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.
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.
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).
Interface Structure Prediction from First-Principles
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.
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.
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 CeO_{2}|ZrO_{2} 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.
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.
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
Giant Mechanocaloric Effects in Fluorite-Structured Superionic Materials.
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
Two Dimensional Ice from First Principles: Structures and Phase Transitions.
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
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.
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.
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.
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.
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.
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).
Two-dimensional boron nitride structures functionalization: first principles studies.
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
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.
Unfolding method for first-principles LCAO electronic structure calculations.
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
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
Crystal Structure Prediction from First Principles: The Crystal Structures of Glycine
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
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.
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).
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
A genetic algorithm for first principles global structure optimization of supported nano structures
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.
Structure of the (111) surface of bismuth: LEED analysis and first-principles calculations
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.
First principles study of structural, electronic and mechanical properties of alkali nitride-KN
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.
First-principles calculation of the structural stability of 6d transition metals
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.
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.
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.
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.
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.
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.
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.
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.%.
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.
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.
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.
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.
Pressure induced structural phase transition of OsB{sub 2}: First-principles calculations
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.
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).
Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations
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.
Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations
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
Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations
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 Co_{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_{11}Zr_{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_{5}Zr phase and larger than that of the low-temperature Co_{5.25}Zr phase. As a result, our calculations provide useful guidelines for further experimental optimization of the magnetic performances of these alloys.
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.
Structural, electronic and mechanical properties of rare earth nitride-ErN: A first principles study
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
Solvation Structure and Dynamics of Ni(2+)(aq) from First Principles.
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
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.
Electronic structures and optical spectra of BaO from first principles
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.
Novel phases of lithium-aluminum binaries from first-principles structural search
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.
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.
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.
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.
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.
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.
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.
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.
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.
Structural stability and electronic properties of InSb nanowires: A first-principles study
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.
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.
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.
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.
Thermodynamic ground state of MgB{sub 6} predicted from first principles structure search methods
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.
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.
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.
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
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
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
Structural stability and electronic properties of β-tetragonal boron: A first-principles study
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.
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.
Determination of structure and properties of molecular crystals from first principles.
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
First-principles material modeling of solid-state electrolytes with the spinel structure.
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
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).
First principles calculations of interlayer exchange coupling in bcc Fe/Cu/Fe structures
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.
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.
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.
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.
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
Prediction of new high pressure structural sequence in thorium carbide: A first principles study
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
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.
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).
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.
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.
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
First-principles study of the crystal and electronic structures of {alpha}-tetragonal boron
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).
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.
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.
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.
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
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.
Substrate-induced structures of bismuth adsorption on graphene: a first principles study.
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
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.
Hydration thermodynamics of pyrochlore structured oxides from TG and first principles calculations.
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
Structure of hydrophobic hydration of benzene and hexafluorobenzene from first principles
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.
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
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
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.
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.
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
First Principles Modeling of Phonon Heat Conduction in Nanoscale Crystalline Structures
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.
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.
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.
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.
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
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.
First principles predictions of van der Waals bonded inorganic crystal structures: Test case, HgCl2
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.
Experimental and first principle studies on electronic structure of BaTiO{sub 3}
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}.
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.
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.
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.
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.
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.
First-Principles Study of Carbon and Vacancy Structures in Niobium
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.
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.
First-Principles Study of Carbon and Vacancy Structures in Niobium
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
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.
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
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.
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.
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.
Thermodynamics of solid electrolytes and related oxide ceramics based on the fluorite structure
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.
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.
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.
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.
First-Principles Mobility Calculations and Atomic-Scale Interface Roughness in Nanoscale Structures
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.
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.
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.
Crystal structure and physical properties of Mo{sub 2}B: First-principle calculations
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.
Investigating the structural evolution of thiolate protected gold clusters from first-principles.
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
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
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.
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.
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.
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.
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.
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
First principle calculations of structural phase transition and electronic properties in AmTe
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.
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.
Large-Scale Computations Leading to a First-Principles Approach to Nuclear Structure
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.
First principle investigation of structural and electronic properties of bulk ZnSe
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.
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.
Bridge Structure for the graphene/Ni(111) system: A first principles study
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
First-principles study on the structural and electronic properties of metallic HfH2 under pressure
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
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
Structure-property relationships of curved aromatic materials from first principles.
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
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).
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.
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.
Structure and density of basaltic melts at mantle conditions from first-principles simulations
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
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.
Thermodynamic stability and structures of iron chloride surfaces: A first-principles investigation
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.
Structure and density of basaltic melts at mantle conditions from first-principles simulations.
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
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.
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.
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.
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
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.
First-principles study of the structure of water layers on flat and stepped Pb electrodes
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
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.
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.
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.
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.
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.
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.
Structural and configurational properties of nanoconfined monolayer ice from first principles.
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
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.
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.
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.
Structure and Properties of the Fe/Y2O3 Interface from First Principles Calculations
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.
Structural and configurational properties of nanoconfined monolayer ice from first principles
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
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.
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.
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
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
Surface structure and hole localization in bismuth vanadate: A first principles study
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.
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.
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.
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.
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.
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.
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.
Effect of tensile strain on the electronic structure of Ge: A first-principles calculation
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.
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.
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.
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.
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.
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
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.
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
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
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.
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
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
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.
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.
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.
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
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
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.
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.
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.
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.
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
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
Structural and magnetic properties of ternary Fe1– xMnxPt nanoalloys from first principles
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
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.
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.
Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7
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
Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7
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
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.
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.
First-principles study of the Pd–Si system and Pd(001)/SiC(001) hetero-structure
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), Pd_{2}Si (P6⁻2m, P6_{3}/mmc, P3m1, P3⁻1m) and Pd_{3}Si (Pnma, P6_{3}22, 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-Pd_{2}Si and D0_{11}-Pd_{3}Si fragments, in agreement with experiment.
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
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.