first-principles calculations. We investigate both dilute and high concentrations of carbon-boron substitutional defects. For the latter purpose, we suggest a superatom's picture of the complex structure and combine it with a special quasirandom structure approach for disorder. In this way, we model a random distribution of high concentrations of the identified low-energy defects: (1) bipolar defects and (2) rotation of icosahedral carbon among the three polar-up sites. Additionally, the substitutional disorder of the icosahedral carbon at all six polar sites, as previously discussed in the literature, is also considered. Two configurational phase transitions from the ordered to the disordered configurations are predicted to take place upon an increase in temperature using a mean-field approximation for the entropy. The first transition, at 870 K, induces substitutional disorder of the icosahedral carbon atoms among the three polar-up sites; meanwhile the second transition, at 2325 K, reveals the random substitution of the icosahedral carbon atoms at all six polar sites coexisting with bipolar defects. Already the first transition removes the monoclinic distortion existing in the ordered ground-state configuration and restore the rhombohedral system (R3m). The restoration of inversion symmetry yielding the full rhombohedral symmetry (R3¯m ) on average, corresponding to what is reported in the literature, is achieved after the second transition. Investigating the effects of high pressure on the configurational stability of the disordered B4C phases reveals a tendency to stabilize the ordered ground-state configuration as the configurationally ordering/disordering transition temperature increases with pressure exerted on B4C. The electronic density of states, obtained from the disordered phases, indicates a sensitivity of the band gap to the degree of configurational

  16. Structure and dynamics of the hydrated magnesium ion and of the solvated magnesium carbonates: insights from first principles simulations.

    PubMed

    Di Tommaso, Devis; de Leeuw, Nora H

    2010-01-28

    We report first principles molecular dynamics simulations based on the density functional theory and the Car-Parrinello method to study the structures and dynamics of the hydrated Mg(2+) ion and of the solvated MgHCO(3)(+) and MgCO(3) complexes in aqueous solution. According to these simulations, the first hydration shell of the hydrated magnesium ion consists of six water molecules, whereas in the solvated magnesium bicarbonate and magnesium carbonate complexes the Mg(2+) is mostly five-coordinated, which indicates that when coordinated to magnesium the HCO(3)(-) and CO(3)(2-) anions reduce its the coordination sphere. Our simulations show that the structures of the most stable monomers of magnesium bi-carbonate and magnesium carbonate in solution are Mg[eta(1)-HCO(3)](H(2)O)(4)(+) and Mg[eta(1)-CO(3)](H(2)O)(4), i.e. the preferred hydration number is four, while the (bi-)carbonate is coordinated to the magnesium in a monodentate mode. The analysis of the exchange processes of the water molecules in the first and second hydration shell of Mg(2+) shows that the HCO(3)(-) or CO(3)(2-) ligands affect the dynamics of the magnesium coordination spheres by making its hydration shell more "labile". Furthermore, molecular dynamics simulations of the non-associated Mg(2+)/Cl(-) pair in water suggest that, despite negligible differences in the coordination spheres of Mg(2+), the chloride anion has a significant influence on the water exchange rates in the second hydration shell of Mg(2+). PMID:20066374

  17. First-principles study on equation of states and electronic structures of shock compressed Ar up to warm dense regime

    NASA Astrophysics Data System (ADS)

    Sun, Huayang; Kang, Dongdong; Dai, Jiayu; Ma, Wen; Zhou, Liangyuan; Zeng, Jiaolong

    2016-03-01

    The equation of states (EOS) and electronic structures of argon with temperatures from 0.02 eV to 3 eV and densities from 0.5 g/cm3 to 5.5 g/cm3 are calculated using the pair potential and many-body potential molecular dynamics and the density functional theory (DFT) molecular dynamics with van der Waals (vdW) corrections. First-principles molecular dynamics is implemented above 2.0 g/cm3. For the cases of low densities below 3 g/cm3, we performed pair potential molecular dynamics in order to obtain the ionic configurations, which are used in density functional theory to calculate the EOS and electronic structures. We checked the validity of different methods at different densities and temperatures, showing their behaviors by comparing EOS. DFT without vdW correction works well above 1 eV and 3.5 g/cm3. Below 1 eV and 2.0 g/cm3, it overestimates the pressure apparently and results in incorrect behaviors of the internal energy. With vdW corrections, the semi-empirical force-field correction (DFT-D2) method gives consistent results in the whole density and temperature region, and the vdW density functional (vdW-DF2) method gives good results below 2.5 g/cm3, but it overestimates the pressure at higher densities. The interactions among the atoms are overestimated by the pair potential above 1 eV, and a temperature dependent scaled pair potential can be used to correct the ionic configurations of the pair potential up to 3 eV. The comparisons between our calculations and the experimental multi-shock compression results show that the Hugoniot line of DFT-D2 and DFT tends to give larger pressure than the results of the self-consistent fluid variational theory, and the difference increases with the density. The electronic energy gap exists for all our cases up to 5.5 g/cm3 and 1 eV. The effect of vdW interactions on the electronic structures are also discussed.

  18. High-pressure U{sub 3}O{sub 8} with the fluorite-type structure

    SciTech Connect

    Zhang, F.X.; Lang, M.; Wang, J.W.; Li, W.X.; Sun, K.; Prakapenka, V.; Ewing, R.C.

    2014-05-01

    A new high-pressure phase of U{sub 3}O{sub 8}, which has a fluorite-type structure, forms at pressures greater than ∼8.1 GPa that was confirmed by in situ x-ray diffraction (XRD) measurements. The fluorite-type U{sub 3}O{sub 8} is stable at pressures at least up to ∼40 GPa and temperatures to 1700 K, and quenchable to ambient conditions. Based on the XRD analysis, there is a huge volume collapse (>20%) for U{sub 3}O{sub 8} during the phase transition and the quenched high-pressure phase is 28% denser than the initial orthorhombic phase at ambient conditions. The high-pressure phase has a very low compressibility comparing with the starting orthorhombic phase. - Graphical abstract: α-U{sub 3}O{sub 8} is in a layered structure with orthorhombic symmetry, at high pressures, it transformed to a fluorite-type cubic structure. There are a lot of defects in the cubic structure, and it is a new kind of hyperstoichiometric uranium oxide, which is stable at ambient conditions. - Highlights: • A new fluorite-type high-pressure phase was found in hyperstoichometric UO{sub 2}+x (x∼0.8). • The new high-pressure structure is quenchable to ambient conditions. • Pressure driven phase transition in orthorhombic U{sub 3}O{sub 8} was first found.

  19. Determination of solubility limit of Sn(4+) in fluorite structured terbia with simultaneous evaluation of photocatalytic function.

    PubMed

    Tripathi, Vikash Kumar; Nagarajan, Rajamani

    2016-07-01

    Although the fluorite structure is highly common among stoichiometric and non-stoichiometric terbia compositions, high pressures are necessary to stabilize SnO2 in the fluorite structure. With this objective, the extent of solubility of Sn(4+) in terbia possessing the fluorite structure has been determined by conducting its synthesis via an epoxide mediated sol-gel method. Up to 40% of Sn(4+) can be incorporated in terbia, which retains its fluorite structure, as concluded from PXRD, FTIR, Raman spectroscopy, FESEM, HR-TEM and SAED measurements. The cubic lattice constant decreases systematically, as inferred from successful Rietveld refinements of PXRD patterns. The stretching vibration of the Tb-O bond is manifested as broad band at 734 cm(-1) for the terbia, and moves to lower wavenumber for the tin substituted samples. The broad band at 611 cm(-1) in the Raman spectrum of terbia became even broader with the maxima shifting towards higher values, which indicated the strain of the lattice and generation of oxygen vacancies with progressive tin substitution. The band gap value increased from 1.78 eV for terbia to 2.05 eV for the 40% tin substituted sample. Emission in the blue region became intense upon tin substitution, which was indicative of increased oxygen vacancies and this has been constructively utilized for environmental remediation as a catalyst to degrade aqueous Rhodamine-6G and Methylene Blue dye solutions. PMID:27328282

  20. Effect of lattice anharmonicity in the structural phase transformation of Laves phase HfV2 alloy: A first-principles investigation

    SciTech Connect

    Krcmar, Maja; Fu, Chong Long

    2013-01-01

    First-principles theory was developed to study the structural phase transformations in the Laves phase HfV2 alloy. We explored the energy landscape and established the role of lattice anharmonicity underlying the structural phase transitions. Our approach is based on a phenomenological Landau theory for the structural phase transition and a mean-field approximation for the free energy. First-principles calculations were utilized to obtain the distortion energy as a function of relevant deformations, and to deduce parameters for constructing the free energy. Our result for the phase transition temperature of HfV2 is in good agreement with experiment. We find that the high-temperature cubic C15 phase is stabilized by the effect of lattice anharmonicity. The theory also predicts an anomalous increase in shear modulus with increasing temperature for systems where the anharmonicity is pronounced.

  1. Alloying effects on structural and thermal behavior of Ti1-xZrxC: A first principles study

    NASA Astrophysics Data System (ADS)

    Chauhan, Mamta; Gupta, Dinesh C.

    2016-05-01

    The formation energy, equilibrium lattice parameter, bulk modulus, Debye temperature and heat capacity at constant volume have been calculated for TiC, ZrC, and their intermediate alloys (Ti1-xZrxC, x = 0,0.25.0.5,0.75,1) using first principles approach. The calculated values of lattice parameter and bulk modulus agree well with the available experimental and earlier theoretical reports. The variation of lattice parameter and bulk modulus with the change in concentration of Zr atom in Ti1-xZrxC has also been reported. The heat capacities of TiC, ZrC, and their intermediate alloys have been calculated by considering both vibrational and electronic contributions.

  2. First-principles study on electronic structure and optical properties of Cu-doped β-Ga2O3

    NASA Astrophysics Data System (ADS)

    Yan, Huiyu; Guo, Yanrui; Song, Qinggong; Chen, Yifei

    2014-02-01

    The electronic and optical properties of the Cu-doped and intrinsic β-Ga2O3 are studied by using the first-principles calculation method. Results show that Cu-doped β-Ga2O3 can be fabricated in experiments. Two acceptor impurity levels are introduced near the top of the valence band by Cu dopant, indicating that Cu-doped gallium oxide is a promising p-type semiconductor. Cu-doped β-Ga2O3 can be used as intermediate band semiconductor in solar cell. Cu dopant induced 100% spin polarization near the Fermi level. The analysis results of optical properties reveal that Cu-doped β-Ga2O3 is a promising potential candidate for p-type ultraviolet (UV) transparent semiconductor.

  3. First-principles study of the pressure and crystal-structure dependences of the superconducting transition temperature in compressed sulfur hydrides

    NASA Astrophysics Data System (ADS)

    Akashi, Ryosuke; Kawamura, Mitsuaki; Tsuneyuki, Shinji; Nomura, Yusuke; Arita, Ryotaro

    2015-06-01

    We calculate the superconducting transition temperatures (Tc) in sulfur hydrides H2S and H3S from first principles using the density functional theory for superconductors. At pressures of ≲150 GPa, the high values of Tc (≥130 K) observed in a recent experiment (A. P. Drozdov, M. I. Eremets, and I. A. Troyan, arXiv:1412.0460) are accurately reproduced by assuming that H2S decomposes into R 3 m H3S and S. For higher pressures, the calculated Tc's for I m 3 ¯m H3S are systematically higher than those for R 3 m H3S and the experimentally observed maximum value (190 K), which suggests the possibility of another higher-Tc phase. We also quantify the isotope effect from first principles and demonstrate that the isotope effect coefficient can be larger than the conventional value (0.5) when multiple structural phases energetically compete.

  4. Adsorption and diffusion of hydrogen on Pd(211) and Pd(111): Results from first-principles electronic structure calculations

    NASA Astrophysics Data System (ADS)

    Hong, Sampyo; Rahman, Talat S.

    2007-04-01

    We have carried out first-principles calculations of H adsorption on Pd(211) using density-functional theory with the generalized gradient approximation in the plane-wave basis to find out that the most preferred is the threefold hollow site on the terrace of Pd(211) with an adsorption energy of 0.52eV : the hcp and fcc sites being almost energetically equally favorable. For subsurface H adsorption on Pd(211), the octahedral site with an adsorption energy of 0.19eV is slightly more favorable than the tetrahedral site (0.18eV) . Our calculated activation energy barrier for H to diffuse from the preferred surface site to the subsurface one on Pd(211) is 0.33eV , as compared with 0.41eV on Pd(111). Thus, there is an enhancement of the probability of finding subsurface hydrogen in Pd(211). Additionally, we find the diffusion barriers for H on the terraces of Pd(211) to be 0.11eV , while that along the step edge to be only 0.05eV and that within the second layer (subsurface) to be 0.15eV .

  5. Strontium ruthenate-anatase titanium dioxide heterojunctions from first-principles: Electronic structure, spin, and interface dipoles

    NASA Astrophysics Data System (ADS)

    Ferdous, Naheed; Ertekin, Elif

    2016-07-01

    The epitaxial integration of functional oxides with wide band gap semiconductors offers the possibility of new material systems for electronics and energy conversion applications. We use first principles to consider an epitaxial interface between the correlated metal oxide SrRuO3 and the wide band gap semiconductor TiO2, and assess energy level alignment, interfacial chemistry, and interfacial dipole formation. Due to the ferromagnetic, half-metallic character of SrRuO3, according to which only one spin is present at the Fermi level, we demonstrate the existence of a spin dependent band alignment across the interface. For two different terminations of SrRuO3, the interface is found to be rectifying with a Schottky barrier of ≈1.3-1.6 eV, in good agreement with experiment. In the minority spin, SrRuO3 exhibits a Schottky barrier alignment with TiO2 and our calculated Schottky barrier height is in excellent agreement with previous experimental measurements. For majority spin carriers, we find that SrRuO3 recovers its exchange splitting gap and bulk-like properties within a few monolayers of the interface. These results demonstrate a possible approach to achieve spin-dependent transport across a heteroepitaxial interface between a functional oxide material and a conventional wide band gap semiconductor.

  6. Temperature-dependence of structural and mechanical properties of TiB{sub 2}: A first principle investigation

    SciTech Connect

    Xiang, Huimin; Feng, Zhihai; Li, Zhongping; Zhou, Yanchun E-mail: yczhou714@gmail.com

    2015-06-14

    High temperature mechanical and thermodynamic properties of TiB{sub 2} are important to its applications as ultrahigh temperature ceramic, which were not well understood. In this study, the thermodynamic and mechanical properties of TiB{sub 2} were investigated by the combination of first principle and phonon dispersion calculations. The thermal expansion of TiB{sub 2} was anisotropic, α{sub c}/α{sub a} is nearly constant (1.46) from 300 K to 1500 K, theoretically. The origination of this anisotropy is the anisotropic compressibility. The heat capacity at constant pressure was estimated from the theoretical entropy and fitted the experimental result quite well when higher-order anharmonic effects were considered. Theoretical isentropic elastic constants and mechanical properties were calculated and their temperature dependence agreed with the existed experiments. From room temperature to 1500 K, the theoretical slope is −0.0211 GPa·K{sup −1}, −0.0155 GPa·K{sup −1}, and −0.0384 GPa·K{sup −1} for B, G, and E, respectively. Our theoretical results highlight the suitability of this method in predicting temperature dependent properties of ultrahigh temperature ceramics and show ability in selecting and designing of novel ultrahigh temperature ceramics.

  7. Rationalization of Hubbard U in CeOx from first principles: Unveiling the role of local structure in screening

    NASA Astrophysics Data System (ADS)

    Lu, Deyu; Liu, Ping

    2014-03-01

    DFT+U method has been widely employed in theoretical studies on various ceria systems to correct the delocalization bias in local and semi-local DFT functionals with moderate computational cost. To rationalize the Hubbard U of Ce 4f, we employed the first principles linear response method to compute Hubbard U for Ce in ceria clusters, bulks, and surfaces. We found that in contrast to the commonly used approach treating U as a constant, the Hubbard U varies in a wide range from 4.1 eV to 6.7 eV, and exhibits a strong correlation with the Ce coordination numbers and Ce-O bond lengths, rather than the Ce 4f valence state. The variation of the Hubbard U can be explained by the changes in the strength of local screening due to O --> Ce intersite transition. Our study represents a systematic, quantitative investigation of the relationship between the Hubbard U and the local atomic arrangement, enabling a DFT+environment-dependent U scheme that can have potential impact on catalysis research of strongly correlated systems. This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

  8. First-principles study of structural, electronic, magnetic and thermodynamic properties of the double perovskite Ba2CeCoO6

    NASA Astrophysics Data System (ADS)

    Xie, Huan-Huan; Gao, Qiang; Li, Lei; Lei, Gang; Hu, Xian-Ru; Deng, Jian-Bo

    2016-07-01

    The structural, electronic and magnetic properties of cubic double perovskite Ba2CeCoO6 were calculated using first-principles full-potential local-orbital minimum-basis method. This compound has a cubic crystal structure with space group Fm 3 bar m. By analysing the band structure we found, at the equilibrium lattice constant, this compound is a half-metal. Moreover, the half-metallicity can be kept under a large range of pressure. Meanwhile, the thermodynamic characters are investigated using the quasi-harmonic Debye model.

  9. Electronic structures and ferromagnetism of SnO{sub 2} (rutile) doped with double-impurities: First-principles calculations

    SciTech Connect

    Fakhim Lamrani, A.; Belaiche, M.; Benyoussef, A.; and others

    2014-01-07

    The electronic and magnetic properties of double-impurities-doped SnO{sub 2} (rutile) are explored using first-principles calculations within the generalized gradient approximation to examine their potential use as spintronic system. Calculations are performed for double impurities (M1 and M2) from M1 = Cr, and M2 = Mn, and Re. The origins of ferromagnetism are shown to be different in the two cases. For Sn{sub 1-2x}Cr{sub x}Mn{sub x}O2, the hybridization between Cr-3d and O-2p results in Cr becoming ferromagnetic with a magnetic moment of about 5.0 μ{sub B} per supercell. The Cr-and Mn-doped SnO{sub 2} system exhibits half-metallic ferromagnetism. The strong ferromagnetic couplings between local magnetic moments can be attributed to p-d hybridization. In contrast, in (Cr, Re) codoped TiO{sub 2}, the local magnetic moments of the impurities and their oxidation states agree with the charge transfer between Cr and Re, which would lead to the ferromagnetic through the double-exchange mechanism in transition metal oxides. Since there are two possible couplings between the impurities, we studied both configurations (ferromagnetic and antiferromagnetic (AF)) for double-impurities-doped SnO{sub 2}. Our calculations show that a ferromagnetic alignment of the spins is energetically always more stable than simple AF arrangements, which makes these materials possible candidates for spin injection in spintronic devices.

  10. Electronic bands, Fermi surface, and elastic properties of new 4.2 K superconductor SrPtAs with a honeycomb structure from first principles calculations

    NASA Astrophysics Data System (ADS)

    Shein, I. R.; Ivanovskii, A. L.

    2011-10-01

    The hexagonal phase SrPtAs (s.g. P6/ mmm; #194) with a honeycomb lattice structure was recently declared as a new low-temperature ( T C ∼ 4.2 K) superconductor. Here, by means of first-principles calculations the optimized structural parameters, electronic bands, Fermi surface, total and partial densities of states, inter-atomic bonding picture, independent elastic constants, bulk and shear moduli for SrPtAs were obtained for the first time and analyzed in comparison with the related layered superconductor SrPt 2As 2.

  11. The effect of Ag adsorption on the structural, electronic, and optical properties of the ZnO (10 1 ̅ 0) surface: A first-principles study

    NASA Astrophysics Data System (ADS)

    Lahmer, M. A.

    2016-09-01

    The effect of the Ag adsorption on the structural, electronic and optical properties of the clean ZnO(10 1 ̅ 0) surface was investigated using the first principles method. The obtained results show that adsorbed Ag atoms transfer charge to the surface which results in a charge accumulation in near-surface region accompanied with a decrease of the work function. On the other hand, our results show that the adsorption of Ag atoms leads also to the new optical absorption peaks in the visible region which could improve ZnO photocatalytical properties.

  12. Room-temperature ferromagnetism in Cr-doped Si achieved by controlling atomic structure, Cr concentration, and carrier densities: A first-principles study

    SciTech Connect

    Wei, Xin-Yuan; Yang, Zhong-Qin; Zhu, Yan; Li, Yun

    2015-04-28

    By using first-principles calculations, we investigated how to achieve a strong ferromagnetism in Cr-doped Si by controlling the atomic structure and Cr concentration as well as carrier densities. We found that the configuration in which the Cr atom occupies the tetrahedral interstitial site can exist stably and the Cr atom has a large magnetic moment. Using this doping configuration, room-temperature ferromagnetism can be achieved in both n-type and p-type Si by tuning Cr concentration and carrier densities. The results indicate that the carrier density plays a crucial role in realizing strong ferromagnetism in diluted magnetic semiconductors.

  13. Origin of structural analogies and differences between the atomic structures of GeSe{sub 4} and GeS{sub 4} glasses: A first principles study

    SciTech Connect

    Bouzid, Assil; Le Roux, Sébastien; Ori, Guido; Boero, Mauro; Massobrio, Carlo

    2015-07-21

    First-principles molecular dynamics simulations based on density functional theory are employed for a comparative study of structural and bonding properties of two stoichiometrically identical chalcogenide glasses, GeSe{sub 4} and GeS{sub 4}. Two periodic cells of 120 and 480 atoms are adopted. Both glasses feature a coexistence of Ge-centered tetrahedra and Se(S) homopolar connections. Results obtained for N = 480 indicate substantial differences at the level of the Se(S) environment, since Ge–Se–Se connections are more frequent than the corresponding Ge–S–S ones. The presence of a more prominent first sharp diffraction peak in the total neutron structure factor of glassy GeS{sub 4} is rationalized in terms of a higher number of large size rings, accounting for extended Ge–Se correlations. Both the electronic density of states and appropriate electronic localization tools provide evidence of a higher ionic character of Ge–S bonds when compared to Ge–Se bonds. An interesting byproduct of these investigations is the occurrence of discernible size effects that affect structural motifs involving next nearest neighbor distances, when 120 or 480 atoms are used.

  14. Origin of structural analogies and differences between the atomic structures of GeSe4 and GeS4 glasses: A first principles study.

    PubMed

    Bouzid, Assil; Le Roux, Sébastien; Ori, Guido; Boero, Mauro; Massobrio, Carlo

    2015-07-21

    First-principles molecular dynamics simulations based on density functional theory are employed for a comparative study of structural and bonding properties of two stoichiometrically identical chalcogenide glasses, GeSe4 and GeS4. Two periodic cells of 120 and 480 atoms are adopted. Both glasses feature a coexistence of Ge-centered tetrahedra and Se(S) homopolar connections. Results obtained for N = 480 indicate substantial differences at the level of the Se(S) environment, since Ge-Se-Se connections are more frequent than the corresponding Ge-S-S ones. The presence of a more prominent first sharp diffraction peak in the total neutron structure factor of glassy GeS4 is rationalized in terms of a higher number of large size rings, accounting for extended Ge-Se correlations. Both the electronic density of states and appropriate electronic localization tools provide evidence of a higher ionic character of Ge-S bonds when compared to Ge-Se bonds. An interesting byproduct of these investigations is the occurrence of discernible size effects that affect structural motifs involving next nearest neighbor distances, when 120 or 480 atoms are used. PMID:26203033

  15. First-principles study on the structure, elastic properties, hardness and electronic structure of TMB{sub 4} (TM=Cr, Re, Ru and Os) compounds

    SciTech Connect

    Pan, Y.; Zheng, W.T.; Guan, W.M.; Zhang, K.H.; Fan, X.F.

    2013-11-15

    The structural formation, elastic properties, hardness and electronic structure of TMB{sub 4} (TM=Cr, Re, Ru and Os) compounds are investigated using first-principles approach. The value of C{sub 22} for these compounds is almost two times bigger than the C{sub 11} and C{sub 33}. The intrinsic hardness, shear modulus and Young's modulus are calculated to be in a sequence of CrB{sub 4}>ReB{sub 4}>RuB{sub 4}>OsB{sub 4}, and the Poisson's ratio and B/G ratio of TMB{sub 4} follow the order of CrB{sub 4}first-principles calculations show that the intrinsic hardness of CrB{sub 4} and ReB{sub 4} are bigger than 40 GPa, which are the potential superhard materials due to the B–B bonds cage structure. Display Omitted - Highlights: • The intrinsic hardness of CrB{sub 4} and ReB{sub 4} is bigger than 40 GPa. • The hardness of TMB{sub 4} is calculated to be in a sequence of CrB{sub 4}>ReB{sub 4}>RuB{sub 4}>OsB{sub 4}. • The trend of hardness for TMB{sub 4} is consistent with the variation of elastic modulus. • The C{sub 22} value of TMB{sub 4} is bigger than that of C{sub 11} and C{sub 33}. • The high hardness of TMB{sub 4} is originated from the B–B bonds cage.

  16. Electronic structure and ground-state properties of Na{sub 2}Po: A first-principles study

    SciTech Connect

    Eithiraj, R. D.

    2015-06-24

    Self-consistent scalar-relativistic band structure calculations have been performed to investigate the electronic structure and ground-state properties of Na{sub 2}Po in cubic antifluorite (anti-CaF{sub 2}-type) structure using the linear muffin-tin orbital in its tight-binding representation (TB-LMTO) method. Ground state properties such as equilibrium lattice constant and bulk modulus were calculated. The results of the electronic structure calculations show that Na{sub 2}Po is direct bandgap semiconductor.

  17. Structure and stability of silicon nanoclusters passivated by hydrogen and oxygen: evolutionary algorithm and first- principles study

    NASA Astrophysics Data System (ADS)

    Baturin, V. S.; Lepeshkin, S. V.; Matsko, N. L.; Uspenskii, Yu A.

    2016-02-01

    We investigate the structural and thermodynamical properties of small silicon clusters. Using the graph theory applied to previously obtained structures of Si10H2m clusters we trace the connection between geometry and passivation degree. The existing data on these clusters and structures of Si10O4n clusters obtained here using evolutionary calculations allowed to analyze the features of Si10H2m clusters in hydrogen atmosphere and Si10O4n clusters in oxygen atmosphere. We have shown the basic differences between structures and thermodynamical properties of silicon clusters, passivated by hydrogen and silicon oxide clusters.

  18. First-principles study of the atomic and electronic structures of crystalline and amorphous B4C

    NASA Astrophysics Data System (ADS)

    Ivashchenko, V. I.; Shevchenko, V. I.; Turchi, P. E. A.

    2009-12-01

    The atomic and electronic structures of crystalline and amorphous B4C were determined within density function theory using the local density approximation and a plane-wave pseudopotential method. For the crystalline phases, chain, and polar structures were considered. The structural parameters were obtained by minimizing the total energy with respect to the size, shape, and internal degrees of freedom of 15 and 45-atom unit cells. The amorphous 120 and 135-atom samples of a-B4C were generated using molecular dynamic simulations in the NVT ensemble using different initial structures. The 120-atom sample was generated from a rhombohedral c-B4C cell, whereas the a-135 sample was obtained from a fcc B4C initial structure that differs essentially from the real crystalline B4C structure. Analysis of the computed results shows that: (i) a random icosahedral network connected with the amorphous B-C matrix is identified in the case of a-B4C ; (ii) carbon clusters are observed in the case of the 120-atom sample of an amorphous matrix; (iii) no chain atoms are found in both amorphous samples that can be explained by their bulk moduli that are lower compared to those of their crystalline counterparts; (iv) the chain and polar B4C phases transform into a-B4C under high pressure; and (v) all crystalline structures studied so far are semiconducting, whereas a-B4C is a semimetal.

  19. Dimorphic HT- and LT-TbTiGe: Electronic and magnetic structures and bonding properties from first principles

    NASA Astrophysics Data System (ADS)

    Matar, Samir F.; Chevalier, Bernard; Etourneau, Jean

    2016-01-01

    TbTiGe intermetallic compound is characterized by temperature dimorphism with different but related crystal structures with ferromagnetic high temperature (HT) form versus antiferromagnetic low temperature (LT) form. Such different structure properties and magnetic behaviors have been addressed based on DFT computations of cohesive energies, charge transfers, mechanical and chemical properties of the two structures. This is particularly illustrated by harder and less ductile LT-form with stronger Ti-Ge bond and larger charge transfer from Tb and Ti on one hand and Ge on the other hand.

  20. Structural and elastic properties of La{sub 2}Mg{sub 17} from first-principles calculations

    SciTech Connect

    Luo, Tao-Peng; Ma, Li; Pan, Rong-Kai; Zhou, Si-Chen; Wang, Hai-Chen; Tang, Bi-Yu

    2013-10-15

    Structural and elastic properties of La{sub 2}Mg{sub 17} with layer structure have been investigated within framework of the density functional theory. Different from the general layer-structured materials, the obtained c/a is less than unity. The calculated elastic constants C{sub 33} is larger than C{sub 11}, being novel in comparison with other alloys with layer structure. The calculated bulk, shear and Young’s modulus of La{sub 2}Mg{sub 17} are higher than other Mg–La alloys with higher La content, implying the stronger covalent bonding. Moreover, the elastic isotropies of La{sub 2}Mg{sub 17} are more excellent. The electronic structure within basal plane is highly symmetric, and the electronic interaction within basal plane is slightly weaker than one between basal planes, which reveal the underlying mechanism for the structural and elastic properties of La{sub 2}Mg{sub 17}. - Graphical abstract: The crystal structure (a) and the atomic positions for (b) (0 0 0 2), (c) (0 0 0 4) and (d) (1 2{sup ¯} 1 0) plane of La{sub 2}Mg{sub 17}. Display Omitted - Highlights: • The c/a of La{sub 2}Mg{sub 17} is anomalously less than unity. • It is novel that for La{sub 2}Mg{sub 17} the elastic constants C{sub 33} is larger than C{sub 11}. • The elastic modulus of La{sub 2}Mg{sub 17} is higher than other Mg–La alloys. • The elastic isotropy of La{sub 2}Mg{sub 17} is excellent. • The electronic structure within basal plane is highly symmetric.

  1. First-principles Exploration of Crystal Structures of Pure Iron at Earth’s Inner Core Conditions

    NASA Astrophysics Data System (ADS)

    Ishikawa, T.; Tsuchiya, T.; Tsuchiya, J.

    2009-12-01

    Determining the structure of Earth’s inner core has been a long standing challenge for geoscience. Iron has been considered as a main composition of the inner core and expected to exist as an iron-nickel-light elements alloy there. In order to get some information about the structure of the inner core, pure iron has been also investigated by many experimental and ab-initio studies. Pure iron has been expected to take the hexagonal close-packed (hcp) structure at the inner core conditions (Jephcoat and Olson, Nature 325, 332-335, 1987.; Mao et al., Nature 396, 741-743, 1998.; Fiquet et al., Science 291, 468-471, 2001.; Uchida et al., J. Geophys. Res. 106, 21799-21810, 2001.). On the other hand, the face-centered cubic (fcc) structure (Mikhaylushkin et al., Phys. Rev. Lett. 99, 165505-165508, 2007.) and the body-centered cubic structure (Vocadlo et al., Phys. Earth Planet In. 170, 52-59, 2008.) have been also considered as the candidate structures. In this study, first we explored the crystal structures of pure iron at 400 GPa by our originally developing algorithm for the structure exploration, Free Energy Surface Trekking (FEST). In FEST, using a minus sign of driving force acting on a simulation cell, we force a system to climb up to a ridge of a free energy surface (ascent-run). Then, flipping the negative driving force to an original one, we make the system go down to a neighboring potential well (descent-run). The more different directions we examine in the ascent-run, the more accurate topography we capture of free energy surface. For the exploration of the ultrahigh-pressure structures of pure iron, we used 16 atoms supercell and explored 64 pathways around the initial local minimum corresponding to hcp. As the result, 30 pathways lead to a complex hcp (chcp) structure, which has ABCACABCBCAB stacking with 12 layers. Other 33 pathways lead to hcp and 1 pathway fcc. The enthalpy of chcp was found higher than that of hcp but only by 4 mRy/atom and lower than

  2. Structural phase stability and bonding behavior of BAlH5(B=Mg,Ba) from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Klaveness, A.; Vajeeston, P.; Ravindran, P.; Fjellvåg, H.; Kjekshus, A.

    2006-03-01

    The ground-state structures of MgAlH5 and BaAlH5 have been subjected to full structural optimization considering 50 different potential atomic arrangements as inputs for accurate density-functional total-energy calculations. The experimentally known crystal structure and structural parameters for BaAlH5 are reproduced, and the crystal structure of MgAlH5 is predicted. At 0K and ambient pressures MgAlH5 and BaAlH5 crystallize in monoclinic ( CaFeF5 type, P21/c ) and orthorhombic (prototype, Pna21 ) structures, respectively. In addition to the ground-state MgAlH5 phase (here designated α-MgAlH5 ), it is also predicted a metastable modification (termed β-MgAlH5 , CaCrF5 type, Cc ). The structures comprise isolated, highly distorted AlH6 octahedra, which form one-dimensional chains along the [001] direction. In α - and β-MgAlH5 these chains are fairly linear, while BaAlH5 exhibits distinct zigzag chains. α-MgAlH5 and BaAlH5 are nonmetallic phases with estimated band gaps of 2.48 and 2.73eV , respectively. Analyses of the density of states, charge density, Mulliken population, and Born effective charge indicate that the interaction between Al and H is polar covalent blended with an ionic woof, while Ba and Mg can be considered as virtually divalent ions.

  3. First-principles predicted low-energy structures of NaSc(BH{sub 4}){sub 4}

    SciTech Connect

    Tran, Huan Doan Amsler, Maximilian; Goedecker, Stefan; Botti, Silvana; Marques, Miguel A. L.

    2014-03-28

    According to previous interpretations of experimental data, sodium-scandium double-cation borohydride NaSc(BH{sub 4}){sub 4} crystallizes in the crystallographic space group Cmcm where each sodium (scandium) atom is surrounded by six scandium (sodium) atoms. A careful investigation of this phase based on ab initio calculations indicates that the structure is dynamically unstable and gives rise to an energetically and dynamically more favorable phase with C222{sub 1} symmetry and nearly identical x-ray diffraction pattern. By additionally performing extensive structural searches with the minima-hopping method we discover a class of new low-energy structures exhibiting a novel structural motif in which each sodium (scandium) atom is surrounded by four scandium (sodium) atoms arranged at the corners of either a rectangle with nearly equal sides or a tetrahedron. These new phases are all predicted to be insulators with band gaps of 7.9–8.2 eV. Finally, we estimate the influence of these structures on the hydrogen-storage performance of NaSc(BH{sub 4}){sub 4}.

  4. First-principles study of structural stability, electronic, optical and elastic properties of binary intermetallic: PtZr

    NASA Astrophysics Data System (ADS)

    Pagare, Gitanjali; Jain, Ekta; Sanyal, S. P.

    2016-05-01

    Structural, electronic, optical and elastic properties of PtZr have been studied using the full-potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). The energy against volume and enthalpy vs. pressure variation in three different structures i.e. B1, B2 and B3 for PtZr has been presented. The equilibrium lattice parameter, bulk modulus and its pressure derivative have been obtained using optimization method for all the three phases. Furthermore, electronic structure was discussed to reveal the metallic character of the present compound. The linear optical properties are also studied under zero pressure for the first time. Results on elastic properties are obtained using generalized gradient approximation (GGA) for exchange correlation potentials. Ductile nature of PtZr compound is predicted in accordance with Pugh's criteria.

  5. First principles simulation of laser-induced periodic surface structure using the particle-in-cell method

    NASA Astrophysics Data System (ADS)

    Mitchell, Robert A.; Schumacher, Douglass W.; Chowdhury, Enam A.

    2015-11-01

    We present our results of a fundamental simulation of a periodic grating structure formation on a copper target during the femtosecond-pulse laser damage process, and compare our results to recent experiment. The particle-in-cell (PIC) method is used to model the initial laser heating of the electrons, a two-temperature model (TTM) is used to model the thermalization of the material, and a modified PIC method is employed to model the atomic transport leading to a damage crater morphology consistent with experimental grating structure formation. This laser-induced periodic surface structure (LIPSS) is shown to be directly related to the formation of surface plasmon polaritons (SPP) and their interference with the incident laser pulse.

  6. Cu/ZnO(0001) under oxidating and reducing conditions: A first-principles survey of surface structures

    NASA Astrophysics Data System (ADS)

    Warschkow, O.; Chuasiripattana, K.; Lyle, M. J.; Delley, B.; Stampfl, C.

    2011-09-01

    We investigate the stability of Cu-exposed ZnO(0001) surface structures in an oxygen environment using density functional theory and the method of ab initio atomistic thermodynamics. A two-dimensional phase diagram is constructed which identifies stable surface structures as a function of the copper and oxygen chemical potentials. Two structures with a (3×3)R30∘ unit cell are found to be prominently stable at intermediate oxygen and copper chemical potentials. These phases are characterized by a single adlayer of Cu4O3 and Cu12O13 stoichiometry on ZnO(0001). We rationalize recent experimental observations in the literature in terms of our results.

  7. A First-Principles Study on the Structural and Electronic Properties of Sn-Based Organic-Inorganic Halide Perovskites

    NASA Astrophysics Data System (ADS)

    Ma, Zi-Qian; Pan, Hui; Wong, Pak Kin

    2016-08-01

    Organic-inorganic halide perovskites have attracted increasing interest on solar-energy harvesting because of their outstanding electronic properties. In this work, we systematically investigate the structural and electronic properties of Sn-based hybrid perovskites MASnX3 and FASnX3 (X = I, Br) based on density-functional-theory calculations. We find that their electronic properties strongly depend on the organic molecules, halide atoms, and structures. We show that there is a general rule to predict the band gap of the Sn-based hybrid perovskite: its band gap increases as the size of halide atom decreases as well as that of organic molecule increase. The band gap of high temperature phase (cubic structure) is smaller than that of low temperature phase (orthorhombic structure). The band gap of tetragonal structure (medium-temperature phase) may be larger or smaller than that of cubic phase, depending on the orientation of the molecule. Tunable band gap within a range of 0.73-1.53 eV can be achieved by choosing halide atom and organic molecule, and controlling structure. We further show that carrier effective mass also reduces as the size of halide atom increases and that of molecule decreases. By comparing with Pb-based hybrid perovskites, the Sn-based systems show enhanced visible-light absorption and carrier mobility due to narrowed band gap and reduced carrier effective mass. These Sn-based organic-inorganic halide perovskites may find applications in solar energy harvesting with improved performance.

  8. Study on structure and properties of transition metal doped BiF3 by first-principles

    NASA Astrophysics Data System (ADS)

    Zhang, Wei; Yin, Jiu-Ren; Wu, Wen-Hu; Xie, Wei; Zhang, Ping; Ding, Yan-Huai

    2016-06-01

    Structure and physical properties of BiF3 doped with M=Cr, Cu, Fe, Mn, Ni, Ti, V and Co are calculated by the DFT+U method. Effect of metal doping on the electronic structure and optical response of host materials BiF3 is investigated systematically. New energy levels are formed and located within the band gap, which could decrease the recombination rate of e-/h+ pairs. Furthermore, transition metal doping extends the optical absorption of BiF3 to the visible spectral region.

  9. Hydrostatic Pressure Effects on Structural and Electronic Properties of ETN and PETN from First-Principles Calculations.

    PubMed

    Fedorov, Igor A; Fedorova, Tatyana P; Zhuravlev, Yuriy N

    2016-05-26

    We studied the structural and electronic properties of pentaerythritol tetranitrate (PETN) and erythritol tetranitrate (ETN) crystals within the framework of density functional theory with van der Waals interactions. The computed lattice parameters have good agreement with experimental data. Electronic and structural properties of the crystals under 0-20 GPa hydrostatic pressure were studied. The parameters of equations of state calculated from the theoretical data show good agreement with experiment within the studied pressure intervals. We have also calculated the detonation velocity and pressure. PMID:27128718

  10. First-Principles Calculations of Structural, Electronic and Optical Properties of CaTiO3 Crystal

    NASA Astrophysics Data System (ADS)

    Medeiros, Subênia; Silva, Jusciane; Albuquerque, Eudenilson; Freire, Valder

    2013-03-01

    The structural, electronic, vibrational, and optical properties of perovskite CaTiO3 in the cubic, orthorhombic, and tetragonal phase are calculated in the framework of density functional theory (DFT) with different exchange-correlation potentials by CASTEP package. The calculated band structure shows an indirect band gap of 1.88 eV at the Γ-R points in the Brillouin zone to the cubic structure, a direct band gap of 2.41 eV at the Γ - Γ points to the orthorhombic structure, and an indirect band gap of 2.31 eV at the M' Γ points to the tetragonal phase. I have concluded that the bonding between Ca and TiO2 is mainly ionic and that the TiO2 entities bond covalently. Unlike some perovskites the CaTiO3 does not exhibit a ferroelectric phase transition down to 4.2 K. It is still known that the CaTiO3 has a static dielectric constant that extrapolates to a value greater than 300 at zero temperature. Our calculated lattice parameters, elastic constants, optical properties, and vibrational frequencies are found to be in good agreement with the available theoretical and experimental values. The results for the effective mass in the electron and hole carriers are also presented in this work.

  11. First principles simulations of structural phase transformations in the solid electrolyte LiBH4 with chemical substitutions

    NASA Astrophysics Data System (ADS)

    Bernstein, Noam; Hoang, Khang; Johannes, Michelle

    2014-03-01

    The proposed hydrogen storage material LiBH4 has been shown to have possible applications as a Li-ion battery solid electrolyte, due to its high Li-ion conductivity over 10-3 S/cm-1 [1], comparable to polymer gel electrolytes. The high conductivity is only observed above a phase transition temperature that is outside of the useful operating range, but doping the material with various substitutions for the Li or BH4 units can bring the phase transition below room temperature. Both smaller and larger substituting species can stabilize the high T structure, indicating that it is not a simple volume effect. We show that variable-cell-shape molecular-dynamics simulations using density functional theory forces and stresses reproduce the structural phase transition. Using umbrella integration to compute the free energy differences between the two structures, we calculate the phase transition temperature and its dependence on substitutional I, Cl, and Na concentrations, and show that they are in very good agreement with experiment. We calculate the effect of K substitution, and predict that it will be even more effective at stabilizing the high T structure. Decomposing the free energy difference changes into enthalpy and entropy contributions shows that the mechanis

  12. Structural, mechanical, and electronic properties of Rh2B and RhB2: first-principles calculations

    PubMed Central

    Chu, Binhua; Li, Da; Tian, Fubo; Duan, Defang; Sha, Xiaojing; Lv, Yunzhou; Zhang, Huadi; Liu, Bingbing; Cui, Tian

    2015-01-01

    The crystal structures of Rh2B and RhB2 at ambient pressure were explored by using the evolutionary methodology. A monoclinic P21/m structure of Rh2B was predicted and donated as Rh2B-I, which is energetically much superior to the previously experimentally proposed Pnma structure. At the pressure of about 39 GPa, the P21/m phase of Rh2B transforms to the C2/m phases. For RhB2, a new monoclinic P21/m phase was predicted, named as RhB2-II, it has the same structure type with Rh2B. Rh2B-I and RhB2-II are both mechanically and dynamically stable. They are potential low compressible materials. The analysis of electronic density of states and chemical bonding indicates that the formation of strong and directional covalent B-B and Rh-B bonds in these compounds contribute greatly to their stabilities and high incompressibility. PMID:26123399

  13. Structural, mechanical, electronic, optical properties and effective masses of CuMO2 (M = Sc, Y, La) compounds: First-principles calculations

    NASA Astrophysics Data System (ADS)

    Liu, Qi-Jun; Zhang, Ning-Chao; Sun, Yan-Yun; Liu, Fu-Sheng; Liu, Zheng-Tang

    2014-05-01

    The structural, elastic, mechanical, electronic, optical properties and effective masses of CuMIIIBO2 (MIIIB = Sc, Y, La) compounds have been investigated by the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory under local density approximation. The equilibrium structural parameters are in good agreement with previous experimental and theoretical data. To our knowledge, there are no available data of elastic constants for comparison. The bulk, shear and Young's modulus, ratio of B/G, Poisson's ratio and Lamé's constants of CuMIIIBO2 have been studied. The electronic structures of CuMIIIBO2 are consistent with other calculations. The population analysis, charge densities and effective masses have been shown and analyzed. The imaginary and real parts of the dielectric function, refractive index and extinction coefficient of CuMIIIBO2 are calculated. The interband transitions to absorption of CuMIIIBO2 have been analyzed.

  14. First-principle investigation of electronic structure, magnetism and phase stability of Heusler-type Pt2-xMn1+xGa alloys

    NASA Astrophysics Data System (ADS)

    Feng, L.; Liu, E. K.; Zhang, W. X.; Wang, W. H.; Wu, G. H.

    2015-03-01

    The electronic structure, magnetism and phase stability of Pt2-xMn1+xGa (x=0, 0.25, 0.5, 0.75, 1) alloys are studied by first-principle calculations. The calculations reveal that a potential magnetic martensitic transformation can be expected in all the series. In addition, a large magnetic-field-induced strain is likely to appear in Pt2-xMn1+xGa (x=0, 0.25, 0.75, 1) alloys. The electronic structure calculations indicate that the tetragonal phase is stabilized upon the distortion because of the pseudogap formation at the Fermi Level. The magnetic structure is also investigated and the total magnetic moment of the tetragonal phase is a little larger than that of the cubic austenite phase in all the series.

  15. The effects of surface bond relaxation on electronic structure of Sb{sub 2}Te{sub 3} nano-films by first-principles calculation

    SciTech Connect

    Li, C. Zhao, Y. F.; Fu, C. X.; Gong, Y. Y.; Chi, B. Q.; Sun, C. Q.

    2014-10-15

    The effects of vertical compressive stress on Sb{sub 2}Te{sub 3} nano-films have been investigated by the first principles calculation, including stability, electronic structure, crystal structure, and bond order. It is found that the band gap of nano-film is sensitive to the stress in Sb{sub 2}Te{sub 3} nano-film and the critical thickness increases under compressive stress. The band gap and band order of Sb{sub 2}Te{sub 3} film has been affected collectively by the surface and internal crystal structures, the contraction ratio between surface bond length of nano-film and the corresponding bond length of bulk decides the band order of Sb{sub 2}Te{sub 3} film.

  16. Electronic structures and magnetism of LaFe2Ge2 and LaFe2Si2: First-principles studies

    NASA Astrophysics Data System (ADS)

    Wang, Guangtao; Shi, Xianbiao

    2016-06-01

    Inspired by the recent discovery of superconductivity in YFe2Ge2, we report first-principles calculations on the electronic structure and magnetic properties of LaFe2Ge2 and LaFe2Si2. We found that LaFe2Ge2 and LaFe2Si2 share similar electronic structure and magnetism properties. In the nonmagnetic state, the density of states at the Fermi level are mostly derived from the dxy, dyz, and dzx orbits, just like the Fe-pnictide superconductors. The band structure and Fermi surfaces exhibit significantly three dimensional character. Our calculations indicate that the ground state of LaFe2Ge2 and LaFe2Si2 is the stripe antiferromagnetic configuration in the a-b plane and stacked antiparallel along the c-axis direction. The overestimate magnetic tendency within calculation indicates these systems nearness to a magnetic quantum critical point (QCP).

  17. Defect formation energy and magnetic structure of shape memory alloys Ni-X-Ga (X=Mn, Fe, Co) by first principle calculation

    NASA Astrophysics Data System (ADS)

    Bai, J.; Raulot, J. M.; Zhang, Y. D.; Esling, C.; Zhao, X.; Zuo, L.

    2010-09-01

    The crystallographic and magnetic structures of the Ni2XGa (X=Mn, Fe, Co), are systematically investigated by means of the first-principles calculations within the framework of density functional theory using the VIENNA AB INITIO SOFTWARE PACKAGE. The formation energies of several kinds of defects (atomic exchange, antisite, vacancy) are estimated. The Ga atoms stabilize the cubic structure, and the effect of X atoms on the structural stability is opposite. For most cases of the site occupation, the excess atoms of the rich component directly occupy the site(s) of the deficient one(s), except for Ga-rich Ni-deficient type. The magnitude of the variation in Ni moments is much larger than that of Mn in defective Ni2XGa. The value of Ni magnetic moment sensitively depends on the distance between Ni and X. Excess Mn could be ferromagnetic or antiferromagnetic, depending on the distance between the neighboring Mn atoms.

  18. A first principles study of the lattice stability of diamond-structure semiconductors under intense laser irradiation

    SciTech Connect

    Feng Shiquan; Zhao Jianling; Cheng Xinlu

    2013-01-14

    Using density-functional linear-response theory, we calculated the phonon dispersion curves for the diamond structural elemental semiconductors of Ge, C and zinc-blende structure semiconductors of GaAs, InSb at different electronic temperatures. We found that the transverse-acoustic phonon frequencies of C and Ge become imaginary as the electron temperature is elevated, which means the lattices of C and Ge become unstable under intense laser irradiation. These results are very similar with previous theoretical and experimental results for Si. For GaAs and InSb, not only can be obtained the similar results for their transverse-acoustic modes, but also their LO-TO splitting gradually decreases as the electronic temperature is increased. It means that the electronic excitation weakens the strength of the ionicity of ionic crystal under intense laser irradiation.

  19. Influence of stresses on structure and properties of Ti and Zr- based alloys from first-principles simulations

    NASA Astrophysics Data System (ADS)

    Barannikova, S. A.; Zharmukhambetova, A. M.; Nikonov, A. Yu; Dmitriev, A. V.; Ponomareva, A. V.; Abrikosov, I. A.

    2015-01-01

    Computer simulations in the framework of the Density Functional Theory have become an established tool for computer simulations of materials properties. In most cases, however, information is obtained at ambient conditions, preventing design of materials for applications at extreme conditions. In this work we employ ab initio calculations to investigate the influence of stresses on structure and stability of Ti-Mo and Zr-Nb alloys, an important class of construction materials. Calculations reproduce known phase stability trends in these systems, and we resolve the controversy regarding the stability of body-centered cubic solid solution in Mo-rich Ti-Mo alloys against the isostructural decomposition. Calculated results are explained in terms of the electronic structure effects, as well as in terms of physically transparent thermodynamic arguments that relate phase stability to deviations of concentration dependence of atomic volume from the linear behavior.

  20. Structural flexibility of 4,4'-methylene diphenyl diisocyanate (4,4'-MDI): evidence from first principles calculations.

    PubMed

    Rodziewicz, Pawel; Goclon, Jakub

    2014-02-01

    A reactant used globally in the production of polyurethane is the molecule 4,4'-methylene diphenyl diisocyanate (4,4'-MDI). The structural flexibility of 4,4'-MDI is one of the most important molecular properties influencing the polymerization process and this property was therefore modeled using density functional theory (DFT) calculations and Car-Parrinello molecular dynamics (MD) simulations. Global and local minima structures were found and confirmed by vibrational analysis. The energy barriers related to rotation of the aromatic rings were estimated by DFT calculations. The stability of global and local minima was verified by Car-Parrinello (MD) runs at finite temperature. The presence of weak C-H⋯π hydrogen bonds was confirmed by atoms in molecules analysis and found to be responsible for the low energy barriers. PMID:24522379

  1. Tuning electronic structure of SnS2 nanosheets by vertical electric field: a first-principles investigation

    NASA Astrophysics Data System (ADS)

    Guo, Peng; Wang, Tianxing; Xia, Congxin; Jia, Yu

    2016-07-01

    Based on density functional theory, we investigated band gap tuning in transition-metal dichalcogenides SnS2 nanosheets by external electric fields applied perpendicular to the layers. We show that the fundamental band gap value of 2H and 4H SnS2 multilayer structures continuously decreases with increasing strength of applied electric fields, eventually rendering them metallic. We interpret our results in the light of the giant Stark effect and obtain a robust relationship, which is essentially characterized by the interlayer spacing, for the rate of band gap change with applied external field. And it is also valid for monolayer structure, though it need very large electric filed to make the gap change.

  2. The structural and electronic properties of cubic AgMO3 (M=Nb, Ta) by first principles calculations

    NASA Astrophysics Data System (ADS)

    Prasad, K. Ganga; Niranjan, Manish K.; Asthana, Saket

    2016-05-01

    We report the electronic structure of the AgMO3(M=Nb, Ta) within the frame work of density functional theory and calculations are performed within the generalized gradient approximation (GGA) by using ultrasoft pseudopotentials. The calculated equilibrium lattice parameters and volumes are extracted from fitting of Birch third order equation of state and which are reasonable agreement with the available experimental results. The density of states,band structure of Ag(Nb,Ta)O3 reveals that the valance bands mostly occupied with O-2p and O-2s states and whereas conduction band occupied with Nb (Ta) 4d(5d) states including less contribution from Ag 5s states.

  3. First-principles calculations of the structural, electronic, optical and thermal properties of the BNxAs1-x alloys

    NASA Astrophysics Data System (ADS)

    Hamioud, L.; Boumaza, A.; Touam, S.; Meradji, H.; Ghemid, S.; El Haj Hassan, F.; Khenata, R.; Omran, S. Bin

    2016-06-01

    The present paper aims to study the structural, electronic, optical and thermal properties of the boron nitride (BN) and BAs bulk materials as well as the BNxAs1-x ternary alloys by employing the full-potential-linearised augmented plane wave method within the density functional theory. The structural properties are determined using the Wu-Cohen generalised gradient approximation that is based on the optimisation of the total energy. For band structure calculations, both the Wu-Cohen generalised gradient approximation and the modified Becke-Johnson of the exchange-correlation energy and potential, respectively, are used. We investigated the effect of composition on the lattice constants, bulk modulus and band gap. Deviations of the lattice constants and the bulk modulus from the Vegard's law and the linear concentration dependence, respectively, were observed for the alloys where this result allows us to explain some specific behaviours in the electronic properties of the alloys. For the optical properties, the calculated refractive indices and the optical dielectric constants were found to vary nonlinearly with the N composition. Finally, the thermal effect on some of the macroscopic properties was predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account.

  4. Local structure of PbMg_1/3Nb_2/3O3 from first principles

    NASA Astrophysics Data System (ADS)

    Tinte, Silvia; Rabe, Karin M.; Vanderbilt, David

    2004-03-01

    The complex perovskite PbMg_1/3Nb_2/3O3 (PMN) is a prototypical member of a family of relaxor-based materials that are of great interest for their enhanced piezoelectric properties. A crucial feature of PMN is its B-site disorder, since Mg^+2 and Nb^+5 have quite different nominal charges and ionic radii. Here, we investigate the local structure of PMN by performing ab-initio calculations of the relaxed structures of several 30-atom supercells with `pseudo-random' B-cation arrangements. We analyse the results using a two-stage approach. We first identify the minimal set of local modes that captures the main features of the structural relaxations. We then analyze the correlations between the input cation arrangements and the observed mode amplitudes. Aside from providing insight into the nature of the local atomic distortions that occur, this analysis lays the foundation for the development of an effective-Hamiltonian approach appropriate to this class of materials.

  5. First-principles study of structural, elastic, electronic and vibrational properties of BiCoO3

    NASA Astrophysics Data System (ADS)

    Koroglu, Ulas; Cabuk, Suleyman; Deligoz, Engin

    2014-08-01

    We used density functional theory (DFT) to study the structural, elastic, electronic, and lattice dynamical properties of tetragonal BiCoO3 applying the “norm-conserving” pseudopotentials within the local spin density approximation (LSDA). The calculated equilibrium lattice parameters and atomic displacements are in agreement with the available experimental and theoretical results. Moreover, the structural stability of tetragonal BiCoO3 were confirmed by the calculated elastic constants. In addition, the elastic properties of polycrystalline aggregates including bulk, shear and Young's moduli, and Poisson's ratio are also determined. The electronic band structure, total and partial density of states (DOS and PDOS) with ferromagnetic spin configuration are obtained. The results show that tetragonal BiCoO3 has an indirect band gap with both up- and down-spin configurations and its bonding behavior is of covalent nature. We compute Born effective charge (BEC) which is found to be quite anisotropic of Bi, Co and O atoms. The infrared and Raman active phonon mode frequencies at the Г point are found. The phonon dispersion curves exhibit imaginary frequencies which lead from the high-symmetry tetragonal phase to low-symmetry rhombohedral phase in BiCoO3. The six independent elastic constants, including bulk, shear and Young's moduli, and Poisson's ratio, complete BEC tensor and phonon dispersion relations in tetragonal BiCoO3 are predicted for the first time. Results of the calculations are compared with the existing experimental and theoretical data.

  6. First-principles prediction of disordering tendencies in complex oxides

    SciTech Connect

    Jiang, Chao; Stanek, Christopher R; Sickafus, Kurt E; Uberuaga, Blas P

    2008-01-01

    The disordering tendencies of a series of zirconate (A{sub 2}Zr{sub 2}O{sub 7}) , hafnate (A{sub 2}Hf{sub 2}O{sub 7}), titanate (A{sub 2}Ti{sub 2}O{sub 7}), and stannate (A{sub 2} Sn{sub 2}O{sub 7}) pyrochlores are predicted in this study using first-principles total energy calculations. To model the disordered (A{sub 1/2}B{sub 1/2})(O{sub 7/8}/V{sub 1/8}){sub 2} fluorite structure, we have developed an 88-atom two-sublattice special quasirandom structure (SQS) that closely reproduces the most important near-neighbor intra-sublattice and inter-sublattice pair correlation functions of the random alloy. From the calculated disordering energies, the order-disorder transition temperatures of those pyrochlores are further predicted and our results agree well with the existing experimental phase diagrams. It is clearly demonstrated that both size and electronic effects play an important role in determining the disordering tendencies of pyrochlore compounds.

  7. Electronic structure and thermoelectric properties of n - and p -type SnSe from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Kutorasinski, K.; Wiendlocha, B.; Kaprzyk, S.; Tobola, J.

    2015-05-01

    We present results of the electronic band structure, Fermi surface, and electron transport property calculations in the orthorhombic n - and p -type SnSe, applying the Korringa-Kohn-Rostoker method and the Boltzmann transport approach. The analysis accounted for the temperature effect on crystallographic parameters in P n m a structure as well as the phase transition to C m C m structure at Tc˜807 K. Remarkable modifications of the conduction and valence bands were noticed upon varying crystallographic parameters within the structure before Tc, while the phase transition mostly leads to the jump in the band-gap value. The diagonal components of the kinetic parameter tensors (velocity, effective mass) and resulting transport quantity tensors [electrical conductivity σ , thermopower S , and power factor (PF)] were computed for a wide range of temperature (15-900 K) and hole (p -type) and electron (n -type) concentrations (1017-1021cm-3 ). SnSe is shown to have a strong anisotropy of the electron transport properties for both types of charge conductivity, as expected for the layered structure, with the generally heavier p -type effective masses compared to n -type ones. Interestingly, p -type SnSe has strongly nonparabolic dispersion relations, with the "pudding-mold-like" shape of the highest valence band. The analysis of σ ,S , and PF tensors indicates that the interlayer electron transport is beneficial for thermoelectric performance in n -type SnSe, while this direction is blocked in p -type SnSe, where in-plane transport is preferred. Our results predict that n -type SnSe is potentially even better thermoelectric material than p -type SnSe. Theoretical results are compared with the single-crystal p -SnSe measurements, and good agreement is found below 600 K. The discrepancy between the computational and experimental data, appearing at higher temperatures, can be explained assuming an increase of the hole concentration versus T , which is correlated with the

  8. The crystal structure and chemical state of aluminum-doped hydroxyapatite by experimental and first principles calculation studies.

    PubMed

    Wang, Ming; Wang, Liping; Shi, Chao; Sun, Tian; Zeng, Yi; Zhu, Yingchun

    2016-08-01

    Aluminum (Al) is a trace element found in hard tissues, and the induction of bone diseases by Al accumulation has generated interest in the role and mechanism of Al in bone metabolism. Because hydroxyapatite (HA) constitutes the main inorganic content of human hard tissues, the biological effect of Al in human hard tissues is closely related to the intrinsic state of Al-doped HA (Al-HA). However, few investigations to date have focused on the crystallography of Al-HA. Herein, we determined the crystallographic characteristics and energy states of Al-HA by conducting theoretical and experimental studies. Al-HA [Ca10-1.5xAlx(PO4)6(OH)2] with a defect structure was synthesized. XRD patterns and morphology images revealed that doping of Al decreased the crystallinity and the HA nanocrystal size. The optimized crystal structure indicated that Al was preferentially substituted for Ca(2) and Ca vacancies appeared at the Ca(2)1 site. Al doping locally distorted the regularity and integrity of the HA crystal structure, leading to the occurrence of Ca(2+) vacancies and the displacement and rotation of OH(-) and [PO4](3-) chains. The total energy of Al-HA increased and the stability decreased. Consequently, Al-HA might be readily degraded by osteoclasts and bone resorption could be accelerated. The destruction and over-resorption of bones caused by excessive Al could result in abnormal bone metabolism. The present findings not only provide the first crystallographic information on the disruptive effects of Al doping in HA but also complement the present understanding of the mechanisms underlying Al-induced bone diseases. PMID:27436334

  9. Mössbauer effect and first principle calculations of the electronic structure and hyperfine interaction parameters of Hf2Fe

    NASA Astrophysics Data System (ADS)

    Belosevic-Cavor, Jelena; Koteski, Vasil; Concas, Giorgio; Cekic, Bozidar; Novakovic, Nikola; Spano, Giorgio

    2005-10-01

    A detailed theoretical study of the structure, electronic properties and the electric field gradients of the Hf2Fe intermetallic compound is presented. Using all-electron full-potential linearized augmented plane wave (FP-LAPW) formalism the equilibrium volume, bulk modulus and electric field gradients are calculated. The obtained results are compared with EFG values inferred from measurements performed using Mössbauer spectroscopy and the earlier reported time differential perturbed angular correlation (TDPAC) measurements. The lattice relaxation and the supercell calculations are found to be essential for the correct interpretation of the experimental results.

  10. Strain effect on electronic structure and thermoelectric properties of orthorhombic SnSe: A first principles study

    NASA Astrophysics Data System (ADS)

    Cuong, Do Duc; Rhim, S. H.; Lee, Joo-Hyong; Hong, Soon Cheol

    2015-11-01

    Strain effect on thermoelectricity of orthorhombic SnSe is studied using density function theory. The Seebeck coefficients are obtained by solving Boltzmann Transport equation (BTE) with interpolated band energies. As expected from the crystal structure, calculated Seebeck coefficients are highly anisotropic, and agree well with experiment. Changes in the Seebeck coefficients are presented, when strain is applied along b and c direction with strength from -3% to +3%, where influence by band gaps and band dispersions are significant. Moreover, for compressive strains, the sign change of Seebeck coefficients at particular direction suggests that the bipolar transport is possible for SnSe.

  11. Strain effect on electronic structure and thermoelectric properties of orthorhombic SnSe: A first principles study

    SciTech Connect

    Cuong, Do Duc; Rhim, S. H. Hong, Soon Cheol; Lee, Joo-Hyong

    2015-11-15

    Strain effect on thermoelectricity of orthorhombic SnSe is studied using density function theory. The Seebeck coefficients are obtained by solving Boltzmann Transport equation (BTE) with interpolated band energies. As expected from the crystal structure, calculated Seebeck coefficients are highly anisotropic, and agree well with experiment. Changes in the Seebeck coefficients are presented, when strain is applied along b and c direction with strength from -3% to +3%, where influence by band gaps and band dispersions are significant. Moreover, for compressive strains, the sign change of Seebeck coefficients at particular direction suggests that the bipolar transport is possible for SnSe.

  12. First principles study on electronic structure and elastic properties of LaCd and LaHg

    SciTech Connect

    Devi, Hansa E-mail: gita-pagare@yahoo.co.in; Pagare, Gitanjali E-mail: gita-pagare@yahoo.co.in; Chouhan, S. S. E-mail: gita-pagare@yahoo.co.in; Sanyal, Sankar P.

    2014-04-24

    Full -potential linearized augmented plane wave method (FP- LAPW) has been used for the comparative study of electronic structure and elastic properties of CsCl-type LaCd and LaHg intermetallic compounds using generalized gradient approximation (GGA). The density of states at the Fermi Level, N (E{sub F}), is found to be 0.06 and 3.03 states/eV for LaCd and LaHg respectively. We report elastic constants for these compounds for the first time. The ductility/brittleness of these compounds has been analyzed using Pugh rule and Cauchy’s pressure.

  13. A comparable study of structural and electrical transport properties of Al and Cu nanowires using first-principle calculations

    SciTech Connect

    Gao, N.; Li, J. C. E-mail: jiangq@jlu.edu.cn; Jiang, Q. E-mail: jiangq@jlu.edu.cn

    2013-12-23

    The structural and quantum transport properties of Al and Cu nanowires with diameters up to 3.6 nm are studied using density functional theory combined with Landauer formalism. Contrary to the classical electronic behavior, the conductance of Al wires is larger than that of Cu. This is mainly attributed to the larger contribution of conductance channels from Al-3p, which is determined by the chemical nature. Meanwhile, the stronger axial contraction of Al wires plays a minor role to conductance. This makes Al wires possible candidate interconnects in integrated circuits.

  14. First principle studies on the electronic structures and absorption spectra in KMgF 3 crystal with fluorine vacancy

    NASA Astrophysics Data System (ADS)

    Cheng, Fang; Liu, Tingyu; Zhang, Qiren; Qiao, Hailin; Zhou, Xiuwen

    2010-08-01

    The experiments indicate that the perfect KMgF 3 crystal has no absorption in the visible range, however the electron irradiation induces a complex absorption spectrum. The absorption spectra can be decomposed by five Gaussian bands peaking at 2.5 eV (488 nm), 3.4 eV (359 nm), 4.2 eV (295 nm), 4.6 eV (270 nm) and 5.2 eV (239 nm), respectively. The purpose of this paper is to seek the origins of the absorption bands. The electronic structures and absorption spectra either for the perfect KMgF 3 or for KMgF 3: VF+ with electrical neutrality have been studied by using density functional theory code CASTEP with the lattice structure optimized. The calculation results predicate that KMgF 3: VF+ also exhibits five absorption bands caused by the existence of the fluorine ion vacancy VF+ and the five absorption bands well coincide with the experimental results. It is believable that the five absorption bands are related to VF+ in KMgF 3 crystal produced by the electron irradiation.

  15. Geometries and electronic structures of the hydrogenated diamond (100) surface upon exposure to active ions: A first principles study

    NASA Astrophysics Data System (ADS)

    Liu, Feng-Bin; Li, Jing-Lin; Chen, Wen-Bin; Cui, Yan; Jiao, Zhi-Wei; Yan, Hong-Juan; Qu, Min; Di, Jie-Jian

    2016-02-01

    To elucidate the effects of physisorbed active ions on the geometries and electronic structures of hydrogenated diamond films, models of HCO 3 - , H3O+, and OH- ions physisorbed on hydrogenated diamond (100) surfaces were constructed. Density functional theory was used to calculate the geometries, adsorption energies, and partial density of states. The results showed that the geometries of the hydrogenated diamond (100) surfaces all changed to different degrees after ion adsorption. Among them, the H3O+ ion affected the geometry of the hydrogenated diamond (100) surfaces the most. This is well consistent with the results of the calculated adsorption energies, which indicated that a strong electrostatic attraction occurs between the hydrogenated diamond (100) surface and H3O+ ions. In addition, electrons transfer significantly from the hydrogenated diamond (100) surface to the adsorbed H3O+ ion, which induces a downward shift in the HOMO and LUMO energy levels of the H3O+ ion. However, for active ions like OH- and HCO 3 - , no dramatic change appears for the electronic structures of the adsorbed ions.

  16. Electronic structure and optical properties of F-doped β-Ga2O3 from first principles calculations

    NASA Astrophysics Data System (ADS)

    Jinliang, Yan; Chong, Qu

    2016-04-01

    The effects of F-doping concentration on geometric structure, electronic structure and optical property of β-Ga2O3 were investigated. All F-doped β-Ga2O3 with different concentrations are easy to be formed under Ga-rich conditions, the stability and lattice parameters increase with the F-doping concentration. F-doped β-Ga2O3 materials display characteristics of the n-type semiconductor, occupied states contributed from Ga 4s, Ga 4p and O 2p states in the conduction band increase with an increase in F-doping concentration. The increase of F concentration leads to the narrowing of the band gap and the broadening of the occupied states. F-doped β-Ga2O3 exhibits the sharp band edge absorption and a broad absorption band. Absorption edges are blue-shifted, and the intensity of broad band absorption has been enhanced with respect to the fluorine content. The broad band absorption is ascribed to the intra-band transitions from occupied states to empty states in the conduction band. Project supported by the Innovation Project of Shandong Graduate Education, China (No. SDYY13093) and the National Natural Science Foundation of China (No. 10974077).

  17. Structural, Thermodynamic, Elastic, and Electronic Properties of α-SnS at High Pressure from First-Principles Investigations

    NASA Astrophysics Data System (ADS)

    Liu, Chun Mei; Xu, Chao; Duan, Man Yi

    2015-10-01

    SnS has potential technical applications, but many of its properties are still not well studied. In this work, the structural, thermodynamic, elastic, and electronic properties of α-SnS have been investigated by the plane wave pseudo-potential density functional theory with the framework of generalised gradient approximation. The calculated pressure-dependent lattice parameters agree well with the available experimental data. Our thermodynamic properties of α-SnS, including heat capacity CP , entropy S, and Gibbs free energy relation of -(GT -H0) curves, show similar growth trends as the experimental data. At T=298.15 K, our CP =52.31 J/mol·K, S=78.93 J/mol·K, and -(GT -H0)=12.03 J/mol all agree very well with experimental data CP =48.77 J/mol·K and 49.25 J/mol·K, S=76.78 J/mol·K, and -(GT -H0)=12.38 J/mol. The elastic constants, together with other elastic properties, are also computed. The anisotropy analyses indicate obvious elastic anisotropy for α-SnS along different symmetry planes and axes. Moreover, calculations demonstrate that α-SnS is an indirect gap semiconductor, and it transforms to semimetal with pressure increasing up to 10.2 GPa. Combined with the density of states, the characters of the band structure have been analysed in detail.

  18. Atomic and Electronic Structure of the P3HT/PCBM Interface From First-Principle Calculations

    NASA Astrophysics Data System (ADS)

    Li, Longhua; Kontsevoi, Oleg; Freeman, Arthur J.

    2013-03-01

    Fundamental research on donor/acceptor (D/A) interfaces of organic photovoltaics (OPV) have drawn immense interest because of their crucial roles in charge separation (CS), charge transfer (CT) and charge recombination (CR). The blend system consisting of regioregular poly(3-hexylthiophene) (rr-P3HT) and fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is a widely investigated binary system. Despite significant efforts that have been done to optimize the OPV, such as the D/A ratio, detailed information on their structure, interfaces, and morphology are far from complete. Additionally, fewer investigations have focused on the elementary charge transfer processes. In this work, such a hetero-interface was carried out by annealing simulation; and then interfacial electronic structure and charge transfer were studied by DFT calculations. The process of PCBM assembly on the P3HT surface were shown and the carrier mobilities could be tuned by PCBM orientations.Our calculations provide an important understanding on the assembly of PCBM and charge transfer at the binary interface. Supported by ANSER, an Energy Frontier Research Center funded by the U.S. Department of Energy.

  19. Prediction of Pressure-Induced Structural Transition and Mechanical Properties of MgY from First-Principles Calculations

    NASA Astrophysics Data System (ADS)

    Pu, Chun-Ying; Xun, Xian-Chao; Song, Hai-Zhen; Zhang, Fei-Wu; Lu, Zhi-Wen; Zhou, Da-Wei

    2016-01-01

    Using the particle swarm optimization algorithm on crystal structure prediction, we first predict that MgY alloy undergoes a first-order phase transition from CsCl phase to P4/NMM phase at about 55 GPa with a small volume collapse of 2.63%. The dynamical stability of P4/NMM phase at 55 GPa is evaluated by the phonon spectrum calculation and the electronic structure is discussed. The elastic constants are calculated, after which the bulk moduli, shear moduli, Young's modui, and Debye temperature are derived. The brittleness/ductile behavior, and anisotropy of two phases under pressure are discussed in details. Our results show that external pressure can change the brittle behavior to ductile at 10 GPa for CsCl phase and improve the ductility of MgY alloy. As pressure increases, the elastic anisotropy in shear of CsCl phase decreases, while that of P4/NMM phase remains nearly constant. The elastic anisotropic constructions of the directional dependences of reciprocals of bulk modulus and Young's modulus are also calculated and discussed. Supported by the Henan Joint Funds of the National Natural Science Foundation of China under Grant Nos. U1304612, U1404608, the National Natural Science Foundation of China under Grant Nos. 51501093, 51374132, and the Special Fund of the Theoretical Physics of China under Grant No. 11247222, Postdoctoral Science Foundation of China under Grant No. 2015M581767, and Young Core Instructor Foundation of Henan Province under Grant No. 2015GGJS-122

  20. Structure and pressure-induced ferroelectric phase transition in antiphase domain boundaries of strontium titanate from first principles

    NASA Astrophysics Data System (ADS)

    Kvasov, Alexander; Tagantsev, Alexander K.; Setter, Nava

    2016-08-01

    In this work, using zero kelvin ab initio calculations, we revisit the structure and ferroelectric phase transition in antiphase domain boundaries (APBs) in SrTiO3 (STO), which has been previously addressed in terms of a phenomenological approach. We confirmed the main qualitative conclusion of the phenomenological results that APBs normal to the rotation axis of the oxygen octahedra ("easy" walls) do not exhibit the transition while those parallel to the rotation axis ("hard" walls) do. However, we found the structure of the hard walls to be close to the Ising type in contrast to the phenomenological prediction of the nearly Néel type. We simulated a pressure-induced phase transition in the hard wall. Combining the results of simulation and experimental data on STO, we evaluated the pressure sensitivity of the ferroelectricity in the hard wall at low temperatures to show that it can be suppressed with very small pressure (a few kbar). We also roughly estimated the ferroelectric transition temperature in the hard wall corroborating the result of the phenomenological treatment.

  1. First-principle study of the electronic structure and magnetism of lithium-adsorbed 3d transition-metal phthalocyanines

    NASA Astrophysics Data System (ADS)

    Wang, M.; Hu, Y.; Zhang, Z.; Li, Y.; Zhou, T.; Ren, J.

    2016-02-01

    Based on density functional theory (DFT) calculations, the electronic structures and magnetic properties of 3d transition-metal phthalocyanine (TMPc, TM = Ti, V, Cr, Mn, Fe, Co, Ni and Cu), as well as Li-adsorbed phthalocyanines have been studied. The results show that the pristine TMPcs all have a good D4h symmetry. When there is one Li atom adsorbed on TMPcs directly over (LiTMPc-α) or slantly above (LiTMPc-β) the TM atoms, the geometries and electronic structures will be changed. For LiTMPc-α systems, the central TM atoms will deviate from the molecular plane and the molecules exhibit good C4v symmetry. LiTMPc-β systems are more stable than LiTMPc-α systems but it do not possess D4h and C4v symmetries. The total and local magnetic moments and the charge transfer are also presented. Finally, by using the orbit mixing and splitting theory under D4h and C4v symmetry, we get the ordering of the energy levels of the central TM atoms.

  2. First-principles study of structure and nonlinear optical properties of CdHg(SCN)4 crystal

    NASA Astrophysics Data System (ADS)

    Zhang, Peng; Kong, Chui-Gang; Zheng, Chao; Wang, Xin-Qiang; Ma, Yue; Feng, Jin-Bo; Jiao, Yu-Qiu; Lu, Gui-Wu

    2015-02-01

    The geometric structure, electronic structure, and optical properties of CdHg(SCN)4 crystal are calculated by using the density functional perturbation theory and Green function screening Coulomb interaction approximation. The band gap of CdHg(SCN)4 crystal is calculated to be 3.198 eV, which is in good agreement with the experimental value 3.265 eV. The calculated second-order nonlinear optical coefficients are d14 = 1.2906 pm/V and d15 = 5.0928 pm/V, which are in agreement with the experimental results (d14 = (1.4 ± 0.6)pm/V and d15 = (6.0 ± 0.9 pm/V). Moreover, it is found that the contribution to the valence band mainly comes from Cd-4d, Hg-5d states, and the contributions to the valence band top and the conduction band bottom predominantly come from C-2p, N-2p, and S-3p states. The second-order nonlinear optical effect of CdHg(SCN)4 crystal results mainly from the internal electronic transition of (SCN)-. Project supported by the National Natural Science Foundation of China (Grant No. 51372140), the Youth Scientist Fund of Shandong Province, China (Grant No. BS2011CL025), and the Basic Discipline Research Fund of China University of Petroleum, Beijing, China (Grant No. 01JB0169).

  3. First-principles investigation of electronic structure and hyperfine properties of heme and nitrosyl-hemoglobin systems

    NASA Astrophysics Data System (ADS)

    Pujari, Minakhi

    This thesis is devoted to study of electronic structures and associated hyperfine properties of molecular systems. The main emphasis of our work is on the electronic structure and hyperfine properties of nitrosyl-hemoglobin and the sensitiveness of the structure to external conditions as this compound is closely related to deoxy-hemoglobin, the most important enzyme of the human body. The other two systems of compounds, sixth group hexafluorides and five-liganded halogen-heme compounds have been studied to test the accuracy of the Hartree-Fock procedure employed in explaining the properties of systems related in different degrees to nitrosyl-hemoglobin. In the hexafluoride systems, the theoretical values obtained in our work for the Nuclear Quadrupole Coupling Constant (NQCC) of 19F*, explain the experimental trend of continuous decrease from lightest to the heaviest systems. This is in keeping with the empirical Townes and Dailey relation and the expected increase in ionicity in going to the heavier systems. In bromo-hemin and iodo-hemin, the magnetic hyperfine properties of 57Fe, 14N, 13C, protons and halogen nuclei were studied. The associated charge and unpaired spin population obtained using their calculated electronic structures indicated more localized charge and spin distribution than were found by the semi-empirical method of Self-Consistent Charge Extended Hückel Procedure. Our results for the hyperfine constants showed satisfactory agreement with available experimental data. The contact and dipolar contribution to the hyperfine constant and their breakdown into direct and exchange polarization contributions were analyzed. The isomer shift at the 57Fe nucleus for both the systems, bromo-hemin and iodo-hemin were also studied and the observed trend was in agreement with that for other related compounds. The studies of the sixth group hexafluorides, and bromo- hemin and iodo-hemin systems, have encouraged us to use the Hartree-Fock Roothaan

  4. First-principles study of the structural and dynamic properties of the liquid and amorphous Li-Si alloys

    NASA Astrophysics Data System (ADS)

    Chiang, Han-Hsin; Lu, Jian-Ming; Kuo, Chin-Lung

    2016-01-01

    We have performed density functional theory calculations and ab initio molecular dynamics to investigate the structures and dynamic properties of the liquid and amorphous LixSi alloys over a range of composition from x = 1.0 - 4.8. Our results show that Si atoms can form a variety of covalently bonded polyanions with diverse local bonding structures in the liquid alloys. Like in c-LiSi, Si atoms can form a continuous bond network in liquid Li1.0Si at 1050 K, while it gradually disintegrates into many smaller Si polyanions as the Li content increases in the alloys. The average sizes of Si polyanions in these liquid alloys were found to be relatively larger than those in their crystalline counterparts, which can even persist in the highly lithiated Li4.81Si alloy at 1500 K. Our results also show that amorphous LixSi alloys have similar local bonding structures but a largely increased short-range order as compared to their liquid counterparts. The differences between the average coordination number of each atomic pair in amorphous solids and that in the liquids are less than 1.1. Furthermore, our calculations reveal that Li and Si atoms can exhibit very distinct dynamic behaviors in the liquids and their diffusivities appear to be largely dependent on the chemical composition of the alloys. The diffusivity of Li was found to increase with the Li content in the alloys primarily because of the reduced interactions between Li and Si atoms, while the Si diffusivity also increases due to the gradual disintegration of the strongly interconnected Si bond network. The diffusivity of Li in amorphous LixSi was predicted to lie in the range between 10-7 and 10-9 cm2/s at 300 K, which is more than 20-fold larger than that of Si over the composition range considered. Our calculations further show that the diffusivities of both Li and Si can increase by two orders of magnitude as x increases from 1.0 to 3.57 in amorphous LixSi, indicating a more profound dependence on the alloy

  5. First-principles study of the structural and dynamic properties of the liquid and amorphous Li-Si alloys.

    PubMed

    Chiang, Han-Hsin; Lu, Jian-Ming; Kuo, Chin-Lung

    2016-01-21

    We have performed density functional theory calculations and ab initio molecular dynamics to investigate the structures and dynamic properties of the liquid and amorphous LixSi alloys over a range of composition from x = 1.0 - 4.8. Our results show that Si atoms can form a variety of covalently bonded polyanions with diverse local bonding structures in the liquid alloys. Like in c-LiSi, Si atoms can form a continuous bond network in liquid Li1.0Si at 1050 K, while it gradually disintegrates into many smaller Si polyanions as the Li content increases in the alloys. The average sizes of Si polyanions in these liquid alloys were found to be relatively larger than those in their crystalline counterparts, which can even persist in the highly lithiated Li4.81Si alloy at 1500 K. Our results also show that amorphous LixSi alloys have similar local bonding structures but a largely increased short-range order as compared to their liquid counterparts. The differences between the average coordination number of each atomic pair in amorphous solids and that in the liquids are less than 1.1. Furthermore, our calculations reveal that Li and Si atoms can exhibit very distinct dynamic behaviors in the liquids and their diffusivities appear to be largely dependent on the chemical composition of the alloys. The diffusivity of Li was found to increase with the Li content in the alloys primarily because of the reduced interactions between Li and Si atoms, while the Si diffusivity also increases due to the gradual disintegration of the strongly interconnected Si bond network. The diffusivity of Li in amorphous LixSi was predicted to lie in the range between 10(-7) and 10(-9) cm(2)/s at 300 K, which is more than 20-fold larger than that of Si over the composition range considered. Our calculations further show that the diffusivities of both Li and Si can increase by two orders of magnitude as x increases from 1.0 to 3.57 in amorphous LixSi, indicating a more profound dependence on the

  6. Multidomains made of different structural phases in multiferroic BiFeO3: A first-principles-based study

    NASA Astrophysics Data System (ADS)

    Wang, Dawei; Salje, Ekhard K. H.; Mi, Shao-Bo; Jia, Chun-Lin; Bellaiche, L.

    2013-10-01

    An effective Hamiltonian scheme is used to reveal the properties of a multidomain structure in BiFeO3 consisting of alternating domains that are initially made of two phases, namely, R3c (ferroelectric with antiphase oxygen octahedral tilting) versus Pnma (antiferroelectric with in-phase and antiphase oxygen octahedral tiltings). These two types of domains dramatically modify their properties as a result of their cohabitation. The weak ferromagnetic vector and polarization rotate, and significantly change their magnitude, in the R3c-like region, while the Pnma-like region becomes polar along the direction of domain propagation. Moreover, the domain walls possess distinct polar and oxygen octahedral tilting patterns that facilitate the transition between these two regions. The studied multidomain is also predicted to exhibit other anomalous properties, such as its strain adopting several plateaus and steps when increasing the magnitude of an applied electric field.

  7. Study of the structure and chemical bonding of crystalline Ge4Sb2Te7 using first principle calculations

    NASA Astrophysics Data System (ADS)

    Singh, Janpreet; Singh, Satvinder; Singh, Gurinder; Kaura, Aman; Tripathi, S. K.

    2016-05-01

    The atomic arrangements and chemical bonding of stable Ge4Sb2Te7 (GeTe rich), a phase-change material, have been investigated by means of ab initio total energy calculations. To study the atomic arrangement, GeTe block is considered into -TeSbTeSbTe- block and -Te-Te- layer in the stacking I and II respectively. The stacking I is energetically more stable than the stacking II. The reason for more stability of the stacking I has been explained. The chemical bonding has been studied with the electronic charge density distribution around the atomic bonds. The quantity of electronic charge loosed or gained by atoms has been calculated using the Bader charge analysis. The metallic character has been studied using band structures calculations. The band gap for the stacking I and II is 0.463 and 0.219 eV respectively.

  8. The formation and phase transition of vortex domain structures in ferroelectric nanodots: First-principles-based simulations

    NASA Astrophysics Data System (ADS)

    Liu, J. Y.; Chen, W. J.; Wang, B.; Zheng, Yue

    2013-07-01

    Effects of size and hydrostatic pressure on the formation of vortex domain structures (VDSs) in ferroelectric nanodots under the open circuit condition have been simulated using the effective Hamiltonian method. Our calculations indicate promising controllability of VDS in the nanodots. Interestingly, it is found that the VDS holds rhombohedral at various sizes and pressures. Furthermore, the paraelectric-ferrotoroidic phase transition temperature (TFD) is very sensitive to size and pressure, manifesting with near linear relationships in most investigated range. At high pressure range, we found that the TFD-pressure curves become nonlinear. It indicates that the traditional Landau potentials with polarization-strain coupling term up to ˜Pɛ2 are not sufficient to describe the high pressure effect and the expansion coefficients should be modified as pressure dependent.

  9. First-Principles Materials Design of High-Performing Bulk Photovoltaics with the Li Nb O3 Structure

    NASA Astrophysics Data System (ADS)

    Young, Steve M.; Zheng, Fan; Rappe, Andrew M.

    2015-11-01

    The bulk photovoltaic effect is a long-known but poorly understood phenomenon. Recently, however, the multiferroic bismuth ferrite has been observed to produce strong photovoltaic response to visible light, suggesting that the effect has been underexploited. Here we present three polar oxides in the Li Nb O3 structure that we predict to have band gaps in the 1-2 eV range and very high bulk photovoltaic response: Pb Ni O3 , Mg1 /2Zn1 /2Pb O3 , and Li Bi O3 . All three have band gaps determined by cations with d10s0 electronic configurations, leading to conduction bands composed of cation s orbitals and O p orbitals. This both dramatically lowers the band gap and increases the bulk photovoltaic response by as much as an order of magnitude over previous materials, demonstrating the potential for high-performing bulk photovoltaics.

  10. First-principles study of structural and elastic properties of CrO2 at high pressure

    NASA Astrophysics Data System (ADS)

    Wu, H. Y.; Chen, Y. H.; Su, X. F.; Deng, C. R.

    2011-09-01

    The structural and elastic properties of CrO2 in the rutile phase under high pressures have been investigated using pseudopotential plane-wave method based on density functional theory. The optimized lattice parameters and the bulk modulus at zero pressure agree well with available experimental and theoretical data. The elastic constants C 11, C 12, C 44, C 33, C 13, and C 66 at zero pressure are calculated to be 359.91, 264.69, 143.28, 309.45, 218.45, and 260.74 GPa, respectively. Elastic constants, bulk modulus, shear modulus, Young's modulus, and Poisson's ratio under pressures are obtained. Our results indicate that the rutile phase is mechanically stable below 11.99 GPa. The elastic anisotropy of rutile phase under pressures has also been predicted.

  11. First Principles Calculations of Structural, Electronic, Thermodynamic and Thermal Properties of BaxSr1-xTe Ternary Alloys

    NASA Astrophysics Data System (ADS)

    Chelli, S.; Meradji, H.; Amara Korba, S.; Ghemid, S.; El Haj Hassan, F.

    2014-12-01

    The structural, electronic thermodynamic and thermal properties of BaxSr1-xTe ternary mixed crystals have been studied using the ab initio full-potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). In this approach, the Perdew-Burke-Ernzerhof-generalized gradient approximation (PBE-GGA) was used for the exchange-correlation potential. Moreover, the recently proposed modified Becke Johnson (mBJ) potential approximation, which successfully corrects the band-gap problem was also used for band structure calculations. The ground-state properties are determined for the cubic bulk materials BaTe, SrTe and their mixed crystals at various concentrations (x = 0.25, 0.5 and 0.75). The effect of composition on lattice constant, bulk modulus and band gap was analyzed. Deviation of the lattice constant from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed for the ternary BaxSr1-xTe alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing, ΔHm as well as the phase diagram. It was shown that these alloys are stable at high temperature. Thermal effects on some macroscopic properties of BaxSr1-xTe alloys were investigated using the quasi-harmonic Debye model, in which the phononic effects are considered.

  12. First-principles study on the structural, electronic, and optical properties of Ca1- x Sr x Se alloys

    NASA Astrophysics Data System (ADS)

    Ahmadian, F.; Salary, A.

    2016-01-01

    The structural, electronic, and optical properties of binary CaSe and SrSe compounds and Ca1- x Sr x Se alloys were studied by using the full potential linearized augmented plane wave (FPLAPW) method within density functional theory (DFT). The band structure calculations showed that the CaSe and the SrSe binary compounds in the rocksalt (RS), zinc-blende (ZB) and wurtzite (WZ) phases were semiconductors while they had a metallic characteristic in the CsCl phase. The lattice constant and bulk modulus values for the Ca1- x Sr x Se alloys in the RS and the ZB phases at different concentrations were calculated and compared with those obtained by using Vegard's law. The energy band gap values in the RS and the ZB phases were estimated for different x values by using both define acronyms the Perdew, Burke, and Ernzerhof (PBE-GGA) and the Engel and Vosko (EV-GGA) schemes, and the results were compared with those obtained by using the empirical electronegativity expression. The band gap bowing parameters were calculated by using quadratic functions and the procedure of Bernard and Zunger to fit the non-linear variation of the band gaps. The static dielectric constant ɛ 1(0) was calculated at different concentrations. The energy loss function L( ω) for the Ca1- x Sr x Se alloys in the RS and the ZB phases has a main peak corresponding to the plasmon frequency. The values of the static refractive index ( n(0)) for the Ca1- x Sr x Se alloys were calculated and compared with the values predicted by using the Moss, Ravindra, and Vandamme models. Finally, the extinction indic incident photon energies. es ( k( ω)) and the reflectivities ( R( ω)) for the Ca1- x Sr x Se alloys were calculated within a wide range of incident photon energies.

  13. Possibility of transforming the electronic structure of one species of graphene adatoms into that of another by application of gate voltage: First-principles calculations

    NASA Astrophysics Data System (ADS)

    Chan, Kevin T.; Lee, Hoonkyung; Cohen, Marvin L.

    2011-10-01

    Graphene provides many advantages for controlling the electronic structure of adatoms and other adsorbates via gating. Using the projected density of states and charge density obtained from first-principles density-functional periodic supercell calculations, we investigate the possibility of performing “alchemy” of adatoms on graphene, i.e., transforming the electronic structure of one species of adatom into that of another species by application of a gate voltage. Gating is modeled as a change in the number of electrons in the unit cell, with the inclusion of a compensating uniform background charge. Within this model and the generalized gradient approximation to the exchange-correlation functional, we find that such transformations are possible for K, Ca, and several transition-metal adatoms. Gate control of the occupation of the p states of In on graphene is also investigated. The validity of the supercell approximation with uniform compensating charge and the model for exchange and correlation is also discussed.

  14. Structural, dynamic, electronic, and vibrational properties of flexible, intermediate, and stressed rigid As-Se glasses and liquids from first principles molecular dynamics

    SciTech Connect

    Bauchy, M.; Kachmar, A.; Micoulaut, M.

    2014-11-21

    The structural, vibrational, electronic, and dynamic properties of amorphous and liquid As{sub x}Se{sub 1-x} (0.10 First Principles Molecular Dynamics. Within the above range of compositions, thresholds, and anomalies are found in the behavior of reciprocal and real space properties that can be correlated to the experimental location of the Boolchand intermediate phase in these glassy networks, observed at 0.27 structural and dynamical atomic-scale fingerprints for the onset of rigidity within the network, while also providing a much more complex picture than the one derived from mean-field approaches of stiffness transitions.

  15. Electronic structures and thermoelectric properties of CuMTe2 (M = Al, Ga, In) copper chalcopyrites: a first-principles study

    NASA Astrophysics Data System (ADS)

    Li, Wenfeng; Yang, Gui; Zhang, Jianwei; Hou, Zhiwei

    2015-12-01

    We investigated the electronic structures and thermoelectric properties of CuMTe2 (M = Al Ga, In) copper chalcopyrites based on first-principles calculations using the Tran-Blaha-modified Becke-Johnson potential, which yielded accurate band gaps for the semiconductors. The band structures show both light and heavy bands near the valence band maximum. Light bands improve mobility, and heavy bands often increase the Seebeck coefficient, so our results suggest that the chalcopyrites should have larger power factors. To calculate the thermoelectric properties of CuMTe2 (M = Al, Ga, In), we used semi-classical Boltzmann transport theory. The results show relatively high Seebeck coefficients and Z e T for low carrier concentrations at a given temperature, and n-type CuAlTe2 appears to be a particularly promising thermoelectric material.

  16. The structural evolution of hydrogenated silicon carbide nanocrystals: an approach from bond energy model, Wang–Landau method and first-principles studies

    NASA Astrophysics Data System (ADS)

    Wang, Ya-Ting; Zhao, Yu-Jun; Yang, Xiao-Bao

    2016-06-01

    The novel properties of nanomaterials are attributed to their variety of structures, while it is a central task to determine the stable configurations under different environment conditions. Exemplified with the hydrogenated cubic silicon carbide nanocrystals (H-SiCNCs), we propose an efficient approach to determine the stable H-SiCNCs by the convex analysis with the possible candidates pre-screened by the Wang–Landau method and a bond energy model, followed by the property analysis from first-principles. We find that the configurations of H-SiCNCs are dominated by the hydrogen and carbon chemical potentials according to the phase diagram, and there are structural transitions with the increasing size from tetrahedron, hexahedron, to octahedron. The energy gaps of tetrahedral H-SiCNCs are larger than that of octahedral ones at similar sizes, and in hexagonal ones there is a charge separation for the highest occupied molecular orbitals and lowest unoccupied molecular orbitals.

  17. Structural, dynamic, electronic, and vibrational properties of flexible, intermediate, and stressed rigid As-Se glasses and liquids from first principles molecular dynamics

    NASA Astrophysics Data System (ADS)

    Bauchy, M.; Kachmar, A.; Micoulaut, M.

    2014-11-01

    The structural, vibrational, electronic, and dynamic properties of amorphous and liquid AsxSe1-x (0.10 First Principles Molecular Dynamics. Within the above range of compositions, thresholds, and anomalies are found in the behavior of reciprocal and real space properties that can be correlated to the experimental location of the Boolchand intermediate phase in these glassy networks, observed at 0.27 structural and dynamical atomic-scale fingerprints for the onset of rigidity within the network, while also providing a much more complex picture than the one derived from mean-field approaches of stiffness transitions.

  18. First-principles study of boron oxygen hole centers in crystals: Electronic structures and nuclear hyperfine and quadrupole parameters

    SciTech Connect

    Li Zucheng; Pan Yuanming

    2011-09-15

    The electronic structures, nuclear hyperfine coupling constants, and nuclear quadrupole parameters of fundamental boron oxygen hole centers (BOHCs) in zircon (ZrSiO{sub 4}, I4{sub 1}/amd) and calcite (CaCO{sub 3}, R3c) have been investigated using ab initio Hartree-Fock (HF) and various density functional theory (DFT) methods based on the supercell models with all-electron localized basis sets. Both exact HF exchange and appropriate correlation functionals are important in describing the BOHCs, and the parameter-free hybrid method based on Perdew, Burke, and Ernzerhof density functionals (PBE0) turns out to be the best DFT method in reproducing the electron paramagnetic resonance (EPR) data. Our results reveal three distinct types of simple-spin (S = 1/2) [BO{sub 3}]{sup 2-} centers in calcite: (i) the classic [BO{sub 3}]{sup 2-} radical with the D{sub 3h} symmetry and the unpaired spin equally distributed on the three oxygen atoms (i.e. the O{sub 3}{sup 5-} type); (ii) the previously reported [BO{sub 2}]{sup 0} center with the unpaired spin equally distributed on two of the three oxygen atoms (O{sub 2}{sup 3-}); and (iii) a new variety with {approx}90% of its unpaired spin localized on one (O{sup -}) of the three oxygen atoms with a long B-O bond (1.44 A). Calculations confirm the unusual [BO{sub 4}]{sup 0} center in zircon and show it to arise from a highly distorted configuration with 90% of the unpaired spin on one oxygen atom that has a considerably longer B-O bond (1.68 A) than its three counterparts (1.45 A). The calculated magnitudes and directions of {sup 11}B and {sup 17}O hyperfine coupling constants and nuclear quadrupole constants for the [BO{sub 4}]{sup 0} center in zircon are in excellent agreement with the 15 K EPR experimental data. These BOHCs are all characterized by a small negative spin density on the central B atom arising from spin polarization. Our calculations also demonstrate that the spin densities on BOHCs are affected substantially by

  19. First principles calculations in iron: structure and mobility of defect clusters and defect complexes for kinetic modelling

    NASA Astrophysics Data System (ADS)

    Fu, Chu Chun; Willaime, F.

    2008-04-01

    Predictive simulations of the defect population evolution in materials under or after irradiation can be performed in a multi-scale approach, where the atomistic properties of defects are determined by electronic structure calculations based on the Density Functional Theory and used as input for kinetic simulations covering macroscopic time and length scales. Recent advances obtained in iron are presented. The determination of the 3D migration of self-interstitial atoms instead of a fast one-dimensional glide induced an overall revision of the widely accepted picture of radiation damage predicted by previously existing empirical potentials. A coupled ab initio and mesoscopic kinetic Monte Carlo simulation provided strong evidence to clarify controversial interpretations of electrical resistivity recovery experiments concerning the mobility of vacancies, self-interstitial atoms, and their clusters. The results on the dissolution and migration properties of helium in α-Fe were used to parameterize Rate Theory models and new inter-atomic potentials, which improved the understanding of fusion reactor materials behavior. Finally, the effects of carbon, present in all steels as the principal hardening element, are also shown. To cite this article: C.C. Fu, F. Willaime, C. R. Physique 9 (2008).

  20. First-principles molecular dynamics study of the structure and dynamic behavior of liquid Li4BN3H10

    NASA Astrophysics Data System (ADS)

    Farrell, David E.; Shin, Dongwon; Wolverton, C.

    2009-12-01

    We have applied density-functional theory based ab initio molecular dynamics to examine Li4BN3H10 at temperatures both above and below the experimental melting point. We examine the structure of the liquid, diffusivity, vibrational spectra and compare to both experimental data and analogous properties from solid-state calculations. We find the following: (1) the liquid state, like the solid state, is primarily a mixture of Li+ , BH4- , and NH2- with ionic interactions between the BH4- and NH2- anions and the Li+ cations. (2) We observe the reaction of two amide anions exchanging hydrogen to form ammonia and an imide anion: 2NH2-→NH3+NH2- . (3) The liquid demonstrates wide bond-angle distributions in the BH4- and NH2- units and thus these anionic units are not simply rigid complexes. (4) The Li+ sublattice disorders before the anionic sublattices and the liquid exhibits very fast Li+ diffusion. We calculate the activation energy and pre-exponential factor for Li+ diffusivity in the liquid to be ˜20kJ/mol and 15×10-4cm2/s , respectively. (5) Finally, we find that the liquid contains the same generic types of vibrational modes as the solid, however the lower-frequency anionic vibration and rotation modes become more prominent with increasing temperature.

  1. Spectroscopic and electronic structure calculation of a potential antibacterial agent incorporating pyrido-dipyrimidine-dione moiety using first principles

    NASA Astrophysics Data System (ADS)

    Fatma, Shaheen; Bishnoi, Abha; Singh, Vineeta; Al-Omary, Fatmah A. M.; El-Emam, Ali A.; Pathak, Shilendra; Srivastava, Ruchi; Prasad, Onkar; Sinha, Leena

    2016-04-01

    Quantum chemical calculations of geometrical structure, energy and vibrational wavenumbers of a novel functionalized pyrido-pyrimidine compound (a prospective antibacterial agent), chemically known as 6-Methyl,13,14,15-Trihydro-14-(4-Nitrophenyl)pyrido[1,2-a:1‧,2‧-a‧] pyrido[2″,3″-d:6″,5″-d‧]dipyrimidine-13,15-dione (C24H16N6O4), were carried out, using B3LYP/6311++G(d,p) method. Comprehensive interpretation of the infrared and Raman spectra of the compound under study is based on potential energy distribution. A good coherence between experimental and theoretical wavenumbers shows the preciseness of the assignments. NLO properties like the dipole moment, polarizability, first static hyperpolarizability and molecular electrostatic potential surface have been calculated to get a better cognizance of the properties of the title compound. Molecular docking results reveal that the title compound exhibit inhibitory activity against Staphylococcus aureus.

  2. First-principles study of the band structure and optical absorption of CuGaS2

    NASA Astrophysics Data System (ADS)

    Aguilera, Irene; Vidal, Julien; Wahnón, Perla; Reining, Lucia; Botti, Silvana

    2011-08-01

    CuGaS2 is the most promising chalcopyrite host for intermediate-band thin-film solar cells. Standard Kohn-Sham density functional theory fails in describing the band structure of chalcopyrite materials, due to the strong underestimation of the band gap and the poor description of p-d hybridization, which makes it inadvisable to use this approach to study the states in the gap induced by doping. We used a state-of-the-art restricted self-consistent GW approach to determine the electronic states of CuGaS2: in the energy range of interest for optical absorption, the GW corrections shift the Kohn-Sham bands almost rigidly, as we proved through analysis of the effective masses, bandwidths, and relative position of the conduction energy valleys. Furthermore, starting from the GW quasiparticle bands, we calculated optical absorption spectra using different approximations. We show that the time-dependent density functional theory can be an efficient alternative to the solution of the Bethe-Salpeter equation when the exchange-correlation kernels derived from the Bethe-Salpeter equation are employed. This conclusion is important for further studies of optical properties of supercells including dopants.

  3. First-principles computation of structural, elastic and magnetic properties of Ni2FeGa across the martensitic transformation.

    PubMed

    Sahariah, Munima B; Ghosh, Subhradip; Singh, Chabungbam S; Gowtham, S; Pandey, Ravindra

    2013-01-16

    The structural stabilities, elastic, electronic and magnetic properties of the Heusler-type shape memory alloy Ni(2)FeGa are calculated using density functional theory. The volume conserving tetragonal distortion of the austenite Ni(2)FeGa find an energy minimum at c/a = 1.33. Metastable behaviour of the high temperature cubic austenite phase is predicted due to elastic softening in the [110] direction. Calculations of the total and partial magnetic moments show a dominant contribution from Fe atoms of the alloy. The calculated density of states shows a depression in the minority spin channel of the cubic Ni(2)FeGa just above the Fermi level which gets partially filled up in the tetragonal phase. In contrast to Ni(2)MnGa, the transition metal spin-down states show partial hybridization in Ni(2)FeGa and there is a relatively high electron density of states near the Fermi level in both phases. PMID:23186622

  4. First-principles study of electronic structure, phonons and electron-phonon interaction in hexagonal PdTe

    NASA Astrophysics Data System (ADS)

    Cao, Jin-Jin; Gou, Xiao-Fan

    2016-01-01

    The electronic structure, phonons and electron-phonon interaction of hexagonal PdTe have been investigated in detail by employing a plane wave pseudopotential method and a linear-response scheme within Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA). Consistent with available theoretical and experimental results, it has been found that the intermediate strength electron-phonon coupling constant (λ) with the calculated value of 0.542 on the LDA and 0.648 on the GGA is due to the coupling of phonons from Pd and Te atoms and electrons from Pd-d and Te-p states. Through comparison, the calculations on the GGA produce better quality than that on the LDA. On the basis of appropriate Coulomb pseudopotential (μ∗) and λ of 0.648 together with experimental Debye temperature (Θ), via the McMillan formula, the superconducting transition temperature with the value of 4.5 K is obtained, same to the experimental value. The results indicate that conventional electron-phonon coupling mechanism can explain the superconductivity in this compound.

  5. Structural and electronic properties of CdS/ZnS core/shell nanowires: A first-principles study

    NASA Astrophysics Data System (ADS)

    Kim, Hyo Seok; Kim, Yong-Hoon

    2015-03-01

    Carrying out density functional theory (DFT) calculation, we studied the relative effects of quantum confinement and strain on the electronic structures of II-IV semiconductor compounds with a large lattice-mismatch, CdS and ZnS, in the core/shell nanowire geometry. We considered different core radii and shell thickness of the CdS/ZnS core/shell nanowire, different surface facets, and various defects in the core/shell interface and surface regions. To properly describe the band level alignment at the core/shell boundary, we adopted the self-interaction correction (SIC)-DFT scheme. Implications of our findings in the context of device applications will be also discussed. This work was supported by the Basic Science Research Grant (No. 2012R1A1A2044793), Global Frontier Program (No. 2013-073298), and Nano-Material Technology Development Program (2012M3A7B4049888) of the National Research Foundation funded by the Ministry of Education, Science and Technology of Korea. Corresponding author

  6. First-principles study of structural, elastic, electronic and thermodynamic properties of topological insulator Bi2Se3 under pressure

    NASA Astrophysics Data System (ADS)

    Gao, Xiang; Zhou, Meng; Cheng, Yan; Ji, Guangfu

    2016-01-01

    The structural, elastic, electronic and thermodynamic properties of the rhombohedral topological insulator Bi2Se3 are investigated by the generalized gradient approximation (GGA) with the Wu-Cohen (WC) exchange-correlation functional. The calculated lattice constants agree well with the available experimental and other theoretical data. Our GGA calculations indicate that Bi2Se3 is a 3D topological insulator with a band gap of 0.287 eV, which are well consistent with the experimental value of 0.3 eV. The pressure dependence of the elastic constants Cij, bulk modulus B, shear modulus G, Young's modulus E, and Poisson's ratio σ of Bi2Se3 are also obtained successfully. The bulk modulus obtained from elastic constants is 53.5 GPa, which agrees well with the experimental value of 53 GPa. We also investigate the shear sound velocity VS, longitudinal sound velocity VL, and Debye temperature ΘE from our elastic constants, as well as the thermodynamic properties from quasi-harmonic Debye model. We obtain that the heat capacity Cv and the thermal expansion coefficient α at 0 GPa and 300 K are 120.78 J mol-1 K-1 and 4.70 × 10-5 K-1, respectively.

  7. First Principles Study of structural characteristics and phase change mechanism of Ge-Sb-Te based materials

    NASA Astrophysics Data System (ADS)

    Park, Hanjin; Kim, Cheol-Woon; Lee, Hyung-June; Song, Hosin; Kwon, Young-Kyun

    Using ab initio density functional theory, we investigate the structural properties and their phase transition mechanism of the crystalline and amorphous phases of Ge-Sb-Te (GST) based phase change materials, which would be utilized for phase change random access memory. Among various stochiometries of GST, we focus on compositions along the (GeTe)n(Sb2Te3)m pseudo-binary line, denoted simply by (n , m) with integer n and m. We explore various GST materials corresponding (n , m) sets including (1,0), (0,1), (1,1), (2,1) and (1,2) by modeling their both phases. Especially, their amorphous phases can be constructed based on experimental data available or molecular dynamics (MD) simulations performing melt-quench processes. To understand the phase transition mechanism, we evaluate their coordination numbers, radial distribution functions, and angle distribution functions, which enables us to identify the characteristic local geometry representing each phase. We further investigate the thermal properties of various phases by evaluating their phonon densities of states obtained by Fourier-transforming the velocity autocorrelation functions calculated directly from our MD simulation.

  8. Hydration Shell Structure and Dynamics of Curium(III) in Aqueous Solution: First Principles and Empirical Studies

    SciTech Connect

    Atta-Fynn, Raymond; Bylaska, Eric J.; Schenter, Gregory K.; De Jong, Wibe A.

    2011-05-12

    Results of ab initio molecular dynamics (AIMD), quantum mechanics/molecular mechanics (QM/MM) and classical molecular dynamics (CMD) simulations of Cm3+ in liquid water at a temperature of 300 K are reported. The AIMD simulation was based on the Car-Parrinello MD scheme and GGA-PBE formulation of density functional theory. Two QM/MM simulations were performed by treating Cm3+ and the water molecules in the first shell quantum mechanically using the PBE (QM/MM-PBE) and the hybrid PBE0 density functionals (QM/MM-PBE0). Two CMD simulations were carried out using ab initio derived pair plus three-body potentials (CMD-3B) and empirical Lennard-Jones pair potential (CMD-LJ). The AIMD and QM/MM-PBE simulations predict average first shell hydration numbers of 8, both of which disagree with recent experimental EXAFS and TRLFS value of 9. On the other hand, the average first shell hydration numbers obtained in the QM/MM-PBE0 and CMD simulations was 9, which agrees with experiment. All the simulations predicted a average first shell and second shell Cm-O bond distances of 2.49-2.53 Å and 4.67-4.75 Å respectively, both of which are in fair agreement with corresponding experimental values of 2.45-2.48 Å and 4.65 Å. The average geometric arrangement of the eight-fold and nine-fold coordinated first shell structures corresponded to the square anti-prism and tricapped trigonal prisms respectively. The second shell hydration number for AIMD QM/MM-PBE, QM/MM-PBE0, CMD-3B, and CMD-LJ, were 15.8, 17.2, 17.7, 17.4, and 16.4 respectively, which indicates second hydration shell over-coordination compared to recent EXAFS experimental value of 13. Save the EXAFS spectra CMD-LJ simulation, all the computed EXAFS spectra agree fairly well with experiment and a clear distinction could not be made between configurations with 8-fold and 9-fold coordinated first shells. The mechanisms responsible for the first shell associative and dissociative ligand exchange in the classical simulations

  9. X-ray absorption and infrared spectra of water and ice: A first-principles electronic structure study

    NASA Astrophysics Data System (ADS)

    Chen, Wei

    Water is of essential importance for chemistry and biology, yet the physics concerning many of its distinctive properties is not well known. In this thesis we present a theoretical study of the x-ray absorption (XA) and infrared (IR) spectra of water in liquid and solid phase. Our theoretical tools are the density functional theory (DFT), Car-Parrinello (CP) molecular dynamics (MD), and the so-called GW method. Since a systematic review of these ab initio methods is not the task of this thesis, we only briefly recall the main concepts of these methods as needed in the course of our exposition. The focus is, instead, an investigation of what is the important physics necessary for a better description of these excitation processes, in particular, core electron excitations (in XA) that reveal the local electronic structure, and vibrational excitations (in IR) associated to the molecular dynamics. The most interesting question we are trying to answer is: as we include better approximations and more complete physical descriptions of these processes, how do the aforementioned spectra reflect the underlying hydrogen-bonding network of water? The first part of this thesis consists of the first four chapters, which focus on the study of core level excitation of water and ice. The x-ray absorption spectra of water and ice are calculated with a many-body approach for electron-hole excitations. The experimental features, even the small effects of a temperature change in the liquid, are reproduced with quantitative detail using molecular configurations generated by ab initio molecular dynamics. We find that the spectral shape is controlled by two major modifications of the short range order that mark the transition from ice to water. One is associated to dynamic breaking of the hydrogen bonds which leads to a strong enhancement of the pre-edge intensity in the liquid. The other is due to densification, which follows the partial collapse of the hydrogen bond network and is

  10. Culturing conceptions: From first principles

    NASA Astrophysics Data System (ADS)

    Roth, Wolff-Michael; Lee, Yew Jin; Hwang, Sungwon

    2008-07-01

    Over the past three decades, science educators have accumulated a vast amount of information on conceptions--variously defined as beliefs, ontologies, cognitive structures, mental models, or frameworks--that generally (at least initially) have been derived from interviews about certain topics. During the same time period, cultural studies has emerged as a field in which everyday social practices are interrogated with the objective to understand culture in all its complexity. Science educators have however yet to ask themselves what it would mean to consider the possession of conceptions as well as conceptual change from the perspective of cultural studies. The purpose of this article is thus to articulate in and through the analysis of an interview about natural phenomenon the first principles of such a cultural approach to scientific conceptions. Our bottom-up approach in fact leads us to develop the kind of analyses and theories that have become widespread in cultural studies. This promises to generate less presupposing and more parsimonious explanations of this core issue within science education than if conceptions are supposed to be structures inhabiting the human mind.

  11. Structure, magnetic, and transport properties of epitaxial ZnFe{sub 2}O{sub 4} films: An experimental and first-principles study

    SciTech Connect

    Jin, Chao; Li, Peng; Mi, Wenbo; Bai, Haili

    2014-06-07

    We investigated the structure, magnetic, and transport properties of ZnFe{sub 2}O{sub 4} (ZFO) by both experimental and first-principles study. The epitaxial ZFO films prepared with various oxygen partial pressures show clear semiconducting behavior and room-temperature ferrimagnetism. A large magnetoresistance of −21.2% was observed at 75 K. The room-temperature ferrimagnetism is induced by the cation disordering. The calculated results indicate that under cation disordering, the ZFO with various oxygen vacancies is a half-metal semiconductor with both possible positive and negative signs of the spin polarization, while ZFO with no oxygen vacancies is an insulator and can be acted as the spin filter layer in spintronic devises.

  12. Rock-salt structure lithium deuteride formation in liquid lithium with high-concentrations of deuterium: a first-principles molecular dynamics study

    NASA Astrophysics Data System (ADS)

    Chen, Mohan; Abrams, T.; Jaworski, M. A.; Carter, Emily A.

    2016-01-01

    Because of lithium’s possible use as a first wall material in a fusion reactor, a fundamental understanding of the interactions between liquid lithium (Li) and deuterium (D) is important. We predict structural and dynamical properties of liquid Li samples with high concentrations of D, as derived from first-principles molecular dynamics simulations. Liquid Li samples with four concentrations of inserted D atoms (LiDβ , β =0.25 , 0.50, 0.75, and 1.00) are studied at temperatures ranging from 470 to 1143 K. Densities, diffusivities, pair distribution functions, bond angle distribution functions, geometries, and charge transfer between Li and D atoms are calculated and analyzed. The analysis suggests liquid-solid phase transitions can occur at some concentrations and temperatures, forming rock-salt LiD within liquid Li. We also observe formation of some D2 molecules at high D concentrations.

  13. First-principles study on the magnetism and electronic structure in 3d transition metal (X=Sc, V, Cr, Mn, Fe, Ni, Cu) doped CoO

    NASA Astrophysics Data System (ADS)

    Liu, R. X.; Wang, X. C.; Chen, G. F.; Yang, B. H.

    2016-03-01

    We have studied the electronic structure and magnetism of the single transitional metal element X=Sc, V, Cr, Mn, Fe, Ni, Cu-doped CoO systems by first-principles calculations. At X=Sc, Cr, Cu, the binding energy of the doped systems is lower than pure CoO, suggesting that these systems are energetically stable. In the Sc, V, Cr, Mn, Fe, Ni, Cu-doped 2×2×2 CoO supercells, the total magnetic moments are 3.03, 5.64, 6.80, 7.70, 6.93, 2.30 and 1.96 μB, respectively. At X=Cr and Fe, the doped CoO systems are half-metallic with a high spin polarization. The large magnetic moment and high spin polarization in the Cr and Fe-doped CoO are important for the design of the spintronic devices.

  14. First-principles investigation on vibrational, anisotropic elastic and thermodynamic properties for L12 structure of Al3Er and Al3Yb under high pressure

    NASA Astrophysics Data System (ADS)

    Zhang, Xudong; Jiang, Wei

    2016-02-01

    To better clarify the physical properties for Al3RE precipitates, first-principles calculations are performed to investigate the vibrational, anisotropic elastic and thermodynamic properties of Al3Er and Al3Yb. The calculated results agree well with available experimental and theoretical ones. The vibrational properties indicate that Al3Er and Al3Yb will keep their dynamical stabilities with L12 structure up to 100 GPa. The elastic constants are satisfied with mechanical stability criteria up to the external pressure of 100 GPa. The mechanical anisotropy is predicted by anisotropic constants AG, AU, AZ and 3D curved surface of Young's modulus. The calculated results show that both Al3Er and Al3Yb are isotropic at zero pressure and obviously anisotropic under high pressure. Further, we systematically investigate the thermodynamic properties and provide the relationships between thermal parameters and pressure. Finally, the pressure-dependent behaviours of density of states, Mulliken charge and bond length are discussed.

  15. Structural studies of TiC{sub 1−x}O{sub x} solid solution by Rietveld refinement and first-principles calculations

    SciTech Connect

    Jiang, Bo Hou, Na; Huang, Shanyan; Zhou, Gege; Hou, Jungang; Cao, Zhanmin; Zhu, Hongmin

    2013-08-15

    The lattice parameters, structural stability and electronic structure of titanium oxycarbides (TiC{sub 1−x}O{sub x}, 0≤x≤1) solid solution were investigated by Rietveld refinement and first-principles calculations. Series of TiC{sub 1−x}O{sub x} were precisely synthesized by sintering process under the vacuum. Rietveld refinement results of XRD patterns show the properties of continuous solid solution in TiC{sub 1−x}O{sub x} over the whole composition range. The lattice parameters vary from 0.4324 nm to 0.4194 nm decreasing with increasing oxygen concentration. Results of first-principles calculations reveal that the disorder C/O structure is stable than the order C/O structure. Further investigations of the vacancy in Ti{sub 1−Va}(C{sub 1−x}O{sub x}){sub 1−Va} solid solution present that the structure of vacancy segregated in TiO-part is more stable than the disorder C/O structure, which can be ascribed to the Ti–Ti bond across O-vacancy and the charge redistributed around Ti-vacancy via the analysis of the electron density difference plots and PDOS. - Graphical abstract: XRD of series of titanium oxycarbides (TiC{sub 1−x}O{sub x}, 0≤x≤1) solid solution prepared by adjusting the proportion of TiO in the starting material. Highlights: • Titanium oxycarbides were obtained by sintering TiO and TiC under carefully controlled conditions. • Rietveld refinement results show continuous solid solution with FCC structure in TiC{sub 1−x}O{sub x}. • The disorder C/O structure is stable than the order C/O structure. • Introduction of vacancy segregated in TiO-part is more stable than disorder C/O structure. • Ti–Ti bond across O-vacancy and the charge redistributed around Ti-vacancy enhance structural stability.

  16. Investigation of structural, surface morphological, optical properties and first-principles study on electronic and magnetic properties of (Ce, Fe)-co doped ZnO

    NASA Astrophysics Data System (ADS)

    Arul Mary, J.; Judith Vijaya, J.; Bououdina, M.; John Kennedy, L.; Daie, J. H.; Song, Y.

    2015-01-01

    We report on the synthesis of ((Zn1-2xCexFex) O (x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05)) nanoparticles via microwave combustion by using urea as a fuel. To understand how the dopant influenced the structural, magnetic and optical properties of nanoparticles, it was characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectra and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Fe co-doped ZnO were probed by first principle calculations. From the analysis of X-ray diffraction, the samples are identified with the wurtzite crystal structure. The change in lattice parameters, micro-strain, and a small shift in XRD peaks confirms the substitution of co dopants into the ZnO lattice. Morphological investigation of the products revealed the existence of irregular shapes, such as spherical, spherodial and hexagonal. DRS measurements showed a decrease in the energy gap with increasing dopants contents, probably due to an increase in the lattice parameters. PL spectra consist of visible emission, due to the electronic defects, which are related to deep level emissions, such as oxide antisite (OZn), interstitial zinc (Zni), interstitial oxygen (Oi) and zinc vacancy (VZn). Magnetic measurements showed a ferromagnetic behavior for all the doped samples at room temperature. The first principle calculation results showed that the Ce governs the stability, while the Fe adjusts the magnetic characteristics in the Ce and Fe co-doped ZnO.

  17. Solid State Structure-Reactivity Studies on Bixbyites, Fluorites and Perovskites Belonging to the Vanadate, Titanate and Cerate Families

    NASA Astrophysics Data System (ADS)

    Shafi, Shahid P.

    This thesis primarily focuses on the systematic understanding of structure-reactivity relationships in two representative systems: bixbyite and related structures as well as indium doped CeO2. Topotactic reaction routes have gained significant attention over the past two decades due to their potential to access kinetically controlled metastable materials. This has contributed substantially to the understanding of solid state reaction pathways and provided first insights into mechanisms. Contrary to the widely used ex-situ methods, in-situ techniques including powder x-ray diffraction and thermogravimetric-differential thermal analysis have been employed extensively throughout this work in order to follow the reaction pathways in real time. Detailed analysis of the AVO3 (A = In, Sc) bixbyite reactivity under oxidative conditions has been carried out and a variety of novel metastable oxygen defect phases have been identified and characterized. The novel metastable materials have oxygen deficient fluorite structures and consequently are potential ion conductors. Structural aspects of the topotactic vs. reconstructive transformations are illustrated with this model system. The structure-reactivity study of AVO3 phases was extended to AVO3 perovskite family. Based on the research methodologies and results from AVO3 bixbyite reactivity studies a generalized mechanistic oxidation pathway has been established with a non-vanadium phase, ScTiO3 bixbyite. However, there is stark contrast in terms of structural stability and features beyond this stability limit during AVO3 and ScTiO3 bixbyite reaction pathways. A series of complex reaction sequences including phase separation and phase transitions were identified during the investigation of ScTiO3 reactivity. The two-step formation pathway for the fluorite-type oxide ion conductor Ce1-xInxO2-delta (0 ≤ x ≤ 0.3) is being reported. The formation of the BaCe1-xInxO 3-delta perovskites and the subsequent CO2-capture reaction

  18. Marcasite osmium nitride with high bulk modulus: First-principles calculations

    NASA Astrophysics Data System (ADS)

    Wang, Yuan Xu; Arai, Masao; Sasaki, Taizo

    2007-02-01

    The authors propose marcasite OsN2 as a structure of the experimentally synthesized orthorhombic OsN2 compound. From the first-principles calculations, they find that the marcasite structure is more stable than the fluorite and pyrite ones and its lattice constants are in good agreement with the experiment. The calculated elastic constants for marcasite OsN2 satisfy the stability condition. The band structure shows that marcasite OsN2 is metallic. There is a pseudogap around the Fermi level of the total density of states of marcasite OsN2, which may contribute to its stability. The appearance of the pseudogap is mainly caused by the anisotropic connectivity of OsN6 octahedra.

  19. First-principles investigation of the structural stability and electronic properties of Pd doped monoclinic Cu6Sn5 intermetallic compounds

    NASA Astrophysics Data System (ADS)

    Shao, Wei-Quan; Lu, Wen-Cai; Chen, Sha-Ou

    2014-12-01

    Tri-layer Au/Pd/Ni(P) films have been widely used as surface finish over the Cu pads in high-end packaging applications. It was found that a thin (Cu,Pd)6Sn5 IMC layer was beneficial in effective reducing inter-diffusion between a Cu substrate and a solder, and therefore the growth of the IMC layer and the EM (electromigration) processes. In this study, the structural properties and phase stability of monoclinic Cu6Sn5-based structures with Pd substitutions were studied by using the first-principles method. The (Cu,Pd)6Sn5 structure with the 4e site substituted by Pd has the lowest heat of formation and is the most stable among (Cu,Pd)6Sn5 structures. Hybridization of Pd-d and Sn-p states is a dominant factor for stability improvement. Moreover, Pd atoms concentration corresponding to the most stable structure of (Cu,Pd)6Sn5 was found to be 1.69 %, which is consistent with the experimental results.

  20. Structural stability and O{sub 2} dissociation on nitrogen-doped graphene with transition metal atoms embedded: A first-principles study

    SciTech Connect

    Yang, Mingye; Wang, Lu E-mail: yyli@suda.edu.cn; Li, Min; Hou, Tingjun; Li, Youyong E-mail: yyli@suda.edu.cn

    2015-06-15

    By using first-principles calculations, we investigate the structural stability of nitrogen-doped (N-doped) graphene with graphitic-N, pyridinic-N and pyrrolic-N, and the transition metal (TM) atoms embedded into N-doped graphene. The structures and energetics of TM atoms from Sc to Ni embedded into N-doped graphene are studied. The TM atoms at N{sub 4}V {sub 2} forming a 4N-centered structure shows the strongest binding and the binding energies are more than 7 eV. Finally, we investigate the catalytic performance of N-doped graphene with and without TM embedding for O{sub 2} dissociation, which is a fundamental reaction in fuel cells. Compared to the pyridinic-N, the graphitic-N is more favorable to dissociate O{sub 2} molecules with a relatively low reaction barrier of 1.15 eV. However, the catalytic performance on pyridinic-N doped structure can be greatly improved by embedding TM atoms, and the energy barrier can be reduced to 0.61 eV with V atom embedded. Our results provide the stable structure of N-doped graphene and its potential applications in the oxygen reduction reactions.

  1. Adatom-induced variations of the atomic and electronic structures of Si(111)3×3-Ag : A first-principles study

    NASA Astrophysics Data System (ADS)

    Jeong, Hojin; Yeom, Han Woong; Jeong, Sukmin

    2008-06-01

    Using a first-principles calculation method, we study the changes in the atomic and electronic structures of the Si(111)3×3-Ag surface (hereafter 3-Ag ) via doping of extra Ag adatoms. We present a structural model for the adatom-induced 21×21 superstructure (21-Ag) , which has three Ag adatoms immersed into the substrate Ag layer within a unit cell. The present structural model reproduces well the measured scanning-tunneling-microscopy images as well as the electronic band structure measured by angle-resolved photoelectron spectroscopy. We find out that the complex band structure seen on the 21-Ag phase basically arises from the band folding of the original surface bands of 3-Ag . The extra Ag adatoms doped on 3-Ag modify only the band alignment without any additional adatom-induced surface state. The almost unoccupied two-dimensional free-electron-like band, generally called S1 , at pristine 3-Ag is gradually filled and shifted downward with an increase in the dopant coverage. As this shifted S1 band crosses other surface bands, it loses its free-electron nature.

  2. Structural stability and O2 dissociation on nitrogen-doped graphene with transition metal atoms embedded: A first-principles study

    NASA Astrophysics Data System (ADS)

    Yang, Mingye; Wang, Lu; Li, Min; Hou, Tingjun; Li, Youyong

    2015-06-01

    By using first-principles calculations, we investigate the structural stability of nitrogen-doped (N-doped) graphene with graphitic-N, pyridinic-N and pyrrolic-N, and the transition metal (TM) atoms embedded into N-doped graphene. The structures and energetics of TM atoms from Sc to Ni embedded into N-doped graphene are studied. The TM atoms at N4V 2 forming a 4N-centered structure shows the strongest binding and the binding energies are more than 7 eV. Finally, we investigate the catalytic performance of N-doped graphene with and without TM embedding for O2 dissociation, which is a fundamental reaction in fuel cells. Compared to the pyridinic-N, the graphitic-N is more favorable to dissociate O2 molecules with a relatively low reaction barrier of 1.15 eV. However, the catalytic performance on pyridinic-N doped structure can be greatly improved by embedding TM atoms, and the energy barrier can be reduced to 0.61 eV with V atom embedded. Our results provide the stable structure of N-doped graphene and its potential applications in the oxygen reduction reactions.

  3. Structure of amorphous GeSe9 by neutron diffraction and first-principles molecular dynamics: Impact of trajectory sampling and size effects.

    PubMed

    Le Roux, Sébastien; Bouzid, Assil; Kim, Kye Yeop; Han, Seungwu; Zeidler, Anita; Salmon, Philip S; Massobrio, Carlo

    2016-08-28

    The structure of glassy GeSe9 was investigated by combining neutron diffraction with density-functional-theory-based first-principles molecular dynamics. In the simulations, three different models of N = 260 atoms were prepared by sampling three independent temporal trajectories, and the glass structures were found to be substantially different from those obtained for models in which smaller numbers of atoms or more rapid quench rates were employed. In particular, the overall network structure is based on Sen chains that are cross-linked by Ge(Se4)1/2 tetrahedra, where the latter are predominantly corner as opposed to edge sharing. The occurrence of a substantial proportion of Ge-Se-Se connections does not support a model in which the material is phase separated into Se-rich and GeSe2-rich domains. The appearance of a first-sharp diffraction peak in the Bhatia-Thornton concentration-concentration partial structure factor does, however, indicate a non-uniform distribution of the Ge-centered structural motifs on an intermediate length scale. PMID:27586930

  4. Energetic stability, oxidation states, and electronic structure of Bi-doped NaTaO3: a first-principles hybrid functional study.

    PubMed

    Joo, Paul H; Behtash, Maziar; Yang, Kesong

    2016-01-14

    We studied the defect formation energies, oxidation states of the dopants, and electronic structures of Bi-doped NaTaO3 using first-principles hybrid density functional theory calculations. Three possible structural models, including Bi-doped NaTaO3 with Bi at the Na site (Bi@Na), with Bi at the Ta site (Bi@Ta), and with Bi at both Na and Ta sites [Bi@(Na,Ta)], are constructed. Our results show that the preferred doping sites of Bi are strongly related to the preparation conditions of NaTaO3. It is energetically more favorable to form a Bi@Na structure under Na-poor conditions, to form a Bi@Ta structure under Na-rich conditions, and to form a Bi@(Na,Ta) structure under mildly Na-rich conditions. The Bi@Na doped model shows an n-type conducting character along with an expected blueshift of the optical absorption edge, in which the Bi atoms exist as Bi(3+) (6s(2)6p(0)). The Bi@Ta doped model has empty gap states consisting of Bi 6s states in its band gap, which can lead to visible-light absorption via the electron transition among the valence band, the conduction band, and the gap states. The Bi dopant is present as a Bi(5+) ion in this model, consistent with the experimental results. In contrast, the Bi@(Na,Ta) doped model has occupied gap states consisting of Bi 6s states in its band gap, and thus visible-light absorption is also expected in this system due to electron excitation from these occupied states to the conduction band, in which the Bi dopants exist as Bi(3+) ions. Our first-principles electronic structure calculations revealed the relationship between the Bi doping sites and the material preparation conditions, and clarified the oxidation states of Bi dopants in NaTaO3 as well as the origin of different visible-light photocatalytic hydrogen evolution behaviors in Bi@Ta and Bi@(Na,Ta) doped NaTaO3. This work can provide a useful reference for preparing a Bi-doped NaTaO3 photocatalyst with desired doping sites. PMID:26646215

  5. Redetermination of the structure of ALa₂WO₇ (A=Ba, Sr) with fluorite-like metal ordering

    SciTech Connect

    Fu, W.T.; IJdo, D.J.W.; Bontenbal, A.

    2013-05-01

    The crystal structures of ALa₂WO₇ (A=Ba, and Sr) at room temperature were re-determined by the Rietveld method using the combined X-ray and neutron powder diffraction data. The compounds are confirmed to be isomorphic, crystallizing in the space group P112₁/b. In ALa₂WO₇ the ordering of metal atoms is fluorite-like, but it differs from that of the fluorite-defect compounds of the formula Ln₃MO₇ (Ln=lanthanide or Y, M=pentavalent metal). The structure of ALa₂WO₇ consists of isolated WO₆ octahedra, whereas in the normal Ln₃MO₇ the MO₆ octahedra share corners forming one-dimensional chains. Although ALa₂WO₇ has a centric space group, La ions are not situate at the centre of symmetry, which explains the ⁵D₀→⁷F₂ transition being dominant in emission spectrum of Eu-doped materials. - Graphical abstract: Schematic drawing of the crystal structures of ALa₂WO₇ (A=Ba, Sr) (left) and Ln₃MO₇ (right) showing the different arrangement of metal octahedra. Note that the example of Ln₃MO₇ is the structure of La₃TaO₇ with the space group Cmcm. Highlights: • The structural ambiguity of BaLa₂WO₇ has been resolved. • The details of the crystal structure of SrLa₂WO₇ are reported for the first time. • ALa₂WO₇ (A=Ba, Sr) closely resemble β-La₃RuO₇ and one of the La₃IrO₇ polymorph.

  6. First-principles calculations of structural, elastic, thermodynamic, and electronic properties of anti-perovskites A III CNi3 (A III = Al, Ga, In)

    NASA Astrophysics Data System (ADS)

    Saadaoui, Fatiha; Driss Khodja, Fatima Zohra; Kadoun, Abd-Ed-Daïm; Driss Khodja, Mohammed; Elias, Abdelkader; Boudali, Abdelkader

    2015-12-01

    We have performed first-principles calculations of structural, elastic, thermodynamic, and electronic properties of anti-perovskites AIIICNi3 (AIII = Al, Ga, In), by using the full-potential linearized augmented plane wave (FP-LAPW) method combined with the quasi-harmonic Debye model. We carried out our calculations within the local density approximation (LDA) and the generalized gradient approximation (GGA-PBE and GGA-PBEsol functionals). Our results constitute interesting first predictions in the case of many elastic parameters of the anti-perovskites AIIICNi3, among them elastic parameters of AlCNi3 and GaCNi3 and some polycrystalline elastic parameters of InCNi3. We also report for the first time calculated values, at ambient conditions, of Grüneisen parameter, thermal expansion coefficient, specific heat at constant pressure, specific heat at constant volume, isothermal bulk modulus, and adiabatic bulk modulus for AlCNi3, GaCNi3, and InCNi3. Band structure, total and partial densities of states, and charge density have been obtained and analyzed. Electronic structure results show metallic behavior for the three compounds. Ni 3 d states play dominant role near the Fermi level and there is a strong hybridization between Ni 3 d and C 2 p states. In addition, as AIIICNi3 synthesized samples are expected to be carbon-deficient, we calculated structural, elastic, and thermodynamic properties of sub-stoichiometric AlC x Ni3 materials.

  7. Structural transformation during Li/Na insertion and theoretical cyclic voltammetry of the δ-NH4V4O10 electrode: a first-principles study.

    PubMed

    Sarkar, Tanmay; Kumar, Parveen; Bharadwaj, Mridula Dixit; Waghmare, Umesh

    2016-04-14

    A double layer δ-NH4V4O10, due to its high energy storage capacity and excellent rate capability, is a very promising cathode material for Li-ion and Na-ion batteries for large-scale renewable energy storage in transportation and smart grids. While it possesses better stability, and higher ionic and electronic conductivity than the most widely explored V2O5, the mechanisms of its cyclability are yet to be understood. Here, we present a theoretical cyclic voltammetry as a tool based on first-principles calculations, and uncover structural transformations that occur during Li(+)/Na(+) insertion (x) into (Lix/Nax)NH4V4O10. Structural distortions associated with single-phase and multi-phase structural changes during the insertion of Li(+)/Na(+), identified through the analysis of voltage profile and theoretical cyclic voltammetry are in agreement with the reported experimental electrochemical measurements on δ-NH4V4O10. We obtain an insight into its electronic structure with a lower band gap that is responsible for the high rate capability of (Lix/Nax) δ-NH4V4O10. The scheme of theoretical cyclic voltammetry presented here will be useful for addressing issues of cyclability and energy rate in other electrode materials. PMID:26996324

  8. Contact Prediction for Beta and Alpha-Beta Proteins Using Integer Linear Optimization and its Impact on the First Principles 3D Structure Prediction Method ASTRO-FOLD

    PubMed Central

    Rajgaria, R.; Wei, Y.; Floudas, C. A.

    2010-01-01

    An integer linear optimization model is presented to predict residue contacts in β, α + β, and α/β proteins. The total energy of a protein is expressed as sum of a Cα – Cα distance dependent contact energy contribution and a hydrophobic contribution. The model selects contacts that assign lowest energy to the protein structure while satisfying a set of constraints that are included to enforce certain physically observed topological information. A new method based on hydrophobicity is proposed to find the β-sheet alignments. These β-sheet alignments are used as constraints for contacts between residues of β-sheets. This model was tested on three independent protein test sets and CASP8 test proteins consisting of β, α + β, α/β proteins and was found to perform very well. The average accuracy of the predictions (separated by at least six residues) was approximately 61%. The average true positive and false positive distances were also calculated for each of the test sets and they are 7.58 Å and 15.88 Å, respectively. Residue contact prediction can be directly used to facilitate the protein tertiary structure prediction. This proposed residue contact prediction model is incorporated into the first principles protein tertiary structure prediction approach, ASTRO-FOLD. The effectiveness of the contact prediction model was further demonstrated by the improvement in the quality of the protein structure ensemble generated using the predicted residue contacts for a test set of 10 proteins. PMID:20225257

  9. Functionalization-induced changes in the structural and physical properties of amorphous polyaniline: a first-principles and molecular dynamics study

    PubMed Central

    Chen, X. P.; Liang, Q. H.; Jiang, J. K.; Wong, Cell K. Y.; Leung, Stanley Y. Y.; Ye, H. Y.; Yang, D. G.; Ren, T. L.

    2016-01-01

    In this paper, we present a first-principles and molecular dynamics study to delineate the functionalization-induced changes in the local structure and the physical properties of amorphous polyaniline. The results of radial distribution function (RDF) demonstrate that introducing -SO3−Na+ groups at phenyl rings leads to the structural changes in both the intrachain and interchain ordering of polyaniline at shorter distances (≤5 Å). An unique RDF feature in 1.8–2.1 Å regions is usually observed in both the interchain and intrachain RDF profiles of the -SO3−Na+ substituted polymer (i.e. Na-SPANI). Comparative studies of the atom-atom pairs, bond structures, torsion angles and three-dimensional structures show that EB-PANI has much better intrachain ordering than that of Na-SPANI. In addition, investigation of the band gap, density of states (DOS), and absorption spectra indicates that the derivatization at ring do not substantially alter the inherent electronic properties but greatly change the optical properties of polyaniline. Furthermore, the computed diffusion coefficient of water in Na-SPANI is smaller than that of EB-PANI. On the other hand, the Na-SPANI shows a larger density than that of EB-PANI. The computed RDF profiles, band gaps, absorption spectra, and diffusion coefficients are in quantitative agreement with the experimental data. PMID:26857962

  10. First-principles prediction of the mechanical properties and electronic structure of ternary aluminum carbide Zr{sub 3}Al{sub 3}C{sub 5}

    SciTech Connect

    Wang Jingyang; Zhou Yanchun; Lin Zhijun; Liao Ting; He Lingfeng

    2006-04-01

    In this paper, we predicted the possible mechanical properties and presented the electronic structure of Zr{sub 3}Al{sub 3}C{sub 5} by means of first-principles pseudopotential total energy method. The equation of state, elastic parameters (including the full set of second order elastic coefficients, bulk and shear moduli, Young's moduli, and Poisson's ratio), and ideal tensile and shear strengths are reported and compared with those of the binary compound ZrC. Furthermore, the bond relaxation and bond breaking under tensile and shear deformation from elasticity to structural instability are illustrated. Because shear induced bond breaking occurs inside the NaCl-type ZrC{sub x} slabs, the ternary carbide is expected to have high hardness and strength, which are related to structural instability under shear deformation, similar to the binary carbide. In addition, mechanical properties are interpreted by analyzing the electronic structure and chemical bonding characteristics accompanying deformation paths. Based on the present results, Zr{sub 3}Al{sub 3}C{sub 5} is predicted to be useful as a hard ceramic for high temperature applications.

  11. Cu-Au, Ag-Au, Cu-Ag, and Ni-Au intermetallics: First-principles study of temperature-composition phase diagrams and structures

    NASA Astrophysics Data System (ADS)

    Ozoliņš, V.; Wolverton, C.; Zunger, Alex

    1998-03-01

    The classic metallurgical systems-noble-metal alloys-that have formed the benchmark for various alloy theories are revisited. First-principles fully relaxed general-potential linearized augmented plane-wave (LAPW) total energies of a few ordered structures are used as input to a mixed-space cluster expansion calculation to study the phase stability, thermodynamic properties, and bond lengths in Cu-Au, Ag-Au, Cu-Ag, and Ni-Au alloys. (i) Our theoretical calculations correctly reproduce the tendencies of Ag-Au and Cu-Au to form compounds and Ni-Au and Cu-Ag to phase separate at T=0 K. (ii) Of all possible structures, Cu3Au (L12) and CuAu (L10) are found to be the most stable low-temperature phases of Cu1-xAux with transition temperatures of 530 K and 660 K, respectively, compared to the experimental values 663 K and ~670 K. The significant improvement over previous first-principles studies is attributed to the more accurate treatment of atomic relaxations in the present work. (iii) LAPW formation enthalpies demonstrate that L12, the commonly assumed stable phase of CuAu3, is not the ground state for Au-rich alloys, but rather that ordered (100) superlattices are stabilized. (iv) We extract the nonconfigurational (e.g., vibrational) entropies of formation and obtain large values for the size-mismatched systems: 0.48 kB/atom in Ni0.5Au0.5 (T=1100 K), 0.37 kB/atom in Cu0.141Ag0.859 (T=1052 K), and 0.16 kB/atom in Cu0.5Au0.5 (T=800 K). (v) Using 8 atom/cell special quasirandom structures we study the bond lengths in disordered Cu-Au and Ni-Au alloys and obtain good qualitative agreement with recent extended x-ray-absorption fine-structure measurements.

  12. First-principles insights into the structure of the incipient magnesium oxide and its instability to decomposition: Oxygen chemisorption to Mg(0001) and thermodynamic stability

    NASA Astrophysics Data System (ADS)

    Francis, M. F.; Taylor, C. D.

    2013-02-01

    In this paper, a detailed density functional theory analysis of oxygen binding to Mg(0001) and subsequent clustering is presented. Oxygen monomer adsorption to Mg(0001) is demonstrated to be subsurface. It is shown that magnesium mediates an attractive oxygen-oxygen interaction which ultimately leads to the formation of hexagonal clusters of O* in the tetrahedral-1 site. The structure, work function, and binding properties of oxygen chemisorbed structures are compared with experiment, which allows the unique identification of the tetrahedral-1 site as the low coverage oxygen binding site and the construction of a picture of the early stages of oxide nuclei formation over magnesium. A model of oxide growth at O*/Mg(0001) is proposed. First-principles thermodynamics analysis is used to describe the surface oxide structures and reveals that surface oxides of intermediate oxygen coverage undergo spinodal decomposition. The thermodynamics of an underlying spinodal create an energetic driving force for decomposition of an oxide surface and renewal of a reactive metal interface that may be important in understanding magnesium corrosion. The implications of the findings are that magnesium unalloyed for oxide behavior will always be highly vulnerable to corrosion.

  13. Insight into structural, mechanical, electronic and thermodynamic properties of intermetallic phases in Zr-Sn system from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Liu, Shuai; Zhan, Yongzhong; Wu, Junyan; Wei, Xuanchen

    2015-11-01

    The structural, phase stabilities, mechanical, electronic and thermodynamic properties of intermetallic phases in Zr-Sn system are investigated by using first-principles method. The equilibrium lattice constants, enthalpy of formation (ΔHform) and elastic constants are obtained and compared with available experimental and theoretical data. The configuration of Zr4Sn is measured with reasonable precision. The ΔHform of five hypothetical structures are obtained in order to find possible metastable phase for Zr-Sn system. The mechanical properties, including bulk modulus, shear modulus, Young's modulus and Poisson's ratio, are calculated by Voigt-Reuss-Hill approximation and the Zr5Sn4 and Zr5Sn3 show excellent mechanical properties. The electronic density of states for Zr5Sn4, Zr5Sn3 and cP8-Zr3Sn are calculated to further investigate the stability of intermetallic compounds. Through the quasi-harmonic Debye model, the Debye temperature, heat capacity and thermal expansion coefficient under temperature of 0-300 K and pressure of 0-50 GPa for Zr5Sn3 and Zr5Sn4 are deeply investigated.

  14. Electronic structures and elastic properties of monolayer and bilayer transition metal dichalcogenides MX2 (M = Mo, W; X = O, S, Se, Te): A comparative first-principles study

    NASA Astrophysics Data System (ADS)

    Zeng, Fan; Zhang, Wei-Bing; Tang, Bi-Yu

    2015-09-01

    First-principle calculations with different exchange-correlation functionals, including LDA, PBE, and vdW-DF functional in the form of optB88-vdW, have been performed to investigate the electronic and elastic properties of two-dimensional transition metal dichalcogenides (TMDCs) with the formula of MX2(M = Mo, W; X = O, S, Se, Te) in both monolayer and bilayer structures. The calculated band structures show a direct band gap for monolayer TMDCs at the K point except for MoO2 and WO2. When the monolayers are stacked into a bilayer, the reduced indirect band gaps are found except for bilayer WTe2, in which the direct gap is still present at the K point. The calculated in-plane Young moduli are comparable to that of graphene, which promises possible application of TMDCs in future flexible and stretchable electronic devices. We also evaluated the performance of different functionals including LDA, PBE, and optB88-vdW in describing elastic moduli of TMDCs and found that LDA seems to be the most qualified method. Moreover, our calculations suggest that the Young moduli for bilayers are insensitive to stacking orders and the mechanical coupling between monolayers seems to be negligible. Project supported by the Construct Program of the Key Discipline in Hunan Province, China and Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, China.

  15. Effect of chemical and hydrostatic pressures on structural and magnetic properties of rare-earth orthoferrites: a first-principles study.

    PubMed

    Zhao, Hong Jian; Ren, Wei; Yang, Yurong; Chen, Xiang Ming; Bellaiche, L

    2013-11-20

    The dependence of structural and magnetic properties of rare-earth orthoferrites (in their Pbnm ground state) on the rare-earth ionic radius is systematically investigated from first principles. The effects of this 'chemical pressure' on lattice constants, Fe-O bond lengths, Fe-O-Fe bond angles and Fe-O bond length splittings are all well reproduced by these ab initio calculations. The simulations also offer novel predictions (on tiltings of FeO6 octahedra, cation antipolar displacements and weak magnetization) to be experimentally checked. In particular, the weak ferromagnetic moment of rare-earth orthoferrites is predicted to be a linear function of the rare-earth ionic radius. Finally, the effects of applying hydrostatic pressure on structural and magnetic behavior of SmFeO3 is also studied. It is found that, unlike previously assumed, hydrostatic pressure typically generates changes in physical properties that are quantitatively and even qualitatively different from those associated with the chemical pressure. PMID:24135000

  16. First-principles calculation of the structural, electronic, elastic, and optical properties of sulfur-doping ε-GaSe crystal

    NASA Astrophysics Data System (ADS)

    Huang, Chang-Bao; Wu, Hai-Xin; Ni, You-Bao; Wang, Zhen-You; Qi, Ming; Zhang, Chun-Li

    2016-08-01

    The structural, electronic, mechanical properties, and frequency-dependent refractive indexes of GaSe1–x S x (x = 0, 0.25, and 1) are studied by using the first-principles pseudopotential method within density functional theory. The calculated results demonstrate the relationships between intralayer structure and elastic modulus in GaSe1–x S x (x = 0, 0.25, and 1). Doping of ε-GaSe with S strengthens the Ga–X bonds and increases its elastic moduli of C 11 and C 66. Born effective charge analysis provides an explanation for the modification of cleavage properties about the doping of ε-GaSe with S. The calculated results of band gaps suggest that the distance between intralayer atom and substitution of SSe, rather than interlayer force, is a key factor influencing the electronic exciton energy of the layer semiconductor. The calculated refractive indexes indicate that the doping of ε-GaSe with S reduces its refractive index and increases its birefringence. Project supported by the National Natural Science Foundation of China (Grant No. 51202250).

  17. First-principles calculations for the structural, elastic and thermodynamic properties of cubic perovskite BaHfO3 under pressure

    NASA Astrophysics Data System (ADS)

    Gu, Fang; Chen, Yun-Yun; Zhang, Xian-Ling; Zhang, Jia-Hong; Liu, Qing-Quan

    2014-10-01

    The structural, single-crystal and polycrystalline elastic and thermodynamic properties of cubic perovskite BaHfO3 under pressure were investigated using the first-principles total energy calculations in the frame of the generalized gradient approximation (GGA) combined with the quasi-harmonic Debye model in which the phonon effects are considered. The calculated ground-state quantities, such as the lattice constant, Young’s modulus, shear modulus, shear and longitudinal sound velocities and Debye temperature, were in reasonable agreement with previous theoretical and experimental data. Based on the elastic constants, bulk modulus, shear modulus and Young’s modulus, the structural stability, hardness, stiffness and the brittle and ductile behaviors, along with the binding characteristic of BaHfO3 under pressure effects, have been discussed. More importantly, the temperature and pressure dependencies of the lattice constant, bulk modulus, the Debye temperature, heat capacities, volume expansion coefficient and lattice thermal conductivity are predicted successfully in the wide temperature and pressure ranges. It was found that the effects of pressure and temperature are inversely proportional. The obtained specific heat capacities at constant pressure, at the thermal expansion coefficient and at the thermal conductivity match well with the experimental data available in the range of 300-1300 K.

  18. First-principles study of the structural, electronic, dynamical, and thermodynamic properties of Li5AlO4

    NASA Astrophysics Data System (ADS)

    Guan, Qiushi; Chen, Xiaojun; Gao, Tao; Xiao, Chengjian; Zhao, Linjie; He, Jianchao; Long, Xinggui

    2015-10-01

    Pentalithium aluminate, Li5AlO4, has attracted increasing attention for its high lithium density and potential uses in tritium breeding materials and thermal batteries. In this work, the structural, electronic, lattice dynamical, and thermodynamic properties of α- and β-phase Li5AlO4 were investigated using first-principles density functional theory. The optimized structural parameters were consistent with the experimental values, with the absolute deviation being less than 2.5%. The indirect band gaps of α- and β-Li5AlO4 were 4.82 and 5.16 eV, respectively, showing that they are insulators. In addition, the vibrational properties of α- and β-Li5AlO4 were computed using density functional perturbation theory. By adding Born effective charges into the phonon calculations, the longitudinal optical-transverse optical (LO-TO) splittings were calculated. The optical modes at the Γ point were categorized as Raman- and IR-active modes. Our results show that β-Li5AlO4 is more polar and anisotropic than α-Li5AlO4. Furthermore, their thermodynamic functions were determined using the calculated phonon density of states. The results were in good agreement with those of previous theoretical studies. The data presented in this work will help in the further characterization of Li5AlO4, which may be valuable for future experimental studies.

  19. Structures and Electronic Properties of Different CH3NH3PbI3/TiO2 Interface: A First-Principles Study

    NASA Astrophysics Data System (ADS)

    Geng, Wei; Tong, Chuan-Jia; Liu, Jiang; Zhu, Wenjun; Lau, Woon-Ming; Liu, Li-Min

    2016-02-01

    Methylammonium lead iodide perovskite, CH3NH3PbI3, has attracted particular attention due to its fast increase in efficiency in dye sensitization TiO2 solid-state solar cells. We performed first-principles calculations to investigate several different types of CH3NH3PbI3/TiO2 interfaces. The interfacial structures between the different terminated CH3NH3PbI3 and phase TiO2 are thoroughly explored, and the calculated results suggest that the interfacial Pb atoms play important roles in the structure stability and electronic properties. A charge transfer from Pb atoms to the O atoms of TiO2 lead to the band edge alignment of Pb-p above Ti-d about 0.4 eV, suggesting a better carries separation. On the other hand, for TiO2, rutile (001) is the better candidate due to the better lattice and atoms arrangement match with CH3NH3PbI3.

  20. Structures and Electronic Properties of Different CH3NH3PbI3/TiO2 Interface: A First-Principles Study.

    PubMed

    Geng, Wei; Tong, Chuan-Jia; Liu, Jiang; Zhu, Wenjun; Lau, Woon-Ming; Liu, Li-Min

    2016-01-01

    Methylammonium lead iodide perovskite, CH3NH3PbI3, has attracted particular attention due to its fast increase in efficiency in dye sensitization TiO2 solid-state solar cells. We performed first-principles calculations to investigate several different types of CH3NH3PbI3/TiO2 interfaces. The interfacial structures between the different terminated CH3NH3PbI3 and phase TiO2 are thoroughly explored, and the calculated results suggest that the interfacial Pb atoms play important roles in the structure stability and electronic properties. A charge transfer from Pb atoms to the O atoms of TiO2 lead to the band edge alignment of Pb-p above Ti-d about 0.4 eV, suggesting a better carries separation. On the other hand, for TiO2, rutile (001) is the better candidate due to the better lattice and atoms arrangement match with CH3NH3PbI3. PMID:26846401

  1. Structures and Electronic Properties of Different CH3NH3PbI3/TiO2 Interface: A First-Principles Study

    PubMed Central

    Geng, Wei; Tong, Chuan-Jia; Liu, Jiang; Zhu, Wenjun; Lau, Woon-Ming; Liu, Li-Min

    2016-01-01

    Methylammonium lead iodide perovskite, CH3NH3PbI3, has attracted particular attention due to its fast increase in efficiency in dye sensitization TiO2 solid-state solar cells. We performed first-principles calculations to investigate several different types of CH3NH3PbI3/TiO2 interfaces. The interfacial structures between the different terminated CH3NH3PbI3 and phase TiO2 are thoroughly explored, and the calculated results suggest that the interfacial Pb atoms play important roles in the structure stability and electronic properties. A charge transfer from Pb atoms to the O atoms of TiO2 lead to the band edge alignment of Pb-p above Ti-d about 0.4 eV, suggesting a better carries separation. On the other hand, for TiO2, rutile (001) is the better candidate due to the better lattice and atoms arrangement match with CH3NH3PbI3. PMID:26846401

  2. First principles studies of structural, electronic, optical, elastic and thermal properties of Ag-chalcopyrites (AgInX2: X=S, Se)

    NASA Astrophysics Data System (ADS)

    Sharma, Sheetal; Verma, A. S.; Jindal, V. K.

    2014-04-01

    First principles calculations for the structural, electronic, optical, elastic and thermal properties of the silver indium dichalcogenides (AgInX2: X=S and Se) have been reported using the full potential linearized augmented plane wave (FP-LAPW) method. In this approach, the recently developed density functional theory of Tran and Blaha is used along with the Wu-Cohen generalized gradient approximation (WC-GGA) for the exchange-correlation potential. Results were given for lattice constants, bulk modulus and its pressure derivative, band structures, dielectric constants and refractive indices. We have also computed the full elastic tensors (C11, C12, C13, C33, C44 and C66). The thermodynamical properties such as thermal expansion, heat capacity, Debye temperature, entropy, Grüneisen parameter, bulk modulus and hardness were calculated employing the quasi-harmonic Debye model at different temperatures (0-800 K) and pressures (0-8 GPa). Most of the investigated parameters are reported for the first time.

  3. Band structure, Fermi surface, elastic, thermodynamic, and optical properties of AlZr 3 , AlCu 3 , and AlCu 2 Zr: First-principles study

    NASA Astrophysics Data System (ADS)

    Parvin, R.; Parvin, F.; Ali, M. S.; Islam, A. K. M. A.

    2016-08-01

    The electronic properties (Fermi surface, band structure, and density of states (DOS)) of Al-based alloys AlM 3 (M = Zr and Cu) and AlCu2Zr are investigated using the first-principles pseudopotential plane wave method within the generalized gradient approximation (GGA). The structural parameters and elastic constants are evaluated and compared with other available data. Also, the pressure dependences of mechanical properties of the compounds are studied. The temperature dependence of adiabatic bulk modulus, Debye temperature, specific heat, thermal expansion coefficient, entropy, and internal energy are all obtained for the first time through quasi-harmonic Debye model with phononic effects for T = 0 K–100 K. The parameters of optical properties (dielectric functions, refractive index, extinction coefficient, absorption spectrum, conductivity, energy-loss spectrum, and reflectivity) of the compounds are calculated and discussed for the first time. The reflectivities of the materials are quite high in the IR–visible–UV region up to ∼ 15 eV, showing that they promise to be good coating materials to avoid solar heating. Some of the properties are also compared with those of the Al-based Ni3Al compound.

  4. Electronic structure and topological features of tin-based binary nanosheets and their hydrogenated/fluorinated derivatives: A first-principles study

    NASA Astrophysics Data System (ADS)

    Wang, Yanli; Ding, Yi

    2016-09-01

    Utilizing first-principles calculations, we have investigated the structural, electronic and topological properties of binary SnSi and SnGe nanosheets as well as their H-/F-derivatives. It is found that all these systems have chair-like buckled configurations with robust structural stabilities. Unlike the elemental group-IV sheets, SnSi and SnGe sheets are narrow-band-gap semiconductors, which have a gapped Dirac cone with massive Fermions. Under the strains, a direct-to-indirect band gap transition and a semiconductor-to-metal transition would occur in these systems. Although these binary systems are trivial band insulators with ℤ2 = 0, their topological features can be altered by the surface decorations. Particularly, the fluorinated SiGe sheet becomes a topological insulator with ℤ2 = 1, and such non-trivial state can also be found in the strained fluorinated SnSi and hydrogenated SnGe sheets. Large non-trivial SOC band gaps of 0.09-0.17 eV are obtained in these 2D topological insulating systems, which will be beneficial for realizing the room-temperature quantum spin Hall effect. Our study demonstrates that binary Sn-based nanomaterials possess tunable electronic and topological properties, which have potential applications in low-power nano-electrics and devices.

  5. Structural and Thermodynamic Properties of TiC x N y O z Solid Solution: Experimental Study and First-Principles Approaches

    NASA Astrophysics Data System (ADS)

    Xiao, Jiusan; Jiang, Bo; Huang, Kai; Jiao, Shuqiang; Zhu, Hongmin

    2016-06-01

    A series of TiC x N y O z solid solutions were synthesized via solid-state reaction and XRD patterns exhibited a single phase of FCC structure over the whole concentration range. The structural and thermodynamic properties of TiC x N y O z solid solutions were studied using experimental method and first-principles calculations. The difference between the calculated and experimental lattice parameters could be attributed to the vacancies segregated in TiO part. The fitting formulae for lattice parameters and mixing enthalpies were firstly given for TiC x N y O z solid solution over the whole concentration range. The obtained thermodynamic data for TiC x N y O z solid solution properly explained the reaction sequence of the carbothermal reduction of TiO2, providing theoretical foundation for TiC x N y O z solid solution as a kind of prospective material for consuming anode utilized in USTB titanium electrolysis process.

  6. Theoretical analysis of structure and formation energy of impurity-doped Mg2Si: Comparison of first-principles codes for material properties

    NASA Astrophysics Data System (ADS)

    Hirayama, Naomi; Iida, Tsutomu; Funashima, Hiroki; Morioka, Shunsuke; Sakamoto, Mariko; Nishio, Keishi; Kogo, Yasuo; Takanashi, Yoshifumi; Hamada, Noriaki

    2015-07-01

    We theoretically investigate the impurity doping effects on the structural parameters such as lattice constant, atomic positions, and site preferences of impurity dopants for Al-doped magnesium silicide (Mg2Si) crystal using the first-principles calculation methods. We present comparison between several codes: ABCAP, Quantum Espresso, and Machikaneyama2002 (Akai KKR), which are based on the full-potential linearized augmented plane-wave method, the pseudopotential method, and KKR/GGA Green’s function method, respectively. As a result, any codes used in the present study exhibit qualitative consistency both in the dependence of the lattice constants on the doping concentration and the energetic preference of the Al atom for the following sites; substitutional Si and Mg sites, and interstitial 4b site; in particular, ABCAP, which is based on the all-electron full-potential method, and Quantum Espresso, which is a code of the pseudopotential method, produce closely-resemble calculation results. We also discuss the effects of local atomic displacement owing to the presence of impurities to the structural parameters of a bulk. Using the analytical method considering the local atomic displacement, moreover, we evaluate the formation energy of Na- and B-doped systems as examples of p-type doping in order to examine the possilbility of realizing p-type Mg2Si.

  7. Structural and Thermodynamic Properties of TiC x N y O z Solid Solution: Experimental Study and First-Principles Approaches

    NASA Astrophysics Data System (ADS)

    Xiao, Jiusan; Jiang, Bo; Huang, Kai; Jiao, Shuqiang; Zhu, Hongmin

    2016-09-01

    A series of TiC x N y O z solid solutions were synthesized via solid-state reaction and XRD patterns exhibited a single phase of FCC structure over the whole concentration range. The structural and thermodynamic properties of TiC x N y O z solid solutions were studied using experimental method and first-principles calculations. The difference between the calculated and experimental lattice parameters could be attributed to the vacancies segregated in TiO part. The fitting formulae for lattice parameters and mixing enthalpies were firstly given for TiC x N y O z solid solution over the whole concentration range. The obtained thermodynamic data for TiC x N y O z solid solution properly explained the reaction sequence of the carbothermal reduction of TiO2, providing theoretical foundation for TiC x N y O z solid solution as a kind of prospective material for consuming anode utilized in USTB titanium electrolysis process.

  8. Core-hole effect on XANES and electronic structure of minor actinide dioxides with fluorite structure

    NASA Astrophysics Data System (ADS)

    Suzuki, Chikashi; Nishi, Tsuyoshi; Nakada, Masami; Akabori, Mitsuo; Hirata, Masaru; Kaji, Yoshiyuki

    2012-02-01

    The authors investigated theoretically core-hole effects on X-ray absorption near-edge structures (XANES) of Np and Am LIII in neptunium dioxide (NpO2) and americium dioxide (AmO2) with CaF2-type crystal lattices using the all-electron full-potential linearized augmented plane-wave (FP-LAPW) method. The peak creation mechanism of XANES was shown by examining the electronic structures of these oxides, which indicated that core-hole screening was more marked for AmO2 than for NpO2 because of the difference in the charge transfer between these oxides. Furthermore, the results of charge density analysis suggested that the white line was assigned to the quasi-bound state composed of the localized Np d or Am d components and O components, and that the tail structure was created as a result of delocalized standing waves between the Np or Am atoms.

  9. Structural, electronic, and magnetic properties of tris(8-hydroxyquinoline)iron(III) molecules and their magnetic coupling with ferromagnetic surface: first-principles study.

    PubMed

    Jiang, W; Zhou, M; Liu, Z; Sun, D; Vardeny, Z V; Liu, F

    2016-05-01

    Using first-principles calculations, we have systematically investigated the structural, electronic, and magnetic properties of facial (fac-) and meridional (mer-) tris(8-hydroxyquinoline)iron(III) (Feq3) molecules and their interaction with ferromagnetic substrate. Our calculation results show that for the isolated Feq3, mer-Feq3 is more stable than the fac-Feq3; both Feq3 isomers have a high spin-state of 5 μ B as the ground state when an on-site Hubbard-U term is included to treat the highly localized Fe 3d electrons; while the standard DFT calculations produce a low spin-state of 1 μ B for mer-Feq3. These magnetic behaviors can be understood by the octahedral ligand field splitting theory. Furthermore, we found that fac-Feq3 has a stronger bonding to the Co surface than mer-Feq3 and an anti-ferromagnetic coupling was discovered between Fe and Co substrate, originating from the superexchange coupling between Fe and Co mediated by the interface oxygen and nitrogen atoms. These findings suggest that Feq3 molecular films may serve as a promising spin-filter material in spintronic devices. PMID:27044670

  10. Structural, electronic, and magnetic properties of tris(8-hydroxyquinoline)iron(III) molecules and their magnetic coupling with ferromagnetic surface: first-principles study

    NASA Astrophysics Data System (ADS)

    Jiang, W.; Zhou, M.; Liu, Z.; Sun, D.; Vardeny, Z. V.; Liu, F.

    2016-05-01

    Using first-principles calculations, we have systematically investigated the structural, electronic, and magnetic properties of facial (fac-) and meridional (mer-) tris(8-hydroxyquinoline)iron(III) (Feq3) molecules and their interaction with ferromagnetic substrate. Our calculation results show that for the isolated Feq3, mer-Feq3 is more stable than the fac-Feq3; both Feq3 isomers have a high spin-state of 5 μ B as the ground state when an on-site Hubbard-U term is included to treat the highly localized Fe 3d electrons; while the standard DFT calculations produce a low spin-state of 1 μ B for mer-Feq3. These magnetic behaviors can be understood by the octahedral ligand field splitting theory. Furthermore, we found that fac-Feq3 has a stronger bonding to the Co surface than mer-Feq3 and an anti-ferromagnetic coupling was discovered between Fe and Co substrate, originating from the superexchange coupling between Fe and Co mediated by the interface oxygen and nitrogen atoms. These findings suggest that Feq3 molecular films may serve as a promising spin-filter material in spintronic devices.

  11. First-principles, structure-based transdermal transport model to evaluate lipid partition and diffusion coefficients of hydrophobic permeants solely from stratum corneum permeation experiments.

    PubMed

    Kushner, Joseph; Deen, William; Blankschtein, Daniel; Langer, Robert

    2007-12-01

    To account for the effect of branched, parallel transport pathways in the intercellular domain of the stratum corneum (SC) on the passive transdermal transport of hydrophobic permeants, we have developed, from first-principles, a new theoretical model-the Two-Tortuosity Model. This new model requires two tortuosity factors to account for: (1) the effective diffusion path length, and (2) the total volume of the branched, parallel transport pathways present in the SC intercellular domain, both of which may be evaluated from known values of the SC structure. After validating the Two-Tortuosity model with simulated SC diffusion experiments in FEMLAB (a finite element software package), the vehicle-bilayer partition coefficient, K(b), and the lipid bilayer diffusion coefficient, D(b), in untreated human SC were evaluated using this new model for two hydrophobic permeants, naphthol (K(b) = 225 +/- 42, D(b) = 1.7 x 10(-7) +/- 0.3 x 10(-7) cm(2)/s) and testosterone (K(b) = 92 +/- 29, D(b) = 1.9 x 10(-8) +/- 0.5 x 10(-8) cm(2)/s). The results presented in this paper demonstrate that this new method to evaluate K(b) and D(b) is comparable to, and simpler than, previous methods, in which SC permeation experiments were combined with octanol-water partition experiments, or with SC solute release experiments, to evaluate K(b) and D(b). PMID:17887175

  12. Structural stabilities, surface morphologies and electronic properties of spinel LiTi2O4 as anode materials for lithium-ion battery: A first-principles investigation

    NASA Astrophysics Data System (ADS)

    Wang, Qi; Yu, Hai-Tao; Xie, Ying; Li, Ming-Xia; Yi, Ting-Feng; Guo, Chen-Feng; Song, Qing-Shan; Lou, Ming; Fan, Shan-Shan

    2016-07-01

    The thermodynamic stabilities, surface morphologies, and electronic structures of the LiTi2O4 compound were investigated by the first-principles methods. The formation enthalpies and lattice constants of LixTi2O4 decrease at first and then increase again. This phenomenon is related to the balance between Lisbnd O attractions and Lisbnd Li repulsions. Population analysis revealed that pure ionic and strong covalent bonds are formed respectively between lithium and oxygen and between titanium and oxygen in LiTi2O4 material. These interactions are very crucial for the thermodynamic stability of the compounds. The surface stability was considered as functions of the chemical potentials, and five terminations, (100)-Ti2O4, (110)-Ti2O4, (210)-Ti2O4, (111)-LiTiO4, and (310)-Ti2O8ones, are dominant in the stability diagram. Our calculation showed that a particle morphology with mono (110) facet can be obtained at Ti- and/or O-moderate conditions, and this morphology will be very helpful for improving the rate performance of the material via reduction of the lithium diffusion distance. Furthermore, partially filled electronic states at the Fermi energy were confirmed for bulk LiTi2O4 and some of the surfaces, and they are responsible for the excellent electronic conductivity of the material. Further calculations showed that the work functions are sensitive to the stoichiometry of the surfaces.

  13. Band crossing in isovalent semiconductor alloys with large size mismatch: First-principles calculations of the electronic structure of Bi and N incorporated GaAs

    NASA Astrophysics Data System (ADS)

    Deng, Hui-Xiong; Li, Jingbo; Li, Shu-Shen; Peng, Haowei; Xia, Jian-Bai; Wang, Lin-Wang; Wei, Su-Huai

    2010-11-01

    For large size- and chemical-mismatched isovalent semiconductor alloys, such as N and Bi substitution on As sites in GaAs, isovalent defect levels or defect bands are introduced. The evolution of the defect states as a function of the alloy concentration is usually described by the popular phenomenological band anticrossing (BAC) model. Using first-principles band-structure calculations we show that at the impurity limit the N- (Bi)-induced impurity level is above (below) the conduction- (valence-) band edge of GaAs. These trends reverse at high concentration, i.e., the conduction-band edge of GaAs1-xNx becomes an N-derived state and the valence-band edge of GaAs1-xBix becomes a Bi-derived state, as expected from their band characters. We show that this band crossing phenomenon cannot be described by the popular BAC model but can be naturally explained by a simple band broadening picture.

  14. Electronic band structure of LiInSe2: A first-principles study using the Tran-Blaha density functional and GW approximation

    NASA Astrophysics Data System (ADS)

    Kosobutsky, A. V.; Basalaev, Yu. M.

    2014-12-01

    Using first-principles theoretical techniques within density functional theory and many-body perturbation theory we investigated the structural and electronic properties of two LiInSe2 crystal modifications, orthorhombic (β-NaFeO2-type) and tetragonal (CuFeS2-type), focusing on the interband transitions and band gaps. It is found that the Tran-Blaha (TB09) functional predicts LiInSe2 to be a direct-gap semiconductor with a significantly larger band gap as compared with that from common local-density and gradient-corrected functionals. The most accurate values of the fundamental energy gaps are calculated within quasiparticle GW approximation and found to be 2.95 eV for the orthorhombic phase and 2.85 eV for the tetragonal one, with equal pressure coefficients of 63 meV/GPa. Our theoretical results eliminate the uncertainty in the band gap of LiInSe2. Moreover, the data obtained define the upper limit of the band gap of solid solutions (Cu,Li)InSe2 and (Ag,Li)InSe2, which can be of interest for applications in optoelectronics.

  15. First-principles study of strain effect on the formation and electronic structures of oxygen vacancy in SrFeO2

    NASA Astrophysics Data System (ADS)

    Wei, Zhang; Jie, Huang

    2016-05-01

    Motivated by recent experimental observations of metallic conduction in the quasi-two-dimensional SrFeO2, we study the epitaxial strain effect on the formation and electronic structures of oxygen vacancy (Vo) by first-principles calculations. The bulk SrFeO2 is found to have the G-type antiferromagnetic ordering (G-AFM) at zero strain, which agrees with the experiment. Under compressive strain the bulk SrFeO2 keeps the G-AFM and has the trend of Mott insulator-metal transition. Different from most of the previous similar work about the strain effect on Vo, both the tensile strain and the compressive strain enhance the Vo formation. It is found that the competitions between the band energies and the electrostatic interactions are the dominant mechanisms in determining the Vo formation. We confirm that the Vo in SrFeO2 would induce the n-type conductivity where the donor levels are occupied by the delocalized d x 2‑y 2 electrons. It is suggested that the vanishing of n-type conductivity observed by the Hall measurement on the strained films are caused by the shift of donor levels into the conduction band. These results would provide insightful information for the realization of metallic conduction in SrFeO2. Project supported by the Creative Plan Project of Nanjing Forest Police College, China (Grant Nos. 201512213045xy and 201512213007x).

  16. First-principles calculations of the electronic structure and magnetic properties of 3d transition-metal impurities in bcc and amorphous iron

    NASA Astrophysics Data System (ADS)

    Kontsevoi, O. Yu.; Gubanov, V. A.

    1995-06-01

    We present the results of the first-principles calculations of electronic structure, magnetic moments, and effective-exchange-interaction parameters for 3d impurities in ferromagnetic bcc and amorphous iron as obtained by the self-consistent tight-binding linear-muffin-tin-orbital recursion method. Impurities in bcc Fe have been modeled both in the single-site approximation and taking into account up to four shells of the nearest-to-the-impurity neighbors. The results for crystalline iron agree well with the previous more precise Korringa-Kohn-Rostoker Green's function calculations [Phys. Rev. B 40, 8203 (1989)], and confirm the sufficient accuracy of the method developed. The perturbations of electronic states for Fe atoms in different coordinational shells around impurity are considered. Peculiarities of impurity electronic states in amorphous Fe and their influence on magnetic behavior of the system are discussed. The role of impurities in possible stabilization of ferromagnetic ordering in amorphous Fe is investigated in terms of effective-exchange-interaction parameters calculated for the nearest-to-impurity host atoms.

  17. Pressure-induced structural and magnetic transitions in the infinite-chains iron oxide Sr2FeO3: a first-principle investigation

    NASA Astrophysics Data System (ADS)

    Gui, Hong; Li, Xin; Zhao, Zhenjie; Xie, Wenhui

    2016-02-01

    The pressure-induced transition of Sr2FeO3 was studied by first-principle calculation using density functional theory with the generalized gradient approximation plus on-site coulomb repulsion method. It shows that Sr2FeO3 exhibits a structure transition from Immm to Ammm and at about 35 GPa and then a spin transition from high spin S  =  2 to intermediate spin S  =  1. And it is also revealed that the pressure leads to a change in the Fe three-dimensional electronic configuration from ({{d}{{z2}}}\\uparrow )1({{d}yz}\\uparrow )1({{d}xz}\\uparrow )1({{d}xy}\\uparrow )1({{d}{{x2}-{{y}2}}}\\uparrow )1 ({{d}{{z2}}}\\downarrow )1 under ambient conditions to ({{d}{{z2}}}\\uparrow )1({{d}yz}\\uparrow )1({{d}xz}\\uparrow )1({{d}xy}\\uparrow )1 ({{d}{{x2}-{{y}2}}}\\uparrow ) δ ({{d}yz}\\downarrow )1 ({{d}{{z2}}}\\downarrow ) σ at high pressure, where δ plus σ equals 1.

  18. Crystallographic, magnetic, and electronic structures of ferromagnetic shape memory alloys Ni2XGa (X=Mn,Fe,Co) from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Bai, J.; Raulot, J. M.; Zhang, Y. D.; Esling, C.; Zhao, X.; Zuo, L.

    2011-01-01

    The crystallographic, magnetic and electronic structures of the ferromagnetic shape memory alloys Ni2XGa (X=Mn, Fe, and Co), are systematically investigated by means of the first-principles calculations within the framework of density functional theory using the VIENNA AB INITIO SOFTWARE PACKAGE. The lattice parameters of both austenitic and martensitic phases in Ni2MnGa have been calculated. The formation energies of the cubic phase of Ni2XGa are estimated, and show a destabilization tendency if Mn atom is substituted by Fe or Co. From Ni2MnGa to Ni2CoGa, the down spin total density of states (DOS) at Fermi level is gradually increasing, whereas that of the up spin part remains almost unchanged. This is the main origin of the difference of the magnetic moment in these alloys. The partial DOS is dominated by the Ni and Mn 3d states in the bonding region below EF. There are two bond types existing in Ni2XGa: one is between neighboring Ni atoms in Ni2MnGa; the other is between Ni and X atoms in Ni2FeGa and Ni2CoGa alloys.

  19. First-principles electronic structure and formation energies of group V and VII impurities in the α-Fe{sub 2}O{sub 3} alloys

    SciTech Connect

    Xia, Congxin; Jia, Yu; Zhang, Qiming

    2014-09-21

    Based on density functional theory, the electronic structures, formation energy, and transition level of the selected group V and VII impurities in α-Fe{sub 2}O{sub 3} are investigated by means of first-principles methods. Numerical results show that the group V and VII atoms-doped α-Fe{sub 2}O{sub 3} can be energetically favorable under the Fe-rich condition. Group V atom substituting O atom can induce the acceptor impurity level, while the deep donor impurity states are formed inside the band gap when group VII atom substitute O atom in the α-Fe{sub 2}O{sub 3}. Moreover, our results show that halogen atom F substituting O atom should be very easy in the α-Fe{sub 2}O{sub 3}. In addition, our results also show that for both group V and VII atom-doped α-Fe{sub 2}O{sub 3}, the upper sides of valence band are modified obviously, while the conduction band edge does not change.

  20. First principle study of structural, electronic and magnetic properties of half-Heusler IrCrZ (Z=Ge, As, sn and sb) compounds

    NASA Astrophysics Data System (ADS)

    Allaf Behbahani, Marzieh; Moradi, Mahmood; Rostami, Mohammad; Davatolhagh, Saeed

    2016-05-01

    First-principle calculations based on the density functional theory for new half-Heusler IrCrZ (Z=Ge, As, Sn and Sb) alloys are performed. It is found that the half-Heusler IrCrGe and IrCrSn compounds have an antiferromagnetic ground state while the ferromagnetic state is more stable than the antiferromagnetic and non-magnetic states for both IrCrAs and IrCrSb compounds. IrCrAs and IrCrSb exhibit half-metallic property with integer magnetic moments of 2.00 μB per formula unit and half-metallic gaps of 0.28 and 0.27 eV at their equilibrium volume, respectively. In addition, the density of states (DOSs) and band structures of IrCrAs and IrCrSb compounds are studied and the origin of their half-metallic gaps are discussed in detail. The estimation of Curie temperatures of IrCrAs and IrCrSb compounds is performed within the mean field approximation (MFA). The Curie temperatures of IrCrAs and IrCrSb are estimated to be 1083 and 1470 K, respectively. The stability of the half-metallicity in IrCrAs and IrCrSb compounds with the variation of lattice constant are also investigated.

  1. First-principles calculation of structural and energetic properties for A2Ti2O7 (A =Lu, Er, Y, Gd, Sm, Nd, La)

    SciTech Connect

    Zhang, ZL; Xiao, H. Y.; Zu, Xiaotao T.; Gao, Fei; Weber, William J.

    2009-04-01

    A first-principles method has been employed to investigate the structural and energetic properties for A2Ti2O7 (A=Lu, Er, Y, Gd, Sm, Nd, La), including the formation energies of the cation antisite-pair, the anion Frenkel pair that defines anion-disorder, and the coupled cation antisite-pair/anion-Frenkel. It is proposed that the interaction may have more significant influence on the radiation resistance behavior of titanate pyrochlores, although the interactions are relatively much stronger than the interactions. It is found that the defect formation energies are not simple functions of the A-site cation radii. The formation energy of the cation antisite-pair increases continuously as the A-site cation varies from Lu to Gd, and then decreases continuously with the variation of the A-site cation from Gd to La, in excellent agreement with the radiation-resistance trend of the titanate pyrochlores. The band gaps in these pyrochlores were also measured, and the band gap widths changed continuously with cation radius.

  2. First-principle study of the electronic band structure and the effective mass of the ternary alloy GaxIn1-xP

    NASA Astrophysics Data System (ADS)

    Yang, H. Q.; Song, T. L.; Liang, X. X.; Zhao, G. J.

    2015-01-01

    In this work, the electronic band structure and the effective mass of the ternary alloy GaxIn1-xP are studied by the first principle calculations. The software QUANTUM ESPRESSO and the generalized gradient approximation (GGA) for the exchange correlations have been used in the calculations. We calculate the lattice parameter, band gap and effective mass of the ternary alloy GaxIn1-xP for the Ga composition x varying from 0.0 to 1.0 by the step of 0.125. The effect of the Ga composition on the lattice parameter and the electronic density of states are discussed. The results show that the lattice parameter varies with the composition almost linearly following the Vegard's law. A direct-to-indirect band-gap crossover is found to occur close to x = 0.7. The effective masses are also calculated at Γ(000) high symmetry point along the [100] direction. The results show that the band gap and the electron effective mass vary nonlinearly with composition x.

  3. Structure, electronic and magnetic properties of hexagonal boron nitride sheets doped by 5d transition metal atoms: First-principles calculations and molecular orbital analysis

    NASA Astrophysics Data System (ADS)

    Zhang, Zhaofu; Geng, Zhaohui; Cai, Danyun; Pan, Tongxi; Chen, Yixin; Dong, Liyuan; Zhou, Tiege

    2015-01-01

    A first-principles calculation based on density functional theory is carried out to reveal the geometry, electronic structures and magnetic properties of hexagonal boron nitride sheets (h-BNSs) doped by 5d transitional mental atoms (Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au and Hg) at boron-site (B5d) and nitrogen-site (N5d). Results of pure h-BNS, h-BNS with B vacancy (VB) and N vacancy (VN) are also given for comparison. It is shown that all the h-BNSs doped with 5d atoms possess a C3v local symmetry except for NLu and NHg which have a clear deviation. For the same 5d dopant, the binding energy of B5d is larger than that of N5d, which indicates the substitution of a 5d atom for B is preferred. The total densities of states are presented, where impurity energy levels exist. Besides, the total magnetic moments (TMMs) change regularly with the increment of the 5d atomic number. Theoretical analyses by molecular orbital under C3v symmetry explain the impurity energy levels and TMMs.

  4. Electronic Structure and Optical Properties of Cu2ZnGeSe4 : First-Principles Calculations and Vacuum-Ultraviolet Spectroscopic Ellipsometric Studies

    NASA Astrophysics Data System (ADS)

    Choi, S. G.; Park, J.-S.; Donohue, A. L.; Christensen, S. T.; To, B.; Beall, C.; Wei, S.-H.; Repins, I. L.

    2015-11-01

    Cu2ZnGeSe4 is of interest for the development of next-generation thin-film photovoltaic technologies. To understand its electronic structure and related fundamental optical properties, we perform first-principles calculations for three structural variations: kesterite, stannite, and primitive-mixed CuAu phases. The calculated data are compared with the room-temperature dielectric function ɛ =ɛ1+i ɛ2 spectrum of polycrystalline Cu2ZnGeSe4 determined by vacuum-ultraviolet spectroscopic ellipsometry in the photon-energy range of 0.7 to 9.0 eV. Ellipsometric data are modeled with the sum of eight Tauc-Lorentz oscillators, and the best-fit model yields the band-gap and Tauc-gap energies of 1.25 and 1.19 eV, respectively. A comparison of overall peak shapes and relative intensities between experimental spectra and the calculated ɛ data for three structural variations suggests that the sample may not have a pure (ordered) kesterite phase. The complex refractive index N =n +i k , normal-incidence reflectivity R , and absorption coefficients α are calculated from the modeled ɛ spectrum, which are also compared with those of Cu2ZnSnSe4 . The spectral features for Cu2ZnGeSe4 appear to be weaker and broader than those for Cu2ZnSnSe4 , which is possibly due to more structural imperfections presented in Cu2ZnGeSe4 than Cu2ZnSnSe4 .

  5. Effect of high hydrostatic pressure on structural stability of Ti{sub 3}GeC{sub 2}: A first-principles investigation

    SciTech Connect

    Cui, Shouxin; Feng, Wenxia; Hu, Haiquan; Zhang, Guiqing; Lv, Zengtao; Gong, Zizheng

    2011-04-15

    An investigation into the structural stability, electronic and elastic properties of Ti{sub 3}GeC{sub 2} under high hydrostatic pressure was conducted using first-principles calculations based on density functional theory (DFT). From the energy and enthalpy calculations, and the variations of elastic constants with pressure, we conclude that {alpha}-Ti{sub 3}GeC{sub 2} is most stable upon compression to 100 GPa, which is not consistent with the nonhydrostatic in situ synchrotron X-ray diffraction studies. The higher structural stability was analyzed in terms of electronic level. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of all polymorphs of Ti{sub 3}GeC{sub 2}. -- Graphical abstract: The less phase stability of {beta}-Ti{sub 3} SiC{sub 2} compared to that of {alpha}-Ti{sub 3} SiC{sub 2} can be interpreted by the states between -6.0 and 3.6 eV shift toward the higher energy region for {beta}-Ti{sub 3} SiC{sub 2} Display Omitted Research highlights: > From the energy and enthalpy calculations of polymorphs for Ti{sub 3}GeC{sub 2}, we can conclude that Ti{sub 3}GeC{sub 2} possessed higher stable structural stability under pressure to 100 GPa, which is not consistent with the nonhydrostatic experiments. > The higher stable structure of Ti{sub 3}GeC{sub 2} is testified by the calculated variations of elastic constants with pressure. > The reasons for this higher stable phase stability were discussed in terms of the electronic level.

  6. The solvation structure of Mg ions in dichloro complex solutions from first-principles molecular dynamics and simulated X-ray absorption spectra.

    PubMed

    Wan, Liwen F; Prendergast, David

    2014-10-15

    The knowledge of Mg solvation structure in the electrolyte is requisite to understand the transport behavior of Mg ions and their dissolution/deposition mechanism at electrolyte/electrode interfaces. In the first established rechargeable Mg-ion battery system [D. Aurbach et al. Nature 2000, 407, 724], the electrolyte is of the dichloro complex (DCC) solution family, Mg(AlCl2BuEt)2/THF, resulting from the reaction of Bu2Mg and EtAlCl2 with a molar ratio of 1:2. There is disagreement in the literature regarding the exact solvation structure of Mg ions in such solutions, i.e., whether Mg(2+) is tetra- or hexacoordinated by a combination of Cl(-) and THF. In this work, theoretical insight into the solvation complexes present is provided based on first-principles molecular dynamics simulations (FPMD). Both Mg monomer and dimer structures are considered in both neutral and positively charged states. We found that, at room temperature, the Mg(2+) ion tends to be tetracoordinated in the THF solution phase instead of hexacoordinated, which is the predominant solid-phase coordination. Simulating the X-ray absorption spectra (XAS) at the Mg K-edge by sampling our FPMD trajectories, our predicted solvation structure can be readily compared with experimental measurements. It is found that when changing from tetra- to hexacoordination, the onset of X-ray absorption should exhibit at least a 1 eV blue shift. We propose that this energy shift can be used to monitor changes in the Mg solvation sphere as it migrates through the electrolyte to electrolyte/electrode interfaces and to elucidate the mechanism of Mg dissolution/deposition. PMID:25243732

  7. Electronic structures of anatase (TiO2)1-x(TaON)x solid solutions: a first-principles study.

    PubMed

    Dang, Wenqiang; Chen, Hungru; Umezawa, Naoto; Zhang, Junying

    2015-07-21

    Sensitizing wide band gap photo-functional materials under visible-light irradiation is an important task for efficient solar energy conversion. Although nitrogen doping into anatase TiO2 has been extensively studied for this purpose, it is hard to increase the nitrogen content in anatase TiO2 because of the aliovalent nitrogen substituted for oxygen, leading to the formation of secondary phases or defects that hamper the migration of photoexcited charge carriers. In this paper, electronic structures of (TiO2)1-x(TaON)x (0 ≤ x ≤ 1) solid solutions, in which the stoichiometry is satisfied with the co-substitution of Ti for Ta along with O for N, are investigated within the anatase crystal structure using first-principles calculations. Our computational results show that the solid solutions have substantially narrower band gaps than TiO2, without introducing any localized energy states in the forbidden gap. In addition, in comparison with the pristine TiO2, the solid solution has a direct band gap when the content of TaON exceeds 0.25, which is advantageous to light absorption. The valence band maximum (VBM) of the solid solutions, which is mainly composed of N 2p states hybridized with O 2p, Ti 3d or Ta 5d orbitals, is higher in energy than that of pristine anatase TiO2 consisting of non-bonding O 2p states. On the other hand, incorporating TaON into TiO2 causes the formation of d-d bonding states through π interactions and substantially lowers the conduction band minimum (CBM) because of the shortened distance between some metal atoms. As a result, the anatase (TiO2)1-x(TaON)x is expected to become a promising visible-light absorber. In addition, some atomic configurations are found to possess exceptionally narrow band gaps. PMID:26096698

  8. Structure and stability of He and He-vacancy clusters at a Σ5(310)/[001] grain boundary in bcc Fe from first-principles.

    PubMed

    Zhang, Lei; Zhang, Ying; Lu, Guang-Hong

    2013-03-01

    We have studied the atomic structure and energetic stability of helium (He) and He-vacancy clusters in an iron (Fe) Σ5(310)/[001] grain boundary (GB) using a first-principles method. The He and He-vacancy clusters in the Fe GB are shown to exhibit high-symmetry structures. The equilibrium He-He distance in the clusters is ~1.70 Å, much smaller than 2.80 Å in the vacuum or 2.94 Å in a face centred cubic (fcc) crystal, indicating the attractive interaction between the He atoms due to the presence of Fe. The charge density surrounding He is demonstrated to decrease with an increasing number of He atoms in the clusters, leading to a positive binding energy of a He atom to the clusters. This suggests He and He-vacancy clusters can energetically trap more He atoms, which is responsible for the growth of the He-related clusters (He and He-vacancy clusters) and thus the He bubbles in the GB. The binding energy of an interstitial He atom to the He-related clusters is found generally lower in the GB than in a bcc crystal. Besides, the binding strengths of small He clusters to the GB and to a vacancy in a bcc matrix are compared, and the latter shows greater trapping strength to an interstitial He and a He(2) cluster. The magnetism of the Fe atoms near the GB as well as its variation caused by the He-related clusters is also investigated. The local magnetic moment variation of the Fe atoms in the system is enhanced to a different extent, depending on the size of the He-related clusters. PMID:23306176

  9. The fluorite related modulated structures of the Gd2(Zr2-xCex)O7 solid solution: An analogue for Pu disposition

    NASA Astrophysics Data System (ADS)

    Reid, D. P.; Stennett, M. C.; Hyatt, N. C.

    2012-07-01

    We present an overview of the Gd2(Zr2-xCex)O7 phase diagram, of interest as a model system for ceramic disposition of Pu (with Ce as a Pu surrogate). The fluorite related structures of this solid solution were determined using a modulated structure approach, to identify the underlying cation and vacancy ordering mechanisms from analysis of key satellite reflections in selected zone axis electron diffraction patterns. This revealed the formation of four structure types: pyrochlore for x<0.25, defect fluorite for 0.5structure for x=1.00, and a C-type structure for x>1.50. X-ray absorption (XAS) and electron energy loss (EELS) spectra confirmed the presence of Ce4+ as the dominant species in compositions across this system, remaining analogous to Pu4+.

  10. Comparative studies of band structures for biaxial (100)-, (110)-, and (111)-strained GeSn: A first-principles calculation with GGA+U approach

    NASA Astrophysics Data System (ADS)

    Huang, Wenqi; Cheng, Buwen; Xue, Chunlai; Liu, Zhi

    2015-10-01

    Experiments and calculations performed in previous studies indicate that compressive strain will increase (100)-strained GeSn's need for Sn to realize a direct bandgap when it is pseudomorphically grown on Ge buffers. To eliminate this negative effect, we systematically investigate the band structures of biaxial (100)-, (110)-, and (111)-strained GeSn using a first-principle calculation combined with supercell models and the GGA+U approach. This method has proven to be efficient and accurate for calculating the properties of GeSn. The calculated lattice constants and elastic constants of Ge and Sn are in good agreement with the experimental results. The crossover value of Sn concentration which is required to change the bandgap of unstrained GeSn from indirect to direct is found to be 8.5%, which is very close to the recent experimental result of 9%. The calculated bandgaps of strained GeSn show that the moving rate of the Γ valley is higher than those of the L and X valleys in (100)- and (110)-strained GeSn. However, the moving rate of the L valley is higher than those of Γ and X valleys in (111)-strained GeSn. Tensile strain has a positive effect on the transition of (100)- and (110)-strained GeSn, changing the bandgap from indirect to direct, whereas compressive strain has a positive effect for (111)-strained GeSn. The use of the (111) orientation can reduce GeSn's need for Sn and greatly increase the energy difference between the L valley and Γ valley. Thus, for strained GeSn grown on Ge buffers, the (111) orientation is a good choice to take advantage of compressive strain.

  11. Electronic structure and thermoelectric properties of (Mg2X)2 / (Mg2Y)2 (X, Y = Si, Ge, Sn) superlattices from first-principle calculations

    NASA Astrophysics Data System (ADS)

    Guo, San-Dong

    2016-05-01

    To identify thermoelectric materials containing abundant, low-cost and non-toxic elements, we have studied the electronic structures and thermoelectric properties of (Mg2X)2/ (Mg2Y)2 (X, Y = Si, Ge, Sn) superlattices with state-of-the-art first-principles calculations using a modified Becke and Johnson (mBJ) exchange potential. Our results show that (Mg2Ge)2/ (Mg2Sn)2 and (Mg2Si)2/ (Mg2Sn)2 are semi-metals using mBJ plus spin-orbit coupling (mBJ + SOC), while (Mg2Si)2/ (Mg2Ge)2 is predicted to be a direct-gap semiconductor with a mBJ gap value of 0.46 eV and mBJ + SOC gap value of 0.44 eV. Thermoelectric properties are predicted by through solving the Boltzmann transport equations within the constant scattering time approximation. It is found that (Mg2Si)2/ (Mg2Ge)2 has a larger Seebeck coefficient and power factor than (Mg2Ge)2/ (Mg2Sn)2 and (Mg2Si)2/ (Mg2Sn)2 for both p-type and n-type doping. The detrimental influence of SOC on the power factor of p-type (Mg2X)2/ (Mg2Y)2 (X, Y = Si, Ge, Sn) is analyzed as a function of the carrier concentration, but there is a negligible SOC effect for n-type. These results can be explained by the influence of SOC on their valence and conduction bands near the Fermi level.

  12. Pressure-induced structural changes in the network-forming isostatic glass GeSe4: An investigation by neutron diffraction and first-principles molecular dynamics

    NASA Astrophysics Data System (ADS)

    Bouzid, Assil; Pizzey, Keiron J.; Zeidler, Anita; Ori, Guido; Boero, Mauro; Massobrio, Carlo; Klotz, Stefan; Fischer, Henry E.; Bull, Craig L.; Salmon, Philip S.

    2016-01-01

    The changes to the topological and chemical ordering in the network-forming isostatic glass GeSe4 are investigated at pressures up to ˜14.4 GPa by using a combination of neutron diffraction and first-principles molecular dynamics. The results show a network built from corner- and edge-sharing Ge(Se1 /2)4 tetrahedra, where linkages by Se2 dimers or longer Sen chains are prevalent. These linkages confer the network with a local flexibility that helps to retain the network connectivity at pressures up to ˜8 GPa, corresponding to a density increase of ˜37 % . The network reorganization at constant topology maintains a mean coordination number n ¯≃2.4 , the value expected from mean-field constraint-counting theory for a rigid stress-free network. Isostatic networks may therefore remain optimally constrained to avoid stress and retain their favorable glass-forming ability over a large density range. As the pressure is increased to around 13 GPa, corresponding to a density increase of ˜49 % , Ge(Se1 /2)4 tetrahedra remain as the predominant structural motifs, but there is an appearance of 5-fold coordinated Ge atoms and homopolar Ge-Ge bonds that accompany an increase in the fraction of 3-fold coordinated Se atoms. The band gap energy decreases with increasing pressure, and midgap states appear at pressures beyond ˜6.7 GPa. The latter originate from undercoordinated Se atoms that terminate broken Sen chains.

  13. Structural, mechanical, thermo-physical and electronic properties of η‧-(CuNi)6Sn5 intermetallic compounds: First-principle calculations

    NASA Astrophysics Data System (ADS)

    Yang, Jian; Huang, Jihua; Fan, Dongyu; Chen, Shuhai; Zhao, Xingke

    2016-05-01

    First-principle calculations have been performed to investigate the structural, mechanical, thermo-physical and electronic properties of η‧-(CuNi)6Sn5 intermetallic compounds. The results indicated that, the doped Ni atom can not only enhance the stability of the η‧-Cu6Sn5, but also improve the mechanical and thermo-physical properties, which are more dependent on the Ni atom doping number than the doping position. In all the η‧-(CuNi)6Sn5, Cu3Ni3Sn5 (Cu1+Cu3 site) shows the best stability, the most excellent deformation resistance and the highest hardness. The Cu6Sn5, Cu3Ni3Sn5, Cu4Ni2Sn5, Cu1Ni5Sn5 and Ni6Sn5 are ductile while the Cu5Ni1Sn5 and Cu4Ni2Sn5 are brittle. The anisotropies of η‧-(CuNi)6Sn5 are all mainly due to the uneven distribution of Young's modulus at (001) planes, moreover, the anisotropy of Cu1Ni5Sn5 (Cu1+Cu2+Cu4 site) is the strongest while that of Ni6Sn5 is the weakest. The calculated Debye temperature and heat capacity showed that Cu4Ni2Sn5 (Cu2 site) possesses the best thermal conductivity (ΘD = 356.9 K) and Cu2Ni4Sn5 (Cu1+Cu2 site) possesses the largest heat capacity. From the electronic property analysis results, the Ni s and Ni p states can replace the Cu s and Cu p states to hybridize with Sn s states at -7.98 eV. Moreover, with the increasing number of the doped Ni atom, the hybridization between Cu d states at different positions is receded, while that between Ni d states is enhanced gradually.

  14. Crystal structure of Sr{sub 6}Y{sub 2}Al{sub 4}O{sub 15}: XRD refinements and first-principle calculations

    SciTech Connect

    Wang Chunhai; Guo Dongfang; Li Zhaofei; Wang Xiaoming; Lin Jianhua; Zeng Zhengzhi; Jing Xiping

    2012-08-15

    The ternary oxide phase Sr{sub 6}Y{sub 2}Al{sub 4}O{sub 15} (SYA) was synthesized and the crystal structure was determined by using the X-ray powder diffraction data. Structure of the phase can be considered as an oxygen-deficient perovskite Sr(Y{sub 1/3}Al{sub 2/3})O{sub 2.5} and has a monoclinic C2 (S.G. No. 5) unit cell with the unit cell parameters: a=17.597(1) A, b=5.7408(1) A, c=7.6860(1) A, {beta}=90.7659(3) Degree-Sign , V{sub cell}=776.37(1) A{sup 3}, Z=2. By bond parameter analysis and first-principle calculations, we confirmed the reasonability of our crystal structure model. According to the calculated band structure, SYA has an indirect band gap {approx}4.3 eV and a direct band gap {approx}4.4 eV, which is wide to be transparent to UV and visible lights. We also synthesized other rare-earth isomorphs Sr{sub 6}Ln{sub 2}Al{sub 4}O{sub 15} (Ln=Tb, Dy, Ho, Er, Tm, Yb and Lu) and obtained their cell parameters. - Graphical abstract: Sr{sub 6}Y{sub 2}Al{sub 4}O{sub 15} has a monoclinic unit cell with space group C2 (5) with unit cell parameters: a=17.597(1) A, b=5.7408(1) A, c=7.6860(1) A, {beta}=90.7659(3) Degree-Sign . Highlights: Black-Right-Pointing-Pointer Sr{sub 6}Y{sub 2}Al{sub 4}O{sub 15} (SYA) has an oxygen-deficient perovskite structure Sr(Y{sub 1/3}Al{sub 2/3})O{sub 2.5}. Black-Right-Pointing-Pointer SYA shows C2 symmetry: a=17.597(1) A, b=5.7408(1) A, c=7.6860(1) A, {beta}=90.7659(3) Degree-Sign . Black-Right-Pointing-Pointer SYA has an indirect band gap {approx}4.3 eV and a direct band gap {approx}4.4 eV.

  15. Structures and chemical bonding of B3O3-/0 and B3O3H-/0: A combined photoelectron spectroscopy and first-principles theory study

    NASA Astrophysics Data System (ADS)

    Zhao, Li-Juan; Tian, Wen-Juan; Ou, Ting; Xu, Hong-Guang; Feng, Gang; Xu, Xi-Ling; Zhai, Hua-Jin; Li, Si-Dian; Zheng, Wei-Jun

    2016-03-01

    We present a combined photoelectron spectroscopy and first-principles theory study on the structural and electronic properties and chemical bonding of B3O3-/0 and B3O3H-/0 clusters. The concerted experimental and theoretical data show that the global-minimum structures of B3O3 and B3O3H neutrals are very different from those of their anionic counterparts. The B3O3- anion is characterized to possess a V-shaped OB-B-BO chain with overall C2v symmetry (1A), in which the central B atom interacts with two equivalent boronyl (B≡O) terminals via B-B single bonds as well as with one O atom via a B=O double bond. The B3O3H- anion has a Cs (2A) structure, containing an asymmetric OB-B-OBO zig-zag chain and a terminal H atom interacting with the central B atom. In contrast, the C2v (1a) global minimum of B3O3 neutral contains a rhombic B2O2 ring with one B atom bonded to a BO terminal and that of neutral B3O3H (2a) is also of C2v symmetry, which is readily constructed from C2v (1a) by attaching a H atom to the opposite side of the BO group. The H atom in B3O3H-/0 (2A and 2a) prefers to interact terminally with a B atom, rather than with O. Chemical bonding analyses reveal a three-center four-electron (3c-4e) π hyperbond in the B3O3H- (2A) cluster and a four-center four-electron (4c-4e) π bond (that is, the so-called o-bond) in B3O3 (1a) and B3O3H (2a) neutral clusters.

  16. Structures and chemical bonding of B3O3 (-/0) and B3O3H(-/0): A combined photoelectron spectroscopy and first-principles theory study.

    PubMed

    Zhao, Li-Juan; Tian, Wen-Juan; Ou, Ting; Xu, Hong-Guang; Feng, Gang; Xu, Xi-Ling; Zhai, Hua-Jin; Li, Si-Dian; Zheng, Wei-Jun

    2016-03-28

    We present a combined photoelectron spectroscopy and first-principles theory study on the structural and electronic properties and chemical bonding of B3O3 (-/0) and B3O3H(-/0) clusters. The concerted experimental and theoretical data show that the global-minimum structures of B3O3 and B3O3H neutrals are very different from those of their anionic counterparts. The B3O3 (-) anion is characterized to possess a V-shaped OB-B-BO chain with overall C2 v symmetry (1A), in which the central B atom interacts with two equivalent boronyl (B≡O) terminals via B-B single bonds as well as with one O atom via a B=O double bond. The B3O3H(-) anion has a Cs (2A) structure, containing an asymmetric OB-B-OBO zig-zag chain and a terminal H atom interacting with the central B atom. In contrast, the C2 v (1a) global minimum of B3O3 neutral contains a rhombic B2O2 ring with one B atom bonded to a BO terminal and that of neutral B3O3H (2a) is also of C2 v symmetry, which is readily constructed from C2 v (1a) by attaching a H atom to the opposite side of the BO group. The H atom in B3O3H(-/0) (2A and 2a) prefers to interact terminally with a B atom, rather than with O. Chemical bonding analyses reveal a three-center four-electron (3c-4e) π hyperbond in the B3O3H(-) (2A) cluster and a four-center four-electron (4c-4e) π bond (that is, the so-called o-bond) in B3O3 (1a) and B3O3H (2a) neutral clusters. PMID:27036442

  17. Structural, energetic and thermodynamic analyses of Ca(BH{sub 4}){sub 2}{center_dot}2NH{sub 3} from first principles calculations

    SciTech Connect

    Yuan Pengfei; Wang Fei; Sun Qiang; Jia Yu; Guo Zhengxiao

    2012-01-15

    Ca(BH{sub 4}){sub 2}{center_dot}2NH{sub 3} is a relatively new compound with potential application in hydrogen storage. Here the fundamental properties of the compound, such as electronic structure, energetic and thermodynamic properties, were comprehensively studied using first-principles calculations. Results from electronic density of states (DOS) and electron localization function (ELF) indicate the covalent bond nature of the N-H bond and the B-H bond. Charge density analyses show weak ionic interactions between the Ca atom and the NH{sub 3} complexes or the (BH{sub 4}){sup -} complexes. The calculated vibration frequencies of B-H and N-H are in good agreement with other theoretical and experimental results. Furthermore, we calculated the reaction enthalpy and reaction Gibbs free energy at a range of temperature 0-700 K. Our results are in good agreement with experimental results in literature. Possible reaction mechanism of the decomposition reaction is proposed. - Graphical Abstract: The crystal structure of this compound and the calculated decomposition reaction free energy for two different reactions: Reac(2):Ca(BH{sub 4}){sub 2} Dot-Operator 2NH{sub 3} Long-Rightwards-Arrow {sup 162 Degree-Sign C}Ca(BH{sub 4}){sub 2} Dot-Operator NH{sub 3}+NH{sub 3} Long-Rightwards-Arrow {sup 230 Degree-Sign C}Ca(BH{sub 4}){sub 2}+2NH{sub 3} Reac(3):Ca(BH{sub 4}){sub 2} Dot-Operator 2NH{sub 3} Long-Rightwards-Arrow {sup 190 Degree-Sign C}1/4Ca(BH{sub 4}){sub 2}+1/4Ca{sub 3}(BN{sub 2}){sub 2}+BN+6H{sub 2}. Highlights: Black-Right-Pointing-Pointer Crystal structure of this compound was studied in detail. Black-Right-Pointing-Pointer Electronic properties were calculated for the first time. Black-Right-Pointing-Pointer Phonon density of states and reaction free energy at different temperatures were first calculated. Black-Right-Pointing-Pointer Possible decomposition mechanism was presented.

  18. First-principles transversal DNA conductance deconstructed

    SciTech Connect

    Zhang, Xiaoguang; Krstic, Predrag; Zikic, Radomir; Wells, Jack C; Fuentes-Cabrera, Miguel A

    2006-01-01

    First-principles calculation of the transverse conductance across DNA fragments placed between gold nanoelectrodes, reveals that such conductance describes electron tunneling that depends critically on geometrical rather than electronic-structure properties. By factoring the first-principles result into two simple and approximately independent tunneling factors, we show that the conductances of the A, C, G, and T fragments differ only because of their sizes: the larger is the DNA base, the smaller is the distance that separates the electrode from the corresponding molecule, and the larger is its conductance. Because the geometrical factors are difficult to control in an experiment, the DC-current measurements across DNA may not be a convenient approach to DNA sequencing.

  19. First principles determination of dislocation properties.

    SciTech Connect

    Hamilton, John C.

    2003-12-01

    This report details the work accomplished on first principles determination of dislocation properties. It contains an introduction and three chapters detailing three major accomplishments. First, we have used first principle calculations to determine the shear strength of an aluminum twin boundary. We find it to be remarkably small ({approx}17 mJ/m{sup 2}). This unexpected result is explained and will likely pertain for many other grain boundaries. Second, we have proven that the conventional explanation for finite grain boundary facets is wrong for a particular aluminum grain boundary. Instead of finite facets being stabilized by grain boundary stress, we find them to originate from kinetic effects. Finally we report on a new application of the Frenkel-Kontorova model to understand reconstructions of (100) type surfaces. In addition to the commonly accepted formation of rectangular dislocation arrays, we find numerous other possible solutions to the model including hexagonal reconstructions and a clock-rotated structure.

  20. Formation, structure and magnetism of the metastable defect fluorite phases AVO{sub 3.5+x} (A=In, Sc)

    SciTech Connect

    Shafi, Shahid P.; Lundgren, Rylan J.; Cranswick, Lachlan M.D.; Bieringer, Mario

    2007-12-15

    We report the preparation and stability of ScVO{sub 3.5+x} and the novel phase InVO{sub 3.5+x}. AVO{sub 3.5+x} (A=Sc, In) defect fluorite structures are formed as metastable intermediates during the topotactic oxidation of AVO{sub 3} bixbyites. The oxidation pathway has been studied in detail by means of thermogravimetric/differential thermal analysis and in-situ powder X-ray diffraction. The oxidation of the bixbyite phase follows a topotactic pathway at temperatures between 300 and 400 deg. C in air/carbon dioxide. The range of accessible oxygen stoichiometries for the AVO{sub 3.5+x} structures following this pathway are 0.00{<=}x{<=}0.22. Rietveld refinements against powder X-ray and neutron data revealed that InVO{sub 3.54} and ScVO{sub 3.70} crystallize in the defect fluorite structure in space group Fm-3 m (227) with a=4.9863(5) and 4.9697(3)A, respectively with A{sup 3+}/V{sup 4+} disorder on the (4a) cation site. Powder neutron diffraction experiments indicate clustering of oxide defects in all samples. Bulk magnetic measurements showed the presence of V{sup 4+} and the absence of magnetic ordering at low temperatures. Powder neutron diffraction experiments confirmed the absence of a long range ordered magnetic ground state. - Graphical abstract: Topotactic oxidation of AVO{sub 3} bixbyite to AVO{sub 3.5} defect fluorite structure followed by in-situ powder X-ray diffraction. The upper structural diagram shows a six coordinated (A/V)-O{sub 6} fragment in bixbyite, the lower structure illustrates the same seven-fold coordinated (A/V)-O{sub 7} cubic environment in the defect fluorite structure.

  1. Structure, Bonding, and Stability of Mercury Complexes with Thiolate and Thioether Ligands from High-Resolution XANES Spectroscopy and First-Principles Calculations.

    PubMed

    Manceau, Alain; Lemouchi, Cyprien; Rovezzi, Mauro; Lanson, Martine; Glatzel, Pieter; Nagy, Kathryn L; Gautier-Luneau, Isabelle; Joly, Yves; Enescu, Mironel

    2015-12-21

    We present results obtained from high energy-resolution L3-edge XANES spectroscopy and first-principles calculations for the structure, bonding, and stability of mercury(II) complexes with thiolate and thioether ligands in crystalline compounds, aqueous solution, and macromolecular natural organic matter (NOM). Core-to-valence XANES features that vary in intensity differentiate with unprecedented sensitivity the number and identity of Hg ligands and the geometry of the ligand environment. Post-Hartree-Fock XANES calculations, coupled with natural population analysis, performed on MP2-optimized Hg[(SR)2···(RSR)n] complexes show that the shape, position, and number of electronic transitions observed at high energy-resolution are directly correlated to the Hg and S (l,m)-projected empty densities of states and occupations of the hybridized Hg 6s and 5d valence orbitals. Linear two-coordination, the most common coordination geometry in mercury chemistry, yields a sharp 2p to 6s + 5d electronic transition. This transition varies in intensity for Hg bonded to thiol groups in macromolecular NOM. The intensity variation is explained by contributions from next-nearest, low-charge, thioether-type RSR ligands at 3.0-3.3 Å from Hg. Thus, Hg in NOM has two strong bonds to thiol S and k additional weak Hg···S contacts, or 2 + k coordination. The calculated stabilization energy is -5 kcal/mol per RSR ligand. Detection of distant ligands beyond the first coordination shell requires precise measurement of, and comparison to, spectra of reference compounds as well as accurate calculation of spectra for representative molecular models. The combined experimental and theoretical approaches described here for Hg can be applied to other closed-shell atoms, such as Ag(I) and Au(I). To facilitate further calculation of XANES spectra, experimental data, a new crystallographic structure of a key mercury thioether complex, Cartesian coordinates of the computed models, and examples of

  2. First-principles study of the structural, electronic, and magnetic properties of double perovskite Sr2FeReO6 containing various imperfections

    NASA Astrophysics Data System (ADS)

    Yan, Zhang; Li, Duan; Vincent, Ji; Ke-Wei, Xu

    2016-05-01

    The structural, electronic, and magnetic properties of double perovskite Sr2FeReO6 containing eight different imperfections of FeRe or ReFe antisites, Fe1–Re1 or Fe1–Re4 interchanges, VFe, VRe, VO or VSr vacancies have been studied by using the first-principles projector augmented wave (PAW) within generalized gradient approximation as well as taking into account the on-site Coulomb repulsive interaction (GGA+U). No obvious structural changes are observed for the imperfect Sr2FeReO6 containing FeRe or ReFe antisites, Fe1–Re1 or Fe1–Re4 interchanges, or VSr vacancy defects. However, the six (eight) nearest oxygen neighbors of the vacancy move away from (close to) VFe or VRe (VO) vacancies. The half-metallic (HM) character is maintained for the imperfect Sr2FeReO6 containing FeRe or ReFe antisites, Fe1–Re4 interchange, VFe, VO or VSr vacancies, while it vanishes when the Fe1–Re1 interchange or VRe vacancy is presented. So the Fe1–Re1 interchange and the VRe vacancy defects should be avoided to preserve the HM character of Sr2FeReO6 and thus usage in spintronic devices. In the FeRe or ReFe antisites, Fe1–Re1 or Fe1–Re4 interchanges cases, the spin moments of the Fe (Re) cations situated on Re (Fe) antisites are in an antiferromagnetic coupling with those of the Fe (Re) cations on the regular sites. In the VFe, VRe, VO, or VSr vacancies cases, a ferromagnetic coupling is obtained within each cation sublattice, while the two cation sublattices are coupled antiferromagnetically. The total magnetic moments μ tot (μ B/f.u.) of the imperfect Sr2FeReO6 containing eight different defects decrease in the sequence of VSr vacancy (3.50), VRe vacancy (3.43), FeRe antisite (2.74), VO vacancy (2.64), VFe vacancy (2.51), ReFe antisite (2.29), Fe1–Re4 interchange (1.96), Fe1–Re1 interchange (1.87), and the mechanisms of the saturation magnetization reduction have been analyzed. Project supported by the National Natural Science Foundation of China (Grant No

  3. High-pressure phases, vibrational properties, and electronic structure of Ne(He)2 and Ar(He)2 : A first-principles study

    NASA Astrophysics Data System (ADS)

    Cazorla, C.; Errandonea, D.; Sola, E.

    2009-08-01

    We have carried out a comprehensive first-principles study of the energetic, structural, and electronic properties of solid rare-gas (RG)-helium binary compounds, in particular, Ne(He)2 and Ar(He)2 , under pressure and at temperatures within the range of 0≤T≤2000K . Our approach is based on density-functional theory and the generalized gradient approximation for the exchange-correlation energy; we rely on total Helmholtz free-energy calculations performed within the quasiharmonic approximation for most of our analysis. In Ne(He)2 , we find that at pressures of around 20 GPa the system stabilizes in the MgZn2 Laves structure, in accordance to what was suggested in previous experimental investigations. In the same compound, we predict a solid-solid phase transition among structures of the Laves family of the type MgZn2→MgCu2 , at a pressure of Pt=120(1)GPa . In Ar(He)2 , we find that the system stabilizes in the MgCu2 Laves phase at low pressures but it transitates toward the AlB2 -type structure by effect of compression at Pt=13.8(4)GPa . The phonon spectra of the Ne(He)2 crystal in the MgZn2 and MgCu2 Laves structures, and that of Ar(He)2 in the AlB2 -type phase, are reported. We observe that the compressibility of RG-RG and He-He bond distances in RG(He)2 crystals is practically identical to that found in respective RG and He pure solids. This behavior emulates that of a system of noninteracting hard spheres in closed-packed configuration and comes to show the relevance of short-range interactions on this type of mixtures. Based on size-ratio arguments and empirical observations, we construct a generalized phase diagram for all RG(He)2 crystals up to a pressure of 200 GPa where we map out systematic structural trends. Excellent qualitative agreement between such generalized phase diagram and accurate ab initio calculations is proved. A similar construction is done for RG(H2)2 crystals; we find that the MgCu2 Laves structure, which has been ignored in all RG

  4. Characterization and photocatalytic activity of Fe- and N-co-deposited TiO{sub 2} and first-principles study for electronic structure

    SciTech Connect

    Yen, Chung-Chih; Wang, Da-Yung; Chang, Li-Shin; Shih, Han C.

    2011-08-15

    Titanium dioxide (TiO{sub 2}), co-deposited with Fe and N, is first implanted with Fe by a metal plasma ion implantation (MPII) process and then annealed in N{sub 2} atmosphere at a temperature regime of 400-600 deg. C. First-principle calculations show that the (Fe, N) co-deposited TiO{sub 2} films produced additional band gap levels at the bottom of the conduction band (CB) and on the top of the valence band (VB). The (Fe, N) co-deposited TiO{sub 2} films were effective in both prohibiting electron-hole recombination and generating additional Fe-O and N-Ti-O impurity levels for the TiO{sub 2} band gap. The (Fe, N) co-deposited TiO{sub 2} has a narrower band gap of 1.97 eV than Fe-implanted TiO{sub 2} (3.14 eV) and N-doped TiO{sub 2} (2.16 eV). A significant reduction of TiO{sub 2} band gap energy from 3.22 to 1.97 eV was achieved, which resulted in the extension of photocatalytic activity of TiO{sub 2} from UV to Vis regime. The photocatalytic activity and removal rate were approximately two-fold higher than that of the Fe-implanted TiO{sub 2} under visible light irradiation. - Graphical abstract: The electronic properties of (Fe, N) co-deposited TiO{sub 2} films determined by theoretical calculations is graphically shown; Fe and N preferentially substitute the Ti and O site, to form impurity level. Highlights: > The MPII produces a simple and low cost process for the fabrication of visible light photocatalysts. > Both theoretical and experimental approaches are used to discuss the relationship between band structure and photocatalysis. > The Fe and N preferentially substitute the Ti and O site, which generate additional Fe-O and N-Ti-O impurity levels for the TiO{sub 2} band gap. > A significant reduction of TiO{sub 2} band gap energy from 3.22 to 1.97 eV was achieved.

  5. First principles calculations for lithiated manganese oxides.

    SciTech Connect

    Prasad, R.

    1998-12-23

    First principles calculations within the local-spin-density-functional theory (LSDFF) framework are presented of densities of electronic states for MnO, LiMnO{sub 2} in the monoclinic and orthorhombic structures, cubic LiMn{sub 2}O{sub 4} spinel and {lambda}-MnO{sub 2} (delithiated spinel), all in antiferromagnetic spin configurations. The changes in energy spectra as the Mn oxidation state varies between 2+ and 4+ are illustrated. Preliminary calculations for Co-doped LiMnO{sub 2} are presented, and the destabilization of a monoclinic relative to a rhombohedral structure is discussed.

  6. Diameter dependence of mechanical, electronic, and structural properties of InAs and InP nanowires: A first-principles study

    NASA Astrophysics Data System (ADS)

    Dos Santos, Cláudia L.; Piquini, Paulo

    2010-02-01

    Semiconductor nanowires (NWs) have ideal morphologies to act as active parts and connections in nanodevices since they naturally restrict the conduction channels and periodicity to one dimension. The advantages from the reduced spatial dimension can be greatly enhanced by wisely selecting the materials composing the NWs, through the knowledge of the properties of their bulk counterparts. NW’s properties can still be tailored by managing (i) internal or intrinsic characteristics as diameters, growth directions, structural phases, and the faceting or saturation of surfaces, and/or (ii) external or extrinsic influences as applied electric, magnetic, thermal, and mechanical fields. Bulk InAs has one of the lowest electron effective-masses among binary III-V semiconducting materials while bulk InP shows excellent optical properties, which make InAs and InP NWs candidates for optoelectronic materials. In this work, we use first-principles calculations to study the structural, electronic, and mechanical properties of [111] zinc-blende InAs and InP NWs as a function of diameter (ranging from 0.5 to 2.0 nm). The influence of external mechanical stress on the electronic properties is also analyzed. The axial lattice constants of the NWs are seen to decrease with decreasing diameter, as a consequence of a shorter surface lattice constant of the NWs, as compared to their bulk values. The Young’s modulus of both InAs and InP NWs is determined to decrease while the Poisson’s ratio to increase with decreasing diameters, with deviations from the bulk Young’s modulus estimated to occur for NWs with diameters lower than 15 nm. The increase in the band-gaps with decreasing diameters is seen to be slower than the expected from simple quantum-mechanical models ( 1/D2 , where D is the diameter), mainly for the smallest (<1.0nm) diameters. The electron effective-masses are seen to increase with decreasing diameters, due to a k -dependent energy shift of the conduction

  7. Decreasing the singlet-triplet gap for thermally activated delayed fluorescence molecules by structural modification on the donor fragment: First-principles study

    NASA Astrophysics Data System (ADS)

    Fan, Jian-zhong; Lin, Li-li; Wang, Chuan-kui

    2016-05-01

    The small energy gap between singlet excitons (S) and triplet excitons (T) of organic molecules is a dominant condition for high efficient thermally activated delayed fluorescence (TADF). In this study, influence of modification in donor groups of a series of molecules on their geometries, S-T energy gaps, and photophysical properties, is investigated based on first-principles calculations. Investigation shows that, as the electron donating ability is increased, both S-T energy gap and overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are decreased. This work provides strategy for designing high efficient and multi-color TADF devices.

  8. First-principle and experiment investigation of MoS2@SnO2 nano-heterogeneous structures with enhanced humidity sensing performance

    NASA Astrophysics Data System (ADS)

    Lei, Xiang; Yu, Ke; Li, Honglin; Tang, Zheng; Guo, Bangjun; Li, Jinzhu; Fu, Hao; Zhang, Qingfeng; Zhu, Ziqiang

    2016-04-01

    In this work, we report the First-principle investigation and synthesis of MoS2@SnO2 heterostructure as high-performance humidity sensor by a two-step hydrothermal method. The first-principles calculations were performed to explain water molecule adsorption mechanism by applying density of state model to simulate the interaction between water molecule and sensing base material. The higher specific surface and the lower adsorption energy theoretically predicted the improvement on humidity sensing performance, which was confirmed by experiments testing. The MoS2@SnO2 heterostructure exhibited promoted humidity sensing characteristics on response time of 53 s and recovery time of 21 s, while switching the humidity between 11% relative humidity (RH) and 95% RH. The corresponding humidity sensing mechanisms of MoS2@SnO2 were elaborately interpreted. This work could bring forward a new design method on practical humidity sensing devices with an excellent stability and fast response by using MoS2@SnO2 heterostructure.

  9. A first principles study of structural stability, electronic structure and mechanical properties of ABeH{sub 3} (A = Li, Na)

    SciTech Connect

    Santhosh, M.; Rajeswarapalanichamy, R.; Priyanga, G. Sudha; Murugan, A.; Kanagaprabha, S.; Iyakutti, K.

    2015-06-24

    Ab initio calculations are performed to investigate the structural stability, electronic structure and mechanical properties of ABeH{sub 3} (A = Li, Na) for three different crystal structures, namely orthorhombic (Pnma), monoclinic (P2{sub 1}/c) and triclinic (P-1) phase. Among the considered structures monoclinic (P2{sub 1}/c) phase is found to be the most stable one for all the three hydrides at ambient condition. The electronic structure reveals that these materials are wide band gap semiconductors. The calculated elastic constants indicate that these materials are mechanically stable at ambient condition.

  10. A first principles study of structural stability, electronic structure and mechanical properties of beryllium alanate BeAlH{sub 5}

    SciTech Connect

    Santhosh, M.; Rajeswarapalanichamy, R. Priyanga, G. Sudha; Cinthia, A. Jemmy; Kanagaprabha, S.; Iyakutti, K.

    2015-06-24

    Ab initio calculations are performed to investigate the structural stability, electronic structure and mechanical properties of BeAlH{sub 5} for monoclinic crystal structures with two different types of space group namely P2{sub 1} and C{sub 2}/c. Among the considered structures monoclinic (P2{sub 1}) phase is found to be the most stable at ambient condition. The structural phase transition from monoclinic (P2{sub 1}) to monoclinic (C{sub 2}/c) phase is observed in BeAlH{sub 5}. The electronic structure reveals that this compound is insulator. The calculated elastic constants indicate that this material is mechanically stable at ambient condition.

  11. Structural and magnetic properties of NiCx and NiNx (x=0 to (1)/(3)) solid solutions from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Fang, C. M.; Sluiter, M. H. F.; van Huis, M. A.; Zandbergen, H. W.

    2012-10-01

    First-principles calculations have been performed for a variety of Ni3X (X = C, N) phases, as well as for NiXy (y = 0 to (1)/(3)) solid solutions to clarify the persistent controversy regarding its magnetic state. The calculations show that the solid solution phases based on hexagonal-close-packed (hcp or ɛ-) Ni have relatively high stability for X concentrations greater than about 0.1 whereas the face-centered-cubic (fcc or γ-) Ni phases are favored for smaller X concentration. Hence, during carburization or nitridization of Ni, a phase transformation is to be expected. In spite of the close-packed nature of both hcp- and fcc-based solid solutions, X quenches the magnetization more effectively in fcc than in hcp-based solid solutions. These findings resolve many apparently contradictory experimental observations concerning C- and N-containing Ni alloys in the literature.

  12. Rationalization of the Hubbard U parameter in CeO{sub x} from first principles: Unveiling the role of local structure in screening

    SciTech Connect

    Lu, Deyu E-mail: pingliu3@bnl.gov; Liu, Ping E-mail: pingliu3@bnl.gov

    2014-02-28

    The density functional theory (DFT)+U method has been widely employed in theoretical studies on various ceria systems to correct the delocalization bias in local and semi-local DFT functionals with moderate computational cost. We present a systematic and quantitative study, aiming to gain better understanding of the dependence of Hubbard U on the local atomic arrangement. To rationalize the Hubbard U of Ce 4f, we employed the first principles linear response method to compute Hubbard U for Ce in ceria clusters, bulks, and surfaces. We found that the Hubbard U varies in a wide range from 4.3 eV to 6.7 eV, and exhibits a strong correlation with the Ce coordination number and Ce–O bond lengths, rather than the Ce 4f valence state. The variation of the Hubbard U can be explained by the changes in the strength of local screening due to O → Ce intersite transitions.

  13. The Effect of Ce-N Codoping on the Electronic Structure and Optical Property of Anatase TiO2: a First-Principles Study

    NASA Astrophysics Data System (ADS)

    Mao, Fei; Hou, Qingyu; Zhao, Chunwang; Guo, Shaoqiang; Zhang, Yue

    2014-01-01

    Based on the first-principles plane wave ultra-soft pseudo potential (USP) method of density function theory pure N and Ce doped and Ce-N codoping anatase TiO2 supercell models were established, respectively, and calculated their energy in this paper. The calculated results show that the three doping systems compared to the pure anatase TiO2 band gap narrowed which results in red-shift of the optical absorption edges and Ce-N codoped anatase TiO2 have the most obvious visible effect. Meanwhile, synergy is very effective for the separation of electron-hole pairs and the electrons have a better lifespan. Research found that the trend of the donor's movements at the shallow level of Ce-N codoped anatase TiO2 is not obvious. This is due to its very thick shell, resulting in shielding effect of the outer layer of the Ce-4f.

  14. First-principles study of locally disordered structures of Mn-induced GaAs(001)-(2 × 2) surface

    NASA Astrophysics Data System (ADS)

    Akaishi, Akira; Funatsuki, Kenta; Ohtake, Akihiro; Nakamura, Jun

    2016-08-01

    Various atomic arrangements of the Mn-induced GaAs(001) surface, consisting of one Ga–As dimer and one Mn atom in the (2 × 2) unit, have been investigated by first-principles calculations. The most stable arrangement is reasonable in view of the classical electrostatic theory. It has been revealed that the topmost Ga–As dimers tend to be aligned along the [1\\bar{1}0] direction, while they are less ordered along the [110] direction. These anisotropic orderings, that is, anisotropic interactions, imply that the Mn atom, which is located between the Ga–As dimers, enhances the local electrostatic interaction between the dimers along the [1\\bar{1}0] direction, as a result of the dielectric anisotropy at the surface.

  15. Chamber Clearing First Principles Modeling

    SciTech Connect

    Loosmore, G

    2009-06-09

    LIFE fusion is designed to generate 37.5 MJ of energy per shot, at 13.3 Hz, for a total average fusion power of 500 MW. The energy from each shot is partitioned among neutrons ({approx}78%), x-rays ({approx}12%), and ions ({approx}10%). First wall heating is dominated by x-rays and debris because the neutron mean free path is much longer than the wall thickness. Ion implantation in the first wall also causes damage such as blistering if not prevented. To moderate the peak-pulse heating, the LIFE fusion chamber is filled with a gas (such as xenon) to reduce the peak-pulse heat load. The debris ions and majority of the x-rays stop in the gas, which re-radiates this energy over a longer timescale (allowing time for heat conduction to cool the first wall sufficiently to avoid damage). After a shot, because of the x-ray and ion deposition, the chamber fill gas is hot and turbulent and contains debris ions. The debris needs to be removed. The ions increase the gas density, may cluster or form aerosols, and can interfere with the propagation of the laser beams to the target for the next shot. Moreover, the tritium and high-Z hohlraum debris needs to be recovered for reuse. Additionally, the cryogenic target needs to survive transport through the gas mixture to the chamber center. Hence, it will be necessary to clear the chamber of the hot contaminated gas mixture and refill it with a cool, clean gas between shots. The refilling process may create density gradients that could interfere with beam propagation, so the fluid dynamics must be studied carefully. This paper describes an analytic modeling effort to study the clearing and refilling process for the LIFE fusion chamber. The models used here are derived from first principles and balances of mass and energy, with the intent of providing a first estimate of clearing rates, clearing times, fractional removal of ions, equilibrated chamber temperatures, and equilibrated ion concentrations for the chamber. These can be used

  16. A first principles molecular dynamics study of lithium atom solvation in binary liquid mixture of water and ammonia: Structural, electronic, and dynamical properties

    NASA Astrophysics Data System (ADS)

    Pratihar, Subha; Chandra, Amalendu

    2011-01-01

    The preferential solvation of solutes in mixed solvent systems is an interesting phenomenon that plays important roles in solubility and kinetics. In the present study, solvation of a lithium atom in aqueous ammonia solution has been investigated from first principles molecular dynamics simulations. Solvation of alkali metal atoms, like lithium, in aqueous and ammonia media is particularly interesting because the alkali metal atoms release their valence electrons in these media so as to produce solvated electrons and metal counterions. In the present work, first principles simulations are performed employing the Car-Parrinello molecular dynamics method. Spontaneous ionization of the Li atom is found to occur in the mixed solvent system. From the radial distribution functions, it is found that the Li ^+ ion is preferentially solvated by water and the coordination number is mostly four in its first solvation shell and exchange of water molecules between the first and second solvation shells is essentially negligible in the time scale of our simulations. The Li ^+ ion and the unbound electron are well separated and screened by the polar solvent molecules. Also the unbound electron is primarily captured by the hydrogens of water molecules. The diffusion rates of Li ^+ ion and water molecules in its first solvation shell are found to be rather slow. In the bulk phase, the diffusion of water is found to be slower than that of ammonia molecules because of strong ammonia-water hydrogen bonds that participate in solvating ammonia molecules in the mixture. The ratio of first and second rank orientational correlation functions deviate from 3, which suggests a deviation from the ideal Debye-type orientational diffusion. It is found that the hydrogen bond lifetimes of ammonia-ammonia pairs is very short. However, ammonia-water H-bonds are found to be quite strong when ammonia acts as an acceptor and these hydrogen bonds are found to live longer than even water-water hydrogen

  17. Water orientation and hydrogen-bond structure at the fluorite/water interface.

    PubMed

    Khatib, Rémi; Backus, Ellen H G; Bonn, Mischa; Perez-Haro, María-José; Gaigeot, Marie-Pierre; Sulpizi, Marialore

    2016-01-01

    Water in contact with mineral interfaces is important for a variety of different processes. Here, we present a combined theoretical/experimental study which provides a quantitative, molecular-level understanding of the ubiquitous and important CaF2/water interface. Our results show that, at low pH, the surface is positively charged, causing a substantial degree of water ordering. The surface charge originates primarily from the dissolution of fluoride ions, rather than from adsorption of protons to the surface. At high pH we observe the presence of Ca-OH species pointing into the water. These OH groups interact remarkably weakly with the surrounding water, and are responsible for the "free OH" signature in the VSFG spectrum, which can be explained from local electronic structure effects. The quantification of the surface termination, near-surface ion distribution and water arrangement is enabled by a combination of advanced phase-resolved Vibrational Sum Frequency Generation spectra of CaF2/water interfaces and state-of-the-art ab initio molecular dynamics simulations which include electronic structure effects. PMID:27068326

  18. Water orientation and hydrogen-bond structure at the fluorite/water interface

    PubMed Central

    Khatib, Rémi; Backus, Ellen H. G.; Bonn, Mischa; Perez-Haro, María-José; Gaigeot, Marie-Pierre; Sulpizi, Marialore

    2016-01-01

    Water in contact with mineral interfaces is important for a variety of different processes. Here, we present a combined theoretical/experimental study which provides a quantitative, molecular-level understanding of the ubiquitous and important CaF2/water interface. Our results show that, at low pH, the surface is positively charged, causing a substantial degree of water ordering. The surface charge originates primarily from the dissolution of fluoride ions, rather than from adsorption of protons to the surface. At high pH we observe the presence of Ca-OH species pointing into the water. These OH groups interact remarkably weakly with the surrounding water, and are responsible for the “free OH” signature in the VSFG spectrum, which can be explained from local electronic structure effects. The quantification of the surface termination, near-surface ion distribution and water arrangement is enabled by a combination of advanced phase-resolved Vibrational Sum Frequency Generation spectra of CaF2/water interfaces and state-of-the-art ab initio molecular dynamics simulations which include electronic structure effects. PMID:27068326

  19. Water orientation and hydrogen-bond structure at the fluorite/water interface

    NASA Astrophysics Data System (ADS)

    Khatib, Rémi; Backus, Ellen H. G.; Bonn, Mischa; Perez-Haro, María-José; Gaigeot, Marie-Pierre; Sulpizi, Marialore

    2016-04-01

    Water in contact with mineral interfaces is important for a variety of different processes. Here, we present a combined theoretical/experimental study which provides a quantitative, molecular-level understanding of the ubiquitous and important CaF2/water interface. Our results show that, at low pH, the surface is positively charged, causing a substantial degree of water ordering. The surface charge originates primarily from the dissolution of fluoride ions, rather than from adsorption of protons to the surface. At high pH we observe the presence of Ca-OH species pointing into the water. These OH groups interact remarkably weakly with the surrounding water, and are responsible for the “free OH” signature in the VSFG spectrum, which can be explained from local electronic structure effects. The quantification of the surface termination, near-surface ion distribution and water arrangement is enabled by a combination of advanced phase-resolved Vibrational Sum Frequency Generation spectra of CaF2/water interfaces and state-of-the-art ab initio molecular dynamics simulations which include electronic structure effects.

  20. Structural determination of fluorite-type oxygen excess uranium oxides using EXAFS spectroscopy

    SciTech Connect

    Jones, D.J.; Roziere, J.; Allen, G.C.; Tempest, P.A.

    1986-06-01

    Extended x-ray absorption fine structure (EXAFS) spectroscopy has been carried out at 77 K at the uranium L/sub III/ edge for UO/sub 2/, ..beta..-U/sub 3/O/sub 7/, and U/sub 4/O/sub 9/ with the aim of determining the structure of these highly defective (oxygen excess) uranium oxide phases, which are of industrial importance. Use has been made of a difference Fourier technique for U/sub 3/O/sub 7/, in which the EXAFS of a perfect lattice model is subtracted. U--O bond lengths calculated from the remaining EXAFS signal, assumed to result only from interstitial oxygens, have been used to determine the atomic coordinates of these interstitials. The analysis of EXAFS data in terms of coordination number has allowed an insight into the defect aggregate arrangement of oxygens in U/sub 3/O/sub 7/ and U/sub 4/O/sub 9/. Furthermore, EXAFS data indicate that the uranium sublattice is perturbed by the incorporation of additional oxygen atoms.

  1. Dynamic compression of water to 700 GPa: single- and double shock experiments on Sandia's Z machine, first principles simulations, and structure of water planets

    NASA Astrophysics Data System (ADS)

    Mattsson, Thomas R.

    2011-11-01

    Significant progress has over the last few years been made in high energy density physics (HEDP) by executing high-precision multi-Mbar experiments and performing first-principles simulations for elements ranging from carbon [1] to xenon [2]. The properties of water under HEDP conditions are of particular importance in planetary science due to the existence of ice-giants like Neptune and Uranus. Modeling the two planets, as well as water-rich exoplanets, requires knowing the equation of state (EOS), the pressure as a function of density and temperature, of water with high accuracy. Although extensive density functional theory (DFT) simulations have been performed for water under planetary conditions [3] experimental validation has been lacking. Accessing thermodynamic states along planetary isentropes in dynamic compression experiments is challenging because the principal Hugoniot follows a significantly different path in the phase diagram. In this talk, we present experimental data for dynamic compression of water up to 700 GPa, including in a regime of the phase-diagram intersected by the Neptune isentrope and water-rich models for the exoplanet GJ436b. The data was obtained on the Z-accelerator at Sandia National Laboratories by performing magnetically accelerated flyer plate impact experiments measuring both the shock and re-shock in the sample. The high accuracy makes it possible for the data to be used for detailed model validation: the results validate first principles based thermodynamics as a reliable foundation for planetary modeling and confirm the fine effect of including nuclear quantum effects on the shock pressure. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. [4pt] [1] M.D. Knudson, D.H. Dolan, and M.P. Desjarlais, SCIENCE

  2. Magnetic properties and structural transitions of fluorite-related rare earth osmates Ln{sub 3}OsO{sub 7} (Ln=Pr, Tb)

    SciTech Connect

    Hinatsu, Yukio; Doi, Yoshihiro

    2013-02-15

    Ternary rare-earth osmates Ln{sub 3}OsO{sub 7} (Ln=Pr, Tb) have been prepared. They crystallize in an ortho-rhombic superstructure of cubic fluorite with space group Cmcm. Both of these compounds undergo a structural phase transition at 130 K (Ln=Pr) and 580 K (Ln=Tb). These compounds show complex magnetic behavior at low temperatures. Pr{sub 3}OsO{sub 7} exhibits magnetic transitions at 8 and 73 K, and Tb{sub 3}OsO{sub 7} magnetically orders at 8 and 60 K. The Os moments become one-dimensionally ordered, and when the temperature is furthermore decreased, it provokes the ordering in the Ln{sup 3+} sublattice that simultaneously becomes three-dimensionally ordered with the Os sublattice. - Graphical abstract: Ternary rare-earth osmates Ln{sub 3}OsO{sub 7} (Ln=Pr, Tb) have been prepared. They crystallize in an orthorhombic superstructure of cubic fluorite with space group Cmcm. Both of these compounds undergo a structural phase transition at 130 K (Ln=Pr) and 580 K (Ln=Tb). These compounds show complex magnetic behavior at low temperatures. Pr{sub 3}OsO{sub 7} exhibits magnetic transitions at 8 and 73 K, and Tb{sub 3}OsO{sub 7} magnetically orders at 8 and 60 K. Highlights: Black-Right-Pointing-Pointer Ternary rare-earth osmates Ln{sub 3}OsO{sub 7} (Ln=Pr, Tb) with an ordered defect-fluorite structure have been prepared. Black-Right-Pointing-Pointer Both of these compounds undergo a structural phase transition at 130 K (Ln=Pr) and 580 K (Ln=Tb). Black-Right-Pointing-Pointer These compounds show complex magnetic behavior at low temperatures due to magnetic ordering of Ln and Os.

  3. Electronic structures and formation energies of pentavalent-ion-doped SnO{sub 2}: First-principles hybrid functional calculations

    SciTech Connect

    Behtash, Maziar; Joo, Paul H.; Nazir, Safdar; Yang, Kesong

    2015-05-07

    We studied the electronic properties and relative thermodynamic stability of several pentavalent-ion (Ta, Nb, P, Sb, and I) doped SnO{sub 2} systems using first-principles hybrid density functional theory calculations, in order to evaluate their potential as transparent conducting oxides (TCOs). I-doped SnO{sub 2}, though conductive, shows a narrowed optical band gap with respect to the undoped system due to the formation of gap states above the valence band. Nb-doped SnO{sub 2} forms localized impurity states below the conduction band bottom, suggesting that the Nb dopant exists as an Nb{sup 4+}-like cation, which is consistent with the recent experimental finding of the formation of the impurity level below the conduction band bottom [Appl. Phys. Express 5, 061201 (2012)]. Ta- and Sb-doped SnO{sub 2} display n-type conductivity, high charge carrier density, and widened optical band gap. P-doped SnO{sub 2} shows similar n-type electronic properties with that of Sb- and Ta-doped systems, and thus P-doped SnO{sub 2} is proposed as a promising candidate TCO for further experimental validation.

  4. Rationalization of the Hubbard U parameter in CeO(x) from first principles: unveiling the role of local structure in screening.

    PubMed

    Lu, Deyu; Liu, Ping

    2014-02-28

    The density functional theory (DFT)+U method has been widely employed in theoretical studies on various ceria systems to correct the delocalization bias in local and semi-local DFT functionals with moderate computational cost. We present a systematic and quantitative study, aiming to gain better understanding of the dependence of Hubbard U on the local atomic arrangement. To rationalize the Hubbard U of Ce 4f, we employed the first principles linear response method to compute Hubbard U for Ce in ceria clusters, bulks, and surfaces. We found that the Hubbard U varies in a wide range from 4.3 eV to 6.7 eV, and exhibits a strong correlation with the Ce coordination number and Ce-O bond lengths, rather than the Ce 4f valence state. The variation of the Hubbard U can be explained by the changes in the strength of local screening due to O → Ce intersite transitions. PMID:24588142

  5. Magnetism driven by non-metal interstitials from first-principles prediction: The case of hydrogen- and fluorine-doped calcium monoxide with rock-salt structure

    NASA Astrophysics Data System (ADS)

    Dong, Shengjie; Zhao, Hui

    2014-12-01

    Our first-principles calculations based on density functional theory confirmed the formation of sp-ferromagnetic states of calcium monoxide with interstitial nonmagnetic F or H atoms. The hydrogen and fluorine interstitials in the oxides were found to be spin polarized and are more stable in the antiferromagnetic state and the ferromagnetic state, respectively. For H-doped CaO, no considerable charge transfer takes place and the spin remains localized on the impurity. For F-doped oxide, the observation may be attributed to the p-p interaction and the charge transfer between the interstitial atom and the neighboring O atoms. We demonstrate that H-doped compound is a potential n-type antiferromagnet, while F-doped material is a potential p-type ferromagnet. The different dopants would induce different magnetic couplings, thus show different ground-state magnetic configurations. The mechanism for the magnetism should be useful for understanding d0 magnetic semiconductors or insulators. The present potential d0 diluted magnetic materials, at least some of them, may be useful in spintronics.

  6. Electronic structure and thermoelectric performance of Zintl compound Sr{sub 3}GaSb{sub 3}: A first-principles study

    SciTech Connect

    Feng Shi, Qing; Li Yan, Yu; Xu Wang, Yuan

    2014-01-06

    By using first-principles method and Boltzmann theory, we simulated the thermoelectric transport properties of p-type and n-type Sr{sub 3}GaSb{sub 3}. It is found that the thermoelectric figure-of merit (ZT) of n-type Sr{sub 3}GaSb{sub 3} is probably better than that of p-type, mainly due to its large band degeneracy. Moreover, a high ZT value of 1.74 at 850 K can be achieved for n-type Sr{sub 3}GaSb{sub 3} along the yy direction, corresponding to the carrier concentration 3.5 × 10{sup 20} e cm{sup −3}. We propose that the high ZT value of experimentally synthesized p-type Sr{sub 3}GaSb{sub 3} is originated from appearing of the larger number of band valley on the top of valence bands.

  7. Structural and electronic properties of half-Heusler alloys PtXBi (with X=Mn, Fe, Co and Ni) calculated from first principles

    NASA Astrophysics Data System (ADS)

    Huang, Wenchao; Wang, Xiaofang; Chen, Xiaoshuang; Lu, Wei; Damewood, L.; Fong, C. Y.

    2015-03-01

    First principles calculations with spin polarization based on density functional theory have been performed on half-Heusler alloys PtXBi, with X=Mn, Fe, Co and Ni, in three different atomic configurations (i.e. α, β, and γ phases). For each configuration, their optimized lattice constants are determined. Electronic and magnetic properties are also investigated. The differences reflect the atomic arrangements of the three phases and varied transition metal elements X. Meanwhile, the possibility of having the integer magnetic moment for each phase is explored. PtMnBi in α phase show half-metallic (HM) properties when its lattice constant is reduced from -3.0% to -11.2% with magnetic moment consistent with the values given by the modified Slater-Pauling rule. Additionally, we examined the effects of the spin-orbit (S-O) interaction on half-metal PtMnBi by comparing the relative shifts of the valence bands and the indirect semiconducting gap with respect to the spin polarized results.

  8. First principles studies on anatase surfaces

    NASA Astrophysics Data System (ADS)

    Selcuk, Sencer

    TiO2 is one of the most widely studied metal oxides from both the fundamental and the technological points of view. A variety of applications have already been developed in the fields of energy production, environmental remediation, and electronics. Still, it is considered to have a high potential for further improvement and continues to be of great interest. This thesis describes our theoretical studies on the structural and electronic properties of anatase surfaces, and their (photo)chemical behavior. Recently much attention has been focused on anatase crystals synthesized by hydrofluoric acid assisted methods. These crystals exhibit a high percentage of {001} facets, generally considered to be highly reactive. We used first principles methods to investigate the structure of these facets, which is not yet well understood. Our results suggest that (001) surfaces exhibit the bulk-terminated structure when in contact with concentrated HF solutions. However, 1x4-reconstructed surfaces, as observed in UHV, become always more stable at the typical temperatures used to clean the as-prepared crystals in experiments. Since the reconstructed surfaces are only weakly reactive, we predict that synthetic anatase crystals with dominant {001} facets should not exhibit enhanced photocatalytic activity. Understanding how defects in solids interact with external electric fields is important for technological applications such as memristor devices. We studied the influence of an external electric field on the formation energies and diffusion barriers of the surface and the subsurface oxygen vacancies at the anatase (101) surface from first principles. Our results show that the applied field can have a significant influence on the relative stabilities of these defects, whereas the effect on the subsurface-to-surface defect migration is found to be relatively minor. Charge carriers play a key role in the transport properties and the surface chemistry of TiO2. Understanding their

  9. Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7

    SciTech Connect

    Aidhy, Dilpuneet S.; Sachan, Ritesh; Zarkadoula, Eva; Pakarinen, Olli; Chisholm, Matthew F.; Zhang, Yanwen; Weber, William J.

    2015-11-10

    The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd2Ti2O7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. Lastly, in view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties.

  10. Iron diffusion from first principles calculations

    NASA Astrophysics Data System (ADS)

    Wann, E.; Ammann, M. W.; Vocadlo, L.; Wood, I. G.; Lord, O. T.; Brodholt, J. P.; Dobson, D. P.

    2013-12-01

    The cores of Earth and other terrestrial planets are made up largely of iron1 and it is therefore very important to understand iron's physical properties. Chemical diffusion is one such property and is central to many processes, such as crystal growth, and viscosity. Debate still surrounds the explanation for the seismologically observed anisotropy of the inner core2, and hypotheses include convection3, anisotropic growth4 and dendritic growth5, all of which depend on diffusion. In addition to this, the main deformation mechanism at the inner-outer core boundary is believed to be diffusion creep6. It is clear, therefore, that to gain a comprehensive understanding of the core, a thorough understanding of diffusion is necessary. The extremely high pressures and temperatures of the Earth's core make experiments at these conditions a challenge. Low-temperature and low-pressure experimental data must be extrapolated across a very wide gap to reach the relevant conditions, resulting in very poorly constrained values for diffusivity and viscosity. In addition to these dangers of extrapolation, preliminary results show that magnetisation plays a major role in the activation energies for diffusion at low pressures therefore creating a break down in homologous scaling to high pressures. First principles calculations provide a means of investigating diffusivity at core conditions, have already been shown to be in very good agreement with experiments7, and will certainly provide a better estimate for diffusivity than extrapolation. Here, we present first principles simulations of self-diffusion in solid iron for the FCC, BCC and HCP structures at core conditions in addition to low-temperature and low-pressure calculations relevant to experimental data. 1. Birch, F. Density and composition of mantle and core. Journal of Geophysical Research 69, 4377-4388 (1964). 2. Irving, J. C. E. & Deuss, A. Hemispherical structure in inner core velocity anisotropy. Journal of Geophysical

  11. Protein Repeats from First Principles.

    PubMed

    Turjanski, Pablo; Parra, R Gonzalo; Espada, Rocío; Becher, Verónica; Ferreiro, Diego U

    2016-01-01

    Some natural proteins display recurrent structural patterns. Despite being highly similar at the tertiary structure level, repeating patterns within a single repeat protein can be extremely variable at the sequence level. We use a mathematical definition of a repetition and investigate the occurrences of these in sequences of different protein families. We found that long stretches of perfect repetitions are infrequent in individual natural proteins, even for those which are known to fold into structures of recurrent structural motifs. We found that natural repeat proteins are indeed repetitive in their families, exhibiting abundant stretches of 6 amino acids or longer that are perfect repetitions in the reference family. We provide a systematic quantification for this repetitiveness. We show that this form of repetitiveness is not exclusive of repeat proteins, but also occurs in globular domains. A by-product of this work is a fast quantification of the likelihood of a protein to belong to a family. PMID:27044676

  12. Protein Repeats from First Principles

    PubMed Central

    Turjanski, Pablo; Parra, R. Gonzalo; Espada, Rocío; Becher, Verónica; Ferreiro, Diego U.

    2016-01-01

    Some natural proteins display recurrent structural patterns. Despite being highly similar at the tertiary structure level, repeating patterns within a single repeat protein can be extremely variable at the sequence level. We use a mathematical definition of a repetition and investigate the occurrences of these in sequences of different protein families. We found that long stretches of perfect repetitions are infrequent in individual natural proteins, even for those which are known to fold into structures of recurrent structural motifs. We found that natural repeat proteins are indeed repetitive in their families, exhibiting abundant stretches of 6 amino acids or longer that are perfect repetitions in the reference family. We provide a systematic quantification for this repetitiveness. We show that this form of repetitiveness is not exclusive of repeat proteins, but also occurs in globular domains. A by-product of this work is a fast quantification of the likelihood of a protein to belong to a family. PMID:27044676

  13. Protein Repeats from First Principles

    NASA Astrophysics Data System (ADS)

    Turjanski, Pablo; Parra, R. Gonzalo; Espada, Rocío; Becher, Verónica; Ferreiro, Diego U.

    2016-04-01

    Some natural proteins display recurrent structural patterns. Despite being highly similar at the tertiary structure level, repeating patterns within a single repeat protein can be extremely variable at the sequence level. We use a mathematical definition of a repetition and investigate the occurrences of these in sequences of different protein families. We found that long stretches of perfect repetitions are infrequent in individual natural proteins, even for those which are known to fold into structures of recurrent structural motifs. We found that natural repeat proteins are indeed repetitive in their families, exhibiting abundant stretches of 6 amino acids or longer that are perfect repetitions in the reference family. We provide a systematic quantification for this repetitiveness. We show that this form of repetitiveness is not exclusive of repeat proteins, but also occurs in globular domains. A by-product of this work is a fast quantification of the likelihood of a protein to belong to a family.

  14. First-principles simulations of thiophene oligomers

    NASA Astrophysics Data System (ADS)

    Scherlis, Damian; Marzari, Nicola

    2003-03-01

    Conducting polymers, extensively investigated for their use in electronic and nanotechnology applications, have recently gained prominence for their possible use as molecular actuators in mechanical and bioengineering devices. We have focused our efforts on thiophene-based compounds, a class of materials that can be designed for high stress generation and large linear displacement (actuation strain), ideally outperforming mammalian muscle. Key features for the development of these materials are the microscopic binding properties of thiophene and thiophene oligomers stacks, where applied electric fields lead to oxidation and enhanced pi-pi bonding. We have completed the structural studies of neutral and charged oligothiophene dimers, in the search for efficient dimerization mechanisms. A comparison between different density-functional and quantum-chemistry approaches is critically presented, as are solvation effects, described in this work with a combination of first-principles molecular dynamics and a QM/MM approach for the solvating medium.

  15. First-principles relativistic calculations of the fine-structure intervals and magnetic dipole transition probabilities in the 1 s sup 2 2 p configuration of the lithium isoelectric sequence

    SciTech Connect

    Das, B.P.; Venugopal, E.P. ); Idrees, M. )

    1990-12-01

    We present the results of our first-principles relativistic calculations of the fine-structure intervals and magnetic dipole transition probabilities for the 1{ital s}{sup 2}2{ital p} configuration of the lithium isoelectronic sequence using a variational approach. The contributions of the Breit interaction and approximate Lamb-shift corrections are incorporated via first-order perturbation theory. Our results of the fine-structure intervals are in good agreement with experiment, but experimental data for the magnetic dipole transition probabilities are not available for comparison with our calculations.

  16. Compositional dependence of the local structure of Se{sub x}Te{sub 1-x} alloys: Electron energy-loss spectra, real-space multiple-scattering calculations, and first-principles molecular dynamics

    SciTech Connect

    Katcho, N. A.; Lomba, E.; Urones-Garrote, E.; Otero-Diaz, L. C.; Landa-Canovas, A. R.

    2006-06-01

    In this work we present an investigation on the composition dependence of the local structure in Se{sub x}Te{sub 1-x} crystalline alloys analyzing their experimental energy-loss spectra with the aid of a real-space multiple-scattering modeling approach and first-principles molecular dynamics. The concourse of this latter technique is essential for a proper modeling of the alloy spectra. From our results, it can be inferred that Se{sub x}Te{sub 1-x} alloys exhibit a high degree of substitutional disorder ruling out the existence of fully ordered alternating copolymer chains of Se and Te atoms.

  17. Compositional dependence of the local structure of SexTe1-x alloys: Electron energy-loss spectra, real-space multiple-scattering calculations, and first-principles molecular dynamics

    NASA Astrophysics Data System (ADS)

    Katcho, N. A.; Lomba, E.; Urones-Garrote, E.; Landa-Cánovas, A. R.; Otero-Díaz, L. C.

    2006-06-01

    In this work we present an investigation on the composition dependence of the local structure in SexTe1-x crystalline alloys analyzing their experimental energy-loss spectra with the aid of a real-space multiple-scattering modeling approach and first-principles molecular dynamics. The concourse of this latter technique is essential for a proper modeling of the alloy spectra. From our results, it can be inferred that SexTe1-x alloys exhibit a high degree of substitutional disorder ruling out the existence of fully ordered alternating copolymer chains of Se and Te atoms.

  18. Lattice dynamics and electronic structure of energetic solids LiN3 and NaN3: A first principles study

    NASA Astrophysics Data System (ADS)

    Ramesh Babu, K.; Vaitheeswaran, G.

    2013-10-01

    We report density functional theory calculations on the crystal structure, elastic, lattice dynamics and electronic properties of iso-structural layered monoclinic alkali azides, LiN3 and NaN3. The effect of van der Waals interactions on the ground- state structural properties is studied by using various dispersion corrected density functionals. Based on the equilibrium crystal structure, the elastic constants, phonon dispersion and phonon density of states of the compounds are calculated. The accurate energy band gaps are obtained by using the recently developed Tran Blaha-modified Becke Johnson (TB-mBJ) functional and found that both the azides are direct band gap insulators.

  19. Stability of MnB2 with AlB2-type structure revealed by first-principles calculations and experiments

    NASA Astrophysics Data System (ADS)

    Gou, Huiyang; Steinle-Neumann, Gerd; Bykova, Elena; Nakajima, Yoichi; Miyajima, Nobuyoshi; Li, Yuan; Ovsyannikov, Sergey V.; Dubrovinsky, Leonid S.; Dubrovinskaia, Natalia

    2013-02-01

    MnB2 with the ReB2-type structure has been theoretically predicted to be a superhard material which could be synthesized at ambient pressure. However, this phase has not been observed experimentally to date. In the present work, we show that even applying moderate pressure does not facilitate the formation of ReB2-structured MnB2. Our high-pressure high-temperature experiments resulted in the synthesis of single crystals of MnB2 with the previously known AlB2-type structure. This is at odds with results from density functional theory-based calculations using the generalized gradient approximation (GGA). The discrepancy with the computational prediction was reconciled by including on-site repulsion (GGA + U), which found the anti-ferromagnetically ordered AlB2-type structure energetically favored over the ReB2-structure for MnB2.

  20. First principles investigations of structural, elastic, dielectric and piezoelectric properties of { Ba,Sr,Pb } TiO3, { Ba,Sr,Pb } ZrO3 and { Ba,Sr,Pb } { Zr,Ti } O3 ceramics

    NASA Astrophysics Data System (ADS)

    Akgenc, Berna; Tasseven, Cetin; Cagin, Tahir

    2015-03-01

    We use first-principle density-functional study of structural, anisotropic mechanical, dielectric and piezoelectric properties of {Ba,Sr,Pb}TiO3, {Ba,Sr,Pb}ZrO3 and {Ba,Sr,Pb}{Zr,Ti}O3 alloys in cubic perovskite structures at zero temperature. Because there is significant interest in finding new piezoelectrics that do not contain toxic elements such as lead. In this study, we compare piezoelectric response of those alloys to synthesize outstanding piezoelectric materials. In perovskite structures, the spontaneous polarization is due to enormous values of Born effective charges computed by linear response within density functional perturbation theory, which are much larger than predicted nominal charge. We deeply investigated the effects of composition, order and site defects structure on piezoelectric constants.