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Sample records for pressure-induced polymorphic transition

  1. First principles study of pressure induced polymorphic phase transition in KNO3

    NASA Astrophysics Data System (ADS)

    Yedukondalu, N.; Vaitheeswaran, G.

    2015-06-01

    We report the structural, elastic, electronic, and vibrational properties of polymorphic phases II and III of KNO3 based on density functional theory (DFT). Using semi-empirical dispersion correction (DFT-D2) method, we predicted the correct thermodynamic ground state of KNO3 and the obtained ground state properties of the polymorphs are in good agreement with the experiments. We further used this method to calculate the elastic constants, IR and Raman spectra, vibrational frequencies and their assignment of these polymorphs. The calculated Tran Blaha-modified Becke Johnson (TB-mBJ) electronic structure shows that both the polymorphic phases are direct band gap insulators with mixed ionic and covalent bonding. Also the TB-mBJ band gaps are improved over standard DFT functionals which are comparable with the available experiments.

  2. First principles study of pressure induced polymorphic phase transition in KNO{sub 3}

    SciTech Connect

    Yedukondalu, N.; Vaitheeswaran, G.

    2015-06-24

    We report the structural, elastic, electronic, and vibrational properties of polymorphic phases II and III of KNO{sub 3} based on density functional theory (DFT). Using semi-empirical dispersion correction (DFT-D2) method, we predicted the correct thermodynamic ground state of KNO{sub 3} and the obtained ground state properties of the polymorphs are in good agreement with the experiments. We further used this method to calculate the elastic constants, IR and Raman spectra, vibrational frequencies and their assignment of these polymorphs. The calculated Tran Blaha-modified Becke Johnson (TB-mBJ) electronic structure shows that both the polymorphic phases are direct band gap insulators with mixed ionic and covalent bonding. Also the TB-mBJ band gaps are improved over standard DFT functionals which are comparable with the available experiments.

  3. Pressure-induced phase transition in pentacene

    NASA Astrophysics Data System (ADS)

    Farina, L.; Brillante, A.; Della Valle, R. G.; Venuti, E.; Amboage, M.; Syassen, K.

    2003-07-01

    We have recently studied two solid phases of bulk pentacene (polymorphs H and C) by means of lattice phonon Raman spectroscopy. The assignment, previously based on lattice dynamics calculations alone, is now verified by X-ray diffraction measurements, conclusively confirming the existence of both polymorphs. Furthermore, Raman phonon spectra indicate a pressure-induced phase transition where the polymorph C (lower density phase) transforms to the H form (higher density phase). The onset pressure for the phase transition is only 0.2 GPa. The phase change is irreversible.

  4. The Pressure-Induced Polymorphic Transformations in Fluconazole.

    PubMed

    Gorkovenko, Ekaterina A; Kichanov, Sergey E; Kozlenko, Denis P; Belushkin, Alexandr V; Wąsicki, Jan; Nawrocik, Wojciech; Mielcarek, Jadwiga; Dubrovinsky, Leonid S; Lathe, Christian; Savenko, Boris N

    2015-12-01

    The structural properties and Raman spectra of fluconazole have been studied by means of X-ray diffraction and Raman spectroscopy at pressures up to 2.5 and 5.5 GPa, respectively. At a pressure of 0.8 GPa, a polymorphic phase transition from the initial form I to a new triclinic form VIII has been observed. At higher pressure of P = 3.2 GPa, possible transformation into another new polymorphic form IX has been detected. The unit cell parameters and volumes, and vibration modes as functions of pressure have been obtained for the different forms of fluconazole. PMID:26367523

  5. Pressure-induced phase transition in CrO2.

    PubMed

    Alptekin, Sebahaddin

    2015-12-01

    The ab initio constant pressure molecular dynamics technique and density functional theory with generalized gradient approximation (GGA) was used to study the pressure-induced phase transition of CrO2. The phase transition of the rutile (P42/mnm) to the orthorhombic CaCl2 (Pnnm) structure at 30 GPa was determined successfully in a constant pressure simulation. This phase transition was analyzed from total energy calculations and, from the enthalpy calculation, occurred at around 17 GPa. Structural properties such as bulk modules, lattice parameters and phase transition were compared with experimental results. The phase transition at 12 ± 3 GPa was in good agreement with experimental results, as was the phase transition from the orthorhombic CaCl2 (Pnnm) to the monoclinic (P21/c) structure also found at 35 GPa. PMID:26541468

  6. Pressure-induced phase transitions and metallization in VO2

    NASA Astrophysics Data System (ADS)

    Bai, Ligang; Li, Quan; Corr, Serena A.; Meng, Yue; Park, Changyong; Sinogeikin, Stanislav V.; Ko, Changhyun; Wu, Junqiao; Shen, Guoyin

    2015-03-01

    We report the results of pressure-induced phase transitions and metallization in VO2 based on synchrotron x-ray diffraction, electrical resistivity, and Raman spectroscopy. Our isothermal compression experiments at room temperature and 383 K show that the room temperature monoclinic phase (M 1 ,P 21/c ) and the high-temperature rutile phase (R ,P 42/m n m ) of VO2 undergo phase transitions to a distorted M 1 monoclinic phase (M 1' ,P 21/c ) above 13.0 GPa and to an orthorhombic phase (CaCl2-like, P n n m ) above 13.7 GPa, respectively. Upon further compression, both high-pressure phases transform into a new phase (phase X ) above 34.3 and 38.3 GPa at room temperature and 383 K, respectively. The room temperature M 1 -M 1' phase transition structurally resembles the R -CaCl2 phase transition at 383 K, suggesting a second-order displacive type of transition. Contrary to previous studies, our electrical resistivity results, Raman measurements, as well as ab initio calculations indicate that the new phase X , rather than the M 1' phase, is responsible for the metallization under pressure. The metallization mechanism is discussed based on the proposed crystal structure.

  7. Elasticity and Pressure-induced Phase Transition in Coesite from Experiments and First Principle Calculations

    NASA Astrophysics Data System (ADS)

    Chen, T.; Wang, X.; Qi, X.; Ma, M.; Xu, Z.; Li, B.

    2015-12-01

    Coesite (space group C2/c) is a high-pressure polymorph of quartz. The behavior of coesite under pressure has long been of interest due to its abundance in the Earth's crust and mantle, and its relative simple chemistry but rich polymorphisms under elevated pressure and/or temperature conditions. A most recent Raman spectroscopy study reported two pressure-induced phase transitions at ~23 (coesite-II) and ~35 GPa, respectively. To further understand the properties of these pressure-induced phase transitions, we conducted X-ray diffraction experiments starting with coesite powder in a diamond anvil cell up to 31 GPa, and performed first-principle calculations on coesite, coesite-II (space group P21/n), and stishovite at 0 K up to 45 GPa. X-ray diffraction data show the formation of coesite-II at pressures above 20 GPa, which is consistent with first principles calculations that the enthalpy of coesite-II becomes lower than that of coesite above 21.4 GPa. Coesite is very anisotropic with the a-axis twice more compressible than the b- and c-axis. By comparison, coesite-II is less anisotropic, with a similar compressibility in a-, b-, and c-axis. As analyzed by a third-order Eulerian finite strain equation of state, the bulk modulus of coesite at 21.4 GPa is 180.6 GPa, and that of coesite-II is 140.8 GPa, indicating that coesite-II is much more compressible than coesite. If coesite-coesite-II transition occurs in cold subduction zones, it will change the elasticity as well as anisotropic properties of the subducted MORB, due to the different compressional behavior between coesite and coesite-II.

  8. Pressure induced phase transitions in ceramic compounds containing tetragonal zirconia

    SciTech Connect

    Sparks, R.G.; Pfeiffer, G.; Paesler, M.A.

    1988-12-01

    Stabilized tetragonal zirconia compounds exhibit a transformation toughening process in which stress applied to the material induces a crystallographic phase transition. The phase transition is accompanied by a volume expansion in the stressed region thereby dissipating stress and increasing the fracture strength of the material. The hydrostatic component of the stress required to induce the phase transition can be investigated by the use of a high pressure technique in combination with Micro-Raman spectroscopy. The intensity of Raman lines characteristic for the crystallographic phases can be used to calculate the amount of material that has undergone the transition as a function of pressure. It was found that pressures on the order of 2-5 kBar were sufficient to produce an almost complete transition from the original tetragonal to the less dense monoclinic phase; while a further increase in pressure caused a gradual reversal of the transition back to the original tetragonal structure.

  9. Pressure-induced reversible phase transition in thiourea dioxide crystal

    SciTech Connect

    Wang, Qinglei; Yan, Tingting; Zhu, Hongyang; Cui, Qiliang; Zou, Bo E-mail: zoubo@jlu.edu.cn; Wang, Kai E-mail: zoubo@jlu.edu.cn

    2015-06-28

    The effect of high pressure on the crystal structure of thiourea dioxide has been investigated by Raman spectroscopy and angle-dispersive X-ray diffraction (ADXRD) in a diamond anvil cell up to 10.3 GPa. The marked changes in the Raman spectra at 3.7 GPa strongly indicated a structural phase transition associated with the distortions of hydrogen bonding. There were no further changes up to the maximum pressure of 10.3 GPa and the observed transition was completely reversible when the system was brought back to ambient pressure. This transition was further confirmed by the changes of ADXRD spectra. The high-pressure phase was indexed and refined to an orthorhombic structure with a possible space group Pbam. The results from the first-principles calculations suggested that this phase transition was mainly related to the changes of hydrogen-bonded networks in thiourea dioxide.

  10. Pressure-induced isostructural transition in PdN2

    SciTech Connect

    Aberg, D; Erhart, P; Crowhurst, J; Zaug, J M; Goncharov, A F; Sadigh, B

    2010-03-05

    We show that a synthesized Pd-N compound crystallize into the pyrite structure by comparison of experimental and calculated Raman intensities. The decreasing Raman intensities with decreasing pressure is explained by a closing of the fundamental band gap. We further discuss the experimental decomposition of this compound at 11 GPa in terms of an isostructural transition within the pyrite structure.

  11. Pressure induced structural phase transition in IB transition metal nitrides compounds

    NASA Astrophysics Data System (ADS)

    Soni, Shubhangi; Kaurav, Netram; Jain, A.; Shah, S.; Choudhary, K. K.

    2015-06-01

    Transition metal mononitrides are known as refractory compounds, and they have, relatively, high hardness, brittleness, melting point, and superconducting transition temperature, and they also have interesting optical, electronic, catalytic, and magnetic properties. Evolution of structural properties would be an important step towards realizing the potential technological scenario of this material of class. In the present study, an effective interionic interaction potential (EIOP) is developed to investigate the pressure induced phase transitions in IB transition metal nitrides TMN [TM = Cu, Ag, and Au] compounds. The long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach with modified ionic charge are properly incorporated in the EIOP. The vdW coefficients are computed following the Slater-Kirkwood variational method, as both the ions are polarizable. The estimated value of the phase transition pressure (Pt) and the magnitude of the discontinuity in volume at the transition pressure are consistent as compared to the reported data.

  12. Pressure induced structural phase transition in IB transition metal nitrides compounds

    SciTech Connect

    Soni, Shubhangi; Kaurav, Netram Jain, A.; Shah, S.; Choudhary, K. K.

    2015-06-24

    Transition metal mononitrides are known as refractory compounds, and they have, relatively, high hardness, brittleness, melting point, and superconducting transition temperature, and they also have interesting optical, electronic, catalytic, and magnetic properties. Evolution of structural properties would be an important step towards realizing the potential technological scenario of this material of class. In the present study, an effective interionic interaction potential (EIOP) is developed to investigate the pressure induced phase transitions in IB transition metal nitrides TMN [TM = Cu, Ag, and Au] compounds. The long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach with modified ionic charge are properly incorporated in the EIOP. The vdW coefficients are computed following the Slater-Kirkwood variational method, as both the ions are polarizable. The estimated value of the phase transition pressure (Pt) and the magnitude of the discontinuity in volume at the transition pressure are consistent as compared to the reported data.

  13. Pressure-Induced Electronic Transition in Black Phosphorus.

    PubMed

    Xiang, Z J; Ye, G J; Shang, C; Lei, B; Wang, N Z; Yang, K S; Liu, D Y; Meng, F B; Luo, X G; Zou, L J; Sun, Z; Zhang, Y; Chen, X H

    2015-10-30

    In a semimetal, both electrons and holes contribute to the density of states at the Fermi level. The small band overlaps and multiband effects engender novel electronic properties. We show that a moderate hydrostatic pressure effectively suppresses the band gap in the elemental semiconductor black phosphorus. An electronic topological transition takes place at approximately 1.2 GPa, above which black phosphorus evolves into a semimetal state that is characterized by a colossal positive magnetoresistance and a nonlinear field dependence of Hall resistivity. The Shubnikov-de Haas oscillations detected in magnetic field reveal the complex Fermi surface topology of the semimetallic phase. In particular, we find a nontrivial Berry phase in one Fermi surface that emerges in the semimetal state, as evidence of a Dirac-like dispersion. The observed semimetallic behavior greatly enriches the material property of black phosphorus and sets the stage for the exploration of novel electronic states in this material. PMID:26565480

  14. Isentropic Compression Loading of HMX and the Pressure-induced Phase Transition at 27 GPa

    SciTech Connect

    Hare, D E; Reisman, D B; Dick, J J; Forbes, J W

    2004-02-25

    The 27 GPa pressure-induced epsilon-phi phase transition in HMX is explored using the Isentropic Compression Experiment (ICE) technique at the Sandia National Laboratories Z-machine facility. Our data indicate that this phase transition is sluggish and if it does occur to any extent under the time scales (200-500 ns) and strain rates (5 x 10{sup 5}) typical of ICE loading conditions, the amount of conversion is small.

  15. Pressure-induced phase transitions in GeS under high pressures

    NASA Astrophysics Data System (ADS)

    Dias, Ranga; Yoo, Choong-Shik

    2012-02-01

    We have studied the pressure-induced structural and electronic phase transitions of layered GeS (Pnma) to 30 GPa, using micro-Raman spectroscopy and electrical resistivity measurements in diamond anvil cells. The result shows a steady decrease in resistivity to that of metal at around 18 GPa. The visual appearance of GeS supports the insulator-metal transition: initially black GeS becomes opaque and eventually reflective with increasing pressure. The Raman result indicates that the metallization is preceded by a structural phase transition, presumably to the previously predicted Cmcm structure.

  16. Pressure-induced irreversible phase transitions of the monoclinic GdOOH nanorods at ambient temperature

    NASA Astrophysics Data System (ADS)

    Zhang, Chuanchao; Dai, Rucheng; Sui, Zhilei; Chen, Qiao; Wang, Zhongping; Yuan, Xiaodong; Zhang, Zengming; Ding, Zejun

    2014-09-01

    The structural transition of monoclinic GdOOH nanorods was studied by using a diamond anvil cell at room temperature with the probe of Eu3+ ion luminescence under pressures up to 21.4 GPa. The changes of luminescence spectra indicated that a pressure-induced phase transition from the monoclinic phase to the high pressure tetragonal phase occurs at 10.7 GPa for GdOOH nanorods, and the monoclinic GdOOH nanorods are gradually transformed into the tetragonal phase with increasing pressure. After releasing of pressure to the ambient, the high pressure tetragonal phase is retained, and the phase transition of GdOOH nanorods is irreversible.

  17. Pressure-induced phase transitions in organic molecular crystals: a combination of x-ray single-crystal and powder diffraction, raman and IR-spectroscopy

    NASA Astrophysics Data System (ADS)

    Boldyreva, E. V.; Sowa, H.; Ahsbahs, H.; Goryainov, S. V.; Chernyshev, V. V.; Dmitriev, V. P.; Seryotkin, Y. V.; Kolesnik, E. N.; Shakhtshneider, T. P.; Ivashevskaya, S. N.; Drebushchak, T. N.

    2008-07-01

    The contribution summarizes the results of recent studies of phase transitions induced by high pressure in a number of molecular organic crystals, such as polymorphs of paracetamol, chlorpropamide, polymorphs of glycine, L- and DL-serine, β-alanine. The main attention is paid to the following topics: (1) Reversible / irreversible transformations; (2) Different behavior of single crystals / powders; (3) The role of pressure-transmitting liquid; (4) The role of the kinetic factors: phase transitions on decompression, or after a long storage at a selected pressure; (5) Isosymmetric phase transitions; (6) The role of the changes in the hydrogen bond networks / intramolecular conformational changes in the phase transitions; (7) Superstructures / nanostructures formed as a result of pressure-induced phase transitions.

  18. Pressure-induced metal-insulator transition in spinel compound CuV 2S 4

    NASA Astrophysics Data System (ADS)

    Okada, H.; Koyama, K.; Hedo, M.; Uwatoko, Y.; Watanabe, K.

    2008-04-01

    In order to investigate the pressure effect on electrical properties of CuV 2S 4, we performed the electrical resistivity measurements under high pressures up to 8 GPa for a high-quality polycrystalline sample. The charge density wave (CDW) transition temperatures increase with increasing pressure. The residual resistivity rapidly increases with increasing pressure over 4 GPa, and the temperature dependence of the electrical resistivity at 8 GPa exhibits a semiconducting behavior below about 150 K, indicating that a pressure-induced metal-insulator transition occurs in CuV 2S 4 at 8 GPa.

  19. Pressure-induced phase transition and superconductivity in YBa2Cu4O8

    NASA Astrophysics Data System (ADS)

    Souliou, S. M.; Subedi, A.; Song, Y. T.; Lin, C. T.; Syassen, K.; Keimer, B.; Le Tacon, M.

    2014-10-01

    We investigate the pressure and temperature dependence of the lattice dynamics of the underdoped, stoichiometric, high-temperature superconductor YBa2Cu4O8 by means of Raman spectroscopy and ab initio calculations. This system undergoes a reversible pressure-induced structural phase transition around 10 GPa to a collapsed orthorhombic structure that is well reproduced by the calculation. The coupling of the B1g-like buckling phonon mode to the electronic continuum is used to probe superconductivity. In the low pressure phase, self-energy effects through the superconducting transition renormalize this phonon, and the amplitude of this renormalization strongly increases with pressure. Whereas our calculation indicates that this mode's coupling to the electronic system is only marginally affected by the structural phase transition, the aforementioned renormalization is completely suppressed in the high pressure phase, demonstrating that under hydrostatic pressures higher than 10 GPa, superconductivity in YBa2Cu4O8 is greatly weakened or obliterated.

  20. Pressure-induced structural transition of CdxZn1-xO alloys

    NASA Astrophysics Data System (ADS)

    Chen, Yabin; Zhang, Shuai; Gao, Weiwei; Ke, Feng; Yan, Jinyuan; Saha, Bivas; Ko, Changhyun; Suh, Joonki; Chen, Bin; Ager, Joel W.; Walukiewicz, Wladek; Jeanloz, Raymond; Wu, Junqiao

    2016-04-01

    CdxZn1-xO alloys, as a transparent conducting oxide, have recently attracted much attention for potential optoelectronic applications. In this letter, we report a hydrostatic pressure-induced phase transition of CdxZn1-xO alloys from the wurtzite to the rocksalt structure and its phase diagram probed using a diamond anvil cell. It is found that the transition pressure, determined by changes in optical and structural properties, depends sensitively on the composition. As the Cd content increases, the critical pressure decreases, until at x = 0.67 where the alloy is intrinsically stable in the rocksalt phase even at ambient pressure. The wurtzite phase is light emitting with a direct bandgap that slightly widens with increasing pressure, while the rocksalt phase has a much wider bandgap that is indirect. The pressure-sensitive light emission and phase transition may find potential applications in fields such as stress sensing and energy storage.

  1. Pressure-induced solidifications of liquid sulfur below and above λ-transition

    NASA Astrophysics Data System (ADS)

    Fei, Tang; Lin-Ji, Zhang; Feng-Liang, Liu; Fei, Sun; Wen-Ge, Yang; Jun-Long, Wang; Xiu-Ru, Liu; Ru, Shen

    2016-04-01

    Two kinds of glassy sulfurs are synthesized by the rapid compression method from liquid sulfur at temperatures below and above the λ -transition point. The glassy sulfur has different colors and transparencies, depending on temperature, which may inherit some structural information from the λ -transition. Raman spectrum studies of these samples show that a large fraction of polymeric chains exist in the glassy sulfur, even in the one solidified from T < T λ . We find that a higher compression rate instead of a higher temperature of the parent liquid captures more polymeric chains. Pressure-induced glassy sulfur presents high thermal stability compared with temperature quenched glassy sulfur and could transform into liquid sulfur directly without crystallization through an abnormal exothermic melting course. High energy x-ray diffraction is utilized to study the local order of the pressure-induced glassy sulfur. Project supported by the Joint Funds of the National Natural Science Foundation of China (Grant No. U1530402), the National Natural Science Foundation of China (Grant No. 11004163), the Fundamental Research Funds for the Central Universities, China (Grant No. 2682014ZT31), the Department of Energy National Nuclear Security Administration (Grant No. DE-NA0001974), and the Department of Energy Basic Energy Sciences (Grant Nos. DE-FG02-99ER45775 and DE-AC02-06CH11357).

  2. Quantum Oscillation Signatures of Pressure-induced Topological Phase Transition in BiTeI

    PubMed Central

    Park, Joonbum; Jin, Kyung-Hwan; Jo, Y. J.; Choi, E. S.; Kang, W.; Kampert, E.; Rhyee, J.-S.; Jhi, Seung-Hoon; Kim, Jun Sung

    2015-01-01

    We report the pressure-induced topological quantum phase transition of BiTeI single crystals using Shubnikov-de Haas oscillations of bulk Fermi surfaces. The sizes of the inner and the outer FSs of the Rashba-split bands exhibit opposite pressure dependence up to P = 3.35 GPa, indicating pressure-tunable Rashba effect. Above a critical pressure P ~ 2 GPa, the Shubnikov-de Haas frequency for the inner Fermi surface increases unusually with pressure, and the Shubnikov-de Haas oscillations for the outer Fermi surface shows an abrupt phase shift. In comparison with band structure calculations, we find that these unusual behaviors originate from the Fermi surface shape change due to pressure-induced band inversion. These results clearly demonstrate that the topological quantum phase transition is intimately tied to the shape of bulk Fermi surfaces enclosing the time-reversal invariant momenta with band inversion. PMID:26522628

  3. Pressure-induced structural transition in copper pyrazine dinitrate and implications for quantum magnetism

    NASA Astrophysics Data System (ADS)

    O'Neal, K. R.; Zhou, J.; Cherian, J. G.; Turnbull, M. M.; Landee, C. P.; Jena, P.; Liu, Z.; Musfeldt, J. L.

    2016-03-01

    We combined synchrotron-based infrared and Raman spectroscopies, diamond anvil cell techniques, and first principles calculations to unveil pressure-induced distortions in quasi-one-dimensional Cu(pyz)(NO3)2. The crossover at 0.7 GPa is local in nature whereas the transition at 5 GPa lowers symmetry from P m n a to P 2221 and is predicted to slightly increase magnetic dimensionality. Comparison with prior magnetoinfrared results reveals the striking role of out-of-plane bending of the pyrazine ligand, a finding that we discuss in terms of the possibility of using pressure to bias the magnetic quantum critical transition in this classic S =1 /2 antiferromagnet.

  4. Pressure induced spin transition revealed by iron M{sub 2,3}-edge spectroscopy

    SciTech Connect

    Nyrow, Alexander; Büning, Thomas; Mende, Kolja; Tolan, Metin; Sternemann, Christian; Hiraoka, Nozomu; Desgreniers, Serge; Wilke, Max

    2014-06-30

    We present a method to characterize pressure induced magnetic high to low spin transition in iron sulphide using x-ray Raman scattering spectroscopy at the iron M{sub 2,3}-edge. The advantage of this method is that the observed spectral changes between pressures of 1.7 GPa and 10.1 GPa can be used with the help of atomic multiplet calculations to determine the crystal field splitting parameters associated with the spin transition. We discuss the potential of this M{sub 2,3}-edge spectroscopy to investigate the irons electronic spin state in-situ at the conditions of the inner Earth, i.e., at high temperature and high pressure, providing exciting opportunities for geophysical and materials science applications.

  5. Pressure-Induced Phase Transition in Hydrogen-Bonded Supramolecular Structure: Guanidinium Nitrate

    SciTech Connect

    Wang, Run; Li, Shourui; Wang, Kai; Duan, Defang; Tang, Lingyun; Cui, Tian; Liu, Bingbing; Cui, Qiliang; Liu, Jing; Zou, Bo; Zou, Guangtian

    2010-08-04

    In situ Raman scattering and synchrotron X-ray diffraction have been used to investigate the effects of high pressure on the structural stability of guanidinium nitrate (C(NH{sub 2}){sub 3}{sup +} {center_dot} NO{sub 3}{sup -}, GN), a representative two-dimensional supramolecular architecture of hydrogen-bonded rosette network. This study has confirmed a structural phase transition observed by Raman scattering and X-ray diffraction at {approx}1 GPa and identified it as a space group change from C2 to P2{sub 1}. The high-pressure phase remained stable up to 22 GPa. We discussed the pressure-induced changes in N-H stretching vibration in Raman spectra and proposed that this phase transition is due to the rearrangements of the hydrogen-bonding networks.

  6. Pressure-induced phonon softenings and the structural and magnetic transitions in CrO2

    NASA Astrophysics Data System (ADS)

    Kim, Sooran; Kim, Kyoo; Kang, Chang-Jong; Min, B. I.

    2012-03-01

    To investigate the pressure-induced structural transitions of chromium dioxide (CrO2), phonon dispersions and total-energy band structures are calculated as a function of pressure. The observed structural transition has been theoretically reproduced at P≈10 GPa from the ground-state tetragonal CrO2 (t-CrO2) of the rutile type to orthorhombic CrO2 (o-CrO2) of the CaCl2 type. The half-metallic property is found to be preserved in o-CrO2. The softening of the Raman-active B1g phonon mode, which is responsible for this structural transition, is demonstrated. The second structural transition is found to occur for P⩾61.1 GPa from ferromagnetic (FM) o-CrO2 to nonmagnetic monoclinic CrO2 (m-CrO2) of the MoO2 type, which is related to the softening mode at q=R((1)/(2),0,(1)/(2)). The third structural transition has been identified at P=88.8 GPa from m-CrO2 to cubic CrO2 of the CaF2 type that is a FM insulator.

  7. Mechanisms for pressure-induced crystal-crystal transition, amorphization, and devitrification of Snl4

    DOE PAGESBeta

    Liu, Hanyu; Tse, John S.; Hu, Michael Y.; Bi, Wenli; Zhao, Jiyong; Alp, E. Ercan; Pasternak, Moshe; Taylor, R. Dean; Lashley, Jason C.

    2015-10-27

    The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigated using first principles molecular dynamics calculations together with high-pressure 119Sn nuclear resonant inelastic x-ray scattering measurements. Above ~8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ~64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI4 under ambient conditions. Although high pressure structures of SnI4 were thought to be determined by random packing of equal-sized spheres,more » we detected electron charge transfer in each phase. As a result, this charge transfer results in a crystal structure packing determined by larger than expected iodine atoms. (C) 2015 AIP Publishing LLC.« less

  8. Observation of the transition state for pressure-induced BO₃→ BO₄ conversion in glass.

    PubMed

    Edwards, Trenton; Endo, Takatsugu; Walton, Jeffrey H; Sen, Sabyasachi

    2014-08-29

    A fundamental mechanistic understanding of the pressure- and/or temperature-induced facile transformation of the coordination environment of boron is important for changing the physical properties of glass. We have used in situ high-pressure (up to 2 gigapascals) boron-11 solid-state nuclear magnetic resonance spectroscopy in combination with ab initio calculations to investigate the nature of the transition state for the pressure-induced BO3→ BO4 conversion in a borosilicate glass at ambient temperature. The results indicate an anisotropic elastic deformation of the BO3 planar triangle, under isotropic stress, into a trigonal pyramid that likely serves as a precursor for the subsequent formation of a BO4 tetrahedron. PMID:25170146

  9. Pressure-induced phase transitions in Pa metal from first-principles theory

    SciTech Connect

    Soederlind, P.; Eriksson, O.

    1997-11-01

    Protactinium metal is shown to undergo a phase transition to the {alpha}-U orthorhombic structure below 1 Mbar pressure. At higher pressures, the bct phase reenters in the phase diagram and at the highest pressures, an ideal hcp structure becomes stable. Hence, Pa undergoes a sequence of transitions; bct{r_arrow}{alpha}-U{r_arrow}bct{r_arrow}hcp, with the first transition taking place at 0.25 Mbar and the subsequent ones above 1 Mbar. The bct{r_arrow}{alpha}-U transition is triggered by the pressure-induced promotion of the spd valence states to 5f states. In this regard, Pa approaches uranium which at ambient conditions has one more 5f electron than Pa at similar conditions. At higher compression of Pa, the 5f band broadens and electrostatic interactions in combination with Born-Mayer repulsion become increasingly important and this drives Pa to gradually more close-packed structures. At ultrahigh pressures, the balance between electrostatic energy, Born-Mayer repulsion, and one-electron band energy stabilizes the hcp (ideal packing) structure. The electrostatic energy and Born-Mayer repulsion rule out open crystal structures under these conditions in Pa and between the close-packed structures, the hcp structure is shown to be stabilized by filling of the 5f band. {copyright} {ital 1997} {ital The American Physical Society}

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

    SciTech Connect

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

    2010-04-15

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

  11. Microscopic origin of pressure-induced isosymmetric transitions in fluoromanganate cryolites

    NASA Astrophysics Data System (ADS)

    Charles, Nenian; Rondinelli, James M.

    2014-09-01

    Using first-principles density functional theory calculations, we investigate the hydrostatic pressure-induced reorientation of the Mn-F Jahn-Teller bond axis in the fluoride cryolite Na3MnF6. We find that a first-order isosymmetric phase transition (IPT) occurs between crystallographically equivalent monoclinic structures at approximately 2.15 GPa, consistent with earlier experimental studies. Mode-crystallography analyses of the pressure-dependent structures in the vicinity of the transition reveal a clear evolution of the Jahn-Teller bond distortions in cooperation with an asymmetrical stretching of the equatorial fluorine atoms in the MnF6 octahedral units. We identify a significant (70%) change in the orbital occupancy of the eg manifold of the 3d4 Mn(III) to be responsible for the transition, stabilizing one monoclinic P21/n variant over the other. The orbital reconstruction as a driving force for the transition is confirmed by analogous calculations of isostructural 3d0 Na3ScF6, which shows no evidence of an IPT up to 6.82 GPa.

  12. Pressure-induced phase transition and structural properties of CrO2

    NASA Astrophysics Data System (ADS)

    Wu, H. Y.; Chen, Y. H.; Deng, C. R.; Su, X. F.

    2012-08-01

    The structural properties and pressure-induced phase transitions of CrO2 have been investigated using the pseudopotential plane-wave method based on the density functional theory (DFT). The rutile-type (P42/mnm), CaCl2-type (Pnnm), pyrite-type (Pā3), and CaF2-type (Fm-3m) phases of CrO2 have been considered. The structural properties such as lattice parameters, bulk moduli and its pressure derivative are consistent with the available experimental data. The second-order phase-transition pressure of CrO2 from the rutile phase to CaCl2 phase is 10.9 GPa, which is in good agreement with the experimental result. The sequence of these phases is rutile-type → CaCl2-type → pyrite-type → CaF2-type with the phase-transition pressures 10.9, 23.9, and 144.5 GPa, respectively. The equation of state of different phases has also been presented. It is more difficult to compress with the increase of pressure for different phases of CrO2.

  13. Pressure-induced transition in Tl{sub 2}MoO{sub 4}

    SciTech Connect

    Machon, Denis; Friese, Karen; Breczewski, Tomasz; Grzechnik, Andrzej

    2010-11-15

    Tl{sub 2}MoO{sub 4} has been studied under high-pressure by X-ray diffraction, Raman spectroscopy, and optical absorption measurements. A first-order phase transition is observed at 3.5{+-}0.5 GPa. The nature (ordered vs. disordered) of the high-pressure phase strongly depends on the local hydrostatic conditions. Optical absorption measurements tend to show that this transition is concomitant with an electronic structure transformation. Prior to the transition, single crystal X-ray diffraction shows that pressure induces interactions between MoO{sub 4} fragments and the Mo coordination number tends to increase. In addition, the stereoactivity of the lone-pair electrons on the three symmetrically independent Tl-sites is not uniform; while for two sites the stereoactivity decreases with increasing pressures for the third site the stereoactivity increases. - Graphical Abstract: (up) Structural evolutions of Tl{sub 2}MoO{sub 4} in the low-pressure phase. (Down) Optical properties of the high-pressure phase as a function of pressure. Display Omitted

  14. Pressure-Induced Structural and Electronic Transition in Sr2ZnWO6 Double Perovskite.

    PubMed

    Li, Nana; Manoun, Bouchaib; Tang, Lingyun; Ke, Feng; Liu, Fengliang; Dong, Haini; Lazor, Peter; Yang, Wenge

    2016-07-01

    High-pressure structural and electrical properties of Sr2ZnWO6 double perovskite were investigated using in situ angle-dispersive synchrotron X-ray diffraction (XRD), Raman, and alternating current (AC) impedance spectroscopy. A structural transition from monoclinic (P21/n) to triclinic (P1̅) phase around 9 GPa was observed due to the pressure-induced distortion of (W, Zn)O6 octahedron. In situ high-pressure Raman spectroscopy showed the increasing interaction among O-W-O in WO6 octahedron with pressure and a transition pressure consistent with the XRD results. From the AC impedance spectroscopy measurements, the resistivity increased steeply by ∼1 order of magnitude around 11 GPa, indicating an electronic transition accompanying the symmetry change. The increase in the interaction among O-W-O enhances the attraction of O(2-) electrons toward W(6+), thus increasing the covalence, which in turn lowers the charge transfer energy between O(2-) and W(6+) and induces the resistivity increase under high pressure. PMID:27308777

  15. Transformation Mechanism and Kinetics for the Pressure-Induced Phase Transition in Shocked CdS

    SciTech Connect

    Gupta, Y.M.; Knudson, M.D.; Kunz, A.B.

    1999-06-24

    The pressure-induced phase transition in CdS was investigated using picosecond time-resolved electronic spectroscopy in plate impact shock wave experiments. Real-time changes in the electronic spectra were observed, with 100 ps time resolution, in single crystals of CdS shocked along the c and a axes to peak stresses between 35 and 90 kbar (above the phase transition stress of approximately 30 kbar measured in continuum studies). When shocked to stresses above approximately 50 kbar along the crystal c axis and 60 to 70 kbar along the crystal a axis, the crystals undergo a very rapid change in electronic structure, indicating that significant structural changes occur within the first 100 ps. These results, along with previous ns continuum measurements, make a strong case for a metastable state during the phase transition in shocked CdS. Ab-initio periodic Hartree-Fock calculations (with DFT correlation corrections) were employed to examine the compression of CdS and to determine a possible lattice structure for the proposed metastable structure. These results, along with details of the transformation kinetics and orientational dependence, will be discussed. Work supported by ONR.

  16. Ab initio study of pressure-induced magnetic transition in manganese pnictides

    NASA Astrophysics Data System (ADS)

    Prathiba, G.; Naanci, B. Anto; Rajagopalan, M.

    2007-02-01

    We report a density functional calculation on the NiAs-type Mn-based pnictides. The total energy as a function of volume is obtained by means of self-consistent tight-binding linear muffin-tin orbital method by performing spin and non-spin polarized calculation. From the present study, we predict a magnetic-phase transition from ferromagnetic (FM) to non-magnetic (NM) around 49 and 35.7 GPa for MnAs and MnSb, respectively. The pressure-induced transition is found to be a second-order transition. The band structure and density of states (DOS) are plotted for FM and NM states. Apart from this the ground-state properties like magnetic moment, lattice parameter and bulk modulus are calculated and are compared with the available results. Under large volume expansion these compounds exist in zinc-blende (ZB) structure, which shows half metallicity. The magnetic moment and equilibrium lattice constants for ZB structure are obtained as well as band structure and DOS are presented.

  17. Pressure-induced phase transitions in acentric BaHf(BO{sub 3}){sub 2}

    SciTech Connect

    Mączka, Mirosław; Szymborska-Małek, Katarzyna; Sousa Pinheiro, Gardenia de; Cavalcante Freire, Paulo Tarso; Majchrowski, Andrzej

    2015-08-15

    High-pressure Raman scattering studies revealed that BaHf(BO{sub 3}){sub 2} is more compressible than calcite-type orthoborates and calcite, aragonite or dolomite carbonates. It undergoes a first-order reversible pressure-induced phase transition in the 3.9–4.4 GPa pressure range. Second structural change is observed at 9.2 GPa. The intermediate phase is most likely trigonal. However, Raman results suggest increase in the number of distinct BO{sub 3} groups from two in the ambient pressure phase to at least three in the intermediate phase. This intermediate phase is also strongly compressible and strong pressure dependence of the lattice modes proves that the main changes under pressure occur within the layers built from BaO{sub 6} and HfO{sub 6} octahedra. The second phase transition leads most likely to lowering of the trigonal symmetry, as evidenced by significant increase of the number of observed bands. The pressure coefficients of the Raman bands of the high-pressure phase are relatively small, suggesting more dense arrangement of the metal–oxygen polyhedra and BO{sub 3} groups in this phase. It is worth noting that the high-pressure phase was not reached in the second compression experiment up to 10 GPa. This behavior can be most likely attributed to worse hydrostatic conditions of the first experiment. - Graphical abstract: Raman spectra of BaHf(BO{sub 3}){sub 2} recorded at different pressures during compression showing onset of pressure-induced phase transitions. - Highlights: • High-pressure Raman spectra were measured for BaHf(BO{sub 3}){sub 2.} • BaHf(BO{sub 3}){sub 2} undergoes a reversible first-order phase transition at 3.9–4.4 GPa into a trigonal phase. • The intermediate trigonal phase is strongly compressible second structural transformation is observed at 9.2 GPa under non-perfect hydrostatic conditions.

  18. Pressure-induced phase transitions in acentric BaHf(BO3)2

    NASA Astrophysics Data System (ADS)

    Mączka, Mirosław; Szymborska-Małek, Katarzyna; Sousa Pinheiro, Gardenia de; Cavalcante Freire, Paulo Tarso; Majchrowski, Andrzej

    2015-08-01

    High-pressure Raman scattering studies revealed that BaHf(BO3)2 is more compressible than calcite-type orthoborates and calcite, aragonite or dolomite carbonates. It undergoes a first-order reversible pressure-induced phase transition in the 3.9-4.4 GPa pressure range. Second structural change is observed at 9.2 GPa. The intermediate phase is most likely trigonal. However, Raman results suggest increase in the number of distinct BO3 groups from two in the ambient pressure phase to at least three in the intermediate phase. This intermediate phase is also strongly compressible and strong pressure dependence of the lattice modes proves that the main changes under pressure occur within the layers built from BaO6 and HfO6 octahedra. The second phase transition leads most likely to lowering of the trigonal symmetry, as evidenced by significant increase of the number of observed bands. The pressure coefficients of the Raman bands of the high-pressure phase are relatively small, suggesting more dense arrangement of the metal-oxygen polyhedra and BO3 groups in this phase. It is worth noting that the high-pressure phase was not reached in the second compression experiment up to 10 GPa. This behavior can be most likely attributed to worse hydrostatic conditions of the first experiment.

  19. The pressure induced B1-B2 phase transition of alkaline halides and alkaline earth chalcogenides. A first principles investigation

    SciTech Connect

    Potzel, Oliver; Taubmann, Gerhard

    2011-05-15

    In this work, we considered the pressure induced B1-B2 phase transition of AB compounds. The DFT calculations were carried out for 11 alkaline halides, 11 alkaline earth chalcogenides and the lanthanide pnictide CeP. For both the B1 and the B2 structures of each compound, the energy was calculated as a function of the cell volume. The transition pressure, the bulk moduli and their pressure derivatives were obtained from the corresponding equations of state. The transition path of the Buerger mechanism was described using roots of the transition matrix. We correlated the computed enthalpies of activation to some structure defining properties of the compounds. A fair correlation to Pearsons hardness of the ions was observed. -- Graphical abstract: Pressure induced transition from the B1 structure (left) via the transition state (middle) to the B2 structure (right). Display Omitted highlights: > Pressure induced phase transitions in AB compounds were considered. > Alkaline halides and alkaline earth chalcogenides were treated. > DFT calculations with periodic boundary conditions were applied. > The transition path was described by roots of the transition matrix. > The enthalpy of activation was calculated for numerous compounds.

  20. Phase stability and pressure-induced structural transitions at zero temperature in ZnSiO3 and Zn2SiO4

    NASA Astrophysics Data System (ADS)

    Karazhanov, S. Zh; Ravindran, P.; Vajeeston, P.; Ulyashin, A. G.; Fjellvåg, H.; Svensson, B. G.

    2009-12-01

    Using density functional total energy calculations the structural phase stability and pressure-induced structural transition in different polymorphs of ZnSiO3 and Zn2SiO4 have been studied. Among the considered monoclinic phase with space groups (P 21/c) and (C 2/c), rhombohedral (R\\bar {3}) and orthorhombic (Pbca) modifications the monoclinic phase (P 21/c) of ZnSiO3 is found to be the most stable one. At high pressure monoclinic ZnSiO3 (C 2/c) can co-exist with orthorhombic (Pbca) modification. Differences in equilibrium volume and total energy of these two polymorphs are very small, which indicates that it is relatively easier to transform between these two phases by temperature, pressure or chemical composition. It can also explain the experimentally established result of metastability of the orthorhombic phase under all conditions. The following sequence of pressure-induced structural phase transitions is found for ZnSiO3 polymorphs: monoclinic (P2_{1}/c) \\to monoclinic (C2/c) \\to rhombohedral (R\\bar {3}) . Among the rhombohedral (R\\bar {3} ), tetragonal (I\\bar {4} 2d) , orthorhombic (Pbca), orthorhombic (Imma), cubic (Fd\\bar {3} m) and orthorhombic (Pbnm) modifications of Zn2SiO4, the rhombohedral phase is found to be the ground state. For this chemical composition of zinc silicate the following sequence of structural phase transitions is found: rhombohedral (R\\bar {3}) \\to tetragonal (I\\bar {4} 2d) \\to orthorhombic (Pbca) \\to orthorhombic (Imma) \\to cubic (Fd\\bar {3} m) \\to orthorhombic (Pbnm). Based on the analogy of crystal structures of magnesium and zinc silicates and using the lattice and positional parameters of Mg2SiO4 as input, structural properties of spinel Zn2SiO4 have also been studied.

  1. Phase stability and pressure-induced structural transitions at zero temperature in ZnSiO(3) and Zn(2)SiO(4).

    PubMed

    Karazhanov, S Zh; Ravindran, P; Vajeeston, P; Ulyashin, A G; Fjellvåg, H; Svensson, B G

    2009-12-01

    Using density functional total energy calculations the structural phase stability and pressure-induced structural transition in different polymorphs of ZnSiO(3) and Zn(2)SiO(4) have been studied. Among the considered monoclinic phase with space groups (P 2(1)/c) and (C 2/c), rhombohedral [Formula: see text] and orthorhombic (Pbca) modifications the monoclinic phase (P 2(1)/c) of ZnSiO(3) is found to be the most stable one. At high pressure monoclinic ZnSiO(3) (C 2/c) can co-exist with orthorhombic (Pbca) modification. Differences in equilibrium volume and total energy of these two polymorphs are very small, which indicates that it is relatively easier to transform between these two phases by temperature, pressure or chemical composition. It can also explain the experimentally established result of metastability of the orthorhombic phase under all conditions. The following sequence of pressure-induced structural phase transitions is found for ZnSiO(3) polymorphs: monoclinic [Formula: see text] monoclinic [Formula: see text] rhombohedral [Formula: see text]. Among the rhombohedral ([Formula: see text]), tetragonal [Formula: see text], orthorhombic (Pbca), orthorhombic (Imma), cubic [Formula: see text] and orthorhombic (Pbnm) modifications of Zn(2)SiO(4), the rhombohedral phase is found to be the ground state. For this chemical composition of zinc silicate the following sequence of structural phase transitions is found: rhombohedral [Formula: see text] tetragonal [Formula: see text] orthorhombic [Formula: see text] orthorhombic (Imma) [Formula: see text] cubic [Formula: see text] orthorhombic (Pbnm). Based on the analogy of crystal structures of magnesium and zinc silicates and using the lattice and positional parameters of Mg(2)SiO(4) as input, structural properties of spinel Zn(2)SiO(4) have also been studied. PMID:21832530

  2. Pressure-Induced Glass Transition Probed via the Mobility of Coumarin 1 Fluorescent Molecule.

    PubMed

    Bonetti, Marco

    2016-05-12

    The route to form a glass is generally achieved upon cooling where the slowing down might be interpreted as the trapping of molecules in potential wells. On the other hand, isothermal compression induces a glassy state by modifying the molecular packing ending in jamming. Here, we focus on how isothermal compression perturbs the mobility of a probe molecule in three different host liquids up to the pressure-induced glass transition. By use of the fluorescence recovery technique, the diffusion of the fluorescent molecule Coumarin 1 (C1) is measured in poly(propylene glycol) (PPG-1000M and -2700M), in the fragile van der Waals propylene carbonate (PC), and in hydrogen-bonded methanol and ethanol. High pressures up to 6 GPa are obtained with a diamond anvil cell. In PC at a pressure ∼1.3 GPa close to the glass-transition pressure, the diffusion coefficient of C1 follows an Arrhenius behavior with an ∼5 orders of magnitude increase of the diffusive time. No decoupling from the Stokes-Einstein equation is noticed. A similar exponential behavior is measured in ethanol and methanol but extended to different pressure ranges up to 2.5 and 6.2 GPa, respectively. In PPG-1000M a decoupling from the Stokes-Einstein relation is observed between 0.3 and 0.8 GPa that could be related to a modification of the interaction between polymer segments and the probe molecule. These results might indicate that interaction between probe and dynamic heterogeneities become less important under applied pressure, unlike in the temperature-induced glass transition. PMID:27110923

  3. Pressure-Induced Phase Transition in N-H···O Hydrogen-Bonded Molecular Crystal Oxamide

    SciTech Connect

    Yan, Tingting; Li, Shourui; Wang, Kai; Tan, Xiao; Jiang, Zhangmei; Yang, Ke; Liu, Bingbing; Zou, Guangtian; Zou, Bo

    2012-11-26

    The effect of high pressure on the structural stability of oxamide has been investigated in a diamond anvil cell by Raman spectroscopy up to ~14.6 GPa and by angle-dispersive X-ray diffraction (ADXRD) up to ~17.5 GPa. The discontinuity in Raman shifts around 9.6 GPa indicates a pressure-induced structural phase transition. This phase transition is confirmed by the change of ADXRD spectra with the symmetry transformation from P1⁻ to P1. On total release of pressure, the diffraction pattern returns to its initial state, implying this transition is reversible. We discuss the pressure-induced variations in N-H stretching vibrations and the amide modes in Raman spectra and propose that this phase transition is attributed to the distortions of the hydrogen-bonded networks.

  4. A Novel Pressure-induced Phase Transition in CaZrO3

    SciTech Connect

    Yang, Xue; Li, Quanjun; Liu, Ran; Liu, Bo; Jiang, Shuqing; Yang, Ke; Liu, Jing; Chen, Zhiqiang; Zou, Bo; Cui, Tian; Liu, Bingbing

    2014-04-14

    The high pressure synchrotron X-ray diffraction measurements on CaZrO3 were carried out in a diamond anvil cell up to 50.1 GPa at room temperature. It was found that the orthorhombic phase can be stable up to 30 GPa. A new pressure-induced phase transition was observed in CaZrO3 beyond 30 GPa. The high pressure structure of CaZrO3 was determined to be a monoclinic phase which is distinct from the high pressure structures that were previously reported for other perovskite oxides. Upon release of pressure, the high pressure phase transforms into the initial orthorhombic phase. Likewise, a fit of the compression data to the third-order Birch–Murnaghan equation of state yields a bulk modulus K0 of 193(14) GPa. We propose that the unique distorted structure probably plays a crucial role in the high pressure behavior of CaZrO3. Especially, the distinct phase transformation may be related to the rotation or tilting of the ZrO6 octahedra.

  5. Pressure-induced reversible amorphization and an amorphous–amorphous transition in Ge2Sb2Te5 phase-change memory material

    PubMed Central

    Sun, Zhimei; Zhou, Jian; Pan, Yuanchun; Song, Zhitang; Mao, Ho-Kwang; Ahuja, Rajeev

    2011-01-01

    Ge2Sb2Te5 (GST) is a technologically very important phase-change material that is used in digital versatile disks-random access memory and is currently studied for the use in phase-change random access memory devices. This type of data storage is achieved by the fast reversible phase transition between amorphous and crystalline GST upon heat pulse. Here we report pressure-induced reversible crystalline-amorphous and polymorphic amorphous transitions in NaCl structured GST by ab initio molecular dynamics calculations. We have showed that the onset amorphization of GST starts at approximately 18 GPa and the system become completely random at approximately 22 GPa. This amorphous state has a cubic framework (c-amorphous) of sixfold coordinations. With further increasing pressure, the c-amorphous transforms to a high-density amorphous structure with trigonal framework (t-amorphous) and an average coordination number of eight. The pressure-induced amorphization is investigated to be due to large displacements of Te atoms for which weak Te–Te bonds exist or vacancies are nearby. Upon decompressing to ambient conditions, the original cubic crystalline structure is restored for c-amorphous, whereas t-amorphous transforms to another amorphous phase that is similar to the melt-quenched amorphous GST. PMID:21670255

  6. Pressure-induced phase transitions in multiferroic RbFe(MoO{sub 4}){sub 2}-Raman scattering study

    SciTech Connect

    Maczka, M.; Ptak, M.; Luz-Lima, C.; Freire, P.T.C.; Paraguassu, W.; Guerini, S.; Hanuza, J.

    2011-10-15

    High pressure Raman scattering experiments were performed on RbFe(MoO{sub 4}){sub 2}. These experiments revealed that two phase transitions take place in RbFe(MoO{sub 4}){sub 2} at very low pressures, i.e. between ambient pressure and 0.2 GPa and between 0.4 and 0.7 GPa. Raman results showed that at the first phase transition the room temperature P3-bar m1 phase transforms into the P3-bar phase, which is also observed at ambient pressure below 190 K. The second pressure-induced phase transition occurs into a low symmetry phase of unknown symmetry. The performed lattice dynamics calculations for the P3-bar m1 phase and ab initio calculation of the structural changes under hydrostatic pressure helped us to get better insights into the mechanism of the observed phase transitions. - Graphical abstract: Raman spectra of RbFe(MoO{sub 4}){sub 2} crystal in the high wavenumber region recorded at different pressures during compression experiment. Highlights: > RbFe(MoO{sub 4}){sub 2} exhibits two pressure-induced phase transitions below 0.7 GPa. > First phase transition is from the P3-bar m1 into P3-bar structure. > Phase transitions in RbFe(MoO{sub 4}){sub 2} are similar as in KFe(MoO{sub 4}){sub 2}. > Transitions' pressures are much lower for the rubidium compound.

  7. Pressure-induced structural phase transition in AlN:Mg and AlN:Co nanowires

    SciTech Connect

    Xu, Yongsheng; Zhu, Hongyang; Ma, Chunli; Zhu, Pinwen; Cong, Ridong; Wu, Xiaoxin; Gao, Wei; Cui, Qiliang

    2013-06-15

    High-pressure behaviors of AlN:Mg and AlN:Co nanowires have been investigated by in situ angle dispersive synchrotron X-ray diffraction up to 41.5 GPa and 38.2 GPa, respectively. Their corresponding pressure-induced wurtzite-to-rocksalt phase transitions start at 17.7 GPa and 15.0 GPa and complete at 33.2 GPa and 31.0 GPa, respectively. The phase-transition routes are not affected by the doped ions, while the phase transition pressures are lower than that of pure AlN nanowires. The distinct high-pressure behaviors are ascribed to the doped ions, which reduce the formation energy of cation vacancies and induce Al vacancies defects together with substitution defects, resulting in lattice distortion and affecting structural stability and phase transition pressure. - Graphical abstract: The high-pressure behaviors of AlN:Mg and AlN:Co nanowires have been investigated by in situ angle dispersive synchrotron X-ray diffraction. - Highlights: • The high-pressure behaviors of AlN:Mg and AlN:Co nanowires have been investigated. • The pressure-induced wurtzite-to-rocksalt phase transitions have been observed. • The phase transition pressures are lower than that of pure AlN nanowires. • The distinct high-pressure behaviors are ascribed to the dopants. • The vacancy defects and substitution defects influence structural stability.

  8. Pressure-induced alpha to omega transition in titanium metal: a systematic study of the effects of uniaxial stress

    SciTech Connect

    Errandonea, D.; Meng, Y.; Somayazulu, M.; Hausermann, D.

    2010-07-13

    The effects of uniaxial stress on the pressure-induced {alpha} {yields} {omega} transition in pure titanium (Ti) are investigated by means of angle dispersive X-ray diffraction in a diamond-anvil cell. Experiments under four different pressure environments reveal that: (1) the onset of the transition depends on the pressure medium used, going from 4.9 GPa (no pressure medium) to 10.5 GPa (argon pressure medium); (2) the {alpha} and {omega} phases coexist over a rather large pressure range, which depends on the pressure medium employed; (3) the hysteresis and quenchability of the {omega} phase is affected by differences in the sample pressure environment; and (4) a short-term laser heating of Ti lowers the {alpha} {yields} {omega} transition pressure. Possible transition mechanisms are discussed in the light of the present results, which clearly demonstrate the influence of uniaxial stress in the {alpha} {yields} {omega} transition.

  9. Pressure Induced Phase Transition (B3-B1) and Elastic Properties of II-Vi ZnSe Semiconductors

    NASA Astrophysics Data System (ADS)

    Varshney, Dinesh

    2012-07-01

    We evolve an effective interionic interaction potential (EIoIP) to investigate the pressure induced phase transitions from Zinc blende (B3) to Rocksalt (B1) structure in ZnSe semiconductor. The developed potential consists of the long-range Coulomb and three-body interactions (TBI) and the Hafemeister and Flygare type short-range (SR) overlap repulsion extended upto the second neighbor ions and the van der Waals (vdW) interaction. The three-body interactions arise from the electron-shell deformation when the nearest-neighbor ions overlap and has been employed for detailed studies of pressure-induced phase-transition behavior of ZnSe semiconductors. Our calculated value of the phase transition pressure (Pt) is higher and the magnitude of the discontinuity in volume at the transition pressure is consistent with reported data. The variation of second-order elastic constants with pressure resembles that observed in some binary semiconductors. It is inferred that the vdW interaction is effective in obtaining the Debye temperature, Gruneisen parameter, thermal expansion coefficient and compressibility. It is argued that the model with TBI (model II) has yielded somewhat more realistic predictions of the phase-transition and high-pressure behavior as compared to usual two-body potentials (model I) based on phenomenological approach.

  10. Pressure-induced development of bonding in NiAs type compounds and polymorphism of NiP

    SciTech Connect

    Dera, Przemyslaw; Lazarz, John D.; Lavina, Barbara

    2011-11-07

    A reversible, displacive, pressure-induced structural phase transition has been found to occur in nickel monophosphide NiP at approximately 3.5 GPa by means of in situ synchrotron single-crystal X-ray diffraction. The new phase, with Pearson symbol oC56, assumes an orthorhombic structure with Cmc2{sub 1} space group and unit cell parameters a=23.801(2) {angstrom}, b=5.9238(6) {angstrom}, and c=4.8479(4) {angstrom} at 5.79 GPa. The high-pressure phase is a superstructure of the ambient, oP16 phase with multiplicity of 3.5. The phosphorous sublattice gradually converts from the net of isolated P{sub 2} dimers found in the ambient NiP, towards zig-zag polymeric P{infinity} chains found in MnP-type structures. The transformation involves development of triatomic phosphorous clusters and interconnected Ni slabs with diamondoid topology. The high-pressure phase, which represents intermediate polymerization step, is a commensurately modulated superstructure of the NiAs aristotype. The phase transformation in NiP bears resemblance to the effect of successive substitution of Si or Ge in place of P found in the series of stoichiometric inhomogeneous linear structures in ternary NiP{sub 1-x}Si{sub x} and NiP{sub 1-x}Ge{sub x} systems.

  11. Pressure-induced phase transitions in rubidium azide: Studied by in-situ x-ray diffraction

    SciTech Connect

    Li, Dongmei; Wu, Xiaoxin; Jiang, Junru; Zhang, Jian; Cui, Qiliang; Zhu, Hongyang; Wang, Xiaoli

    2014-08-18

    We present the in-situ X-ray diffraction studies of RbN{sub 3} up to 42.0 GPa at room temperature to supplement the high pressure exploration of alkali azides. Two pressure-induced phase transitions of α-RbN{sub 3} → γ-RbN{sub 3} → δ-RbN{sub 3} were revealed at 6.5 and 16.0 GPa, respectively. During the phase transition of α-RbN{sub 3} → γ-RbN{sub 3}, lattice symmetry decreases from a fourfold to a twofold axis accompanied by a rearrangement of azide anions. The γ-RbN{sub 3} was identified to be a monoclinic structure with C2/m space group. Upon further compression, an orthogonal arrangement of azide anions becomes energetically favorable for δ-RbN{sub 3}. The compressibility of α-RbN{sub 3} is anisotropic due to the orientation of azide anions. The bulk modulus of α-RbN{sub 3} is 18.4 GPa, quite close to those of KN{sub 3} and CsN{sub 3}. By comparing the phase transition pressures of alkali azides, their ionic character is found to play a key role in pressure-induced phase transitions.

  12. Ab initio molecular dynamics simulation of pressure-induced zinc blende to rocksalt phase transition in SiC

    SciTech Connect

    Xiao, Haiyan J.; Gao, Fei; Zu, Xiaotao T.; Weber, William J.

    2009-06-17

    The high-pressure induced phase transformation from the zinc blende to rocksalt structure in SiC has been studied by ab initio molecular dynamics. The simulations show that SiC passes through tetragonal and then monoclinic intermediate states before finally forming the rock salt structure at 160 GPa. The mechanism for this phase transformation agrees well with recent ab initio MD simulations, in which the applied pressure was as high as ~600 GPa, but in the present study the transformation occurs at much lower pressure. It is found that the phase transition has to overcome an energy barrier of 0.44 eV/pair.

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

    NASA Astrophysics Data System (ADS)

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

    2012-01-01

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

  14. RAYLEIGH-TAYLOR STRENGTH EXPERIMENTS OF THE PRESSURE-INDUCED alpha->epsilon->alpha' PHASE TRANSITION IN IRON

    SciTech Connect

    Belof, J L; Cavallo, R M; Olson, R T; King, R S; Gray, G T; Holtkamp, D B; Chen, S R; Rudd, R E; Barton, N R; Arsenlis, A; Remington, B A; Park, H; Prisbrey, S T; Vitello, P A; Bazan, G; Mikaelian, K O; Comley, A J; Maddox, B R; May, M J

    2011-08-10

    We present here the first dynamic Rayleigh-Taylor (RT) strength measurement of a material undergoing solid-solid phase transition. Iron is quasi-isentropically driven across the pressure-induced bcc ({alpha}-Fe) {yields} hcp ({var_epsilon}-Fe) phase transition and the dynamic strength of the {alpha}, {var_epsilon} and reverted {alpha}{prime} phases have been determined via proton radiography of the resulting Rayleigh-Taylor unstable interface between the iron target and high-explosive products. Simultaneous velocimetry measurements of the iron free surface yield the phase transition dynamics and, in conjunction with detailed hydrodynamic simulations, allow for determination of the strength of the distinct phases of iron. Forward analysis of the experiment via hydrodynamic simulations reveals significant strength enhancement of the dynamically-generated {var_epsilon}-Fe and reverted {alpha}{prime}-Fe, comparable in magnitude to the strength of austenitic stainless steels.

  15. Pressure-induced hydration and order-disorder transition in a synthetic potassium gallosilicate zeolite with gismondine topology.

    PubMed

    Lee, Yongjae; Kim, Sun Jin; Kao, Chi-Chang; Vogt, Thomas

    2008-03-01

    Two high-pressure phases of a potassium gallosilicate with a gismondine framework (K-GaSi-GIS) were characterized using Rietveld refinements of in-situ high-pressure, high-resolution synchrotron X-ray powder diffraction data. The observed response of the K-GaSi-GIS framework under hydrostatic pressure is a gradual flattening of the so-called "double crankshaft" structural chain units. At pressures below 1.0(1) GPa, additional water molecules from the hydrostatic pressure-transmitting medium are inserted into the potassium-water guest network ("pressure-induced hydration") resulting in a "super-hydrated" high-pressure phase I. As the flattening of the double crankshaft structural units in the GIS framework continues above 1.6 GPa, the ellipticity of the cross-linking 8-ring windows is reduced below a certain threshold, and a disordering of the potassium-water guest structure along the 8-ring channel, characteristic of a disordered high-pressure phase II, is observed. The concerted framework distortion and guest network disordering accommodates the increased hydration level while maintaining the seven-fold coordination environment of the potassium cations to framework oxygen atoms and water molecules. We have thus established the atomistic details of a guest-host order-disorder transition under pressure-induced hydration conditions in a zeolite with GIS framework and compared it to other zeolites during pressure-induced hydration. We find that the structural changes mediated by the extra-framework cations and their coordination environment under PIH conditions are at the core of these different mechanisms and are driving the changes in the ellipticity of pore openings, order-disorder and disorder-order transitions, and framework distortions. PMID:18266365

  16. Enhanced electron transport in Nb-doped TiO2 nanoparticles via pressure-induced phase transitions.

    PubMed

    Lü, Xujie; Yang, Wenge; Quan, Zewei; Lin, Tianquan; Bai, Ligang; Wang, Lin; Huang, Fuqiang; Zhao, Yusheng

    2014-01-01

    Anatase TiO2 is one of the most important energy materials but suffers from poor electrical conductivity. Nb doping has been considered as an effective way to improve its performance in the applications of photocatalysis, solar cells, Li batteries, and transparent conducting oxide films. Here, we report the further enhancement of electron transport in Nb-doped TiO2 nanoparticles via pressure-induced phase transitions. The phase transition behavior and influence of Nb doping in anatase Nb-TiO2 have been systematically investigated by in situ synchrotron X-ray diffraction and Raman spectroscopy. The bulk moduli are determined to be 179.5, 163.3, 148.3, and 139.0 GPa for 0, 2.5, 5.0, and 10.0 mol % Nb-doped TiO2, respectively. The Nb-concentration-dependent stiffness variation has been demonstrated: samples with higher Nb concentrations have lower stiffness. In situ resistance measurements reveal an increase of 40% in conductivity of quenched Nb-TiO2 in comparison to the pristine anatase phase. The pressure-induced conductivity evolution is discussed in detail in terms of the packing factor model, which provides direct evidence for the rationality of the correlation of packing factors with electron transport in semiconductors. Pressure-treated Nb-doped TiO2 with unique properties surpassing those in the anatase phase holds great promise for energy-related applications. PMID:24320708

  17. Pressure-induced pseudoatom bonding collapse and isosymmetric phase transition in Zr{sub 2}Cu: First-principles predictions

    SciTech Connect

    Ning, Jinliang; Zhang, Xinyu E-mail: riping@ysu.edu.cn; Zhang, Suhong; Sun, Na; Wang, Limin; Ma, Mingzhen; Liu, Riping E-mail: riping@ysu.edu.cn

    2013-12-21

    The structural evolution of tetragonal Zr{sub 2}Cu has been investigated under high pressures up to 70 GPa by means of density functional theory. Our calculations predict a pressure-induced isosymmetric transition where the tetragonal symmetry (I4/mmm) is retained during the entire compression as well as decompression process while its axial ratio (c/a) undergoes a transition from ∼3.5 to ∼4.2 at around 35 GPa with a hysteresis width of about 4 GPa accompanied by an obvious volume collapse of 1.8% and anomalous elastic properties such as weak mechanical stability, dramatically high elastic anisotropy, and low Young's modulus. Crystallographically, the tetragonal axial ratio shift renders this transition analogous to a simple bcc-to-fcc structural transition, which implies it might be densification-driven. Electronically, the ambient Zr{sub 2}Cu is uncovered with an intriguing pseudo BaFe{sub 2}As{sub 2}-type structure, which upon the phase transition undergoes an electron density topological change and collapses to an atomic-sandwich-like structure. The pseudo BaFe{sub 2}As{sub 2}-type structure is demonstrated to be shaped by hybridized dxz + yz electronic states below Fermi level, while the high pressure straight Zr-Zr bonding is accommodated by electronic states near Fermi level with dx{sup 2} − y{sup 2} dominant features.

  18. Pressure-induced Phase Transition in Thiol-capped CdTe Nanoparticles

    SciTech Connect

    Wu, F; Zaug, J; Young, C; Zhang, J Z

    2006-11-29

    Phase transitions for CdTe nanoparticles (NPs) under high pressure up to 37.0 GPa have been studied using fluorescence measurements. The phase transition from cinnarbar to rocksalt phase has been observed in CdTe NPs solution at 5.8 GPa, which is much higher than the phase transition pressure of bulk CdTe (3.8 GPa) and that of CdTe NPs in solid form (0.8 GPa). CdTe NPs solution therefore shows elevated phase transition pressure and enhanced stability against pressure compared with bulk CdTe and CdTe NPs in solid forms. The enhanced stability of CdTe NPs solution has been attributed to possible shape change in the phase transition and/or inhomogeneous strains in nanoparticle solutions.

  19. Mechanisms for pressure-induced crystal-crystal transition, amorphization, and devitrification of SnI{sub 4}

    SciTech Connect

    Liu, H.; Tse, J. S.; Hu, M. Y.; Bi, W.; Zhao, J.; Alp, E. E.; Pasternak, M.; Taylor, R. D.; Lashley, J. C.

    2015-10-28

    The pressure-induced amorphization and subsequent recrystallization of SnI{sub 4} have been investigated using first principles molecular dynamics calculations together with high-pressure {sup 119}Sn nuclear resonant inelastic x-ray scattering measurements. Above ∼8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ∼64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI{sub 4} under ambient conditions. Although high pressure structures of SnI{sub 4} were thought to be determined by random packing of equal-sized spheres, we detected electron charge transfer in each phase. This charge transfer results in a crystal structure packing determined by larger than expected iodine atoms.

  20. Mechanisms for pressure-induced crystal-crystal transition, amorphization, and devitrification of Snl4

    SciTech Connect

    Liu, Hanyu; Tse, John S.; Hu, Michael Y.; Bi, Wenli; Zhao, Jiyong; Alp, E. Ercan; Pasternak, Moshe; Taylor, R. Dean; Lashley, Jason C.

    2015-10-27

    The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigated using first principles molecular dynamics calculations together with high-pressure 119Sn nuclear resonant inelastic x-ray scattering measurements. Above ~8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ~64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI4 under ambient conditions. Although high pressure structures of SnI4 were thought to be determined by random packing of equal-sized spheres, we detected electron charge transfer in each phase. As a result, this charge transfer results in a crystal structure packing determined by larger than expected iodine atoms. (C) 2015 AIP Publishing LLC.

  1. Structural response in FeCl2 (iron chloride) to pressure-induced electro-magnetic transitions

    SciTech Connect

    Taylor, R D; Rozenberg, G Kh; Pasternak, M P; Gorodetsky, P; Xu, W M; Dubrovinsky, L S; Le Bihan, T L

    2009-01-01

    High pressure (HP) synchrotron x-ray diffraction studies were carried out in FeCl{sub 2} together with resistivity (R) studies, at various temperatures and pressures to 65 GPa using diamond anvil cells. This work follows a previous HP {sup 57}Fe Mossbauer study in which two pressure-induced (PI) electronic transitions were found interpreted as: (i) quenching of the orbital-term contribution to the hyperfine field concurring with a tilting of the magnetic moment by 55 degrees and (ii) collapse of the magnetism concurring with a sharp decrease of the isomer shift (IS). The R(P,T) studies affirm that the cause the collapse of the magnetism is a PI p-d correlation breakdown, leading to an insulator-metal transition at {approx}45 GPa and is not due to a spi-Ir,crossover (S=2 {yields} S=0). The structure response to the pressure evolution of the two electronic phase transitions starting at low pressures (LP), through an intermediate phase (IP) 30-57 GPa, and culminating in a high-pressure phase (HP), P >32 GPa, can clearly be quantified. The IP-HP phases coexist through the 32-57 GPa range in which the HP abundance increases monotonically at the expense of the IP phase. At the LP-IP interface no volume change is detected, yet the c-axis increases and the a-axis shrinks by 0.21 Angstroms and 0.13 Angstroms, respectively. The fit of the equation of state of the combined LP-IP phases yields a bulk modulus K{sub 0} = 35.3(1.8) GPa. The intralayer CI-CI distances increases, but no change is observed in Fe-CI bond-length nor are there substantial changes in the interlayer spacing. The pressure-induced electronic IP-HP transition leads to a first-order structural phase transition characterized by a decrease in Fe-CI bond length and an abrupt drop in V(P) by {approx}3.5% accompanying the correlation breakdown. In this transition no symmetry change is detected,and the XRD data could be satisfactorily fitted with the CdI{sub 2} structure. The bulk modulus of the HP phase is

  2. Structure and pressure-induced ferroelectric phase transition in antiphase domain boundaries of strontium titanate from first principles

    NASA Astrophysics Data System (ADS)

    Kvasov, Alexander; Tagantsev, Alexander K.; Setter, Nava

    2016-08-01

    In this work, using zero kelvin ab initio calculations, we revisit the structure and ferroelectric phase transition in antiphase domain boundaries (APBs) in SrTiO3 (STO), which has been previously addressed in terms of a phenomenological approach. We confirmed the main qualitative conclusion of the phenomenological results that APBs normal to the rotation axis of the oxygen octahedra ("easy" walls) do not exhibit the transition while those parallel to the rotation axis ("hard" walls) do. However, we found the structure of the hard walls to be close to the Ising type in contrast to the phenomenological prediction of the nearly Néel type. We simulated a pressure-induced phase transition in the hard wall. Combining the results of simulation and experimental data on STO, we evaluated the pressure sensitivity of the ferroelectricity in the hard wall at low temperatures to show that it can be suppressed with very small pressure (a few kbar). We also roughly estimated the ferroelectric transition temperature in the hard wall corroborating the result of the phenomenological treatment.

  3. Pressure Induced Phase Transitions In SmVO4: An In-Situ Raman Study

    NASA Astrophysics Data System (ADS)

    Patel, Nishant N.; Garg, Alka B.; Meenakshi, S.; Pandey, K. K.; Wani, B. N.; Sharma, S. M.

    2010-12-01

    High pressure room temperature Raman investigation on SmVO4 was carried out up to 19 GPa. The ambient zircon phase was observed to remain stable up to 5.8 GPa. At higher pressure two structural phase transitions were observed at 6.8 GPa and 15.9 GPa respectively. The second phase transition was found to be reversible whereas the intermediate phase was retained on complete pressure release.

  4. Pressure Induced Phase Transitions In SmVO{sub 4}: An In-Situ Raman Study

    SciTech Connect

    Patel, Nishant N.; Garg, Alka B.; Meenakshi, S.; Pandey, K. K.; Sharma, S. M.; Wani, B. N.

    2010-12-01

    High pressure room temperature Raman investigation on SmVO{sub 4} was carried out up to 19 GPa. The ambient zircon phase was observed to remain stable up to 5.8 GPa. At higher pressure two structural phase transitions were observed at 6.8 GPa and 15.9 GPa respectively. The second phase transition was found to be reversible whereas the intermediate phase was retained on complete pressure release.

  5. Pressure-induced shape change of phospholipid vesicles: implication of compression and phase transition.

    PubMed

    Perrier-Cornet, J-M; Baddóuj, K; Gervais, P

    2005-04-01

    A microscopic study has allowed the analysis of modifications of various shapes acquired by phospholipid vesicles during a hydrostatic pressure treatment of up to 300 MPa. Giant vesicles of dimyristoylphosphatidylcholine / phosphatidylserine (DMPC/PS) prepared at 40 degrees C mainly presented a shape change resembling budding during pressure release. This comportment was reinforced by the incorporation of 1,2-dioleyl-sn-glycero-3-phosphatidylethanolamine (DOPE) or by higher temperature (60 degrees C) processing. The thermotropic main phase transition (L alpha to P beta') of the different vesicles prepared was determined under pressure through a spectrofluorimetric study of 6-dodecanoyl-2-dimethylamino-naphtalene (Laurdan) incorporated into the vesicles' bilayer. This analysis was performed by microfluorescence observation of single vesicles. The phase transition was found to begin at about 80 MPa and 120 MPa for DMPC/PS vesicles at, respectively, 40 degrees C and 60 degrees C. At 60 degrees C the liquid-to-gel transition phase was not complete within 250 MPa. Addition of DMPE at 40 degrees C does not significantly shift the onset boundary of the phase transition but extends the transition region. At 40 degrees C, the gel phase was obtained at, respectively, 110 MPa and 160 MPa for DMPC/PS and DMPC/PS/DOPE vesicles. In comparing volume data obtained from image analysis and Laurdan signal, we assume the shape change is a consequence of the difference between lateral compressibility of the membrane and bulk water. The phase transition contributes to the membrane compression but seems not necessary to induce shape change of vesicles. The high compressibility of the L alpha phase at 60 degrees C allows induction on DMPC/PS vesicles of a morphological transition without phase change. PMID:16245032

  6. Pressure-Induced Tricritical Behavior of the SmA-Cho Transition in COC

    NASA Astrophysics Data System (ADS)

    Shichijyo, Shiro; Okamoto, Toshiyuki; Takemura, Tetuo

    1982-09-01

    The tricritical nature of the smectic-A-cholesteric (SmA-Cho) transition in cholesteryl oleyl carbonate (COC) was investigated by several methods. Differential thermal analysis (DTA) and volume measurement showed the existence of a tricritical point at 3.1± 0.1× 103 kg/cm2 and 74°C, where the first-order transition converted to the second-order one. The dynamic properties were studied by ultrasonic measurement, and the differences in the transition processes were observed with a polarized microscope. Spherulites were observed in the smectic-A phase above the tricritical point. The structure of the spherulites and the morphology effects are discussed.

  7. 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.

  8. Pressure-induced magnetic transitions with change of the orbital configuration in dimerised systems

    PubMed Central

    Korotin, Dmitry M.; Anisimov, Vladimir I.; Streltsov, Sergey V.

    2016-01-01

    We suggest a possible scenario for magnetic transition under pressure in dimerised systems where electrons are localised on molecular orbitals. The mechanism of transition is not related with competition between kinetic energy and on-site Coulomb repulsion as in Mott-Hubbard systems, or between crystal-field splitting and intra-atomic exchange as in classical atomic spin-state transitions. Instead, it is driven by the change of bonding-antibonding splitting on part of the molecular orbitals. In the magnetic systems with few half-filled molecular orbitals external pressure may result in increase of the bonding-antibonding splitting and localise all electrons on low-lying molecular orbitals suppressing net magnetic moment of the system. We give examples of the systems, where this or inverse transition may occur and by means of ab initio band structure calculations predict that it can be observed in α−MoCl4 at pressure P ~ 11 GPa. PMID:27189206

  9. Pressure-induced structural transitions in BN from ab initio metadynamics

    NASA Astrophysics Data System (ADS)

    Hromadová, Liliana; Martoňák, Roman

    2011-12-01

    We report here results of ab initio metadynamics simulations of structural transitions in boron nitride at high pressures. Transitions starting from sp2 bonded (graphite-like) structures are studied in a temperature range from 300 to 3000 K and pressures from 20 to 31 GPa. Rhombohedral boron nitride (r-BN) was found to directly transform at all temperatures into cubic boron nitride (c-BN). Hexagonal boron nitride (h-BN) transforms at T<700 K into wurtzite boron nitride (w-BN). At higher temperatures we found a possible transformation pathway resulting in the fully tetrahedrally (sp3) bonded metastable structure. This structure is tetragonal (P42/mnm) and is an analog of the “bct C4” (I4/mmm) structure recently discussed for carbon. The P42/mnm structure has been predicted theoretically for BN but so far not reported experimentally. We calculate structural, elastic, and electronic properties of this structure and discuss the transition mechanism. We also study the transitions at extreme pressures in the tera-pascal range starting from sp3 bonded c-BN and w-BN structures.

  10. Pressure-induced magnetic transitions with change of the orbital configuration in dimerised systems

    NASA Astrophysics Data System (ADS)

    Korotin, Dmitry M.; Anisimov, Vladimir I.; Streltsov, Sergey V.

    2016-05-01

    We suggest a possible scenario for magnetic transition under pressure in dimerised systems where electrons are localised on molecular orbitals. The mechanism of transition is not related with competition between kinetic energy and on-site Coulomb repulsion as in Mott-Hubbard systems, or between crystal-field splitting and intra-atomic exchange as in classical atomic spin-state transitions. Instead, it is driven by the change of bonding-antibonding splitting on part of the molecular orbitals. In the magnetic systems with few half-filled molecular orbitals external pressure may result in increase of the bonding-antibonding splitting and localise all electrons on low-lying molecular orbitals suppressing net magnetic moment of the system. We give examples of the systems, where this or inverse transition may occur and by means of ab initio band structure calculations predict that it can be observed in α‑MoCl4 at pressure P ~ 11 GPa.

  11. Pressure induced magnetic and semiconductor–metal phase transitions in Cr2MoO6

    NASA Astrophysics Data System (ADS)

    San-Dong, Guo

    2016-05-01

    We investigate magnetic ordering and electronic structures of Cr2MoO6 under hydrostatic pressure. To overcome the band gap problem, the modified Becke and Johnson exchange potential is used to investigate the electronic structures of Cr2MoO6. The insulating nature at the experimental crystal structure is produced, with a band gap of 1.04 eV, and the magnetic moment of the Cr atom is 2.50 μ B, compared to an experimental value of about 2.47 μ B. The calculated results show that an antiferromagnetic inter-bilayer coupling–ferromagnetic intra-bilayer coupling to a ferromagnetic inter-bilayer coupling–antiferromagnetic intra-bilayer coupling phase transition is produced with the pressure increasing. The magnetic phase transition is simultaneously accompanied by a semiconductor–metal phase transition. The magnetic phase transition can be explained by the Mo–O hybridization strength, and ferromagnetic coupling between two Cr atoms can be understood by empty Mo-d bands perturbing the nearest O-p orbital. Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. 2015XKMS073).

  12. Pressure-induced magnetic transitions with change of the orbital configuration in dimerised systems.

    PubMed

    Korotin, Dmitry M; Anisimov, Vladimir I; Streltsov, Sergey V

    2016-01-01

    We suggest a possible scenario for magnetic transition under pressure in dimerised systems where electrons are localised on molecular orbitals. The mechanism of transition is not related with competition between kinetic energy and on-site Coulomb repulsion as in Mott-Hubbard systems, or between crystal-field splitting and intra-atomic exchange as in classical atomic spin-state transitions. Instead, it is driven by the change of bonding-antibonding splitting on part of the molecular orbitals. In the magnetic systems with few half-filled molecular orbitals external pressure may result in increase of the bonding-antibonding splitting and localise all electrons on low-lying molecular orbitals suppressing net magnetic moment of the system. We give examples of the systems, where this or inverse transition may occur and by means of ab initio band structure calculations predict that it can be observed in α-MoCl4 at pressure P ~ 11 GPa. PMID:27189206

  13. Pressure-induced magneto-structural transition in iron via a modified solid-state nudged elastic band method

    NASA Astrophysics Data System (ADS)

    Zarkevich, Nikolai A.; Johnson, Duane D.

    2015-03-01

    Materials under pressure may exhibit critical electronic and structural transitions that affect equation of states, as known for superconductors and the magneto-structural transformations of iron with both geophysical and planetary implications. While experiments often use constant-pressure (diamond-anvil cell, DAC) measurements, many theoretical results address a constant-volume transitions, which avoid issues with magnetic collapse but cannot be directly compared to experiment. We establish a modified solid-state nudge elastic band (MSS-NEB) method to handle magnetic systems that may exhibit moment (and volume) collapse during transformation. We apply it to the pressure-induced transformation in iron between the low-pressure body-centered cubic (bcc) and the high-pressure hexagonal close-packed (hcp) phases, find the bcc-hcp equilibrium coexistence pressure and a transitional pathway, and compare to shock and DAC experiments. We use methods developed with support by the U.S. Department of Energy (DE-FG02-03ER46026 and DE-AC02-07CH11358). Ames Laboratory is operated for the DOE by Iowa State University under contract DE-AC02-07CH11358.

  14. A pressure-induced, magnetic transition in pyrrhotite: Implications for the formation pressure of meteorites and diamonds

    NASA Astrophysics Data System (ADS)

    Gilder, S. A.; Egli, R.; Hochleitner, R.; Roud, S. C.; Volk, M.; Le Goff, M.; de Wit, M.

    2010-12-01

    Meteorites and diamonds encounter high-pressures during their geologic histories. These materials commonly contain magnetic inclusions of pyrrhotite, and because magnetic properties are sensitive to strain, pyrrhotite can potentially record the shock or formation pressures of its host. Moreover, pyrrhotite undergoes a pressure-induced phase transition between 1.6 and 6.2 GPa, but the magnetic signature of this transition is poorly known. Here we report magnetic measurements performed at high-pressures on single and multi-domain pyrrhotite. A magnetic hysteresis model based on our observations suggests that multidomain pyrrhotite transforms into single domain-like material, and once in the single domain state, hysteresis loops become progressively squarer and then squatter with increasing pressure, until they ultimately collapse approaching the paramagnetic state at the transition. The ratio of the bulk magnetic coercive force to magnetic remanence for pure pyrrhotite is reversible with pressure and follows a logarithmic law as a function of pressure, which can be used as a magnetic barometer for natural systems.

  15. Plasma-assisted synthesis and pressure-induced structural transition of single-crystalline SnSe nanosheets.

    PubMed

    Zhang, Jian; Zhu, Hongyang; Wu, Xiaoxin; Cui, Hang; Li, Dongmei; Jiang, Junru; Gao, Chunxiao; Wang, Qiushi; Cui, Qiliang

    2015-06-28

    Two-dimensional tin selenide (SnSe) nanosheets were synthesized using a plasma-assisted direct current arc discharge method. The structural characterization indicates that the nanosheets are single-crystalline with an average thickness of ~25 nm and a lateral dimension of 500 nm. The high pressure behaviors of the as-synthesized SnSe nanosheets were investigated by in situ high-pressure synchrotron angle-dispersive X-ray diffraction and Raman scattering up to ~30 GPa in diamond anvil cells at room temperature. A second-order isostructural continuous phase transition (Pnma → Cmcm) was observed at ~7 GPa, which is considerably lower than the transition pressure of bulk SnSe. The reduction of transition pressure is induced by the volumetric expansion with softening of the Poisson ratio and shear modulus. Moreover, the measured zero-pressure bulk modulus of the SnSe nanosheets coincides with bulk SnSe. This abnormal phenomenon is attributed to the unique intrinsic geometry in the nanosheets. The high-pressure bulk modulus is considerably higher than the theoretical value. The pressure-induced morphology change should be responsible for the improved bulk modulus. PMID:26269801

  16. Theoretical investigation of pressure-induced structural transitions in americium using GGA+U and hybrid density functional theory methods

    NASA Astrophysics Data System (ADS)

    Verma, Ashok K.; Modak, P.; Sharma, Surinder M.; Svane, A.; Christensen, N. E.; Sikka, S. K.

    2013-07-01

    First-principles calculations have been performed for americium (Am) metal using the generalized gradient approximation + orbital-dependent onsite Coulomb repulsion via Hubbard interaction (GGA+U) and hybrid density functional theory (HYB-DFT) methods to investigate various ground state properties and pressure-induced structural transitions. Both methods yield equilibrium volume and bulk modulus in good agreement with the experimental results. The GGA+spin orbit coupling+U method reproduced all structural transitions under pressure correctly, but the HYB-DFT method failed to reproduce the observed Am-I to Am-II transition. Good agreement was found between calculated and experimental equations of states for all phases, but the first three phases need larger U (α) parameters (where α represents the fraction of Hartree-Fock exchange energy replacing the DFT exchange energy) than the fourth phase in order to match the experimental data. Thus, neither the GGA+U nor the HYB-DFT methods are able to describe the energetics of Am metal properly in the entire pressure range from 0 GPa to 50 GPa with a single choice of their respective U and α parameters. Low binding-energy peaks in the experimental photoemission spectrum at ambient pressure relate, for some parameter choices, well to peak positions in the calculated density of states function of Am-I.

  17. Pressure-induced normal-incommensurate and incommensurate-commensurate phase transitions in CrOCl

    PubMed Central

    Bykov, Maxim; Bykova, Elena; Dubrovinsky, Leonid; Hanfland, Michael; Liermann, Hanns-Peter; van Smaalen, Sander

    2015-01-01

    The high-pressure behavior of layered CrOCl is shown to be governed by non-bonded interactions between chlorine atoms in relation to a rigid framework composed of Cr and O atoms. The competition between optimizing intra- and interlayer Cl–Cl distances and the general trend towards denser packing defines a novel mechanism for high-pressure phase transitions of inorganic materials. CrOCl possesses an incommensurate phase for 16–51 GPa. Single-crystal x-ray diffraction in a diamond anvil cell provides an accurate description of the evolution of the incommensurate wave with pressure. It thus demonstrates a continuous increase of the amplitude up to 30 GPa, followed by a decrease of the wavelength until a lock-in transition occurs at 51 GPa. PMID:25999303

  18. Pressure-Induced Phase Transition in a Molecule-Based Magnet with Interpenetrating Sublattices

    SciTech Connect

    Fishman, Randy Scott; Shum, William W.; Miller, Joel S.

    2010-01-01

    The molecule-based magnet [Ru2(O2CMe)4]3[Cr(CN)6] contains two interpenetrating sublattices with each sublattice moment confined by anisotropy to a cubic diagonal. At ambient pressure, a field of about 850 Oe rotates the antiferromagnetically-coupled sublattice moments towards the field direction, producing a wasp-waisted magnetization curve. Up to 7 kbar, the sublattice moments increase with pressure due to the enhanced exchange coupling between the Cr and Ru2 spins on each sublattice. Above 7 kbar, the sublattice moment drops by about half and the the linear susceptibility of each sublattice along the cubic diagonal increases dramatically. The most likely explanation for this phase transition is that a high- to low-spin transition on each Ru2 complex reverses and lowers the net spin of each sublattice.

  19. Pressure Induced Metal-Nonmetal and FCC-BCC Transitions in Calcium*

    NASA Astrophysics Data System (ADS)

    Wang, G. M.; Blaisten-Barojas, E.; Papaconstantopoulos, D. A.

    2001-04-01

    The band structure of fcc and bcc calcium at different densities is obtained with the Augmented Plane Wave (APW) method using a soft-core approximation and Gaspar-Kohn-Sham potential. A tight-binding(TB) model is then built successfully to reproduce the first principles band structure and density of states. Properties examined within TB include bulk modulus, elastic constants, metal-nonmetal transition and fcc to bcc structural transition under pressure. Results are in an excellent agreement with experimental observations. Several dynamical properties of calcium under pressure are further explored with TB molecular dynamics at finite temperature. *Work supported in part by the Office of Naval Research grant N00014-98-1-0832

  20. Spectroscopy of the pressure-induced virtual phase transition in Hg2I2 crystals

    NASA Astrophysics Data System (ADS)

    Markov, Yu. F.; Mirovitskii, V. Yu.; Roginskii, E. M.

    2015-03-01

    Raman spectra of incipient ferroelastics, namely, Hg2I2 crystals, have been investigated over a wide range of hydrostatic pressures. Linear pressure dependences of phonon frequencies have been obtained at P < P c ). It has been found that jumps and breaks are observed in these dependences at the phase transition point P c = 9 kbar. The Grüneisen parameters of the Hg2I2 crystals have been determined and discussed. The Raman spectra of the ferroelastic phase ( P > P c ) in different polarizations exhibit excitation of acoustic vibrations from the X point of the Brillouin zone boundary, including transverse acoustic (TA1 and TA2) and longitudinal acoustic (LA) phonons. The Raman spectra of the ferroelastic phase have been interpreted based on the analysis of the experimental results, and a model of the phase transition in these crystals has been proposed.

  1. High pressure induced phase transition and superdiffusion in anomalous fluid confined in flexible nanopores

    SciTech Connect

    Bordin, José Rafael; Krott, Leandro B. Barbosa, Marcia C.

    2014-10-14

    The behavior of a confined spherical symmetric anomalous fluid under high external pressure was studied with Molecular Dynamics simulations. The fluid is modeled by a core-softened potential with two characteristic length scales, which in bulk reproduces the dynamical, thermodynamical, and structural anomalous behavior observed for water and other anomalous fluids. Our findings show that this system has a superdiffusion regime for sufficient high pressure and low temperature. As well, our results indicate that this superdiffusive regime is strongly related with the fluid structural properties and the superdiffusion to diffusion transition is a first order phase transition. We show how the simulation time and statistics are important to obtain the correct dynamical behavior of the confined fluid. Our results are discussed on the basis of the two length scales.

  2. Pressure-induced Mott transition in an organic superconductor with a finite doping level.

    PubMed

    Oike, H; Miyagawa, K; Taniguchi, H; Kanoda, K

    2015-02-13

    We report the pressure study of a doped organic superconductor with a Hall coefficient and conductivity measurements. We find that maximally enhanced superconductivity and a marginal-Fermi liquid appear around a certain pressure where mobile carriers increase critically, suggesting a possible quantum phase transition between strongly and weakly correlated regimes. This observation points to the presence of a criticality in Mottness for a doped Mott insulator with tunable correlation. PMID:25723239

  3. New pressure-induced phase transitions of L-threonine crystal: A Raman spectroscopic study

    NASA Astrophysics Data System (ADS)

    Holanda, R. O.; Lima, J. A.; Freire, P. T. C.; Melo, F. E. A.; Mendes Filho, J.; Polian, A.

    2015-07-01

    L-threonine crystal was studied by Raman spectroscopy under pressure in the spectral range from 50 to 3300 cm-1. The pressure range of a previous study has been extended from 4 to 27.0 GPa. Modifications in the whole spectrum give us evidence of three structural phase transitions undergone by this amino acid as well as two conformational change. The classification of the vibrational modes and the behavior of their frequencies as a function of the pressure are presented.

  4. Pressure induced tetragonal to monoclinic transition in RbN3 studied from first principles theory

    NASA Astrophysics Data System (ADS)

    Vaitheeswaran, G.; Babu, K. Ramesh

    2014-04-01

    Alkali metal azides are well known for their application as explosives and gas generators. They are used as precursors in synthesis of polymeric nitrogen, an ultimate green high energy density material. Among the alkali metal azides, rubidium azide RbN3 crystallizes in tetragonal structure with linear azide ions arranged in layers and binds through weak dispersive interactions. In this present work, we have studied the structural stability, electronic structure and optical properties of solid RbN3 by using van der Waals corrected density functional theory. We find that the ambient tetragonal structure undergoes a structural transition to monoclinic structure at 0.72 GPa, which is in good agreement with the experimental transition pressure of less than 1 GPa. The phonon frequencies at the gamma point are calculated and found that the lattice mode Eg softens under pressure which may supports the structural phase transition. The electronic band structure and optical properties are calculated by using Tran Blaha-modified Becke Johnson (TB-mBJ) functional and found that solid RbN3 is an insulator with a gap of 5.976 eV and the optical absorption starts with the UV light of wave length 207.5 nm.

  5. Pressure-Induced Irreversible Phase Transition in the Energetic Material Urea Nitrate

    NASA Astrophysics Data System (ADS)

    Li, Shourui; Zou, Bo

    2013-06-01

    The behavior of energetic material Urea Nitrate ((NH2)2 COH+ . NO3-,UN) has been investigated up to the pressure of ~26 GPa. UN exhibits the typical supramolecular structure with uronium cation and nitrate anion held together by multiple hydrogen bonds in the layer. Both Raman and XRD data provide obvious evidence for the distorted phase transition in the pressure range ~9-15 GPa. Further analysis indicates phase II has Pc symmetry. The mechanism for the phase transition involves collapse of the initial 2D supramolecular structure to 3D hydrogen-bonded networks in phase Pc. Importantly, the transition is irreversible and leads to a large reduction in volume on release of pressure. The density in phase Pc has been increased by ~11.8% compared to the phase P21/ c under ambient conditions and therefore phase Pc is expected to have much higher detonation power. This study opens new opportunities for preparing energetic materials with high density combining supramolecular chemistry with high-pressure techniques. Corresponding author. E-mail: zoubo@jlu.edu.cn This work is supported by National Science Foundation of China (NSFC) (Nos. 91227202, and 21073071).

  6. Pressure induced metallization with absence of structural transition in layered molybdenum diselenide

    SciTech Connect

    Zhao, Zhao; Zhang, Haijun; Yuan, Hongtao; Wang, Shibing; Lin, Yu; Zeng, Qiaoshi; Xu, Gang; Liu, Zhenxian; Solanki, G. K.; Patel, K. D.; Cui, Yi; Hwang, Harold Y.; Mao, Wendy L.

    2015-06-19

    Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ~60 GPa using multiple experimental techniques and ab-initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.

  7. Pressure induced metallization with absence of structural transition in layered molybdenum diselenide

    DOE PAGESBeta

    Zhao, Zhao; Zhang, Haijun; Yuan, Hongtao; Wang, Shibing; Lin, Yu; Zeng, Qiaoshi; Xu, Gang; Liu, Zhenxian; Solanki, G. K.; Patel, K. D.; et al

    2015-06-19

    Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ~60 GPa using multiple experimental techniques and ab-initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunablemore » transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.« less

  8. Pressure induced metallization with absence of structural transition in layered molybdenum diselenide

    PubMed Central

    Zhao, Zhao; Zhang, Haijun; Yuan, Hongtao; Wang, Shibing; Lin, Yu; Zeng, Qiaoshi; Xu, Gang; Liu, Zhenxian; Solanki, G. K.; Patel, K. D.; Cui, Yi; Hwang, Harold Y.; Mao, Wendy L.

    2015-01-01

    Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ∼60 GPa using multiple experimental techniques and ab-initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides. PMID:26088416

  9. Pressure induced magneto-structural phase transitions in layered RMn2X2 compounds (invited)

    NASA Astrophysics Data System (ADS)

    Kennedy, Shane; Wang, Jianli; Campbell, Stewart; Hofmann, Michael; Dou, Shixue

    2014-05-01

    We have studied a range of pseudo-ternaries derived from the parent compound PrMn2Ge2, substituting for each constituent element with a smaller one to contract the lattice. This enables us to observe the magneto-elastic transitions that occur as the Mn-Mn nearest neighbour distance is reduced and to assess the role of Pr on the magnetism. Here, we report on the PrMn2Ge2-xSix, Pr1-xYxMn2Ge2, and PrMn2-xFexGe2 systems. The pressure produced by chemical substitution in these pseudo-ternaries is inherently non-uniform, with local pressure variations dependent on the local atomic distribution. We find that concentrated chemical substitution on the R or X site (e.g., in Pr0.5Y0.5Mn2Ge2 and PrMn2Ge0.8Si1.2) can produce a separation into two distinct magnetic phases, canted ferromagnetic and canted antiferromagnetic, with a commensurate phase gap in the crystalline lattice. This phase gap is a consequence of the combination of phase separation and spontaneous magnetostriction, which is positive on transition to the canted ferromagnetic phase and negative on transition to the canted antiferromagnetic phase. Our results show that co-existence of canted ferromagnetic and antiferromagnetic phases depends on chemical pressure from the rare earth and metalloid sites, on local lattice strain distributions and on applied magnetic field. We demonstrate that the effects of chemical pressure bear close resemblance to those of mechanical pressure on the parent compound.

  10. The atomistic simulation of pressure-induced phase transition in uranium mononitride

    NASA Astrophysics Data System (ADS)

    Tseplyaev, V. I.; Starikov, S. V.

    2015-11-01

    Phase transition in uranium mononitride (UN) at high pressure has been studied using molecular dynamics. At low pressure, UN has the cubic structure like NaCl (with the space group Fm3̅m). The research based on Gibbs energy calculation shows that cubic UN turns into rhombohedral face-centered structure (with the space group R3̅m) at pressure about 32 GPa. It is shown that parameters of R3̅m-structure change at increasing of the pressure. At various pressures, the parameters of structures with isotropic stress tensor are different.

  11. Pressure induced phase transition and thermo-physical properties in LuX (X = N, P)

    NASA Astrophysics Data System (ADS)

    Sahoo, B. D.; Mukherjee, D.; Joshi, K. D.; Kaushik, T. C.; Gupta, Satish C.

    2016-04-01

    Detailed total energy calculations have been performed in lutetium pnictides (LuX, where X = N, P) to understand their high pressure structural stability. In LuN, the ambient rocksalt type structure (B1 phase) transforms to a tetragonal structure (B10 phase) at ∼240 GPa; whereas in LuP the orthorhombic structure (B33, space group Cmcm) emerges as a high pressure structure above 48 GPa. Both the transitions are found to be of first-order type with volume discontinuities of ∼6% and 8.2%, respectively. The high pressure phases B10 and B33 are found to be stable up to 400 GPa, respectively. Further, the structural stability predicted from static lattice calculations has been supported by lattice dynamical stability analysis. The present calculations rule out the B1 to B2 (CsCl type) structural phase transitions predicted to occur at 241 GPa in LuN and at 98 GPa in LuP by previous all-electron calculations (Gupta and Bhat 2013 J. Mol. Model 19 5343–54). The temperature dependence of several thermo-physical properties such as volume, bulk modulus, specific heat and thermal expansion coefficient of the rocksalt structure of these compounds calculated in the present study, using quasi-harmonic approximation, awaits confirmation by experimental studies.

  12. Pressure-induced kinetics of the α to ω transition in zirconium

    SciTech Connect

    Jacobsen, M. K.; Velisavljevic, N.; Sinogeikin, S. V.

    2015-07-14

    Diamond anvil cells (DAC) coupled with x-ray diffraction (XRD) measurements are one of the primary techniques for investigating structural stability of materials at high pressure-temperature (P-T) conditions. DAC-XRD has been predominantly used to resolve structural information at set P-T conditions and, consequently, provides P-T phase diagram information on a broad range of materials. With advances in large scale synchrotron x-ray facilities and corresponding x-ray diagnostic capabilities, it is now becoming possible to perform sub-second time resolved measurements on micron sized DAC samples. As a result, there is an opportunity to gain valuable information about the kinetics of structural phase transformations and extend our understanding of material behavior at high P-T conditions. Using DAC-XRD time resolved measurements, we have investigated the kinetics of the α to ω transformation in zirconium. We observe a clear time and pressure dependence in the martensitic α-ω transition as a function of pressure-jump, i.e., drive pressure. The resulting data are fit using available kinetics models, which can provide further insight into transformation mechanism that influence transformation kinetics. Our results help shed light on the discrepancies observed in previous measurements of the α-ω transition pressure in zirconium.

  13. Pressure-Induced Antifluorite-to-Anticotunnite Phase Transition in Lithium Oxide

    SciTech Connect

    Lazicki, A; Yoo, C; Evans, W J; Pickett, W E

    2006-04-12

    Using synchrotron angle-dispersive x-ray diffraction (ADXD) and Raman spectroscopy on samples of Li{sub 2}O pressurized in a diamond anvil cell, we observed a reversible phase change from the cubic antifluorite ({alpha}, Fm-3m) to orthorhombic anticotunnite ({beta}, Pnma) phase at 50({+-}5) GPa at ambient temperature. This transition is accompanied by a relatively large volume collapse of 5.4 ({+-}0.8)% and large hysteresis upon pressure reversal (P{sub down} at {approx} 25 GPa). Contrary to a recent study, our data suggest that the high-pressure {beta}-phase (B{sub o} = 188 {+-} 12 GPa) is substantially stiffer than the low-pressure {alpha}-phase (B{sub o} = 90 {+-} 1 GPa). A relatively strong and pressure-dependent preferred orientation in {beta}-Li{sub 2}O is observed. The present result is in accordance with the systematic behavior of antifluorite-to-anticotunnite phase transitions occurring in the alkali-metal sulfides.

  14. Pressure-Induced Phase Transitions of Hydrophobically Solvated Block-Copolymer Solutions

    SciTech Connect

    Osaka, Noboru; Shibayama, Mitsuhiro

    2006-02-03

    The structures of poly(2-(2-ethoxy)ethoxyethyl vinyl ether)-block-poly(2-methoxyethyl vinyl ether) in D{sub 2}O have been investigated with small-angle neutron scattering (SANS) as a function of temperature T and pressure P. At ambient pressure, the solution underwent a two-step transition at 40 and 65 deg. C, both of which were convex-upward functions of P having a maximum around P{sub 0}{approx_equal}150 MPa. The first transition was assigned to a microphase separation to form a bcc structure, and the second was to a macrophase separation. Pressurizing at 28 deg. C resulted in a macrophase separation with divergence at 350 MPa. At 45 deg. C, a reentrant microphase separation was observed by increasing P. Differences in the states of hydrophobic solvation in the low (PP{sub 0}) are discussed based on the SANS structure factors.

  15. X-ray diagnosis of the pressure induced Mott nonmetal-metal transition.

    PubMed

    Lévy, A; Dorchies, F; Benuzzi-Mounaix, A; Ravasio, A; Festa, F; Recoules, V; Peyrusse, O; Amadou, N; Brambrink, E; Hall, T; Koenig, M; Mazevet, S

    2012-02-01

    The evolution of the K-edge x-ray absorption near-edge spectroscopy (XANES) spectrum is investigated for an aluminum plasma expanding from the solid density down to 0.5  g/cm{3}, with temperatures lying from 5 down to 2 eV. The dense plasma is generated by nanosecond laser-induced shock compression. These conditions correspond to the density-temperature region where a metal-nonmetal transition occurs as the density decreases. This transition is directly observed in XANES spectra measurements through the progressive formation of a preedge structure for densities around 1.6  g/cm{3}. Ab initio calculations based on density functional theory and a jellium model have been efficiently tested through direct comparison with the experimental measurements and show that this preedge corresponds to the relocalization of the 3p atomic orbital as the system evolves from a dense plasma toward a partially ionized atomic fluid. PMID:22400937

  16. A pressure-induced, magnetic transition in pyrrhotite: Implications for the formation pressure of meteorites and diamonds

    NASA Astrophysics Data System (ADS)

    Gilder, S. A.; Egli, R.; Hochleitner, R.; Roud, S. C.; Volk, M. W. R.; Le Goff, M.; de Wit, M.

    2012-04-01

    Meteorites and diamonds encounter high pressures during their formation or subsequent evolution. These materials sometimes contain magnetic inclusions of pyrrhotite. Because magnetic properties are sensitive to strain, pyrrhotite can potentially record the shock or formation pressures of its host. Moreover, pyrrhotite undergoes a pressure-induced phase transition between 1.6 and 6.2 GPa, but the magnetic signature of this transition is poorly known. Here we report room temperature magnetic measurements on multi- and single domain pyrrhotite under non-hydrostatic pressure up to 4.5 GPa. We find that the ratio of magnetic coercivity and remanence follows a logarithmic law with respect to pressure, which can potentially be used as a geobarometer. Due to the greater thermal expansion of pyrrhotite with respect to diamond, pyrrhotite inclusions in diamond experience a confining pressure at the Earth's surface. Applying our experimentally derived magnetic geobarometer to pyrrhotite-bearing diamonds from Botswana and the Central African Republic suggests the pressures of the pyrrhotite inclusions in the diamonds range from 1.3 to 2.1 GPa. These overpressures constrain the mantle source pressures from 5.4 to 9.5 GPa, depending on which bulk modulus and thermal expansion coefficients of the two phases are used. We are now trying to develop magnetic barometers on other magnetic phases to apply to meteorites, ultimately to constrain the minimum pressure in which the meteorite formed and, hence, information regarding the planetesmal's size, and/or depth, in which the meteorite was derived.

  17. Pressure Induced Structural Phase Transition in Actinide Monophospides: Ab Initio Calculations

    NASA Astrophysics Data System (ADS)

    Makode, Chandrabhan; Sanyal, Sankar P.

    2011-07-01

    The structural and electronic properties of monophospides of Thorium, Uranium and Neptunium have been investigated using tight binding linear muffin-in-orbital (TB-LMTO) method within the local density approximation (LDA). From present study with the help of total energy calculations it is found that ThP, UP and NpP are stable in NaCl- type structure under ambient pressure. The structure stability of ThP, UP and NpP changes under the application of pressure. We predict a structural phase transition from NaCl-type (B1-phase) structure to CsCl-type (B2-phase) structure for these phospides in the pressure range of 37.0-24.0 GPa (ThP to NpP). The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.

  18. Pressure-Induced Transition of Bilayers in a Nonionic Surfactant Solution.

    PubMed

    Takano, Tetsuo; Kawabata, Youhei; Suzuki, Takuro; Kato, Tadashi

    2015-09-01

    Pressure effects on the bilayers of polyoxyethylene type nonionic surfactant in water have been investigated by means of small- and wide-angle X-ray scattering. It has been found that the Krafft transition from the micellar phase to the lamellar gel phase (Lβ) is induced by pressure. By further pressurizing, the lamellar structural parameters, such as the repeat distance d and Caillé parameter η, discontinuously decrease after taking a maximum. All the SAXS and WAXS results revealed that the Lβ phase is transformed into the higher-ordered lamellar crystal phase (Lc). On the basis of these observations, we have made the T-C and T-P phase diagrams. PMID:26237248

  19. Pressure induced structural phase transition in actinide mono-bismuthides: Ab initio calculations

    NASA Astrophysics Data System (ADS)

    Pataiya, J.; Makode, C.; Aynyas, M.; Sanyal, Sankar P.

    2013-06-01

    The structural and electronic properties of mono-bismuthides of Plutonium and Americium have been investigated using tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). From present study with the help of total energy calculations it is found that PuBi and AmBi are stable in NaCl - type structure under ambient pressure. The structure stability of PuBi and AmBi changes under the application of pressure. We predict a structural phase transition from NaCl-type (B1-phase) structure to CsCl-type (B2-phase) structure for these phospides in the pressure range of 45 - 4.5 GPa for PuBi and AmBi respectively. The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.

  20. Pressure-induced electronic phase transition in compound EuCu2Ge2

    NASA Astrophysics Data System (ADS)

    Geondzhian, A. Y.; Yaroslavtsev, A. A.; Alekseev, P. A.; Chernikov, R. V.; Gaynanov, B. R.; Baudelet, F.; Nataf, L.; Menushenkov, A. P.

    2016-05-01

    We report the high-pressure XANES study of the electronic phase transition from 4f7 to 4f 6 configuration of europium in the rare-earth compound EuCu2Ge2. The hydrostatic pressure dependence of the europium valence was obtained in a wide pressure range (1-30 GPa) at room temperature. It was found that upon the pressure increase above 20 GPa the europium valence does not reach the integer value +3 but stabilizes at 2.87. The experimental results were supported by the band structure calculations in the framework of DFT, which allowed us to discuss the features of 3d-4f hybridization in this system. The study also compares the mechanisms of external and “chemical” pressure by the Si substitution in Ge site in series EuCu2(SixGe1-x)2.

  1. Comprehensive characterization of temperature- and pressure-induced bilayer phase transitions for saturated phosphatidylcholines containing longer chain homologs.

    PubMed

    Goto, Masaki; Endo, Takuya; Yano, Takahiro; Tamai, Nobutake; Kohlbrecher, Joachim; Matsuki, Hitoshi

    2015-04-01

    Complete elucidation of the phase behavior of phospholipid bilayers requires information on the subtransition from the lamellar crystal (Lc) phase to the gel phase. However, for bilayers of saturated diacylphosphatidylcholines (CnPCs), especially longer chain homologs, equilibration in the Lc phase is known to be very slow. In this study, bilayer phase transitions of three CnPCs with longer acyl chains, C19PC, C20PC and C21PC, were observed by differential scanning calorimetry under atmospheric pressure and by light-transmittance measurements under high pressure. Using lipid samples treated by thermal annealing enabled the observation of the sub-, pre- and main transitions of the C19PC and C20PC bilayers under atmospheric pressure. Only the pre- and main transitions could be observed for the C21PC bilayer due to very slow kinetics of the Lc phase formation for lipids with long acyl chains. The temperature and pressure phase diagrams constructed and phase-transitions quantities (enthalpy, entropy and volume changes) evaluated for these bilayers were compared with one another and with those of bilayers of the CnPC homologs examined in previous studies. These results allowed us (1) to clarify the temperature- and pressure-dependent phase sequence and phase stability of the CnPC (n=12-22) bilayers as a function of the hydrophobicity of the molecules, (2) to prove the presence of a shorter and a longer limit (n=13 and 21) in the acyl chain length for the pressure-induced bilayer interdigitation and (3) to reveal the chain-length dependence of the thermodynamic quantities of the subtransitions including the volume change. PMID:25779604

  2. Pressure-Induced Structural Phase Transition in CeNi: X-ray and Neutron Scattering Studies and First-Principles Calculations

    DOE PAGESBeta

    Mirmelstein, A.; Podlesnyak, Andrey A.; dos Santos, Antonio M.; Ehlers, Georg; Kerbel, O.; Matvienko, V.; Sefat, A. S.; Saporov, B.; Halder, G. J.; Tobin, J. G.

    2015-08-03

    The pressure-induced structural phase transition in the intermediate-valence compound CeNi has been investigated by x-ray and neutron powder diffraction techniques. It is shown that the structure of the pressure-induced CeNi phase (phases) can be described in terms of the Pnma space group. Equations of state for CeNi on both sides of the phase transition are derived and an approximate P-T phase diagram is suggested for P<8 GPa and T<300 K. The observed Cmcm→Pnma structural transition is then analyzed using density functional theory calculations, which successfully reproduce the ground state volume, the phase transition pressure, and the volume collapse associated withmore » the phase transition.« less

  3. Pressure-Induced Structural Phase Transition in CeNi: X-ray and Neutron Scattering Studies and First-Principles Calculations

    SciTech Connect

    Mirmelstein, A.; Podlesnyak, Andrey A.; dos Santos, Antonio M.; Ehlers, Georg; Kerbel, O.; Matvienko, V.; Sefat, A. S.; Saporov, B.; Halder, G. J.; Tobin, J. G.

    2015-08-03

    The pressure-induced structural phase transition in the intermediate-valence compound CeNi has been investigated by x-ray and neutron powder diffraction techniques. It is shown that the structure of the pressure-induced CeNi phase (phases) can be described in terms of the Pnma space group. Equations of state for CeNi on both sides of the phase transition are derived and an approximate P-T phase diagram is suggested for P<8 GPa and T<300 K. The observed Cmcm→Pnma structural transition is then analyzed using density functional theory calculations, which successfully reproduce the ground state volume, the phase transition pressure, and the volume collapse associated with the phase transition.

  4. Pressure-induced phase transition in synthetic trioctahedral Rb-mica

    NASA Astrophysics Data System (ADS)

    Comodi, P.; Drábek, M.; Montagnoli, M.; Rieder, M.; Weiss, Z.; Zanazzi, P. F.

    The crystal structure of a synthetic Rb analog of tetra-ferri-annite (Rb-TFA) 1M with the composition Rb0.99Fe2+3.03(Fe3+ 1.04 Si2.96)O10.0(OH)2.0 was determined by the single-crystal X-ray diffraction method. The structure is homooctahedral (space group C2/m) with M1 and M2 occupied by divalent iron. Its unit cell is larger than that of the common potassium trioctahedral mica, and similar lateral dimensions of the tetrahedral and octahedral sheets allow a small tetrahedral rotation angle α=2.23(6)°. Structure refinements at 0.0001, 1.76, 2.81, 4.75, and 7.2 GPa indicate that in some respects the Rb-TFA behaves like all other micas when pressure increases: the octahedra are more compressible than the tetrahedra and the interlayer is four times more compressible than the 2:1 layer. However, there is a peculiar behavior of the tetrahedral rotation angle α: at lower pressures (0.0001, 1.76, 2.81 GPa), it has positive values that increase with pressure [from 2.23(6)° to 6.3(4)°] as in other micas, but negative values -7.5(5)° and -8.5(9)° appear at higher pressures, 4.75 and 7.2 GPa, respectively. This structural evidence, together with electrostatic energy calculations, shows that Rb-TFA has a Franzini A-type 2:1 layer up to at least 2.81 GPa that at higher pressure yields to a Franzini B-type layer, as shown by the refinements at 4.75 and 7.2 GPa. The inversion of the α angle is interpreted as a consequence of an isosymmetric displacive phase transition from A-type to B-type structure between 2.81 and 4.75 GPa. The compressibility of the Rb-TFA was also investigated by single-crystal X-ray diffraction up to a maximum pressure of 10 GPa. The lattice parameters reveal a sharp discontinuity between 3.36 and 3.84 GPa, which was associated with the phase transition from Franzini-A to Franzini-B structure.

  5. Pressure-induced phase transitions of AX2-type iron pnictides: an ab initio study

    NASA Astrophysics Data System (ADS)

    Wu, X.; Steinle-Neumann, G.; Qin, S.; Kanzaki, M.; Dubrovinsky, L.

    2009-05-01

    An investigation into the high-pressure behavior of AX2-type iron pnictides was conducted using first-principles calculations based on density functional theory within the generalized gradient approximation. Our results demonstrate that a phase transition from the marcasite to the CuAl2 occurs at 108 GPa for FeP2, at 92 GPa for FeAs2, and at 38 GPa for FeSb2, accompanying a semiconductor-to-metal crossover. A linear relationship between bulk moduli and the inverse specific volume is proposed to be B0 = 17 498/V0-45.9 GPa for the marcasite-type phase and B0 = 31 798/V0-67.5 GPa for the CuAl2-type phase. According to the observed structural evolutions, we claim that the regular marcasite transforms to the CuAl2-type phase and the anomalous marcasite transforms to the pyrite-type phase at high pressures.

  6. Pressure induced structural phase transition and electronic properties of actinide monophospides: Ab-initio calculations

    NASA Astrophysics Data System (ADS)

    Makode, Chandrabhan; Sanyal, Sankar P.

    2011-09-01

    We have investigated the structural and electronic properties of monophospides of thorium, uranium and neptunium. The total energy as a function of volume is obtained by means of the self-consistent tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). From the present study with the help of total energy calculations it is found that ThP, UP and NpP are stable in NaCl-type structure at ambient pressure. The structural stability of ThP, UP and NpP changes under the application of pressure. We predict a structural phase transition from NaCl-type (B 1-phase) structure to CsCl-type (B 2-phase) structure for these phospides in the pressure range of 37.0-24.0 GPa (ThP-NpP). We also calculate lattice parameter ( a0), bulk modulus ( B0), band structure and density of states. From energy band diagram it is observed that ThP, UP and NpP exhibit metallic behavior. The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.

  7. Pressure responses of portlandite and H-D isotope effects on pressure-induced phase transitions

    NASA Astrophysics Data System (ADS)

    Iizuka, Riko; Kagi, Hiroyuki; Komatsu, Kazuki; Ushijima, Daichi; Nakano, Satoshi; Sano-Furukawa, Asami; Nagai, Takaya; Yagi, Takehiko

    2011-12-01

    The pressure responses of portlandite and the isotope effect on the phase transition were investigated at room temperature from single-crystal Raman and IR spectra and from powder X-ray diffraction using diamond anvil cells under quasi-hydrostatic conditions in a helium pressure-transmitting medium. Phase transformation and subsequent peak broadening (partial amorphization) observed from the Raman and IR spectra of Ca(OH)2 occurred at lower pressures than those of Ca(OD)2. In contrast, no isotope effect was found on the volume and axial compressions observed from powder X-ray diffraction patterns. X-ray diffraction lines attributable to the high-pressure phase remained up to 28.5 GPa, suggesting no total amorphization in a helium pressure medium within the examined pressure region. These results suggest that the H-D isotope effect is engendered in the local environment surrounding H(D) atoms. Moreover, the ratio of sample-to-methanol-ethanol pressure medium (i.e., packing density) in the sample chamber had a significant effect on the increase in the half widths of the diffraction lines, even at pressures below the hydrostatic limit of the pressure medium.

  8. Pressure-induced oversaturation and phase transition in zeolitic imidazolate frameworks with remarkable mechanical stability.

    PubMed

    Zhao, Pu; Bennett, Thomas D; Casati, Nicola P M; Lampronti, Giulio I; Moggach, Stephen A; Redfern, Simon A T

    2015-03-14

    Zeolitic imidazolate frameworks (ZIFs) 7 and 9 are excellent candidates for CO2 adsorption and storage. Here, high-pressure X-ray diffraction is used to further understand their potential in realistic industrial applications. ZIF-7 and ZIF-9 are shown be able to withstand high hydrostatic pressures whilst retaining their porosity and structural integrity through a new ferroelastic phase transition. This stability is attributed to the presence of sterically large organic ligands. Results confirm the notable influence of guest occupancy on the response of ZIFs to pressure; oversaturation of ZIFs with solvent molecules greatly decreases their compressibility and increases their resistance to amorphisation. By comparing the behaviours of both ZIFs under high pressure, it is demonstrated that their mechanical stability is not affected by metal substitution. The evacuated ZIF-7 phase, ZIF-7-II, is shown to be able to recover to the ZIF-7 structure with excellent resistance to pressure. Examining the pressure-related structural behaviours of ZIF-7 and ZIF-9, we have assessed the great industrial potential of ZIFs. PMID:25649463

  9. Theoretical investigation of La monopnictides: Electronic properties and pressure-induced phase transition

    SciTech Connect

    Yan, X. Z.; Chen, Y. M.; Kuang, X. Y.; Xiang, S. K.

    2014-08-28

    The NaCl-type La monopnictides are proper reference materials for the study of strongly correlated rare-earth pnictides. Yet, despite the simple crystal structure of this system, traditional density functional theory (DFT) calculations have dramatic failures in describing their electronic properties: DFT severely underestimates the band gaps and thus predicts incorrect transport characters of them. Here, we perform a corrected DFT calculation to rectify this failure. Our results show that LaN, LaP, and LaAs are semiconductor with band gaps of 0.82, 0.25, and 0.12 eV, respectively, and LaSb is semimetallic with an overlap of conduction and valence bands approximately 0.28 eV, in agreement with the available experiments. Additionally, under high-pressure, we find that LaN displays a new sequence of phase-transition, B1 → anti-B10 → B2, which is different from the previous theoretical predictions but consistent with the recent experiment.

  10. Mercury Fluorides under high pressure: Hg as a pressure-induced transition metal

    NASA Astrophysics Data System (ADS)

    Botana, Jorge; Wang, Xiaoli; Yang, Dadong; Ling, Haiqing; Ma, Yangming; Miao, Mao-Sheng

    2014-03-01

    Hg has recently been found experimentally to be capable of forming a chemical compound, HgF4, where it behaves as a transition metal, with an oxidation number of IV, but this molecule is very short lived. In this work we present theoretical evidence obtained through ab initio calculations that higher oxidation states than II can be stabilized in crystalline form for Hg, under extreme pressure. We have performed a structural search and optimization by means of Particle Swarm Optimization and Density Functional Theory for the crystalline series of HgFn (n=3,4,5,6), and then used those data to draw the phase diagram of the equilibrium among those stoichiometries and HgF2 and F2. We have found that from 0 to 38 GPa only the mixture of HgF2 and F2 phases is thermodynamically stable. HgF3 and HgF4 have been found to be thermodynamically stable in different pressure ranges (from 73 GPa to at least 500 GPa and from 38 GPa to 200 GPa , respectively). We have also found that the HgF3 crystal shows a very interesting band structure that suggests it could be a transparent conductor.

  11. Picosecond Electronic Spectroscopy to Determine the Transformation Mechanism for the Pressure-Induced Phase Transition in Shocked CdS

    SciTech Connect

    Knudson, M.D.; Gupta, Y.M.; Kunz, A.B.

    1999-07-21

    Plate impact, shock wave experiments provide a unique method to investigate the time-dependent mechanisms and the kinetics associated with pressure-induced phenomena, such as chemical reactions and phase transformations. The very rapid and well defined loading conditions associated with plate-impact experiments permit real-time examination of the shock-induced changes. Further, the ability to propagate the shock wave along various crystallographic directions provides the means to perform careful analysis of the stress and orientational dependence. Recently, an experimental method has been developed to observe real-time changes in the absorption transmission of materials, with 100 or 200 ps resolution, in single-event, plate impact shock experiments [1-4]. These data can provide useful information regarding the material under investigation. In particular, the dependence of the absorption edge on photon energy can distinguish between direct and indirect electronic transitions, and can provide an estimate of the band-gap energy of the material [5]. Along with ab-initio techniques to calculate the electronic structure of a crystalline system, this electronic information can be used to gain insight regarding the crystal structure. As described in Ref. [1,2,4] the wurtzite-to-rocksalt phase transition in cadmium sulfide (CdS) is well suited to investigation through the use of fast electronic spectroscopy; the wurtzite and rocksalt phases exhibit a direct and indirect band gap with band gap energies of 2.5 and 1.5-1.7 eV, respectively [6-8]. The intent of this work was to use picosecond electronic spectroscopy and ab-initio methods to examine the real-time structural changes that occur in the initial stages of the shock-induced wurtzite-to-rocksalt phase transition in single crystal CdS.

  12. Pressure induced phase transitions and metallization of a neutral radical conductor.

    PubMed

    Wong, Joanne W L; Mailman, Aaron; Lekin, Kristina; Winter, Stephen M; Yong, Wenjun; Zhao, Jianbao; Garimella, Subrahmanyam V; Tse, John S; Secco, Richard A; Desgreniers, Serge; Ohishi, Yasuo; Borondics, Ferenc; Oakley, Richard T

    2014-01-22

    The crystal structure and charge transport properties of the prototypal oxobenzene-bridged 1,2,3-bisdithiazolyl radical conductor 3a are strongly dependent on pressure. Compression of the as-crystallized α-phase, space group Fdd2, to 3-4 GPa leads to its conversion into a second or β-phase, in which F-centering is lost. The space group symmetry is lowered to Pbn2₁, and there is concomitant halving of the a and b axes. A third or γ-phase, also space group Pbn2₁, is generated by further compression to 8 GPa. The changes in packing that accompany both phase transitions are associated with an "ironing out" of the ruffled ribbon-like architecture of the α-phase, so that consecutive radicals along the ribbons are rendered more nearly coplanar. In the β-phase the planar ribbons are propagated along the b-glides, while in the γ-phase they follow the n-glides. At ambient pressure 3a is a Mott insulator, displaying high but activated conductivity, with σ(300 K) = 6 × 10(-3) S cm(-1) and E(act) = 0.16 eV. With compression beyond 4 GPa, its conductivity is increased by 3 orders of magnitude, and the thermal activation energy is reduced to zero, heralding the formation of a metallic state. High pressure infrared absorption and reflectivity measurements are consistent with closure of the Mott-Hubbard gap near 4-5 GPa. The results are discussed in the light of DFT calculations on the molecular and band electronic structure of 3a. The presence of a low-lying LUMO in 3a gives rise to high electron affinity which, in turn, creates an electronically much softer radical with a low onsite Coulomb potential U. In addition, considerable crystal orbital (SOMO/LUMO) mixing occurs upon pressurization, so that a metallic state is readily achieved at relatively low applied pressure. PMID:24400662

  13. A coupled cluster and Møller-Plesset perturbation theory study of the pressure induced phase transition in the LiH crystal

    SciTech Connect

    Grüneis, Andreas

    2015-09-14

    We employ Hartree–Fock, second-order Møller-Plesset perturbation, coupled cluster singles and doubles (CCSD) as well as CCSD plus perturbative triples (CCSD(T)) theory to study the pressure induced transition from the rocksalt to the cesium chloride crystal structure in LiH. We show that the calculated transition pressure converges rapidly in this series of increasingly accurate many-electron wave function based theories. Using CCSD(T) theory, we predict a transition pressure for the structural phase transition in the LiH crystal of 340 GPa. Furthermore, we investigate the potential energy surface for this transition in the parameter space of the Buerger path.

  14. Pressure-induced structural phase transition, elastic and thermodynamic properties of ReC under high pressure

    NASA Astrophysics Data System (ADS)

    Lei, Hui-Ru; Zhu, Jun; Hao, Yan-Jun; Zhang, Lin; Zhao, Yu-Xin; Zhan, Guo-Fu

    2015-10-01

    The pressure-induced structural phase transition of rhenium monocarbon (ReC) is investigated via the projector augmented wave (PAW) method with the generalized gradient approximation (GGA). Using the first-principles calculations, the equilibrium structural parameters of ReC in rocksalt (NaCl), cesium chloride (CsCl), zinc blende (ZB), wurtzite (WZ), nickel arsenide (NiAs) and tungsten carbide (WC) types are successfully obtained, and the results are well consistent with other theoretical data. It is firstly noted that WC-ReC translates into CsCl-ReC at 510.50 GPa by analyzing the enthalpy difference versus pressure. From the calculated elastic constants, the aggregate elastic modulus (B, G, E), the Poisson's ratio (σ) and the Debye temperature ΘD of WC-type are also derived. It is observed that all the data of WC-ReC obtained increase monotonically with increasing pressure. Meanwhile, the thermodynamic properties of WC-ReC under high temperature and high pressure are investigated applying nonempirical Debye model in the quasi-harmonic approximation.

  15. Mechanism of Pressure-Induced Phase Transitions, Amorphization, and Absorption-Edge Shift in Photovoltaic Methylammonium Lead Iodide.

    PubMed

    Szafrański, Marek; Katrusiak, Andrzej

    2016-09-01

    Our single-crystal X-ray diffraction study of methylammonium lead triiodide, MAPbI3, provides the first comprehensive structural information on the tetragonal phase II in the pressure range to 0.35 GPa, on the cubic phase IV stable between 0.35 and 2.5 GPa, and on the isostructural cubic phase V observed above 2.5 GPa, which undergoes a gradual amorphization. The optical absorption study confirms that up to 0.35 GPa, the absorption edge of MAPbI3 is red-shifted, allowing an extension of spectral absorption. The transitions to phases IV and V are associated with the abrupt blue shifts of the absorption edge. The strong increase of the energy gap in phase V result in a spectacular color change of the crystal from black to red around 3.5 GPa. The optical changes have been correlated with the pressure-induced strain of the MAPbI3 inorganic framework and its frustration, triggered by methylammonium cations trapped at random orientations in the squeezed voids. PMID:27538989

  16. Charge disproportionation and the pressure-induced insulator–metal transition in cubic perovskite PbCrO3

    PubMed Central

    Cheng, Jinguang; Kweon, K. E.; Larregola, S. A.; Ding, Yang; Shirako, Y.; Marshall, L. G.; Li, Z.-Y.; Li, X.; dos Santos, António M.; Suchomel, M. R.; Matsubayashi, K.; Uwatoko, Y.; Hwang, G. S.; Goodenough, John B.; Zhou, J.-S.

    2015-01-01

    The perovskite PbCrO3 is an antiferromagnetic insulator. However, the fundamental interactions leading to the insulating state in this single-valent perovskite are unclear. Moreover, the origin of the unprecedented volume drop observed at a modest pressure of P = 1.6 GPa remains an outstanding problem. We report a variety of in situ pressure measurements including electron transport properties, X-ray absorption spectrum, and crystal structure study by X-ray and neutron diffraction. These studies reveal key information leading to the elucidation of the physics behind the insulating state and the pressure-induced transition. We argue that a charge disproportionation 3Cr4+ → 2Cr3+ + Cr6+ in association with the 6s-p hybridization on the Pb2+ is responsible for the insulating ground state of PbCrO3 at ambient pressure and the charge disproportionation phase is suppressed under pressure to give rise to a metallic phase at high pressure. The model is well supported by density function theory plus the correlation energy U (DFT+U) calculations. PMID:25624483

  17. Pressure-induced semimetal-semiconductor transition and enhancement of thermoelectric performance in α-MgAgSb

    NASA Astrophysics Data System (ADS)

    Miao, Naihua; Zhou, Jian; Sa, Baisheng; Xu, Bin; Sun, Zhimei

    2016-05-01

    Comparable to bismuth telluride, α-MgAgSb-based materials (α-MAS) have been investigated recently as promising candidates for room-temperature thermoelectric energy harvesting and thus various efforts have been devoted to the enhancement of their thermoelectric performance. By utilizing first-principles density functional calculations and Boltzmann transport theory, we report that the thermoelectric properties of α-MAS can be dramatically improved with the application of hydrostatic pressure. This is attributed to a pressure-induced semimetal to semiconductor transition in α-MAS. With the benefit of this pressure-tunable behaviour, the Seebeck coefficient of α-MAS can be manipulated flexibly. Furthermore, we found that, through the combination of applying pressure and p-type doping, the optimal thermoelectric power factor and figure of merit of α-MAS can be enhanced remarkably by 110% at 550 K compared with the intrinsic case. Our results provide an interesting insight and a feasible guideline for the improvement of the thermoelectric properties of α-MAS related materials.

  18. Pressure-induced magnetic transition and sound velocities of Fe(3)c : implications for carbon in the earth's inner core.

    SciTech Connect

    Gao, L.; Chen, B.; Wang, J.; Alp, E.E.; Zhao, J.; Lerche, M.; Sturhahn, W.; Scott, H.P.; Huang, F.; Ding, Y.; Sinogeikin, S.V.; Lundstrom, C.C.; Bass, J.D.; Li, J.; X-Ray Science Division; Univ. of Illinois; Carnegie Inst. of Washington; Indiana Univ.

    2008-09-11

    We have carried out nuclear resonant scattering measurements on {sup 57}Fe-enriched Fe{sub 3}C between 1 bar and 50 GPa at 300 K. Synchrotron Moessbauer spectra reveal a pressure-induced magnetic transition in Fe{sub 3}C between 4.3 and 6.5 GPa. On the basis of our nuclear resonant inelastic X-ray scattering spectra and existing equation-of-state data, we have derived the compressional wave velocity V{sub p} and shear wave velocity V{sub s} for the high-pressure nonmagnetic phase, which can be expressed as functions of density ({rho}): V{sub p} (km/s) = -3.99 + 1.29{rho}(g/cm{sup 3}) and V{sub s} (km/s) = 1.45 + 0.24{rho}(g/cm{sup 3}). The addition of carbon to iron-nickel alloy brings density, V{sub p} and V{sub s} closer to seismic observations, supporting carbon as a principal light element in the Earth's inner core.

  19. Anatomy of a pressure-induced, ferromagnetic-to-paramagnetic transition in pyrrhotite: Implications for the formation pressure of diamonds

    NASA Astrophysics Data System (ADS)

    Gilder, Stuart A.; Egli, Ramon; Hochleitner, Rupert; Roud, Sophie C.; Volk, Michael W. R.; Le Goff, Maxime; de Wit, Maarten

    2011-10-01

    Meteorites and diamonds encounter high pressures during their formation or subsequent evolution. These materials commonly contain magnetic inclusions of pyrrhotite. Because magnetic properties are sensitive to strain, pyrrhotite can potentially record the shock or formation pressures of its host. Moreover, pyrrhotite undergoes a pressure-induced phase transition between 1.6 and 6.2 GPa, but the magnetic signature of this transition is poorly known. Here we report room temperature magnetic measurements on multidomain and single-domain pyrrhotite under nonhydrostatic pressure. Magnetic remanence in single-domain pyrrhotite is largely insensitive to pressure until 2 GPa, whereas the remanence of multidomain pyrrhotite increases 50% over that of initial conditions by 2 GPa, and then decreases until only 33% of the original remanence remains by 4.5 GPa. In contrast, magnetic coercivity increases with increasing pressure to 4.5 GPa. Below ˜1.5 GPa, multidomain pyrrhotite obeys Néel theory with a positive correlation between coercivity and remanence; above ˜1.5 GPa, it behaves single domain-like yet distinctly different from uncompressed single-domain pyrrhotite. The ratio of magnetic coercivity and remanence follows a logarithmic law with respect to pressure, which can potentially be used as a geobarometer. Owing to the greater thermal expansion of pyrrhotite with respect to diamond, pyrrhotite inclusions in diamonds experience a confining pressure at Earth's surface. Applying our experimentally derived magnetic geobarometer to pyrrhotite-bearing diamonds from Botswana and the Central African Republic suggests the pressures of the pyrrhotite inclusions in the diamonds range from 1.3 to 2.1 GPa. These overpressures constrain the mantle source pressures from 5.4 to 9.5 GPa, depending on which bulk modulus and thermal expansion coefficients of the two phases are used.

  20. Pressure-induced phase transition in Bi2Se3 at 3 GPa: electronic topological transition or not?

    PubMed

    Bera, Achintya; Pal, Koushik; Muthu, D V S; Waghmare, U V; Sood, A K

    2016-03-16

    In recent years, a low pressure transition around P3 GPa exhibited by the A2B3-type 3D topological insulators is attributed to an electronic topological transition (ETT) for which there is no direct evidence either from theory or experiments. We address this phase transition and other transitions at higher pressure in bismuth selenide (Bi2Se3) using Raman spectroscopy at pressure up to 26.2 GPa. We see clear Raman signatures of an isostructural phase transition at P2.4 GPa followed by structural transitions at ∼ 10 GPa and 16 GPa. First-principles calculations reveal anomalously sharp changes in the structural parameters like the internal angle of the rhombohedral unit cell with a minimum in the c/a ratio near P3 GPa. While our calculations reveal the associated anomalies in vibrational frequencies and electronic bandgap, the calculated Z2 invariant and Dirac conical surface electronic structure remain unchanged, showing that there is no change in the electronic topology at the lowest pressure transition. PMID:26881905

  1. Pressure-induced phase transition in Bi2Se3 at 3 GPa: electronic topological transition or not?

    NASA Astrophysics Data System (ADS)

    Bera, Achintya; Pal, Koushik; Muthu, D. V. S.; Waghmare, U. V.; Sood, A. K.

    2016-03-01

    In recent years, a low pressure transition around P∼ 3 GPa exhibited by the {{A}2}{{B}3} -type 3D topological insulators is attributed to an electronic topological transition (ETT) for which there is no direct evidence either from theory or experiments. We address this phase transition and other transitions at higher pressure in bismuth selenide (Bi2Se3) using Raman spectroscopy at pressure up to 26.2 GPa. We see clear Raman signatures of an isostructural phase transition at P∼ 2.4 GPa followed by structural transitions at  ∼10 GPa and 16 GPa. First-principles calculations reveal anomalously sharp changes in the structural parameters like the internal angle of the rhombohedral unit cell with a minimum in the c/a ratio near P∼ 3 GPa. While our calculations reveal the associated anomalies in vibrational frequencies and electronic bandgap, the calculated {{{Z}}2} invariant and Dirac conical surface electronic structure remain unchanged, showing that there is no change in the electronic topology at the lowest pressure transition.

  2. Pressure-induced electronic and magnetic phase transitions in a Mott insulator: Ti-doped C a3R u2O7 bilayer ruthenate

    NASA Astrophysics Data System (ADS)

    Zou, T.; Cao, H. B.; Liu, G. Q.; Peng, J.; Gottschalk, M.; Zhu, M.; Zhao, Y.; Leão, J. B.; Tian, W.; Mao, Z. Q.; Ke, X.

    2016-07-01

    We report the hydrostatic pressure-induced electronic and magnetic phase transitions in a Mott insulator, a bilayer ruthenate C a3(Ru0.97Ti0.03 ) 2O7 , via electronic transport and single crystal neutron diffraction measurements. The system undergoes an insulator-metal transition at a very small hydrostatic pressure ≈0.04 GPa, followed by a magnetic phase transition around 0.3 GPa, suggesting that the low energy charge fluctuation and magnetic ordering couple to the pressure separately in this compound. The a b initio calculations show that the suppressed Ru O6 flattening induced by the pressure reduces the orbital polarization and gives rise to an insulator-metal transition preceding the magnetic phase transition.

  3. A coupled cluster and Møller-Plesset perturbation theory study of the pressure induced phase transition in the LiH crystal.

    PubMed

    Grüneis, Andreas

    2015-09-14

    We employ Hartree-Fock, second-order Møller-Plesset perturbation, coupled cluster singles and doubles (CCSD) as well as CCSD plus perturbative triples (CCSD(T)) theory to study the pressure induced transition from the rocksalt to the cesium chloride crystal structure in LiH. We show that the calculated transition pressure converges rapidly in this series of increasingly accurate many-electron wave function based theories. Using CCSD(T) theory, we predict a transition pressure for the structural phase transition in the LiH crystal of 340 GPa. Furthermore, we investigate the potential energy surface for this transition in the parameter space of the Buerger path. PMID:26374010

  4. Exact matrix treatment of an osmotic ensemble model of adsorption and pressure induced structural transitions in metal organic frameworks.

    PubMed

    Dunne, Lawrence J; Manos, George

    2016-03-14

    Here we present an exactly treated quasi-one dimensional statistical mechanical osmotic ensemble model of pressure and adsorption induced breathing structural transformations of metal-organic frameworks (MOFs). The treatment uses a transfer matrix method. The model successfully reproduces the gas and pressure induced structural changes which are observed experimentally in MOFs. The model treatment presented here is a significant step towards analytical statistical mechanical treatments of flexible metal-organic frameworks. PMID:26514851

  5. X-Ray Diffraction and Mössbauer Spectroscopy Studies of Pressure-Induced Phase Transitions in a Mixed-Valence Trinuclear Iron Complex.

    PubMed

    Madsen, Solveig R; Gunnlaugsson, Haraldur P; Moggach, Stephen A; Eikeland, Espen; Wu, Lai-Chin; Leupold, Olaf; Overgaard, Jacob; Iversen, Bo B

    2016-07-01

    The mixed-valence complex Fe3 O(cyanoacetate)6 (H2 O)3 (1) has been studied by single-crystal X-ray diffraction analysis at pressures up to 5.3(1) GPa and by (synchrotron) Mössbauer spectroscopy at pressures up to 8(1) GPa. Crystal structure refinements were possible up to 4.0(1) GPa. In this pressure range, 1 undergoes two pressure-induced phase transitions. The first phase transition at around 3 GPa is isosymmetric and involves a 60° rotation of 50 % of the cyanoacetate ligands. The second phase transition at around 4 GPa reduces the symmetry from rhombohedral to triclinic. Mössbauer spectra show that the complex becomes partially valence-trapped after the second phase transition. This sluggish pressure-induced valence-trapping is in contrast to the very abrupt valence-trapping observed when compound 1 is cooled from 130 to 120 K at ambient pressure. PMID:27245642

  6. Temperature and pressure-induced valence transitions in YbNi2Ge2 and YbPd2Si2

    NASA Astrophysics Data System (ADS)

    Yamaoka, Hitoshi; Jarrige, Ignace; Tsujii, Naohito; Lin, Jung-Fu; Hiraoka, Nozomu; Ishii, Hirofumi; Tsuei, Ku-Ding

    2010-07-01

    We have measured the temperature and pressure-induced Yb valence transitions in tetragonal YbNi2Ge2 and YbPd2Si2 using x-ray absorption spectroscopy in the partial fluorescence yield mode and resonant x-ray emission spectroscopy. A temperature dependence of the Yb valence on the order of 0.1 has been measured, consistent with the magnetic-susceptibility study. The crossover from the low-temperature state having a stronger mixed valence to a high-temperature local moment behavior is analyzed within the Anderson impurity model. Pressure-induced second-order valence transitions are observed for both compounds with a more gradual transition in YbPd2Si2 than that of YbNi2Ge2 . The mean valences are slightly less than 3+ at ambient pressure but increase with applying pressure. Small variations in the Yb valence on the order of 0.03-0.05 can result in drastic change in the physical properties such as magnetic order and transport properties. Our results show that the Yb valence is noninteger around the quantum critical point.

  7. The pressure-induced ringwoodite to Mg-perovskite and periclase post-spinel phase transition: a Bader's topological analysis of the ab initio electron densities

    NASA Astrophysics Data System (ADS)

    Parisi, Filippo; Sciascia, Luciana; Princivalle, Francesco; Merli, Marcello

    2012-02-01

    In order to characterize the pressure-induced decomposition of ringwoodite (γ-Mg2SiO4), the topological analysis of the electron density ρ( r), based upon the theory of atoms in molecules (AIM) developed by Bader in the framework of the catastrophe theory, has been performed. Calculations have been carried out by means of the ab initio CRYSTAL09 code at the HF/DFT level, using Hamiltonians based on the Becke- LYP scheme containing hybrid Hartree-Fock/density functional exchange-correlation terms. The equation of state at 0 K has been constructed for the three phases involved in the post-spinel phase transition (ringwoodite → Mg-perovskite + periclase) occurring at the transition zone-lower mantel boundary. The topological results show that the decomposition of the ringwoodite at high pressures is caused by a conflict catastrophe. Furthermore, topological evidences of the central role played by the oxygen atoms to facilitate the pressure-induced ringwoodite decomposition and the subsequent phase transition have been noticed.

  8. An X-ray diffraction study of pressure-induced phase transitions in Bi{sub 2}MoO{sub 6}

    SciTech Connect

    Scott, Paul R.; Crow, J.A.; Maczka, M.; Kruger, M.B.

    2012-10-15

    Synchrotron based X-ray diffraction through a diamond anvil cell was used to determine the equations of state and pressure-induced phase transitions in Bi{sub 2}MoO{sub 6}. It was observed that Bi{sub 2}MoO{sub 6} undergoes a phase transformation at {approx}6.8 GPa. The high-pressure phase can be indexed to the orthorhombic structure and the transition is reversible on decompression from {approx}47 GPa. The bulk moduli of the low and high-pressure phases were calculated, while holding K Prime =4, to be: K=51{+-}1 GPa and K=141.5 {+-}0.1 GPa, respectively. - Graphical abstract: The material Bi{sub 2}MoO{sub 6} was placed inside a diamond anvil cell and then studied under high pressure at beamline X17C of the National Synchrotron Light source. X-ray diffraction data was analyzed using the Rietveld method. Highlights: Black-Right-Pointing-Pointer A high-pressure study of bismuth molybdate was performed. Black-Right-Pointing-Pointer Pressure-induced phase transitions were observed. Black-Right-Pointing-Pointer The low pressure phase bulk modulus was calculated to be K=51{+-}1 GPa. Black-Right-Pointing-Pointer The high pressure phase bulk modulus was calculated to be B=141.5{+-}0.1 GPa.

  9. Pressure-induced isostructural phase transition and correlation of FeAs coordination with the superconducting properties of 111-type Na

    SciTech Connect

    Liu, Qingqing; Yu, Xiaohui; Wang, Xiancheng; Deng, Zheng; Lv, Yuxi; Zhu, Jinlong; Zhang, Sijia; Liu, Haozhe; Yang, Wenge; Wang, Lin; Mao, Hokwang; Shen, Guoyin; Lu, Zhong-Yi; Ren, Yang; Chen, Zhiqiang; Lin, Zhijun; Zhao, Yusheng; Jin, Changqing

    2011-05-25

    The effect of pressure on the crystalline structure and superconducting transition temperature (T{sub c}) of the 111-type Na{sub 1–x}FeAs system using in situ high-pressure synchrotron X-ray powder diffraction and diamond anvil cell techniques is studied. A pressure-induced tetragonal to tetragonal isostructural phase transition was found. The systematic evolution of the FeAs{sub 4} tetrahedron as a function of pressure based on Rietveld refinements on the powder X-ray diffraction patterns was obtained. The nonmonotonic T{sub c}(P) behavior of Na{sub 1–x}FeAs is found to correlate with the anomalies of the distance between the anion (As) and the iron layer as well as the bond angle of As–Fe–As for the two tetragonal phases. This behavior provides the key structural information in understanding the origin of the pressure dependence of T{sub c} for 111-type iron pnictide superconductors. A pressure-induced structural phase transition is also observed at 20 GPa.

  10. Study of pressure induced polyamorphic transition in Ce-based ternary BMG using in situ x-ray scattering and electrical conductivity measurement

    NASA Astrophysics Data System (ADS)

    Chen, J.; Ma, C.; Tang, R.; Li, L.; Liu, H.; Gao, C.; Yang, W.

    2015-12-01

    In situ high energy x-ray scattering and electrical conductivity measurements on Ce70Al10Cu20 bulk metallic glass have been conducted using a diamond anvil cell (DAC) in conjunction with synchrotron x-rays or a laboratory electrical measurement system. The relative volumetric change (V/V0) as a function of pressure is inferred using the first sharp diffraction peak (FSDP) and the universal fractional noncubic power law[1]. The result indicates a pressure-induced polyamorphic transition at about 4 GPa in the ternary system. While the observed pressure of such polyamorphic transition in the Ce-base binary BMG is not very sensitive to its composition based on some of the previous studies[2, 3], this study indicates that such transition pressure increases considerably when a new component is added to the system. In the electrical conductivity measurement, a significant resistance change was observed in the pressure range coupled to polyamorphic transition. More discussions will be given regarding the electrical conductivity behavior of this system under high pressure to illustrate the delocalization of 4f electrons as the origin of the observed polyamorphic transition. References: 1. Zeng Q, Kono Y, Lin Y, Zeng Z, Wang J, Sinogeikin SV, Park C, Meng Y, Yang W, Mao H-K (2014) Universal fractional noncubic power law for density of metallic glasses. Physical Review Letters 112: 185502-185502 2. Zeng Q-S, Ding Y, Mao WL, Yang W, Sinogeikin SV, Shu J, Mao H-K, Jiang JZ (2010) Origin of pressure-induced polyamorphism in Ce75Al25 metallic glass. Physical Review Letters 104: 105702-105702 3. Sheng HW, Liu HZ, Cheng YQ, Wen J, Lee PL, Luo WK, Shastri SD, Ma E (2007) Polyamorphism in a metallic glass. Nature Materials DOI: 10.1038/nmat1839.

  11. High-pressure induced phase transitions of Y[subscript 2]O[subscript 3] and Y[subscript 2]O[subscript 3]:Eu[superscript 3+

    SciTech Connect

    Wang, Lin; Pan, Yuexiao; Ding, Yang; Yang, Wenge; Mao, Wendy L.; Sinogeikin, Stanislav V.; Meng, Yue; Shen, Guoyin; Mao, Ho-kwang

    2009-02-23

    We investigated high-pressure induced phase transitions in Y{sub 2}O{sub 3} and Eu-doped Y{sub 2}O{sub 3} (Y{sub 2}O:Eu{sup 3+}) using angular dispersive synchrotron x-ray diffraction, Raman spectroscopy, and photoluminescence (PL). With increasing pressure, we observed a series of phase transformations in Y{sub 2}O{sub 3}:Eu{sup 3+}, which followed a structure sequence of cubic {yields} monoclinic {yields} hexagonal, while Y{sub 2}O{sub 3} followed a sequence of cubic {yields} hexagonal. During decompression, both hexagonal structured Y{sub 2}O{sub 3} and Y{sub 2}O{sub 3}:Eu{sup 3+} transformed into monoclinic phases which were quenchable back to ambient pressure. Raman and PL measurements shed additional light on the different phase transition behavior in these two samples.

  12. Rivastigmine hydrogen tartrate polymorphs: Solid-state characterisation of transition and polymorphic conversion via milling

    NASA Astrophysics Data System (ADS)

    Amaro, Maria Inês; Simon, Alice; Cabral, Lúcio Mendes; de Sousa, Valéria Pereira; Healy, Anne Marie

    2015-11-01

    Rivastigmine (RHT) is an active pharmaceutical ingredient that is used for the treatment of mild to moderately severe dementia in Alzheimer's disease, and is known to present two polymorphic forms and to amorphise upon granulation. To date there is no information in the scientific or patent literature on polymorphic transition and stability. Hence, the aim of the current study was to gain a fundamental understanding of the polymorphic forms by (1) evaluating RHT thermodynamic stability (monotropy or enantiotropy) and (2) investigating the potential for polymorphic transformation upon milling. The two polymorphic and amorphous forms were characterised using X-ray powder diffractometry, thermal analyses, infra-red spectroscopy and water sorption analysis. The polymorphic transition was found to be spontaneous (ΔG0 < 0) and exothermic (ΔH0 < 0), indicative of a monotropic polymorph pair. The kinetic studies showed a fast initial polymorphic transition characterised by a heterogeneous nucleation, followed by a slow crystal growth. Ball milling can be used to promote the polymorphic transition and for the production of RHT amorphous form.

  13. Pressure-induced structural, electronic, and magnetic phase transitions in FeCl2 studied by x-ray diffraction and resistivity measurements

    NASA Astrophysics Data System (ADS)

    Rozenberg, G. Kh.; Pasternak, M. P.; Gorodetsky, P.; Xu, W. M.; Dubrovinsky, L. S.; Le Bihan, T.; Taylor, R. D.

    2009-06-01

    High-pressure (HP) synchrotron x-ray diffraction (XRD) studies were carried out in FeCl2 (TN≈24K) together with resistivity (R) studies at various temperatures and pressures to 65 GPa using diamond-anvil cells. This work follows a previous HP F57e Mössbauer study in which two pressure-induced (PI) electronic transitions were found interpreted as: (i) quenching of the orbital-term contribution to the hyperfine field concurring with a tilting of the magnetic moment by 55° , and (ii) collapse of the magnetism concurring with a sharp decrease in the isomer shift. The R(P,T) studies affirm that the cause of the collapse of the magnetism is a PI p-d correlation breakdown, leading to an insulator-metal transition at ˜45GPa and is not due to a spin crossover (S=2→S=0) . The structure response to the pressure evolution of the two electronic phase transitions starting at low pressures (LP), through an intermediate phase (IP) 30-57 GPa, and culminating in a high-pressure phase, P>32GPa , can clearly be quantified. The IP-HP phases coexist through the 32-57 GPa range in which the HP abundance increases monotonically at the expense of the IP phase. At the LP-IP interface no volume change is detected, yet the c axis increases and the a axis shrinks by 0.21 and 0.13Å , respectively. The fit of the equation of state of the combined LP-IP phases yields a bulk modulus K0=35.3(1.8)GPa . The intralayer Cl-Cl distances increase but no change is observed in Fe-Cl bond length nor are there substantial changes in the interlayer spacing. The pressure-induced electronic IP-HP transition leads to a first-order structural phase transition characterized by a decrease in Fe-Cl bond length and an abrupt drop in V(P) by ˜3.5% accompanying the correlation breakdown. In this transition no symmetry change is detected and the XRD data could be satisfactorily fitted with the CdI2 structure. The bulk modulus of the HP phase is practically the same as that of the LP-IP phases suggesting

  14. Structural properties of pressure-induced structural phase transition of Si-doped GaAs by angular-dispersive X-ray diffraction

    NASA Astrophysics Data System (ADS)

    Lin, Kung-Liang; Lin, Chih-Ming; Lin, Yu-Sheng; Jian, Sheng-Rui; Liao, Yen-Fa; Chuang, Yu-Chun; Wang, Chuan-Sheng; Juang, Jenh-Yih

    2016-02-01

    Pressure-induced phase transitions in n-type silicon-doped gallium arsenide (GaAs:Si ) at ambient temperature were investigated by using angular-dispersive X-ray diffraction (ADXRD) under high pressure up to around 18.6 (1) GPa, with a 4:1 (in volume ratio) methanol-ethanol mixture as the pressure-transmitting medium. In situ ADXRD measurements revealed that n-type GaAs:Si starts to transform from zinc- blende structure to an orthorhombic structure [GaAs-II phase], space group Pmm2, at 16.4 (1) GPa. In contrast to previous studies of pure GaAs under pressure, our results show no evidence of structural transition to Fmmm or Cmcm phase. The fitting of volume compression data to the third-order Birch-Murnaghan equation of state yielded that the zero-pressure isothermal bulk moduli and the first-pressure derivatives were 75 (3) GPa and 6.4 (9) for the B3 phase, respectively. After decompressing to the ambient pressure, the GaAs:Si appears to revert to the B3 phase completely. By fitting to the empirical relations, the Knoop microhardness numbers are between H PK = 6.21 and H A = 5.85, respectively, which are substantially smaller than the values of 7-7.5 for pure GaAs reported previously. A discontinuous drop in the pressure-dependent lattice parameter, N- N distances, and V/ V 0 was observed at a pressure of 11.5 (1) GPa, which was tentatively attributed to the pressure-induced dislocation activities in the crystal grown by vertical gradient freeze method.

  15. Theoretical study of the pressure-induced B3-B1 phase transition in Cd1-xMnxTe

    NASA Astrophysics Data System (ADS)

    Hao, Jun-Hua; Wang, Yu-Fang; Jin, Qing-Hua

    2013-02-01

    The high pressure phase transition in Cd1-xMnxTe (0 ≤ x ≤ 0.5), which is from the cubic zinc-blende structure (B3) to the NaCl structure (B1), is investigated by using first principles spin-polarized LCAO calculations based on the density functional theory (DFT) formalism. The calculations indicate that the transition pressure of the B3-to-B1 structural phase transformation depends on the Mn content of the sample. This result is consistent with the expectation that the substitution of Cd by Mn in CdTe tends to perturb the tetrahedral coordination geometry and thereby to destabilize the B3 structure. Several structural properties (equilibrium lattice constant, bulk modulus, transition pressure, etc.) of Cd1-xMnxTe (x = 0.0, 0.25 and 0.5) CdTe have been calculated, which are in agreement with the previous results.

  16. Pressure-induced changes in the electron density distribution in α-Ge near the α-β transition

    NASA Astrophysics Data System (ADS)

    Li, Rui; Liu, Jing; Bai, Ligang; Tse, John S.; Shen, Guoyin

    2015-08-01

    Electron density distributions in α-Ge have been determined under high pressure using maximum entropy method with structure factors obtained from single crystal synchrotron x-ray diffraction in a diamond anvil cell. The results show that the sp3 bonding is enhanced with increasing pressure up to 7.7(1) GPa. At higher pressures but below the α-β transition pressure of 11.0(1) GPa, the sp3-like electron distribution progressively weakens with a concomitant increase of d-orbitals hybridization. The participation of d-orbitals in the electronic structure is supported by Ge Kβ2 (4p-1s) x-ray emission spectroscopy measurements showing the reduction of 4s character in the valence band at pressures far below the α-β transition. The gradual increase of d-orbitals in the valence level in the stability field of α-Ge is directly related to the eventual structural transition.

  17. Pressure-Induced Mott Transition Followed by a 24-K Superconducting Phase in BaFe2S3

    NASA Astrophysics Data System (ADS)

    Yamauchi, Touru; Hirata, Yasuyuki; Ueda, Yutaka; Ohgushi, Kenya

    2015-12-01

    We performed high-pressure study for a Mott insulator BaFe2S3 , by measuring dc resistivity and ac susceptibility up to 15 GPa. We found that the antiferromagnetic insulating state at the ambient pressure is transformed into a metallic state at the critical pressure, Pc=10 GPa , and the superconductivity with the optimum Tc=24 K emerges above Pc. Furthermore, we found that the metal-insulator transition (Mott transition) boundary terminates at a critical point around 10 GPa and 75 K. The obtained pressure-temperature (P -T ) phase diagram is similar to those of the organic and fullerene compounds; namely, BaFe2S3 is the first inorganic superconductor in the vicinity of bandwidth control type Mott transition.

  18. Pressure-Induced Phase Transition in Guanidinium Perchlorate: A Supramolecular Structure Directed by Hydrogen Bonding and Electrostatic Interactions

    SciTech Connect

    Li, Shourui; Li, Qian; Wang, Kai; Tan, Xiao; Zhou, Mi; Li, Bing; Liu, Bingbing; Zou, Guangtian; Zou, Bo

    2012-01-20

    In situ Raman spectroscopy and synchrotron X-ray diffraction (XRD) experiments have been performed to investigate the response of guanidinium perchlorate (C(NH{sub 2}){sub 3}{sup +} {center_dot} ClO{sub 4}{sup -}, GP) to high pressures of {approx}11 GPa. GP exhibits a typical supramolecular structure of two-dimensional (2D) hydrogen-bonded ionic networks at ambient conditions. A subtle phase transition, accompanied by the symmetry transformation from R3m to C2, has been confirmed by obvious changes in both Raman and XRD patterns at 4.5 GPa. The phase transition is attributed to the competition between hydrogen bonds and close packing of the supramolecular structure at high pressure. Hydrogen bonds have been demonstrated to evolve into a distorted state through the phase transition, accompanied by the reduction in separation of oppositely charged ions in adjacent sheet motifs. A detailed mechanism of the phase transition, as well as the cooperativity between hydrogen bonding and electrostatic interactions, is discussed by virtue of the local nature of the structure.

  19. Pressure-induced structural and magnetic transitions in the infinite-chains iron oxide Sr2FeO3: a first-principle investigation

    NASA Astrophysics Data System (ADS)

    Gui, Hong; Li, Xin; Zhao, Zhenjie; Xie, Wenhui

    2016-02-01

    The pressure-induced transition of Sr2FeO3 was studied by first-principle calculation using density functional theory with the generalized gradient approximation plus on-site coulomb repulsion method. It shows that Sr2FeO3 exhibits a structure transition from Immm to Ammm and at about 35 GPa and then a spin transition from high spin S  =  2 to intermediate spin S  =  1. And it is also revealed that the pressure leads to a change in the Fe three-dimensional electronic configuration from ({{d}{{z2}}}\\uparrow )1({{d}yz}\\uparrow )1({{d}xz}\\uparrow )1({{d}xy}\\uparrow )1({{d}{{x2}-{{y}2}}}\\uparrow )1 ({{d}{{z2}}}\\downarrow )1 under ambient conditions to ({{d}{{z2}}}\\uparrow )1({{d}yz}\\uparrow )1({{d}xz}\\uparrow )1({{d}xy}\\uparrow )1 ({{d}{{x2}-{{y}2}}}\\uparrow ) δ ({{d}yz}\\downarrow )1 ({{d}{{z2}}}\\downarrow ) σ at high pressure, where δ plus σ equals 1.

  20. Pressure-induced phase transitions of exposed curved surface nano-TiO2 with high photocatalytic activity

    NASA Astrophysics Data System (ADS)

    Huang, Yanwei; Chen, Fengjiao; Li, Xin; Yuan, Ye; Dong, Haini; Samanta, Sudeshna; Yu, Zhenhai; Rahman, Saqib; Zhang, Jun; Yang, Ke; Yan, Shuai; Wang, Lin

    2016-06-01

    We report a unique phase transition in compressed exposed curved surface nano-TiO2 with high photocatalytic activity using in situ synchrotron X-ray diffraction and Raman Spectroscopy. High-pressure studies indicate that the anatase phase starts to transform into baddeleyite phase upon compression at 19.4 GPa, and completely transforms into the baddeleyite phase above 24.6 GPa. Upon decompression, the baddeleyite phase was maintained until the pressure was released to 6.4 GPa and then transformed into the α-PbO2 phase at 2.7 GPa. Together with the results of high-resolution transmission electron microscopy and the pressure-volume relationship, this phase transition's characteristics during the compression-decompression cycle demonstrate that the truncated biconic morphology possessed excellent stability. This study may provide an insight to the mechanisms of stability for high photocatalytic activity of nano-TiO2.

  1. Pressure induced tetragonal to monoclinic transition in RbN{sub 3} studied from first principles theory

    SciTech Connect

    Vaitheeswaran, G. Babu, K. Ramesh

    2014-04-24

    Alkali metal azides are well known for their application as explosives and gas generators. They are used as precursors in synthesis of polymeric nitrogen, an ultimate green high energy density material. Among the alkali metal azides, rubidium azide RbN{sub 3} crystallizes in tetragonal structure with linear azide ions arranged in layers and binds through weak dispersive interactions. In this present work, we have studied the structural stability, electronic structure and optical properties of solid RbN{sub 3} by using van der Waals corrected density functional theory. We find that the ambient tetragonal structure undergoes a structural transition to monoclinic structure at 0.72 GPa, which is in good agreement with the experimental transition pressure of less than 1 GPa. The phonon frequencies at the gamma point are calculated and found that the lattice mode Eg softens under pressure which may supports the structural phase transition. The electronic band structure and optical properties are calculated by using Tran Blaha-modified Becke Johnson (TB-mBJ) functional and found that solid RbN{sub 3} is an insulator with a gap of 5.976 eV and the optical absorption starts with the UV light of wave length 207.5 nm.

  2. Pressure-induced structural transition of OsN2 and effect of metallic bonding on its hardness

    NASA Astrophysics Data System (ADS)

    Wang, Zhen-Hua; Kuang, Xiao-Yu; Zhong, Ming-Min; Lu, Peng; Mao, Ai-Jie; Huang, Xiao-Fen

    2011-09-01

    Using first-principles calculations, the elastic constant, structural phase transition and effect of metallic bonding on the hardness of OsN2 under high pressure are investigated by means of the pseudopotential plane-waves method. Five candidate structures are chosen to investigate for OsN2, namely, the pyrite, CoSb2-type, marcasite, simple hexagonal and tetragonal structures. A comparison among the formation energies of OsN2 explains the synthesis of OsN2 marcasite under high pressure. On the basis of the third-order Birch-Murnaghan equation of states, the transition pressure Pt (Pt=223 GPa) between the marcasite and simple tetragonal phase is determinated. Elastic constants, shear modulus, Young's modulus, Poisson's ratio and Debye temperature are derived. The calculated values are, generally speaking, in good agreement with experiments and other theoretical calculations. Our calculation indicates that the N-N bond length is one determinative factor for the ultrahigh bulk moduli of the heavy-transition-metal dinitrides. Moreover, based on Mulliken overlap population analysis in first-principles technique, a semiempirical method to evaluate the hardness of multicomponent crystals with partial metallic bonding is presented. The effect of metallic bonding on the hardness of OsN2 is investigated and the hardness shows a gradual decrease rather than increase under compression, which is different from diamond. This is a quantitative investigation on the structural properties of OsN2, and it still awaits experimental confirmation.

  3. Pressure-induced changes in the electron density distribution in α-Ge near the α-β transition

    SciTech Connect

    Li, Rui; Liu, Jing; Bai, Ligang; Shen, Guoyin; Tse, John S.

    2015-08-17

    Electron density distributions in α-Ge have been determined under high pressure using maximum entropy method with structure factors obtained from single crystal synchrotron x-ray diffraction in a diamond anvil cell. The results show that the sp{sup 3} bonding is enhanced with increasing pressure up to 7.7(1) GPa. At higher pressures but below the α-β transition pressure of 11.0(1) GPa, the sp{sup 3}-like electron distribution progressively weakens with a concomitant increase of d-orbitals hybridization. The participation of d-orbitals in the electronic structure is supported by Ge Kβ{sub 2} (4p-1s) x-ray emission spectroscopy measurements showing the reduction of 4s character in the valence band at pressures far below the α-β transition. The gradual increase of d-orbitals in the valence level in the stability field of α-Ge is directly related to the eventual structural transition.

  4. Revisit of Pressure-Induced Phase Transition in PbSe: Crystal Structure, and Thermoelastic and Electrical Properties.

    PubMed

    Wang, Shanmin; Zang, Chengpeng; Wang, Yongkun; Wang, Liping; Zhang, Jianzhong; Childs, Christian; Ge, Hui; Xu, Hongwu; Chen, Haiyan; He, Duanwei; Zhao, Yusheng

    2015-05-18

    Lead selenide, PbSe, an important lead chalcogenide semiconductor, has been investigated using in-situ high-pressure/high-temperature synchrotron X-ray diffraction and electrical resistivity measurements. For the first time, high-quality X-ray diffraction data were collected for the intermediate orthorhombic PbSe. Combined with ab initio calculations, we find a Cmcm, InI-type symmetry for the intermediate phase, which is structurally more favorable than the anti-GeS-type Pnma. At room temperature, the onset of the cubic-orthorhombic transition was observed at ∼3.5 GPa with a ∼3.4% volume reduction. At an elevated temperature of 1000 K, the reversed orthorhombic-to-cubic transition was observed at 6.12 GPa, indicating a positive Clapeyron slope for the phase boundary. Interestingly, phase-transition induced elastic softening in PbSe was also observed, which can be mainly attributed to the loosely bonded trigonal prisms along the b-axis in the Cmcm structure. In a comparison with the cubic phase, orthorhombic PbSe exhibits a large negative pressure dependence of electrical resistivity. In addition, thermoelastic properties of orthorhombic PbSe have been derived from isothermal compression data, such as the temperature derivative of bulk modulus and thermally induced pressure. PMID:25938257

  5. Pressure induced magneto-structural phase transitions in layered RMn{sub 2}X{sub 2} compounds (invited)

    SciTech Connect

    Kennedy, Shane; Wang, Jianli; Campbell, Stewart; Hofmann, Michael; Dou, Shixue

    2014-05-07

    We have studied a range of pseudo-ternaries derived from the parent compound PrMn{sub 2}Ge{sub 2}, substituting for each constituent element with a smaller one to contract the lattice. This enables us to observe the magneto-elastic transitions that occur as the Mn-Mn nearest neighbour distance is reduced and to assess the role of Pr on the magnetism. Here, we report on the PrMn{sub 2}Ge{sub 2−x}Si{sub x}, Pr{sub 1−x}Y{sub x}Mn{sub 2}Ge{sub 2}, and PrMn{sub 2−x}Fe{sub x}Ge{sub 2} systems. The pressure produced by chemical substitution in these pseudo-ternaries is inherently non-uniform, with local pressure variations dependent on the local atomic distribution. We find that concentrated chemical substitution on the R or X site (e.g., in Pr{sub 0.5}Y{sub 0.5}Mn{sub 2}Ge{sub 2} and PrMn{sub 2}Ge{sub 0.8}Si{sub 1.2}) can produce a separation into two distinct magnetic phases, canted ferromagnetic and canted antiferromagnetic, with a commensurate phase gap in the crystalline lattice. This phase gap is a consequence of the combination of phase separation and spontaneous magnetostriction, which is positive on transition to the canted ferromagnetic phase and negative on transition to the canted antiferromagnetic phase. Our results show that co-existence of canted ferromagnetic and antiferromagnetic phases depends on chemical pressure from the rare earth and metalloid sites, on local lattice strain distributions and on applied magnetic field. We demonstrate that the effects of chemical pressure bear close resemblance to those of mechanical pressure on the parent compound.

  6. Pressure-induced phase transition and electrical properties of thermoelectric Al-doped Mg{sub 2}Si

    SciTech Connect

    Zhao, Jianbao; Tse, John S.; Liu, Zhenxian; Gordon, Robert A.; Takarabe, Kenichi; Reid, Joel

    2015-10-14

    A recent study has shown the thermoelectric performance of Al-doped Mg{sub 2}Si materials can be significantly enhanced at moderate pressure. To understand the cause of this phenomenon, we have performed in situ angle dispersive X-ray diffraction and infrared reflectivity measurements up to 17 GPa at room temperature. Contrary to previous experiment, using helium as a pressure transmission medium, no structural transformation was observed in pure Mg{sub 2}Si. In contrast, a phase transition from cubic anti-fluorite (Fm-3m) to orthorhombic anti-cotunnite (Pnma) was observed in the Al-doped sample at 10 GPa. Infrared reflectivity measurements show the electrical conductivity increases with pressure and is further enhanced after the phase transition. The electron density of states at the Fermi level computed form density functional calculations predict a maximum thermoelectric power factor at 1.9 GPa, which is in good agreement with the experimental observation.

  7. Pressure-induced semiconductor-to-metal transition in Mg2Sn with the modified Becke-Johnson potential

    NASA Astrophysics Data System (ADS)

    Guo, San-Dong

    2015-03-01

    We investigate the energy band gap, dielectric functions and thermoelectric properties of Mg2Sn at hydrostatic pressure by using a modified Becke and Johnson exchange potential. It is very interesting that the energy band gap first increases with increasing pressure, and then decreases. The phonon calculations prove that no structural phase transition under the considered pressure is produced. When the pressure reaches 5.6 GPa, the energy band gap attains the biggest value, which is also a critical pressure with the Mg s-character near the high symmetry X-point transforming from the first conduction band to the second one. When the pressure increases to 50.7 GPa, the energy band gap closes, leading to a semiconductor-to-semimetal transition. As the pressure increases, the main peaks of the real and imaginary part of the dielectric functions of Mg2Sn move toward the high-energy region. The Seebeck coefficient and power factor for p-type doping change little with increasing pressure, but for n-type they vary greatly. The change trend of the Seebeck coefficient and power factor for n-type doping as a function of pressure is conic, whose critical pressure is just 5.6 GPa.

  8. Pressure-induced phase transition in La1–xSmxO0.5F0.5BiS2

    DOE PAGESBeta

    Fang, Y.; Yazici, D.; White, B. D.; Maple, M. B.

    2015-09-15

    Electrical resistivity measurements on La1–xSmxO0.5F0.5BiS2 (x = 0.1, 0.3, 0.6, 0.8) have been performed under applied pressures up to 2.6 GPa from 2 K to room temperature. The superconducting transition temperature Tc of each sample significantly increases at a Sm-concentration dependent pressure Pt, indicating a pressure-induced phase transition from a low-Tc to a high-Tc phase. At ambient pressure, Tc increases dramatically from 2.8 K at x = 0.1 to 5.4 K at x = 0.8; however, the Tc values at P > Pt decrease slightly with x and Pt shifts to higher pressures with Sm substitution. In the normal state,more » semiconducting-like behavior is suppressed and metallic conduction is induced with increasing pressure in all of the samples. Furthermore, these results suggest that the pressure dependence of Tc for the BiS2-based superconductors is related to the lattice parameters at ambient pressure and enable us to estimate the evolution of Tc for SmO0.5F0.5BiS2 under pressure.« less

  9. Pressure-induced phase transition in La1 -xSmxO0.5F0.5BiS2

    NASA Astrophysics Data System (ADS)

    Fang, Y.; Yazici, D.; White, B. D.; Maple, M. B.

    2015-09-01

    Electrical resistivity measurements on La1 -xSmxO0.5F0.5BiS2 (x =0.1 ,0.3 ,0.6 ,0.8 ) have been performed under applied pressures up to 2.6 GPa from 2 K to room temperature. The superconducting transition temperature Tc of each sample significantly increases at a Sm-concentration-dependent pressure Pt, indicating a pressure-induced phase transition from a low-Tc to a high-Tc phase. At ambient pressure, Tc increases dramatically from 2.8 K at x =0.1 to 5.4 K at x =0.8 ; however, the Tc values at P >Pt decrease slightly with x , and Pt shifts to higher pressures with Sm substitution. In the normal state, semiconducting-like behavior is suppressed, and metallic conduction is induced with increasing pressure in all of the samples. These results suggest that the pressure dependence of Tc for the BiS2-based superconductors is related to the lattice parameters at ambient pressure and enable us to estimate the evolution of Tc for SmO0.5F0.5BiS2 under pressure.

  10. Pressure-induced structural phase transition of dense droplet microemulsions studied by small-angle x-ray scattering

    NASA Astrophysics Data System (ADS)

    Seto, Hideki; Nagao, Michihiro; Kawabata, Youhei; Takeda, Takayoshi

    2001-11-01

    A small-angle x-ray scattering (SAXS) study of dense water-in-oil droplet microemulsions composed of water, decane, and AOT [sodium bis(2-ethylhexyl) sulfosuccinate] was performed in order to clarify phase behavior with applied pressure and the corresponding structural phase transitions. SAXS spectra were collected for pressures between ambient pressure (0.1 MPa) and 80 MPa and droplet volume fraction, φ, from 0.40 to 0.70. With increasing φ, the mean radius of droplets decreased slightly and polydispersity increased. With increasing pressure, the droplet structure transformed to a two-phase system with coexistence of lamellar and droplet structures, independent of the droplet volume fraction. These results suggest that, with increasing pressure, the increasing inter-droplet attractive force controls the pressure variation of the structure.

  11. Pressure-induced phase transitions and structural properties of CoF2: An ab-initio molecular dynamics study

    NASA Astrophysics Data System (ADS)

    Kürkçü, Cihan; Merdan, Ziya; Öztürk, Hülya

    2016-04-01

    The crystal structure of CoF2 was studied theoretically using first-principles density functional theory (DFT) methods within the generalized gradient approximation (GGA) and local density approximation (LDA) under rapid hydrostatic pressure up to 144 GPa. CoF2 undergoes a structural phase transformation from the rutile-type tetragonal parent phase with space group P42/mnm to the CaCl2-type orthorhombic parent phase with space group Pnnm at 64 GPa with GGA and at 96 GPa with LDA methods. Another phase transformation occurs from the CaCl2-type structure to monoclinic parent phase with space group P21/c at 96 GPa with a GGA method. These phase transitions are also studied by enthalpy and total energy calculations. According to these calculations, we obtained the first phase transformation at about 6.5 GPa both GGA and LDA methods and the later phase transformation at about 45 GPa with the GGA method.

  12. Pressure-induced phase transitions of AX(2)-type iron pnictides: an ab initio study.

    PubMed

    Wu, X; Steinle-Neumann, G; Qin, S; Kanzaki, M; Dubrovinsky, L

    2009-05-01

    An investigation into the high-pressure behavior of AX(2)-type iron pnictides was conducted using first-principles calculations based on density functional theory within the generalized gradient approximation. Our results demonstrate that a phase transition from the marcasite to the CuAl(2) occurs at 108 GPa for FeP(2), at 92 GPa for FeAs(2), and at 38 GPa for FeSb(2), accompanying a semiconductor-to-metal crossover. A linear relationship between bulk moduli and the inverse specific volume is proposed to be B(0) = 17 498/V(0)-45.9 GPa for the marcasite-type phase and B(0) = 31 798/V(0)-67.5 GPa for the CuAl(2)-type phase. According to the observed structural evolutions, we claim that the regular marcasite transforms to the CuAl(2)-type phase and the anomalous marcasite transforms to the pyrite-type phase at high pressures. PMID:21825462

  13. Pressure-induced phonon softening and electronic topological transition in HgBa{sub 2}CuO{sub 4}

    SciTech Connect

    Novikov, D.L.; Katsnelson, M.I.; Yu, J.; Postnikov, A.V.; Freeman, A.J.

    1996-07-01

    Total energy local density calculations for the effects of pressure on the lattice parameters, bond lengths, electronic structure, and {ital A}{sub 1{ital g}} phonon frequency in HgBa{sub 2}CuO{sub 4} have been carried out in order to understand the role of pressure in increasing the {ital T}{sub {ital c}} of mercury-based superconductors. Theoretically determined zero-pressure lattice parameters and phonon frequencies are found to be in good agreement with experiment. An electronic topological transition is found to occur when the van Hove singularity (vHS) is shifted close to the vicinity of {ital E}{sub {ital F}} by pressure which causes considerable phonon softening and anomalous behavior of the {ital c}-axis length, the Hg-O(2) bond, and the Ba {ital z} coordinate. A set of experiments that might be able to detect the presence of the vHS close to {ital E}{sub {ital F}} is proposed. {copyright} {ital 1996 The American Physical Society.}

  14. Prediction of Pressure-Induced Structural Transition and Mechanical Properties of MgY from First-Principles Calculations

    NASA Astrophysics Data System (ADS)

    Pu, Chun-Ying; Xun, Xian-Chao; Song, Hai-Zhen; Zhang, Fei-Wu; Lu, Zhi-Wen; Zhou, Da-Wei

    2016-01-01

    Using the particle swarm optimization algorithm on crystal structure prediction, we first predict that MgY alloy undergoes a first-order phase transition from CsCl phase to P4/NMM phase at about 55 GPa with a small volume collapse of 2.63%. The dynamical stability of P4/NMM phase at 55 GPa is evaluated by the phonon spectrum calculation and the electronic structure is discussed. The elastic constants are calculated, after which the bulk moduli, shear moduli, Young's modui, and Debye temperature are derived. The brittleness/ductile behavior, and anisotropy of two phases under pressure are discussed in details. Our results show that external pressure can change the brittle behavior to ductile at 10 GPa for CsCl phase and improve the ductility of MgY alloy. As pressure increases, the elastic anisotropy in shear of CsCl phase decreases, while that of P4/NMM phase remains nearly constant. The elastic anisotropic constructions of the directional dependences of reciprocals of bulk modulus and Young's modulus are also calculated and discussed. Supported by the Henan Joint Funds of the National Natural Science Foundation of China under Grant Nos. U1304612, U1404608, the National Natural Science Foundation of China under Grant Nos. 51501093, 51374132, and the Special Fund of the Theoretical Physics of China under Grant No. 11247222, Postdoctoral Science Foundation of China under Grant No. 2015M581767, and Young Core Instructor Foundation of Henan Province under Grant No. 2015GGJS-122

  15. Pressure-Induced Slip-System Transition in Forsterite: Single-Crystal Rheological Properties at Mantle Pressure and Temperature

    SciTech Connect

    Raterron,P.; Chen, J.; Li, L.; Weidner, D.; Cordier, P.

    2007-01-01

    Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented Mg2SiO4 olivine (Fo100) single crystals, at pressure (P) ranging from 2.1 to 7.5 GPa, in the temperature (T) range 1373-1677 K, and in dry conditions. These experiments were designed to investigate the effect of pressure on olivine dislocation slip-system activities, responsible for the lattice-preferred orientations observed in the upper mantle. Two compression directions were tested, promoting either [100] slip alone or [001] slip alone in (010) crystallographic plane. Constant applied stress ({sigma}) and specimen strain rates (Formula) were monitored in situ using time-resolved X-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation. A slip transition with increasing pressure, from a dominant [100]-slip to a dominant [001]-slip, is documented. Extrapolation of the obtained rheological laws to upper-mantle P, T, and {sigma} conditions, suggests that [001]-slip activity becomes comparable to [100]-slip activity in the deep upper mantle, while [001] slip is mostly dominant in subduction zones. These results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle, and for the 'puzzling' seismic-anisotropy anomalies commonly observed in subduction zones.

  16. Pressure-induced phase-transition and improvement of the microdielectric properties in yttrium-doped SrZrO3

    NASA Astrophysics Data System (ADS)

    Dai, Lidong; Wu, Lei; Li, Heping; Hu, Haiying; Zhuang, Yukai; Liu, Kaixiang

    2016-06-01

    In this study, the effect of pressure on undoped and 5% yttrium-doped SrZrO3 (SZY0 and SZY5) were conducted from the ambient condition to ∼25 \\text{GPa} with a diamond anvil cell. The comparison of the high-pressure Raman spectra of SZY0 and SZY5 indicate that SZY0 displays a rigid structure without any structural modification, whereas for SZY5 a structural transition at ∼15 \\text{GPa} is revealed. Some characteristic physical parameters such as bulk conductivities, grain boundary conductivities, Warburg diffusion coefficient, transference number and bulk relaxation frequency were determined by the high-pressure impedance spectroscopy data. An obvious discontinuous inflexion point (at ∼13 \\text{GPa} ) for SZY5 sample is observable which was also verified the phase-transition of the Raman spectroscopy results. A mixed conduction mechanism for both SZY0 and SZY5 are coexisting for both SZY0 and SZY5 in a wide pressure range. The pressure-induced phase-transition of SZY5 would result in an inversion of conduction mechanism that is characterized by the dominant charge carriers transformation from electron to ion. The Maxwell-Wagner relaxation arising at the interfaces of grain and grain boundary indicates that Y-doping and pressure could make the ions diffusion much easier through the boundaries and finally enhance the dielectric performance of the sample. It is suggested that pressure could be a useful tool to manipulate the microstructure and dielectric performance of polycrystal through altering the grain boundary distribution.

  17. Pressure-induced structural changes and insulator-metal transition in layered bismuth triiodide, BiI3: a combined experimental and theoretical study.

    PubMed

    Devidas, T R; Chandra Shekar, N V; Sundar, C S; Chithaiah, P; Sorb, Y A; Bhadram, V S; Chandrabhas, N; Pal, K; Waghmare, U V; Rao, C N R

    2014-07-01

    Noting that BiI3 and the well-known topological insulator (TI) Bi2Se3 have the same high symmetry parent structures, and that it is desirable to find a wide-band gap TI, we determine here the effects of pressure on the structure, phonons and electronic properties of rhombohedral BiI3. We report a pressure-induced insulator-metal transition near 1.5 GPa, using high pressure electrical resistivity and Raman measurements. X-ray diffraction studies, as a function of pressure, reveal a structural peculiarity of the BiI3 crystal, with a drastic drop in c/a ratio at 1.5 GPa, and a structural phase transition from rhombohedral to monoclinic structure at 8.8 GPa. Interestingly, the metallic phase, at relatively low pressures, exhibits minimal resistivity at low temperatures, similar to that in Bi2Se3. We corroborate these findings with first-principles calculations and suggest that the drop in the resistivity of BiI3 in the 1-3 GPa range of pressure arises possibly from the appearance of an intermediate crystal phase with a lower band-gap and hexagonal crystal structure. Calculated Born effective charges reveal the presence of metallic states in the structural vicinity of rhombohedral BiI3. Changes in the topology of the electronic bands of BiI3 with pressure, and a sharp decrease in the c/a ratio below 2 GPa, are shown to give rise to changes in the slope of phonon frequencies near that pressure. PMID:24934819

  18. Giant Volume Change and Topological Gaps in Temperature- and Pressure-Induced Phase Transitions: Experimental and Computational Study of ThMo2 O8.

    PubMed

    Xiao, Bin; Kegler, Philip; Gesing, Thorsten M; Robben, Lars; Blanca-Romero, Ariadna; Kowalski, Piotr M; Li, Yan; Klepov, Vladislav; Bosbach, Dirk; Alekseev, Evgeny V

    2016-01-18

    By applying high temperature (1270 K) and high pressure (3.5 GPa), significant changes occur in the structural volume and crystal topology of ThMo2 O8 , allowing the formation of an unexpected new ThMo2 O8 polymorph (high-temperature/high-pressure (HT/HP) orthorhombic ThMo2 O8 ). Compared with the other three ThMo2 O8 polymorphs prepared at the ambient pressure (monoclinic, orthorhombic, and hexagonal phases), the molar volume for the quenched HT/HP-orthorhombic ThMo2 O8 is decreased by almost 20 %. As a result of such a dramatic structural transformation, a permanent high-pressure quenchable state is able to be sustained when the pressure is released. The crystal structures of the three ambient ThMo2 O8 phases are based on three-dimensional (3D) frameworks constructed from corner-sharing ThOx (x=6, 8, or 9) polyhedra and MoO4 tetrahedra. The HT/HP-orthorhombic ThMo2 O8 , however, crystallizes in a novel structural topology, exhibiting very dense arrangements of ThO11 and MoO4+1 polyhedra connecting along the crystallographic c axis. The phase transitions among all four of these ThMo2 O8 polymorphs are unveiled and fully characterized with regard to the structural transformation, thermal stability, and vibrational properties. The complementary first principles calculations of Gibbs free energies reveal the underlying energetics of the phase transition, which support the experimental findings. PMID:26626413

  19. Pressure-induced phase transition of Fe{sub 2}TiO{sub 4}: X-ray diffraction and Moessbauer spectroscopy

    SciTech Connect

    Wu Ye; Wu Xiang; Qin Shan

    2012-01-15

    X-ray diffraction and Moessbauer spectroscopy were employed to investigate structural stability of Fe{sub 2}TiO{sub 4} under high pressure. Measurements were performed up to about 24 GPa at room temperature using diamond anvil cell. Experimental results demonstrate that Fe{sub 2}TiO{sub 4} undergoes a series of phase transitions from cubic (Fd3-bar m) to tetragonal (I4{sub 1}/amd) at 8.7 GPa, and then to orthorhombic structure (Cmcm) at 16.0 GPa. The high-pressure phase (Cmcm) of Fe{sub 2}TiO{sub 4} is kept on decompression to ambient pressure. In all polymorphs of Fe{sub 2}TiO{sub 4}, iron cations present a high-spin ferrous property without electric charge exchange with titanium cations at high pressure supported by Moessbauer evidences. - Graphical abstract: A series of phase transition of Fe{sub 2}TiO{sub 4} occurs from cubic (a) to tetragonal (b and c) then to orthorhombic phase (d-f) at high pressure. Highlights: Black-Right-Pointing-Pointer High pressure behaviors of Fe{sub 2}TiO{sub 4} were investigated. Black-Right-Pointing-Pointer Phase transitions were observed from cubic to tetragonal and then to orthorhombic. Black-Right-Pointing-Pointer Orthorhombic phase can be kept on decompression. Black-Right-Pointing-Pointer In all polymorphs of Fe{sub 2}TiO{sub 4}, iron ions are ferrous with high-spin state.

  20. Raman scattering studies of pressure-induced phase transitions in perovskite formates [(CH3)2NH2][Mg(HCOO)3] and [(CH3)2NH2][Cd(HCOO)3

    NASA Astrophysics Data System (ADS)

    Mączka, M.; Almeida da Silva, T.; Paraguassu, W.; Pereira da Silva, K.

    2016-03-01

    Pressure-dependent Raman studies were preformed on two dimethylammonium metal formates, [(CH3)2NH2][Mg(HCOO)3] (DMMg) and [(CH3)2NH2][Cd(HCOO)3] (DMCd). They revealed three pressure-induced transitions in the DMMg near 2.2, 4.0 and 5.6 GPa. These transitions are associated with significant distortion of the anionic framework and the phase transition at 5.6 GPa has also great impact on the DMA+ cation. The DMCd undergoes two pressure-induced phase transitions. The first transition occurred between 1.2 and 2.0 GPa and the second one near 3.6 GPa. The first transition leads to subtle structural changes associated with distortion of anionic framework and the later leads to significant distortion of the framework. In contrast to the DMMg, the third transition associated with distortion of DMA+ cation is not observed for the DMCd up to 7.8 GPa. This difference can be most likely associated with larger volume of the cavity occupied by DMA+ cation in the DMCd and thus weaker interactions between anionic framework and DMA+ cations.

  1. Polymorphic transitions of diborane at sub- and near-megabar pressures

    NASA Astrophysics Data System (ADS)

    Torabi, Amin; Murli, Chitra; Song, Yang; Staroverov, Viktor N.

    2015-09-01

    Recent theoretical investigations of high-pressure structures of diborane have yielded many intriguing predictions which have so far remained untested due to challenges of acquiring experimental data at extreme pressures. Here we report new pressure-induced polymorphic transformations of crystalline diborane observed between 36 and 88 GPa by in situ Raman spectroscopy and interpreted using electronic structure calculations. Two previously unknown phase transitions are identified near 42 and 57 GPa, as evidenced by significant changes in the Raman profiles. The corresponding new phases, labeled IV and V, consist of B2H6 molecules and have triclinic unit cells (P), as deduced through evolutionary structure search and comparison of experimental and simulated Raman spectra. Density-functional calculations suggest that, at pressures above 110 GPa, phase V will form new molecular structures consisting of one-dimensional (BH3)n chains and will become metallic near 138 GPa. Our findings make a significant contribution to the elucidation of the structures and properties of diborane in the near-megabar pressure region.

  2. Polymorphism of iron at high pressure: A 3D phase-field model for displacive transitions with finite elastoplastic deformations

    NASA Astrophysics Data System (ADS)

    Vattré, A.; Denoual, C.

    2016-07-01

    A thermodynamically consistent framework for combining nonlinear elastoplasticity and multivariant phase-field theory is formulated at large strains. In accordance with the Clausius-Duhem inequality, the Helmholtz free energy and time-dependent constitutive relations give rise to displacive driving forces for pressure-induced martensitic phase transitions in materials. Inelastic forces are obtained by using a representation of the energy landscape that involves the concept of reaction pathways with respect to the point group symmetry operations of crystal lattices. On the other hand, additional elastic forces are derived for the most general case of large strains and rotations, as well as nonlinear, anisotropic, and different elastic pressure-dependent properties of phases. The phase-field formalism coupled with finite elastoplastic deformations is implemented into a three-dimensional Lagrangian finite element approach and is applied to analyze the iron body-centered cubic (α-Fe) into hexagonal close-packed (ɛ-Fe) phase transitions under high hydrostatic compression. The simulations exhibit the major role played by the plastic deformation in the morphological and microstructure evolution processes. Due to the strong long-range elastic interactions between variants without plasticity, a forward α → ɛ transition is energetically unfavorable and remains incomplete. However, plastic dissipation releases considerably the stored strain energy, leading to the α ↔ ɛ ↔α‧ (forward and reverse) polymorphic phase transformations with an unexpected selection of variants.

  3. Pressure-induced zircon-type to scheelite-type phase transitions in YbPO{sub 4} and LuPO{sub 4}

    SciTech Connect

    Zhang, F.X. Lang, M.; Ewing, R.C. Lian, J.; Wang, Z.W.; Hu, J.; Boatner, L.A.

    2008-10-15

    The tetragonal orthophosphates, YbPO{sub 4} and LuPO{sub 4}, were studied by in situ X-ray diffraction (XRD) at pressures up to 52 and 43 GPa, respectively. A reversible phase transition from the zircon structure-type to the scheelite structure-type was found at {approx}22 GPa for YbPO{sub 4} and 19 GPa for LuPO{sub 4}. Coinciding with the transition from the zircon structure-type to the scheelite structure-type, there is a {approx}10% reduction in volume and a significant increase in the bulk modulus for both compounds. - Graphical abstract: The tetragonal orthophosphates, YbPO{sub 4} and LuPO{sub 4}, show reversible phase transitions from the zircon structure-type to the scheelite structure-type at {approx}22 and 19 Gpa, respectively. Coinciding with the phase transition, there is a {approx}10% reduction in unit cell volume.

  4. Pressure-Induced Phase Transitions in Ammonium Squarate: A Supramolecular Structure Based on Hydrogen-Bonding and [pi]-Stacking Interactions

    SciTech Connect

    Li, Shourui; Wang, Kai; Zhou, Mi; Li, Qian; Liu, Bingbing; Zou, Guangtian; Zou, Bo

    2012-02-06

    We report the results of high-pressure Raman and X-ray diffraction measurements performed on ammonium squarate ((NH{sub 4}){sub 2}C{sub 4}O{sub 4}, AS), a representative supramolecular architecture based on hydrogen bonding and {pi}-stacking interactions, at various pressures up to 19 GPa. Two phase transitions at 2.7 GPa and in the pressure range of 11.1-13.6 GPa were observed. Both Raman and XRD results provide convincing evidence for these two phase transitions. The first phase transition is attributed to the rearrangements of hydrogen-bonding networks, resulting in the symmetry transformation from P2{sub 1}/c to P1. The second one, which is identified as an order-disorder phase transition, arises from significant modifications of squarate rings and random orientations of NH{sub 4}{sup +} cations. The cooperative effects between hydrogen-bonding and {pi}-stacking interactions, as well as mechanisms for the phase transitions, are discussed by virtue of the local structure of AS.

  5. A shock pressure induced phase transition from liquid to solid of cyclohexane using time-resolved coherent anti-Stokes Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Oguchi, Shiro; Sato, Akira; Kondo, Ken-Ichi; Nakamura, Kazutaka

    2007-06-01

    The liquid-solid phase transition of cyclohexane has been studied under laser shock compression up to 3.8 GPa by using nanosecond time-resolved Coherent Anti-stokes Raman Spectroscopy (CARS) and laser shock compression. The shock wave is generated by irradiation of 10 ns pulsed laser beam on the plasma confinement target and its pressure is estimated from a particle velocity, which is measured by optically recording velocity interferometer system (ORVIS). Higher frequency shift of the Raman peaks (ring-breathing, C-C stretching, and CH2 twist modes) was observed at high pressure. At 3.8 GPa, splitting of the peak (CH2 twist mode) due to change in symmetry of surrounding molecules, which corresponds to phase transition to solid IV, was observed at delay time of 20 ns. Rapid liquid-solid phase transition has been directly observed to occur within 20 ns.

  6. Pressure-Induced Zircon-Type to Scheelite-Type Phase Transition in Orthophosphates YbPO4 and LuPO4

    SciTech Connect

    Zhang, F.; Maik, L; Ewing, R; Lian, J; Wang, Z; Hu, J; Boatner, L

    2008-01-01

    The tetragonal orthophosphates, YbPO4 and LuPO4, were studied by in situ X-ray diffraction (XRD) at pressures up to 52 and 43 GPa, respectively. A reversible phase transition from the zircon structure-type to the scheelite structure-type was found at not, vert, similar22 GPa for YbPO4 and 19 GPa for LuPO4. Coinciding with the transition from the zircon structure-type to the scheelite structure-type, there is a not, vert, similar 10% reduction in volume and a significant increase in the bulk modulus for both compounds.

  7. Predicted pressure-induced spin and electronic transition in double perovskite R2CoMnO6 (R = rare-earth ion)

    NASA Astrophysics Data System (ADS)

    Zhao, Hong Jian; Zhou, Haiyang; Chen, Xiang Ming; Bellaiche, L.

    2015-06-01

    Specific first-principles calculations are performed to predict structural, magnetic and electronic properties of seven double perovskite R2CoMnO6 materials, with R being a rare-earth ion, under hydrostatic pressure. All these compounds are found to undergo a first-order transition from a high spin (HS) to low spin (LS) state at a critical pressure (whose value is dependent on the R ion). Such transition not only results in a significant volume collapse but also yields a dramatic change in electronic structure. More precisely, the HS-to-LS transition is accompanied by a transition from an insulator to a half-metallic state in the R2CoMnO6 compounds having the largest rare-earth ionic radius (i.e., Nd, Sm, Gd and Tb) while it induces a change from an insulator to a semiconductor having a narrow band gap for the smallest rare-earth ions (i.e., R = Dy, Ho and Er). Experiments are called for to confirm these predictions.

  8. Pressure-induced magnetic, structural, and electronic phase transitions in LaFeO{sub 3}: A density functional theory (generalized gradient approximation) + U study

    SciTech Connect

    Javaid, Saqib; Javed Akhtar, M.

    2014-07-14

    We have investigated the behavior of orthoferrite LaFeO{sub 3} at ambient conditions and under pressure using DFT (generalized gradient approximation (GGA)) + U approach. Ground state electronic (band gap) and magnetic properties are considerably improved due to the Hubbard correction. Moreover, the experimentally observed pressure-driven phase transition, namely, the simultaneous occurrence of spin crossover, isostructural volume collapse, and drastic reduction in electrical resistance (electronic phase transition) is nicely described by GGA + U calculations. In particular, despite a sharp drop in resistance, a small band gap still remains in the low spin state indicating an insulator to semiconductor phase transition, in good agreement with the experiments but in contrast to GGA, which predicts metallic behavior in low spin state. We discuss the origin of variation in electronic structure of LaFeO{sub 3} in low spin state as obtained from GGA to GGA + U methods. These results emphasize the importance of correlation effects in describing the pressure-driven phase transition in LaFeO{sub 3} and other rare-earth orthoferrites.

  9. Pressure-induced shift of Tc and structural transition in "122" type pnictide superconductor Ca0.34Na0.66Fe2As2

    NASA Astrophysics Data System (ADS)

    Zhang, Sijia; Zhao, Kan; Yu, Xiaohui; Zhu, Jinlong; Liu, Qingqing; Wang, Xiancheng; Feng, Shaomin; Chen, Zhiqiang; Zhao, Yusheng; Jin, Changqing

    2016-07-01

    The effect of pressure on superconductivity of "122" type Ca1-xNaxFe2As2 (x=0.66) single crystal is investigated through the temperature dependence of resistance measurement. Optimal Na doped (Ca0.34Na0.66)Fe2As2 shows a superconducting transition with Tc ˜ 33 K at ambient pressure. With application of pressure, Tc decreases nearly linearly with dTc/dP ˜ - 1.7K/GPa at pressures lower than 2 GPa, and disappears gradually at higher pressure. The disappearance of superconductivity is also companied with the recovery of standard Fermi liquid behaviors of the normal-state transport properties. Moreover, (Ca0.34Na0.66)Fe2As2 exhibits a tetragonal (T) to collapsed-tetragonal (cT) transition at about 3 GPa. The evolution of non-Fermi liquid behaviors and superconductivity under pressure are both related to the interband fluctuations.

  10. The effect of shear deformations on the transition onset pressure of the bcc to hcp pressure induced martensitic phase transformation in iron.

    NASA Astrophysics Data System (ADS)

    Caspersen, K.; Lew, A.; Ortiz, M.; Carter, E.

    2003-12-01

    At a pressure of approximately 13 GPa iron undergoes a martensitic phase transition from ground state ferro-magnetic bcc to a non-magnetic hcp structure. The exact transformation varies between experiments and is postulated to have a strong dependence on shear stresses during the loading process. To study this shear dependence we have developed a multi-scale model of iron, in which we employ a quantum mechanics based free energy, a kinematically compatible spinodal decomposition of phases, and a dependence on the bcc{<->}hcp transition path(s). Using this model we see that that the predicted transformation pressure for pure hydrostatic compression is much higher than expected, however with the inclusion of small initial shear deformations we see the predicted transformation pressure drop considerably and into the experimentally determined pressure range.

  11. Pressure-induced tuning of phase transition and role of disorder in electrical transport properties of β-SrxV6O15

    NASA Astrophysics Data System (ADS)

    Akrap, Ana; Barišić, Neven; Gaal, Richard; Forró, László

    2007-12-01

    We report the resistivity and thermoelectric power of β-SrxV6O15 , for various stoichiometries, 0.6⩽x⩽1 , and under pressures up to 1.7GPa . The pristine system (x=1) exhibits a semiconductor-insulator transition at 155K , which is evidenced in both resistivity and thermopower and is probably induced by charge ordering. We observe a pronounced change in the nature of the phase transition under pressure and we attribute it to the tuning of the nearest neighbor Coulomb interaction V . At ambient pressure, as the system moves away from stoichiometry to x<1 , disorder is introduced into the strontium sublattice and the phase transition is immediately suppressed. The temperature dependence of the thermoelectric power gradually weakens as the system moves away from x=1 , indicating the importance of disorder. While for x<1 compound thermoelectric power shows evidence of a localized contribution to the conduction, which may involve polaronic effects, the activation energies speak against small polarons in the pristine x=1 compound. We explain our results in a model of conduction through localized states in the off-stoichiometric systems and of thermally activated conduction in the pristine system.

  12. Pressure-induced Pbca-P21/c phase transition of natural orthoenstatite: The effect of high temperature and its geophysical implications

    NASA Astrophysics Data System (ADS)

    Zhang, Jin S.; Reynard, Bruno; Montagnac, Gilles; Bass, Jay D.

    2014-03-01

    In-situ high-pressure (P) high-temperature (T) Raman spectroscopy has been used to investigate the effect of temperature on the high-pressure phase transition of Mg-rich orthoenstatite (OEN) to a newly-discovered P21/c phase (HPCEN2) up to 673 K and 18.2(10) GPa. Two natural orthoenstatite samples were used in this study: near end-member Mg orthoenstatite (Zabargad Island, Egypt), and Al + Fe-bearing orthoenstatite (San Carlos, Arizona). For San Carlos OEN (SC-OEN), the experiment was performed at room temperature, 373, 573 and 673 K; For Zabargad Island OEN (Zabg OEN), experiments were performed at 573 and 673 K. The three phases OEN, HPCEN2, and another high-pressure phase with space group C2/c (denoted by HPCEN) are readily distinguished by a characteristic doublet, triplet, or singlet, respectively, in the 660-680 cm-1 range. Similarly, splitting of a peak near 1100 cm-1 is indicative of an OEN → HPCEN2 transition. For both samples, no phase other than OEN and HPCEN2 was observed within the investigated P-T range. The recovered products after slow cooling for over 24 h from 673 K and 16.6(9) GPa were OEN. The Clapeyron slope (dP/dT) of this transition is bracketed between +0.020 to -0.0026 GPa/K for Zabg-OEN, and +0.0023 to -0.0049 GPa/K for SC-OEN. Our results suggest a possible stability field for HPCEN2 at the bottom of the upper mantle.

  13. A DFT study of pressure-induced phase transitions, structural and electronic properties of Cu2ZnSnS4

    NASA Astrophysics Data System (ADS)

    Zhao, Yifen; Li, Decong; Liu, Zuming

    2016-06-01

    The structural properties, phase transitions, and electronic structures of Cu2ZnSnS4 (CZTS) in the three structures have been researched using the first-principles density functional theory (DFT). The results indicate that the energies of stannite (ST) and pre-mixed Cu-Au (PMCA) CZTS are higher than those of kesterite (KS) CZTS, indicating that the KS CZTS is more stable. We found the phase transition pressure between the KS and ST structures of CZTS is about 32 GPa. Moreover, for KS- and PMCA-CZTS, there exists in the mischcrystal phase between 52 GPa and 65 GPa. The band structures show that the KS- and ST-CZTS are direct band gap semiconductors. The band gaps of three-type CZTS increase with increasing pressure, and the maximum band gap of KS and ST structures for CZTS occurs at 50 GPa. However, PMCA CZTS possesses metal property. Furthermore, the PMCA CZTS translates from metal to the indirect semiconductor with increasing pressure. The results play an important role in future experimental and theoretical work for CZTS materials.

  14. Pressing Induced Polymorphic Phase Transition in Submicron-Sized Gamma-Hmx

    NASA Astrophysics Data System (ADS)

    Lee, K.-Y.; Moore, D. S.

    2007-12-01

    Using Raman spectroscopy, a novel submicron-sized HMX (sm-HMX) was determined to be both the gamma polymorph and stable with respect to conversion to beta-HMX under ambient conditions for at least a year. Pressing of sm-HMX powder in a small diameter pellet press at pressures from 10,000 psi to 31,000 psi and 1 to 5 minute hold times was found to promote the gamma to beta polymorphic phase transition. The fraction converted and rate of conversion versus time after pellet removal from the press, measured using Raman spectroscopy, fit a sigmoidal curve, indicating nucleation and growth as a possible polymorphic transition mechanism.

  15. Pressure induced magnetic phase transition in RhFe3N and IrFe3N: An ab-initio study

    NASA Astrophysics Data System (ADS)

    Puvaneswari, S.; Rajeswarapalanichamy, R.; Manikandan, M.

    2016-05-01

    The structural, electronic, elastic and magnetic properties of RhFe3N and IrFe3N are investigated using ab-initio calculations based on density functional theory as implemented in VASP code within the gradient generalized approximation. The non-spin polarized and spin polarized calculations are performed for these nitrides at normal and high pressures. It is found that these ternary nitrides are stable in ferromagnetic state at normal pressure. The lattice constant and bulk modulus values are calculated. The electronic structure reveals that these nitrides are metallic at normal pressure. The calculated elastic constants indicate that they are mechanically stable at ambient pressure. Ferromagnetic to nonmagnetic phase transition is observed in RhFe3N and IrFe3N at high pressure. Ferromagnetism is quenched in these nitrides at high pressure.

  16. Pressure-induced metallization of silane

    SciTech Connect

    Chen,X.; Struzhkin, V.; Song, Y.; Goncharov, A.; Ahart, M.; Liu, Z.; Mao, H.; Hemley, R.

    2008-01-01

    There is a great interest in electronic transitions in hydrogen-rich materials under extreme conditions. It has been recently suggested that the group IVa hydrides such as methane (CH4), silane (SiH4), and germane (GeH4) become metallic at far lower pressures than pure hydrogen at equivalent densities because the hydrogen is chemically compressed in group IVa hydride compounds. Here we report measurements of Raman and infrared spectra of silane under pressure. We find that SiH4 undergoes three phase transitions before becoming opaque at 27-30 GPa. The vibrational spectra indicate the material transforms to a polymeric (framework) structure in this higher pressure range. Room-temperature infrared reflectivity data reveal that the material exhibits Drude-like metallic behavior above 60 GPa, indicating the onset of pressure-induced metallization.

  17. Experimental Deformation of Olivine Crystals at Mantle P and T: Evidences for a Pressure-Induced Slip Transition and Implications for Upper-Mantle Seismic Anisotropy and Low Viscosity Zone

    NASA Astrophysics Data System (ADS)

    Raterron, P.; Chen, J.; Geenen, T.; Girard, J.

    2009-04-01

    Recent developments in high-pressure deformation devices coupled with synchrotron radiation allow investigating the rheology of mantle minerals and aggregates at the extreme pressure (P) and temperature (T) of their natural occurrence in the Earth. This is particularly true in the case of olivine, which rheology has been recently investigated in the Deformation-DIA apparatus (D-DIA, see Wang et al., 2003, Rev. Scientific Instr., 74, 3002) at upper-mantle P and T conditions. Olivine deforms by dislocation creep in the shallow upper-mantle, as revealed by the seismic velocity anisotropy observed in this region. The attenuation of seismic anisotropy at depth greater than 200 km is interpreted as a pressure-induced change in olivine main deformation mechanism. It was first attributed to a transition from dislocation creep to diffusion creep (Karato and Wu, 1993, Science, 260, 771). This interpretation has been challenged by deformation data obtained at high pressure (P > 3 GPa) in the dislocation creep regime (Couvy et al., 2004, EJM, 16, 877; Raterron et al., 2007, Am. Miner., 92, 1436; Raterron et al., 2009, PEPI, 72, 74), which support a second interpretation: a transition in olivine dominant dislocation slip, from [100] slip at low P to [001] slip at high P (e.g., Mainprice et al., 2005, Nature, 433, 731). Such a P -induced [100]/[001] slip transition is also supported by recent theoretical studies based on first-principle calculations of olivine dislocation slips (Durinck et al., 2005, PCM, 32, 646; Durinck et al., 2007, Eur. J. Mineral., 19, 631). In order to further constrain the effect of pressure on olivine slip system activities, deformation experiments were carried out in poor water condition at P > 5 GPa and T =1400˚ C, on pure forsterite (Fo100) and San Carlos olivine crystals, using the D-DIA at the X17B2 beamline of the NSLS (Upton, NY, USA). Crystals were oriented in order to active either [100] slip alone or [001] slip alone in (010) plane, or both

  18. Pressing induced polymorphic phase transition in submicron-sized gamma-HMX

    NASA Astrophysics Data System (ADS)

    Moore, David; Lee, Kien-Yin

    2007-06-01

    Submicron HMX has been produced and characterized to be less sensitive than impact standard HMX in small-scale sensitivity tests. The sm-HMX was found to be the gamma polymorph and to be stable under ambient conditions for at least a year. Pressing of sm-HMX in a small diameter pellet press at pressures from 10 000 psi to 31 000 psi and 1 to 5 minute hold times was found to promote the gamma to beta polymorphic phase transition. The fraction converted and rate of conversion versus time after pellet removal from the press were found to fit a sigmoidal curve, indicating nucleation and growth as a possible polymorphic transition mechanism.

  19. An Exercise in X-Ray Diffraction Using the Polymorphic Transition of Nickel Chromite.

    ERIC Educational Resources Information Center

    Chipman, David W.

    1980-01-01

    Describes a laboratory experiment appropriate for a course in either x-ray crystallography or mineralogy. The experiment permits the direct observation of a polymorphic transition in nickel chromite without the use of a special heating stage or heating camera. (Author/GS)

  20. Shock induced polymorphic transition in quartz, carbon, and boron nitride

    NASA Technical Reports Server (NTRS)

    Tan, Hua; Ahrens, Thomas J.

    1990-01-01

    The model proposed by Ahrens (1988) to explain the mechanism of the polymorphism in silicates is revised, and the revised model is applied to the quartz/stishovite, graphite/diamond, and graphite-boron nitride (g-BN) phase transformations. In this model, a key assumption is that transformation to a high-density amorphous or possibly liquid phase which rapidly crystallized to the high-pressure phase is triggered by the high temperatures in the shear band and upon crossing the metastable extension of a melting curve. Good agreement between the calcualted results and published data is obtained. The present theory predicts the standard entropy for cubic BN to be 0.4-0.5 J/g K.

  1. Polymorphic transitions in n-hydrocarbon-water and n-alcohol-water binary systems

    NASA Astrophysics Data System (ADS)

    Mirgorod, Yu. A.

    2014-08-01

    The mixing of hydrocarbons and alcohols in an excess of water is explained by polymorphic transitions similar to crystallization in an ensemble of water clusters. Enthalpies of transitions of 4.90 ± 0.07 and 2.2 ± 0.3 kJ/mol are obtained for solutions of hydrocarbons and alcohols in an excess of water, respectively. It is concluded that the mixing of water in an excess of hydrocarbons and alcohols is similar to evaporation (the breaking of H-bonds) with an enthalpy of 34 ± 1.4 kJ/mol. It is established that a polymorphic transition occurs between two binodals, and is accompanied by the emergence of microphases (concentration fluctuations) of alcohols in water. Binodals and spinodals in an excess of water and alcohol coincide for butyl and other higher alcohols.

  2. Combined synchrotron XRD/Raman measurements: in situ identification of polymorphic transitions during crystallization processes.

    PubMed

    Klimakow, Maria; Leiterer, Jork; Kneipp, Janina; Rössler, Ernst; Panne, Ulrich; Rademann, Klaus; Emmerling, Franziska

    2010-07-01

    A combination of two analytical methods, time-resolved X-ray diffraction (XRD) and Raman spectroscopy, is presented as a novel tool for crystallization studies. An acoustic levitator was employed as sample environment. This setup enables the acquisition of XRD and Raman data in situ simultaneously within a 20 s period and hence permits investigation of polymorphic phase transitions during the crystallization process in different solvents (methanol, ethanol, acetone, dichloromethane, acetonitrile). These real time measurements allow the determination of the phase content from the onset of the first crystalline molecular assemblies to the stable system. To evaluate the capability of this approach, the setup was applied to elucidate the crystallization process of the polymorphic compound nifedipine. The results indicate the existence of solvent-dependent transient phases during the crystallization process. The quality of the data allowed the assignment of the lattice constants of the hitherto unknown crystal structure of the beta-polymorph. PMID:20222693

  3. Polymorphism in α-sexithiophene crystals: relative stability and transition path.

    PubMed

    Klett, Bernhard; Cocchi, Caterina; Pithan, Linus; Kowarik, Stefan; Draxl, Claudia

    2016-06-01

    We present a joint theoretical and experimental study to investigate polymorphism in α-sexithiophene (6T) crystals. By means of density-functional theory calculations, we clarify that the low-temperature phase is favorable over the high-temperature one, with higher relative stability up to 50 meV per molecule. This result is in agreement with our thermal desorption measurements. We also propose a transition path between the high- and low-temperature 6T polymorphs, estimating an upper bound for the energy barrier of about 1 eV per molecule. The analysis of the electronic properties of the investigated 6T crystal structures complements our study. PMID:27181997

  4. Polymorphic phase transitions in systems evolving in a two-dimensional discrete space.

    PubMed

    Gadomski, A

    1999-08-01

    Polymorphic phase transitions in systems evolving in a two-dimensional discrete space have been studied. The driving force of the transitions appears to be a difference between two main energetic contributions: one, related to the thermal activation of the process, and another, being of quantum nature. The former (high temperature limit) is naturally assigned to the expansion (melting) part of the transition, while the latter (low temperature limit) has much in common with the contraction (solidification) part. Between the two main physical states distinguished, there exists a certain state, corresponding to a discontinuity point (pole) in the morphological phase diagram, represented by the well-known Bose-Einstein (Planck) formula, in which the system blows up. This point is related to an expected situation in which the contour of the object under investigation stands for the Brownian or purely diffusional path, with the fractal dimension dw=2, and the situation can be interpreted as some emergence of an intermediate "tetratic" phase. This, in turn, recalls a certain analogy to the equilibrium (order-disorder) phase transition of Kosterlitz-Thouless type, characteristic of, e.g., rough vs rigid interfaces in a two-dimensional space, with some disappearance of interface correlation length at dw=2. Otherwise, the contours of the objects are equivalent to fractional Brownian paths either in superlinear or "turbulent" (dw<2; the expansion case), or sublinear, viz., anomalously slow (dw>2; the contraction case) regimes, respectively. It is hoped that the description offered will serve to reflect properly the main subtleties of the dynamics of the polymorphic transitions in complex "soft-matter" systems, like formation of lipid mesomorphs or diffusional patterns, with nonzero line tension effect. PMID:11969883

  5. Improved tabletability after a polymorphic transition of delta-mannitol during twin screw granulation.

    PubMed

    Vanhoorne, V; Bekaert, B; Peeters, E; De Beer, T; Remon, J-P; Vervaet, C

    2016-06-15

    In most formulations processed via continuous twin screw granulation microcrystalline cellulose (MCC) and/or lactose are used as excipients, but mannitol is also a preferred excipient for wet granulation and tableting due to its non-hygroscopicity and inertness. Therefore, the aim of the current study was to investigate the influence of process parameters on critical quality attributes of granules (moisture content, solid state, morphology, size distribution, specific surface area, friability, flowability and hygroscopicity) and tablets (tensile strength and friability) after twin screw granulation of δ-mannitol. The δ-polymorph was selected since a moisture-induced transformation to β-mannitol was observed during batch wet granulation, which exhibited a unique morphology with a large surface area and improved tabletability. A full factorial experimental design was performed, varying screw speed (400-900rpm), granulation temperature (25-40°C), number of kneading elements (6 or 12) and liquid-to-solid (L/S) ratio, on the granulation unit of a ConsiGma™-25 line (a continuous powder-to-tablet manufacturing system). After tray drying the granules were milled and tableted. The results showed that the polymorphic transition from δ- to β-mannitol also occurred during twin screw granulation, although the residence time and L/S ratios were much lower in continuous twin screw granulation compared to batch processing. However, the polymorphic transition was not complete in all experiments and depended on the L/S ratio, screw speed and number of kneading elements. Nevertheless all granules exhibited the unique morphology linked to the polymorphic transition and had a superior tabletability compared to granules produced with β-mannitol as starting material. This was attributed to enhanced plastic deformation of the granules manufactured using δ-mannitol as starting material. In addition, it was concluded that mannitol was granulated via a different mechanism than

  6. Atomistic pathways of the pressure-induced densification of quartz

    NASA Astrophysics Data System (ADS)

    Liang, Yunfeng; Miranda, Caetano R.; Scandolo, Sandro

    2015-10-01

    When quartz is compressed at room temperature it retains its crystal structure at pressures well above its stability domain (0-2 GPa), and collapses into denser structures only when pressure reaches 20 GPa. Depending on the experimental conditions, pressure-induced densification can be accompanied by amorphization; by the formation of crystalline, metastable polymorphs; and can be preceded by the appearance of an intermediate phase, quartz II, with unknown structure. Based on molecular dynamic simulations, we show that this rich phenomenology can be rationalized through a unified theoretical framework of the atomistic pathways leading to densification. The model emphasizes the role played by the oxygen sublattice, which transforms from a bcc-like order in quartz into close-packed arrangements in the denser structures, through a ferroelastic instability of martensitic nature.

  7. Structural study of the Eu3+ environments in fluorozirconate glasses: Role of the temperature-induced and the pressure-induced phase transition processes in the development of a rare earth's local structure model

    NASA Astrophysics Data System (ADS)

    Muñoz-Santiuste, Juan E.; Rodríguez-Mendoza, Ulises R.; González-Platas, Javier; Lavín, Víctor

    2009-04-01

    The correlation between the optical properties of the Eu3+ ions and their local structures in fluorozirconate glasses and glass-ceramics have been analyzed by means of steady-state and time-resolved site-selective laser spectroscopies. Changes in the crystal-field interaction, ranging from weak to medium strength values, are observed monitoring the luminescence and the lifetime of the Eu3+ ions in different local environments in the glass. As key roles in this study, the Eu3+ luminescence in the thermally-induced crystallization of the glass and the pressure-induced amorphization of the crystalline phase of the glass-ceramic experimentally states the existence of a parent local structure for the Eu3+ ions in the glass, identified as the EuZrF7 crystalline phase. Starting from the ab initio single overlap model, crystal-field calculations have been performed in the glass and the glass-ceramic. From the site-selective measurements, the crystal-field parameters sets are obtained, giving a suitable simulation of the F7J (J =0-6) Stark energy level diagram for the Eu3+ ions in the different environments present in the fluorozirconate glass. A simple geometrical model based on a continuous distortion of the parent structure is proposed for the distribution of local environments of the Eu3+ ions in the fluorozirconate glass.

  8. Pressure induced hyperfine shift and broadening rates of the 52S1/2 →62P1/2 and 52S1/2 →62P3/2 transitions of rubidium with He, Ar, CH4, and C2H6

    NASA Astrophysics Data System (ADS)

    Guy, M. R.; Guild, E. M.; Young, J. W.; Sheets, I. O.; Pitz, G. A.

    2016-01-01

    Pressure induced broadening and spectral shift rates have been resolved for the two ground state hyperfine lines in the 52S1/2 →62P1/2 and 52S1/2 →62P3/2 of 85Rb and 87Rb in the presence of 5-50 Torr of helium, argon, methane, and ethane. Broadening rates averaged over the hyperfine components, for the P1/2 transition for He, Ar, CH4, and C2H6 are 60.2, 45.2, 63.8, and 60.4 MHz/Torr, respectively and 50.4, 42.4, 62.0, and 60.4 MHz/Torr for the P3/2 transition. Average spectral shift rates for the P1/2 transition were found to be 15.7, -11.5, -18.1, and -19.9 MHz/Torr for He, Ar, CH4, and C2H6, respectively, and 3.1, -12.6,-21.8, and -19.9 MHz/Torr for the P3/2 transition. Deviation in broadening and shift rates between hyperfine lines was found to be as high as 16 and 7.6 MHz/Torr respectively.

  9. Pressure-induced phase transformation of In2Se3

    NASA Astrophysics Data System (ADS)

    Rasmussen, Anya; Teklemichael, Samuel; Mafi, Elham; Gu, Yi; McCluskey, Matthew

    2013-06-01

    Phase-change memory, with fast read-write speeds and small dimensions, will soon replace flash memory in our cell phones and tablets. This type of memory relies on phase change materials like indium selenide, In2Se3, a III-VI semiconductor that exists in multiple crystalline phases. To achieve controlled switching between phases, it is important to understand both the thermal and elastic properties of In2Se3. Using synchrotron x-ray diffraction and a diamond-anvil cell, a pressure-induced phase transition in powder In2Se3 from the α phase to β phase was discovered at 0.7 GPa. This pressure is an order of magnitude lower than phase-transition pressures in most semiconductors. Raman spectroscopy experiments confirm this result. The bulk moduli are reported for both α and β phases, and the c / a ratio for the β phase is shown to have a nonlinear dependence on pressure.

  10. Pressure-Induced Order in the Gapped Quantum Magnet DTN

    NASA Astrophysics Data System (ADS)

    Mannig, Alexandra; Moeller, Johannes; Zheludev, Andrey; Garlea, V. Ovidiu; Dela Cruz, Clarina; Guguchia, Zurab; Khasanov, Rustem; Morenzoni, Elvezio

    We present muon-spin relaxation, neutron diffraction and magnetic susceptibility data under applied hydrostatic pressure on the organometallic S = 1 quantum magnet NiCl2 . 4 [ SC(NH2)2 ] . The material consists of weakly coupled antiferromagnetic chains and has a spin gap resulting from a large single-ion anisotropy. Our muon spin rotation experiments provide local field dependencies on temperature as well as pressure and allow for the mapping of a detailed phase diagram up to 22kbar. Thus, we demonstrate that the compound may be driven through two subsequent pressure-induced transitions into apparently distinct magnetically ordered phases. Neutron diffraction and susceptibility measurements support those results and show the potential of low-pressure transitions to be investigated by various techniques.

  11. Irreversibility of Pressure Induced Boron Speciation Change in Glass

    PubMed Central

    Smedskjaer, Morten M.; Youngman, Randall E.; Striepe, Simon; Potuzak, Marcel; Bauer, Ute; Deubener, Joachim; Behrens, Harald; Mauro, John C.; Yue, Yuanzheng

    2014-01-01

    It is known that the coordination number (CN) of atoms or ions in many materials increases through application of sufficiently high pressure. This also applies to glassy materials. In boron-containing glasses, trigonal BO3 units can be transformed into tetrahedral BO4 under pressure. However, one of the key questions is whether the pressure-quenched CN change in glass is reversible upon annealing below the ambient glass transition temperature (Tg). Here we address this issue by performing 11B NMR measurements on a soda lime borate glass that has been pressure-quenched at ~0.6 GPa near Tg. The results show a remarkable phenomenon, i.e., upon annealing at 0.9Tg the pressure-induced change in CN remains unchanged, while the pressurised values of macroscopic properties such as density, refractive index, and hardness are relaxing. This suggests that the pressure-induced changes in macroscopic properties of soda lime borate glasses compressed up to ~0.6 GPa are not attributed to changes in the short-range order in the glass, but rather to changes in overall atomic packing density and medium-range structures. PMID:24442182

  12. Irreversibility of pressure induced boron speciation change in glass.

    PubMed

    Smedskjaer, Morten M; Youngman, Randall E; Striepe, Simon; Potuzak, Marcel; Bauer, Ute; Deubener, Joachim; Behrens, Harald; Mauro, John C; Yue, Yuanzheng

    2014-01-01

    It is known that the coordination number (CN) of atoms or ions in many materials increases through application of sufficiently high pressure. This also applies to glassy materials. In boron-containing glasses, trigonal BO3 units can be transformed into tetrahedral BO4 under pressure. However, one of the key questions is whether the pressure-quenched CN change in glass is reversible upon annealing below the ambient glass transition temperature (Tg). Here we address this issue by performing (11)B NMR measurements on a soda lime borate glass that has been pressure-quenched at ~0.6 GPa near Tg. The results show a remarkable phenomenon, i.e., upon annealing at 0.9Tg the pressure-induced change in CN remains unchanged, while the pressurised values of macroscopic properties such as density, refractive index, and hardness are relaxing. This suggests that the pressure-induced changes in macroscopic properties of soda lime borate glasses compressed up to ~0.6 GPa are not attributed to changes in the short-range order in the glass, but rather to changes in overall atomic packing density and medium-range structures. PMID:24442182

  13. Electrochemically-induced reversible transition from the tunneled to layered polymorphs of manganese dioxide

    NASA Astrophysics Data System (ADS)

    Lee, Boeun; Yoon, Chong Seung; Lee, Hae Ri; Chung, Kyung Yoon; Cho, Byung Won; Oh, Si Hyoung

    2014-08-01

    Zn-ion batteries are emerging energy storage systems eligible for large-scale applications, such as electric vehicles. These batteries consist of totally environmentally-benign electrode materials and potentially manufactured very economically. Although Zn/α-MnO2 systems produce high energy densities of 225 Wh kg-1, larger than those of conventional Mg-ion batteries, they show significant capacity fading during long-term cycling and suffer from poor performance at high current rates. To solve these problems, the concrete reaction mechanism between α-MnO2 and zinc ions that occur on the cathode must be elucidated. Here, we report the intercalation mechanism of zinc ions into α-MnO2 during discharge, which involves a reversible phase transition of MnO2 from tunneled to layered polymorphs by electrochemical reactions. This transition is initiated by the dissolution of manganese from α-MnO2 during discharge process to form layered Zn-birnessite. The original tunneled structure is recovered by the incorporation of manganese ions back into the layers of Zn-birnessite during charge process.

  14. Crystalline Morphology and Polymorphic Phase Transitions in Electrospun Nylon-6 Nanofibers

    SciTech Connect

    Liu,Y.; Cui, L.; Guan, F.; Gao, Y.; Hedin, N.; Zhu, L.; Fong, H.

    2007-01-01

    Uniform nylon-6 nanofibers with diameters around 200 nm were prepared by electrospinning. Polymorphic phase transitions and crystal orientation of nylon-6 in unconfined (i.e., as-electrospun) and a high T{sub g} (340 C) polyimide confined nanofibers were studied. Similar to melt-spun nylon-6 fibers, electrospun nylon-6 nanofibers also exhibited predominant, metastable {gamma}-crystalline form, and the {gamma}-crystal (chain) axes preferentially oriented parallel to the fiber axis. Upon annealing above 150 C, {gamma}-form crystals gradually melted and recrystallized into thermodynamically stable {alpha}-form crystals, which ultimately melted at 220 C. Release of surface tension accompanied this melt-recrystallization process, as revealed by differential scanning calorimetry. For confined nanofibers, both the melt-recrystallization and surface tension release processes were substantially depressed; {gamma}-form crystals did not melt and recrystallize into {alpha}-form crystals until 210 C, only 10 C below the T{sub m} at 220 C. After complete melting of nanoconfined crystals at 240 C and recrystallization at 100 C, only {alpha}-form crystals oriented perpendicular to the nanofiber axis were obtained. In the polyimide-confined nanofibers, the Brill transition (from the monoclinic {alpha}-form to a high-temperature monoclinic form) was observed at 180-190 C, which was at least 20 C higher than that in unconfined nylon-6 at {approx}160 C. This, again, was attributed to the confinement effect.

  15. On the high-temperature phase transitions of CsH2PO4: A polymorphic transition? A transition to a superprotonic conducting phase?

    NASA Astrophysics Data System (ADS)

    Ortiz, E.; Vargas, R. A.; Mellander, B.-E.

    1999-03-01

    X-ray diffraction, thermogravimetric (TGA), differential scanning calorimetric (DSC), and impedance analysis were used to study the reported high-temperature phase transitions at 107, 149, 230, and 256 °C in crystals of cesium dihydrogen phosphate, CsH2PO4 (CDP). Our results show strong evidence that at all these temperatures, the observed DSC or differential thermal analysis (DTA) endothermic effects appear only as a consequence of a dehydration process starting on the surface of the crystal. Our results thus show that the reported transition at 230 °C is not a polymorphic transition. This means that the monoclinic symmetry, stable at room temperature, with space group P21/m-C2k2, is maintained up to the final decomposition. Moreover, since we have not found any evidence for the existence of a superprotonic high-temperature phase above 230 °C, the high conductivity above 230 °C is thus only a consequence of the dehydration of the crystal surface.

  16. Pressure-induced transformations in molecular crystals

    SciTech Connect

    Taylor, R.D.; Hearne, G.R. |; Pasternak, M.P.

    1995-09-01

    A review is given on the unique features of the Moessbauer spectroscopy (MS) which by virtue of the quadrupole interaction and the lattice dynamics allows one to characterize some structural properties in the pressure-induced amorphous state of molecular crystals. Experiments were performed in GeI{sub 4}, SnI{sub 4} and SnBr{sub 4} by means of {sup 119}Sn and {sup 129}I MS with pressures to 35 GPa at cryogenic temperatures using diamond anvil cells.

  17. Critical behavior of resistivity in the pressure-induced first to second order transition in Pr0.6Ca0.4Mn0.96B0.04O3 (B=Co and Cr) polycrystals

    NASA Astrophysics Data System (ADS)

    Thiyagarajan, R.; Mahendiran, R.; Arumugam, S.

    2015-06-01

    We have investigated the hydrostatic pressure (P) dependence of the resistivity on Pr0.6Ca0.4Mn0.96B0.04O3 (B = Co and Cr) polycrystals. At ambient pressure, the temperature dependence of resistivity [ρ(T)] of both the samples show a first order paramagnetic insulator-ferromagnetic metallic transition at T=TIM. The application of P on both the samples increases the TIM, reduces the resistivity, and suppresses the hysteresis width, indicating a crossover from first to second order transition. The critical pressure, where the first-second order crossover takes place, are 2.02 and 2.40 GPa for Co and Cr doped samples respectively. The critical property of both systems around second order transition is investigated using Fisher-Langer relation and Suezaki-Mori method. The estimated critical exponents are close to the three-dimensional Heisenberg model for the Co doped sample suggesting short range interaction, and the exponents for the Cr doped sample follow the mean field theory suggesting long range ferromagnetic order. Further, the application of P suppresses the high temperature resistivity by reducing high temperature polarons in the case of the Cr doped sample, but it does not happen for the Co doped sample. The application of P helps to examine the stability of polarons in the high temperature regime.

  18. A kinetic and thermodynamic study of seratrodast polymorphic transition by isothermal microcalorimetry.

    PubMed

    Urakami, Koji; Beezer, Anthony E

    2003-05-12

    The development of isothermal microcalorimetry to a study of the kinetic and thermodynamics of polymorphic transitions in seratrodast ((+/-)-7-(3,5,6-trimethyl-1,4-benzoquinon-2-yl)-7-phenylheptanoic acid) Form II is reported. Sieved samples of Form II were allowed to convert to Form I, in a reaction vessel of an isothermal microcalorimeter, under 13, 31, 63 and 93% relative humidity (RH) between 48 and 65 degrees C. The power (Phi, in Watts) versus time curves from the microcalorimeter were integrated into the heat output (q, in Joules) versus time curves to yield fractional extent of Form I converted versus time curves. The change in enthalpy (-5.70 kJmol(-1)) agreed very closely with that obtained by differential scanning calorimetry and solution calorimetry, which indicated that the power measured by the microcalorimeter was due only to the Form II-to-Form I transition. Application of the theoretical kinetic method [J. Am. Ceram. Soc. 55 (1972) 74] revealed that the transition took place via a two-dimensional growth of nuclei mechanism at all the studied relative humidities and temperatures. The rate constant increased with increasing RH and temperature, and with decreasing the particle size of sample. The activation energies obtained from Arrhenius plots were 292, 290, 280 and 284 kJmol(-1), and the extrapolated rate constants at 25 degrees C were also 3.01 x 10(-10), 3.11 x 10(-10), 9.65 x 10(-10) and 3.84 x 10(-9)s(-1) for 13, 31, 63 and 93% RH, respectively. PMID:12711181

  19. Nature of Pressure-induced Insulating States in Simple Metals

    NASA Astrophysics Data System (ADS)

    Naumov, Ivan; Hemley, Russell

    As experimentally established, all the alkali metals and heavy alkaline earth metals (Ca, Sr and Ba) become progressively less conductive on compression, at least up to some critical limit over a broad pressure range. Of these metals, Li and Na clearly undergo pressure-induced metal-insulator transitions, which may also be called reverse Mott transitions. Here, using group theory arguments and first-principles calculations, we show that such transitions can be understood in terms of band representations introduced by Zak. The valence bands in the insulating states are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms. The character of the Wannier functions is closely related to the degree of s-p(-d) hybridization and reflects multi-center chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as re-entrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of semimetallic phases with flat surface states. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been identified or reported (e.g., Li, Na, and Ca). This research was supported by EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DESC0001057.

  20. Crystalline Morphology and Polymorphic Phase Transitions in Electrospun Nylon 6 Nanofibers.

    PubMed

    Liu, Yi; Cui, Li; Guan, Fangxiao; Gao, Yi; Hedin, Nyle E; Zhu, Lei; Fong, Hao

    2007-01-01

    Uniform nylon 6 nanofibers with diameters around 200 nm were prepared by electrospinning. Polymorphic phase transitions and crystal orientation of nylon 6 in unconfined (i.e., as-electrospun) and a high T(g) (340 degrees C) polyimide confined nanofibers were studied. Similar to melt-spun nylon 6 fibers, electrospun nylon 6 nanofibers also exhibited predominant, meta-stable gamma crystalline form, and the gamma-crystal (chain) axes preferentially oriented parallel to the fiber axis. Upon annealing above 150 degrees C, gamma-form crystals gradually melted and recrystallized into the thermodynamically stable alpha-form crystals, which ultimately melted at 220 degrees C. Release of surface tension accompanied this melt-recrystallization process, as revealed by differential scanning calorimetry. For confined nanofibers, both the melt-recrystallization and surface tension release processes were substantially depressed; gamma-form crystals did not melt and recrystallize into alpha-form crystals until 210 degrees C, only 10 degrees C below the T(m) at 220 degrees C. After complete melting of nano-confined crystals at 240 degrees C and recrystallization at 100 degrees C, only alpha-form crystals oriented perpendicular to the nanofiber axis were obtained. In the polyimide-confined nanofibers, the Brill transition (from the monoclinic alpha-form to a high temperature monoclinic form) was observed at 180-190 degrees C, which was at least 20 degrees C higher than that in unconfined nylon 6 at approximately 160 degrees C. This, again, was attributed to the confinement effect. PMID:18698379

  1. Crystalline Morphology and Polymorphic Phase Transitions in Electrospun Nylon 6 Nanofibers

    PubMed Central

    Liu, Yi; Cui, Li; Guan, Fangxiao; Gao, Yi; Hedin, Nyle E.; Zhu, Lei; Fong, Hao

    2008-01-01

    Uniform nylon 6 nanofibers with diameters around 200 nm were prepared by electrospinning. Polymorphic phase transitions and crystal orientation of nylon 6 in unconfined (i.e., as-electrospun) and a high Tg (340 °C) polyimide confined nanofibers were studied. Similar to melt-spun nylon 6 fibers, electrospun nylon 6 nanofibers also exhibited predominant, meta-stable γ crystalline form, and the γ-crystal (chain) axes preferentially oriented parallel to the fiber axis. Upon annealing above 150 °C, γ-form crystals gradually melted and recrystallized into the thermodynamically stable α-form crystals, which ultimately melted at 220 °C. Release of surface tension accompanied this melt-recrystallization process, as revealed by differential scanning calorimetry. For confined nanofibers, both the melt-recrystallization and surface tension release processes were substantially depressed; γ-form crystals did not melt and recrystallize into α-form crystals until 210 °C, only 10 °C below the Tm at 220 °C. After complete melting of nano-confined crystals at 240 °C and recrystallization at 100 °C, only α-form crystals oriented perpendicular to the nanofiber axis were obtained. In the polyimide-confined nanofibers, the Brill transition (from the monoclinic α-form to a high temperature monoclinic form) was observed at 180–190 °C, which was at least 20 °C higher than that in unconfined nylon 6 at approximately 160 °C. This, again, was attributed to the confinement effect. PMID:18698379

  2. Pressure-Induced Foaming of Metals

    NASA Astrophysics Data System (ADS)

    García-Moreno, Francisco; Mukherjee, Manas; Jiménez, Catalina; Banhart, John

    2015-05-01

    Pressure-induced foaming (PIF) of metals is a foaming technique in which blowing agent free compacted metal powders are foamed. The method consists of heating hot-compacted metallic precursors to above their melting temperature under gas overpressure and foaming them by pressure release. This study focuses on PIF of Al99.7 and AlSi7 alloys under both air or Ar and overpressures up to 9 bar. In situ x-ray radioscopy allows us to follow the foaming process and to perform quantitative analyses of expansion, foam morphology, and coalescence rate. Mass spectrometry helps to identify hydrogen as the foaming gas. Adsorbates on the former powder particles are found to be the primary gas source. Various advantages of this new method are identified and discussed.

  3. Elasticity of Hydrous Olivine Polymorphs: Implications for Seismic Structure of the Transition Zone

    NASA Astrophysics Data System (ADS)

    Duffy, T. S.; Mao, Z.; Jacobsen, S. D.; Jiang, F.; Smyth, J. R.; Holl, C. M.; Frost, D. J.

    2007-12-01

    The presence of water in the upper mantle and transition zone has the potential to explain various phenomena such as shear velocity anomalies or uplift and broadening of the 410-km discontinuity. The presence of H2O in the transition zone has also been frequently invoked to reconcile laboratory elasticity data on olivine polymorphs with seismic data for the amplitude of the 410-km discontinuity (Li et al., 2001; Chambers et al., 2005). Recently, we have measured the single-crystal elastic properties of hydrous olivine (Jacobsen et al., 2006) and a suite of hydrous wadsleyites (Mao et al., 2007a) at ambient conditions and one hydrous wadsleyite composition (0.84 wt% H2O) up to 12 GPa (Mao et al., 2007b). These data provide new constraints on elastic moduli and their pressure derivatives for hydrous olivine and wadsleyite. Using this data, we first examine the effect of H2O on bulk sound velocities under transition zone conditions because anelastic effects can be neglected in this case. At 410 km depth (~13.8 GPa, along a 1400°C adiabat), the bulk sound velocity of wadsleyite with 1 wt% H2O is 3.1% lower than for dry wadsleyite. Comparison of the seismic velocity jump across the 410-km discontinuity with the measured velocity contrast between wadsleyite and olivine provides a means to estimate the olivine abundance at 410-km depth. For mantle wadsleyite with 0.1-0.2 wt% H2O (Huang et al., 2005) and using experimentally determined olivine- wadsleyite H2O partition coefficients, the olivine abundance is found to be 40%, much lower than a pyrolite model. In order for a pyrolite composition to satisfy the seismic data, 1.2 wt. % H2O is needed in wadsleyite- a value greater than its maximum solubility under these conditions. The anomalously steep seismic gradient in the transition zone has been another feature of the region that has long defied explanation. We show that the seismic gradient can be matched if there is a gradient in H2O concentration across the transition

  4. Pressure-induced non-superconducting phase of β-Na0.33V2O5 and the mechanism of high-pressure phase transitions in β-Na0.33V2O5 and β-Li0.33V2O5 at room temperature

    NASA Astrophysics Data System (ADS)

    Grzechnik, A.; Ueda, Y.; Yamauchi, T.; Hanfland, M.; Hering, P.; Potapkin, V.; Friese, K.

    2016-01-01

    The crystal structure of β-Na0.33V2O5 (C2/m, Z  =  6) has been studied on compression to 19 GPa at room temperature using synchrotron single-crystal diffraction in a diamond anvil cell. The vanadate bronze undergoes a phase transition to a non-superconducting phase at about 12 GPa due to changes of polyhedral connectivities in the vanadate framework and due to ordering of the Na+ cations. This novel structure (Cm, Z  =  6) is interpreted as an intermediate stage in the sequence of pressure-induced transformations in the β-A 0.33V2O5 bronzes (A: Li, Na) at room temperature. This study reveals the close relation between the loss of the two-leg ladder V-V system and non-superconducting state of the β-A 0.33V2O5 materials.

  5. Pressure-induced non-superconducting phase of β-Na0.33V2O5 and the mechanism of high-pressure phase transitions in β-Na0.33V2O5 and β-Li0.33V2O5 at room temperature.

    PubMed

    Grzechnik, A; Ueda, Y; Yamauchi, T; Hanfland, M; Hering, P; Potapkin, V; Friese, K

    2016-01-27

    The crystal structure of β-Na0.33V2O5 (C2/m, Z  =  6) has been studied on compression to 19 GPa at room temperature using synchrotron single-crystal diffraction in a diamond anvil cell. The vanadate bronze undergoes a phase transition to a non-superconducting phase at about 12 GPa due to changes of polyhedral connectivities in the vanadate framework and due to ordering of the Na(+) cations. This novel structure (Cm, Z  =  6) is interpreted as an intermediate stage in the sequence of pressure-induced transformations in the β-A0.33V2O5 bronzes (A: Li, Na) at room temperature. This study reveals the close relation between the loss of the two-leg ladder V-V system and non-superconducting state of the β-A0.33V2O5 materials. PMID:26702603

  6. The impact of room temperature polymorphism in K doped NaTaO3 on structural phase transition behaviour

    NASA Astrophysics Data System (ADS)

    Arulnesan, Shamanthini William; Kayser, Paula; Kennedy, Brendan J.; Knight, Kevin S.

    2016-06-01

    Temperature dependent high resolution neutron diffraction studies demonstrate that the sequence and temperatures of the crystallographic phase transitions in NaTaO3 are not impacted by doping with 1% K to form Na0.99K0.01TaO3. Rietveld analysis of the neutron diffraction data shows the structural transitions to be: Pbnm ↔ 723       K Cmcm ↔ 803   K P 4 / nbm ↔ 893   K Pm 3 bar m . The two orthorhombic polymorphs, Pbnm and Cmcm, differ fundamentally in the distortion and tilting of the octahedra, such that they cannot be obtained from each other via a continuous phase transition resulting in their co-existence between RT and 723 K. Chemical doping, does however, dramatically impact on the amount of the metastable Cmcm phase observed at room temperature.

  7. Trichoscopy of Noncicatricial Pressure-induced Alopecia Resembling Alopecia Areata.

    PubMed

    Papaiordanou, Francine; da Silveira, Bruno Rebelo Lages; Piñeiro-Maceira, Juan; Pirmez, Rodrigo

    2016-01-01

    Pressure-induced alopecia is an unusual cause of hair loss, and reports of its trichoscopic features are scarce. In this paper, we describe a case of pressure-induced alopecia in which trichoscopic and histopathological findings overlap with those described for alopecia areata. PMID:27601865

  8. Trichoscopy of Noncicatricial Pressure-induced Alopecia Resembling Alopecia Areata

    PubMed Central

    Papaiordanou, Francine; da Silveira, Bruno Rebelo Lages; Piñeiro-Maceira, Juan; Pirmez, Rodrigo

    2016-01-01

    Pressure-induced alopecia is an unusual cause of hair loss, and reports of its trichoscopic features are scarce. In this paper, we describe a case of pressure-induced alopecia in which trichoscopic and histopathological findings overlap with those described for alopecia areata. PMID:27601865

  9. Pressure-induced recovery of Fourier's law in one-dimensional momentum-conserving systems

    NASA Astrophysics Data System (ADS)

    Sato, Dye SK

    2016-07-01

    We report the two typical models of normal heat conduction in one-dimensional momentum-conserving systems. They show the Arrhenius and the non-Arrhenius temperature dependence. We construct the two corresponding phenomenologies, transition-state theory of thermally activated dissociation and the pressure-induced crossover between two fixed points in fluctuating hydrodynamics. Compressibility yields the ballistic fixed point, whose scaling is observed in Fermi-Pasta-Ulam (FPU) β lattices.

  10. Pressure-induced recovery of Fourier's law in one-dimensional momentum-conserving systems.

    PubMed

    Sato, Dye Sk

    2016-07-01

    We report the two typical models of normal heat conduction in one-dimensional momentum-conserving systems. They show the Arrhenius and the non-Arrhenius temperature dependence. We construct the two corresponding phenomenologies, transition-state theory of thermally activated dissociation and the pressure-induced crossover between two fixed points in fluctuating hydrodynamics. Compressibility yields the ballistic fixed point, whose scaling is observed in Fermi-Pasta-Ulam (FPU) β lattices. PMID:27575085

  11. Pressure-induced metallization of molybdenum disulfide.

    PubMed

    Chi, Zhen-Hua; Zhao, Xiao-Miao; Zhang, Haidong; Goncharov, Alexander F; Lobanov, Sergey S; Kagayama, Tomoko; Sakata, Masafumi; Chen, Xiao-Jia

    2014-07-18

    X-ray diffraction, Raman spectroscopy, and electrical conductivity measurements of molybdenum disulfide MoS(2) are performed at pressures up to 81 GPa in diamond anvil cells. Above 20 GPa, we find discontinuous changes in Raman spectra and x-ray diffraction patterns which provide evidence for isostructural phase transition from 2H(c) to 2H(a) modification through layer sliding previously predicted theoretically. This first-order transition, which is completed around 40 GPa, is characterized by a collapse in the c-lattice parameter and volume and also by changes in interlayer bonding. After the phase transition completion, MoS(2) becomes metallic. The reversibility of the phase transition is identified from all these techniques. PMID:25083660

  12. High-Pressure Induced New Phases and Properties in Typical Molecular Systems

    NASA Astrophysics Data System (ADS)

    Cui, Tian

    2013-06-01

    High pressure introduces new phases by the rearrangement of atoms and reconfigurations of electronic states in materials, often with new physical and chemical phenomena. Study of the new phases in typical molecular systems under high pressure is an interesting subject, such as energy storage materials of solid hydrogen and polymeric nitrogen, hydrogen-rich compound with high-Tc superconductivity under high pressure, high pressure induced metallization of hydrogen, etc. High-pressure structures and pressure-induced phase transitions in the typical molecular solids, such as solid iodine, CHBr3, N2/CN, HBr/HCl, hydrogen-rich compounds (H2S, ZrH2, AsH3, BaReH9, etc.), and group IVA hydrides (Si2H6, Ge2H6, Sn2H6, etc.) are investigated extensively by means of first-principles density functional theory and extensive prediction strategies (molecular dynamics simulation, simulated annealing, soft mode phase transition, random structure-searching method and evolutionary methodology etc.). The new structures and new properties derived from pressure-induced phase transitions in these typical molecular systems have been observed. It is showed that high pressure provides a path for producing new materials with new properties.

  13. Pressure-induced amorphization of La{sub 1/3}TaO{sub 3}

    SciTech Connect

    Noked, O.; Melchior, A.; Shuker, R.; Livneh, T.; Steininger, R.; Kennedy, B.J.; Sterer, E.

    2013-06-15

    La{sub 1/3}TaO{sub 3}, an A-site cation deficient perovskite, has been studied under pressure by synchrotron X-ray powder diffraction and Raman spectroscopy. It undergoes irreversible pressure induced amorphization at P=18.5 GPa. An almost linear unit cell volume decrease vs. pressure is observed from ambient pressure up to the phase transition. The Raman spectroscopy also shows amorphization at the same pressure, with positive shifts of all modes as a function of pressure. The pressure dependence of the E{sub g} and A{sub 1g} Raman modes arising from the octahedral oxygen network is discussed. - Graphical abstract: La{sub 1/3}Tao{sub 3} exhibits linear pressure–volume relation until irreversible pressure induced amorphization at 18.5 Gpa. - Highlights: • La{sub 1/3}TaO{sub 3} has been studied under pressure by synchrotron XRD and Raman spectroscopy. • La{sub 1/3}TaO{sub 3} undergoes irreversible pressure induced amorphization around 18.5 GPa. • The transition is manifested in both XRD and Raman measurements. • A linear P–V relation is observed from ambient pressure up to the phase transition.

  14. Pressure induced reactions amongst calcium aluminate hydrate phases

    SciTech Connect

    Moon, Ju-hyuk; Oh, Jae Eun; Balonis, Magdalena; Glasser, Fredrik P.; Clark, Simon M.; Monteiro, Paulo J.M.

    2011-06-15

    The compressibilities of two AFm phases (straetlingite and calcium hemicarboaluminate hydrate) and hydrogarnet were obtained up to 5 GPa by using synchrotron high-pressure X-ray powder diffraction with a diamond anvil cell. The AFm phases show abrupt volume contraction regardless of the molecular size of the pressure-transmitting media. This volume discontinuity could be associated to a structural transition or to the movement of the weakly bound interlayer water molecules in the AFm structure. The experimental results seem to indicate that the pressure-induced dehydration is the dominant mechanism especially with hygroscopic pressure medium. The Birch-Murnaghan equation of state was used to compute the bulk modulus of the minerals. Due to the discontinuity in the pressure-volume diagram, a two stage bulk modulus of each AFm phase was calculated. The abnormal volume compressibility for the AFm phases caused a significant change to their bulk modulus. The reliability of this experiment is verified by comparing the bulk modulus of hydrogarnet with previous studies.

  15. Polymorphic single crystal {r_reversible} single crystal transition in K{sub 0.975}Rb{sub 0.025}NO{sub 3}

    SciTech Connect

    Asadov, Yu. G. Nasirov, E. V.

    2010-09-15

    Polymorphic transformations in K{sub 0.975}Rb{sub 0.025}NO{sub 3} single crystals have been investigated by optical microscopy and X-ray diffraction. The equilibrium temperature between modifications II and III has been determined. It is established that the crystal growth at II {r_reversible} III polymorphic transitions is accompanied by the formation and growth of daughter-modification nuclei in the matrix crystal.

  16. Pressure-induced phase transformations during femtosecond-laser doping of silicon

    NASA Astrophysics Data System (ADS)

    Smith, Matthew J.; Lin, Yu-Ting; Sher, Meng-Ju; Winkler, Mark T.; Mazur, Eric; Gradečak, Silvija

    2011-09-01

    Silicon hyperdoped with chalcogens via femtosecond-laser irradiation exhibits unique near-unity sub-bandgap absorptance extending into the infrared region. The intense light-matter interactions that occur during femtosecond-laser doping produce pressure waves sufficient to induce phase transformations in silicon, resulting in the formation of metastable polymorphic phases, but their exact formation mechanism and influence on the doping process are still unknown. We report direct observations of these phases, describe their formation and distribution, and consider their potential impact on sub-bandgap absorptance. Specifically, the transformation from diamond cubic Si-I to pressure-induced polymorphic crystal structures (amorphous Si, Si-XII, and Si-III) during femtosecond-laser irradiation was investigated using scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. Amorphous Si, Si-XII, and Si-III were found to form in femtosecond-laser doped silicon regardless of the presence of a gaseous or thin-film dopant precursor. The rate of pressure loading and unloading induced by femtosecond-laser irradiation kinetically limits the formation of pressure-induced phases, producing regions of amorphous Si 20 to 200 nm in size and nanocrystals of Si-XII and Si-III. The surface texturing that occurs during femtosecond-laser irradiation produces inhomogeneous pressure distributions across the surface and causes delayed development of high-pressure silicon polymorphs over many laser pulses. Finally, we find that the polymorph phases disappear during annealing more rapidly than the sub-bandgap absorptance decreases, enabling us to decouple these two processes through post-treatment annealing.

  17. Pressure-induced polyamorphism in salty water.

    PubMed

    Bove, L E; Klotz, S; Philippe, J; Saitta, A M

    2011-03-25

    We investigated the metastable phase diagram of an ionic salt aqueous solution, LiCl:6D₂O, at high pressure and low temperature by neutron diffraction measurements and computer simulations. We show that the presence of salt triggers a stepwise transformation, under annealing at high pressure, to a new very high-density amorphous form. The transition occurs abruptly at 120 K and 2 GPa, is reversible, and is characterized by a sizeable enthalpy release. Simulations suggest that the polyamorphic transition is linked to a local structural reorganization of water molecules around the Li ions. PMID:21517327

  18. Evidence for charge-trapping inducing polymorphic structural-phase transition in pentacene.

    PubMed

    Ando, Masahiko; Kehoe, Tom B; Yoneya, Makoto; Ishii, Hiroyuki; Kawasaki, Masahiro; Duffy, Claudia M; Minakata, Takashi; Phillips, Richard T; Sirringhaus, Henning

    2015-01-01

    Trapped-charge-induced transformation of pentacene polymorphs is observed by using in situ Raman spectroscopy and molecular dynamics simulations reveal that the charge should be localized in pentacene molecules at the interface with static intermolecular disorder along the long axis. Quantum chemical calculations of the intermolecular transfer integrals suggest the disorder to be large enough to induce Anderson-type localization. PMID:25382806

  19. Pressure-induced exotic states in rare earth hexaborides

    NASA Astrophysics Data System (ADS)

    Sun, Liling; Wu, Qi

    2016-08-01

    Finding the exotic phenomena in strongly correlated electron systems (SCESs) and understanding the corresponding microphysics have long been the research frontiers of condensed matter physics. The remarkable examples for the intriguing phenomena discovered in past years include unconventional superconductivity, heavy Fermion behaviors, giant magneto-resistance and so on. A fascinating type of rare earth hexaboride RB6 (R  =  Sm, Yb, Eu and Ce) belongs to a strongly correlated electron system (SCES), but shows unusual ambient-pressure and high-pressure behaviors beyond the phenomena mentioned above. Particularly, the recent discovery of the coexistence of an unusual metallic surface state and an insulating bulk state in SmB6, known to be a Kondo insulator decades ago, by theoretical calculations and many experimental measurements creates new interest for the investigation of the RB6. This significant progress encourages people to revisit the RB6 with an attempt to establish a new physics that links the SCES and the unusual metallic surface state which is a common feature of a topological insulator (TI). It is well known that pressure has the capability of tuning the electronic structure and modifying the ground state of solids, or even inducing a quantum phase transition which is one of the kernel issues in studies of SCESs. In this brief review, we will describe the progress in high pressure studies on the RB6 based on our knowledge and research interests, mainly focusing on the pressure-induced phenomena in YbB6 and SmB6, especially on the quantum phase transitions and their connections with the valence state of the rare earth ions. Moreover, some related high-pressure results obtained from CeB6 and EuB6 are also included. Finally, a summary is given in the conclusions and perspectives section.

  20. Pressure-induced exotic states in rare earth hexaborides.

    PubMed

    Sun, Liling; Wu, Qi

    2016-08-01

    Finding the exotic phenomena in strongly correlated electron systems (SCESs) and understanding the corresponding microphysics have long been the research frontiers of condensed matter physics. The remarkable examples for the intriguing phenomena discovered in past years include unconventional superconductivity, heavy Fermion behaviors, giant magneto-resistance and so on. A fascinating type of rare earth hexaboride RB6 (R  =  Sm, Yb, Eu and Ce) belongs to a strongly correlated electron system (SCES), but shows unusual ambient-pressure and high-pressure behaviors beyond the phenomena mentioned above. Particularly, the recent discovery of the coexistence of an unusual metallic surface state and an insulating bulk state in SmB6, known to be a Kondo insulator decades ago, by theoretical calculations and many experimental measurements creates new interest for the investigation of the RB6. This significant progress encourages people to revisit the RB6 with an attempt to establish a new physics that links the SCES and the unusual metallic surface state which is a common feature of a topological insulator (TI). It is well known that pressure has the capability of tuning the electronic structure and modifying the ground state of solids, or even inducing a quantum phase transition which is one of the kernel issues in studies of SCESs. In this brief review, we will describe the progress in high pressure studies on the RB6 based on our knowledge and research interests, mainly focusing on the pressure-induced phenomena in YbB6 and SmB6, especially on the quantum phase transitions and their connections with the valence state of the rare earth ions. Moreover, some related high-pressure results obtained from CeB6 and EuB6 are also included. Finally, a summary is given in the conclusions and perspectives section. PMID:27376406

  1. Structure family and polymorphous phase transition in the compounds with soft sublattice: Cu2Se as an example.

    PubMed

    Qiu, Wujie; Lu, Ping; Yuan, Xun; Xu, Fangfang; Wu, Lihua; Ke, Xuezhi; Liu, Huili; Yang, Jiong; Shi, Xun; Chen, Lidong; Yang, Jihui; Zhang, Wenqing

    2016-05-21

    Quite a few interesting but controversial phenomena, such as simple chemical composition but complex structures, well-defined high-temperature cubic structure but intriguing phase transition, coexist in Cu2Se, originating from the relatively rigid Se framework and "soft" Cu sublattice. However, the electrical transport properties are almost uninfluenced by such complex substructures, which make Cu2Se a promising high-performance thermoelectric compound with extremely low thermal conductivity and good power factor. Our work reveals that the crystal structure of Cu2Se at the temperature below the phase-transition point (∼400 K) should have a group of candidate structures that all contain a Se-dominated face-centered-cubic-like layered framework but nearly random site occupancy of atoms from the "soft" Cu sublattice. The energy differences among those structures are very low, implying the coexistence of various structures and thus an intrinsic structure complexity with a Se-based framework. Detailed analyses indicate that observed structures should be a random stacking of those representative structure units. The transition energy barriers between each two of those structures are estimated to be zero, leading to a polymorphous phase transition of Cu2Se at increasing temperature. Those are all consistent with experimental observations. PMID:27208953

  2. Structure family and polymorphous phase transition in the compounds with soft sublattice: Cu2Se as an example

    NASA Astrophysics Data System (ADS)

    Qiu, Wujie; Lu, Ping; Yuan, Xun; Xu, Fangfang; Wu, Lihua; Ke, Xuezhi; Liu, Huili; Yang, Jiong; Shi, Xun; Chen, Lidong; Yang, Jihui; Zhang, Wenqing

    2016-05-01

    Quite a few interesting but controversial phenomena, such as simple chemical composition but complex structures, well-defined high-temperature cubic structure but intriguing phase transition, coexist in Cu2Se, originating from the relatively rigid Se framework and "soft" Cu sublattice. However, the electrical transport properties are almost uninfluenced by such complex substructures, which make Cu2Se a promising high-performance thermoelectric compound with extremely low thermal conductivity and good power factor. Our work reveals that the crystal structure of Cu2Se at the temperature below the phase-transition point (˜400 K) should have a group of candidate structures that all contain a Se-dominated face-centered-cubic-like layered framework but nearly random site occupancy of atoms from the "soft" Cu sublattice. The energy differences among those structures are very low, implying the coexistence of various structures and thus an intrinsic structure complexity with a Se-based framework. Detailed analyses indicate that observed structures should be a random stacking of those representative structure units. The transition energy barriers between each two of those structures are estimated to be zero, leading to a polymorphous phase transition of Cu2Se at increasing temperature. Those are all consistent with experimental observations.

  3. Pressure-induced amorphous-to-amorphous reversible transformation in Pr{sub 75}Al{sub 25}

    SciTech Connect

    Lin, C. L.; Ahmad, A. S.; Lou, H. B.; Wang, X. D.; Cao, Q. P.; Jiang, J. Z.; Li, Y. C.; Liu, J.; Hu, T. D.; Zhang, D. X.

    2013-12-07

    A pressure-induced amorphous-to-amorphous reversible transformation was revealed in Pr{sub 75}Al{sub 25} metallic glass (MG) using in situ high-pressure synchrotron x-ray diffraction technique. The transition began at about 21 GPa with a ∼ 5% volume collapse and ended at about 35 GPa. This transition is reversible with hysteresis. Based on the high-pressure behaviors of Ce-based metallic glasses and Pr metal here, we suggest that the pressure-induced polyamorphic transition in Pr{sub 75}Al{sub 25} MG stems from 4f-electron delocalization of Pr metal which leads to abrupt change in bond shortening. These results obtained here provide new insights into the underlying mechanism of the amorphous-to-amorphous phase transition in metallic glasses and will trigger more theoretical and experimental investigations for such transition.

  4. On the pressure induced phase of Na{sub 2}CsC{sub 60}

    SciTech Connect

    Morosin, B.; Schirber, J.E.; Jorgensen, J.D.; Kwei, G.H.; Yildirim, T. |; Fischer, J.E.

    1996-06-01

    Neutron powder diffraction at pressures to 6 kbar in gaseous Ne has been used to study the pressure-induced phase transition and compressibilities of Na{sub 2}CsC{sub 60}. The pressure-induced phase can be achieved by compression to about 5 kbar at room temperature. If cooled, this phase can be retained below 200 K upon release of the pressure. The structure is orthorhombic as previously reported (but may differ in its detailed crystal structure) with lattice constants near 80 K and ambient pressure of a=9.385 A, b=10.06 A, and c=14.36 A. Corresponding linear compressibilities are 0.0004, 0014, and 0.0017 kbar{sup -1}, respectively. Identical pressure temperature cycling results in a superconductor with an unexpectedly low pressure dependence for {Tc} while in this phase. Models for the superconducting behavior of this compound are discussed.

  5. Pressure-induced superconductivity in topological parent compound Bi2Te3

    SciTech Connect

    Zhang, J. L.; Zhang, S. J.; Weng, H. M.; Zhang, W.; Yang, L. X.; Liu, Q. Q.; Feng, S. M.; Wang, X. C.; Yu, R. C.; Cao, L. Z.; Wang, L.; Yang, W. G.; Liu, H. Z.; Zhao, W. Y.; Zhang, S. C.; Dai, X.; Fang, Z.; Jin, C. Q.

    2011-01-04

    We report a successful observation of pressure-induced superconductivity in a topological compound Bi₂Te₃ with Tc of ~3 K between 3 to 6 GPa. The combined high-pressure structure investigations with synchrotron radiation indicated that the superconductivity occurred at the ambient phase without crystal structure phase transition. The Hall effects measurements indicated the hole-type carrier in the pressure-induced superconducting Bi₂Te₃ single crystal. Consequently, the first-principles calculations based on the structural data obtained by the Rietveld refinement of X-ray diffraction patterns at high pressure showed that the electronic structure under pressure remained topologically nontrivial. The results suggested that topological superconductivity can be realized in Bi₂Te₃ due to the proximity effect between superconducting bulk states and Dirac-type surface states. We also discuss the possibility that the bulk state could be a topological superconductor.

  6. Order-parameter-aided temperature-accelerated sampling for the exploration of crystal polymorphism and solid-liquid phase transitions

    SciTech Connect

    Yu, Tang-Qing Vanden-Eijnden, Eric; Chen, Pei-Yang; Chen, Ming; Samanta, Amit; Tuckerman, Mark

    2014-06-07

    The problem of predicting polymorphism in atomic and molecular crystals constitutes a significant challenge both experimentally and theoretically. From the theoretical viewpoint, polymorphism prediction falls into the general class of problems characterized by an underlying rough energy landscape, and consequently, free energy based enhanced sampling approaches can be brought to bear on the problem. In this paper, we build on a scheme previously introduced by two of the authors in which the lengths and angles of the supercell are targeted for enhanced sampling via temperature accelerated adiabatic free energy dynamics [T. Q. Yu and M. E. Tuckerman, Phys. Rev. Lett. 107, 015701 (2011)]. Here, that framework is expanded to include general order parameters that distinguish different crystalline arrangements as target collective variables for enhanced sampling. The resulting free energy surface, being of quite high dimension, is nontrivial to reconstruct, and we discuss one particular strategy for performing the free energy analysis. The method is applied to the study of polymorphism in xenon crystals at high pressure and temperature using the Steinhardt order parameters without and with the supercell included in the set of collective variables. The expected fcc and bcc structures are obtained, and when the supercell parameters are included as collective variables, we also find several new structures, including fcc states with hcp stacking faults. We also apply the new method to the solid-liquid phase transition in copper at 1300 K using the same Steinhardt order parameters. Our method is able to melt and refreeze the system repeatedly, and the free energy profile can be obtained with high efficiency.

  7. Order-parameter-aided temperature-accelerated sampling for the exploration of crystal polymorphism and solid-liquid phase transitions

    PubMed Central

    Yu, Tang-Qing; Chen, Pei-Yang; Chen, Ming; Samanta, Amit; Vanden-Eijnden, Eric; Tuckerman, Mark

    2014-01-01

    The problem of predicting polymorphism in atomic and molecular crystals constitutes a significant challenge both experimentally and theoretically. From the theoretical viewpoint, polymorphism prediction falls into the general class of problems characterized by an underlying rough energy landscape, and consequently, free energy based enhanced sampling approaches can be brought to bear on the problem. In this paper, we build on a scheme previously introduced by two of the authors in which the lengths and angles of the supercell are targeted for enhanced sampling via temperature accelerated adiabatic free energy dynamics [T. Q. Yu and M. E. Tuckerman, Phys. Rev. Lett. 107, 015701 (2011)]. Here, that framework is expanded to include general order parameters that distinguish different crystalline arrangements as target collective variables for enhanced sampling. The resulting free energy surface, being of quite high dimension, is nontrivial to reconstruct, and we discuss one particular strategy for performing the free energy analysis. The method is applied to the study of polymorphism in xenon crystals at high pressure and temperature using the Steinhardt order parameters without and with the supercell included in the set of collective variables. The expected fcc and bcc structures are obtained, and when the supercell parameters are included as collective variables, we also find several new structures, including fcc states with hcp stacking faults. We also apply the new method to the solid-liquid phase transition in copper at 1300 K using the same Steinhardt order parameters. Our method is able to melt and refreeze the system repeatedly, and the free energy profile can be obtained with high efficiency. PMID:24907992

  8. APOE and COMT polymorphisms are complementary biomarkers of status, stability, and transitions in normal aging and early mild cognitive impairment

    PubMed Central

    Dixon, Roger A.; DeCarlo, Correne A.; MacDonald, Stuart W. S.; Vergote, David; Jhamandas, Jack; Westaway, David

    2014-01-01

    Objective: Research has reported associations among selected genetic susceptibility biomarkers and risk of (a) normal cognitive aging decrements, (b) established mild cognitive impairment (MCI), and (c) sporadic Alzheimer's disease (AD). In focusing on the transitional normal-to-early MCI phase, we examine associations among three theoretically relevant polymorphisms (APOE [rs429358, rs7412], BDNF [rs6265], COMT [rs4680]) and both baseline cognitive status (MCI vs. normal aging) and two-wave (four-year) longitudinal stability or change profiles. The latter included three profiles: (a) stable as normal aging, (b) stable or chronic impairment (MCI-to-MCI), and (c) emergence of impairment (normal-to-MCI). Method: Genotyped older adults (n = 237 at baseline; age range = 64–91; 62% women) from the Victoria Longitudinal Study were examined for (a) independent and interactive associations of three genetic polymorphisms with (b) two objectively classified cognitive status groups (not-impaired controls (NIC) and MCI) at (c) both baseline and across a two-wave (four-year) longitudinal interval. Results: First, logistic regression revealed that the presence of at least one APOE ε4 allele (the risk factor for AD) was linked to greater baseline risk of objective MCI. Second, multinomial logistic regression revealed that (a) the presence of an APOE ε4 allele was associated with an increased risk of 4-year MCI status stability (chronicity), and (b) the COMT homozygous risk genotype (G/G or Val/Val) was associated with an increased risk of both MCI-to-MCI stability (chronicity) and emerging NIC-to-MCI conversion. Discussion: Both chronicity and emergence of objectively classified early cognitive impairment may be genetically heterogeneous phenomena, with influences from a panel of both normal cognitive aging (COMT) and AD-related (APOE) polymorphisms. PMID:25249975

  9. Effect of CD44 gene polymorphisms on risk of transitional cell carcinoma of the urinary bladder in Taiwan.

    PubMed

    Weng, Wei-Chun; Huang, Yu-Hui; Yang, Shun-Fa; Wang, Shian-Shiang; Kuo, Wu-Hsien; Hsueh, Chao-Wen; Huang, Ching-Hsuan; Chou, Ying-Erh

    2016-05-01

    The carcinogenesis of transitional cell carcinoma (TCC) of the urinary bladder involves etiological factors, such as ethnicity, the environment, genetics, and diet. Cluster of differentiation (CD44), a well-known tumor marker, plays a crucial role in regulating tumor cell differentiation and metastasis. This study investigated the effect of CD44 single nucleotide polymorphisms (SNPs) on TCC risk and clinicopathological characteristics. Five SNPs of CD44 were analyzed through real-time polymerase chain reaction in 275 patients with TCC and 275 participants without cancer. In this study, we observed that CD44 rs187115 polymorphism carriers with the genotype of at least one G were associated with TCC risk. Furthermore, TCC patients who carried at least one G allele at CD44 rs187115 had a higher stage risk than did patients carrying the wild-type allele (p < 0.05). In addition, The AATAC or GACGC haplotype among the five CD44 sites was also associated with a reduced risk of TCC. In conclusion, our results suggest that CD44 SNPs influence the risk of TCC. Patients with CD44 rs187115 variant genotypes (AG + GG) exhibited a higher risk of TCC; these patients may possess chemoresistance to developing late-stage TCC compared with those with the wild-type genotype. The CD44 rs187115 SNP may predict poor prognosis in patients with TCC. PMID:26662954

  10. Polymorphic transition of solid-fats dispersed systems — its characterization by a novel method and scanning electron microscopy observation

    NASA Astrophysics Data System (ADS)

    Hirokawa, Norio; Ueda, Masahiro; Harano, Yoshio

    1994-08-01

    Solid-fats dispersed systems, such as margarine, butter and cacao-butter, were characterized by a novel method based on liquid permeation under pressure, for the simultaneous measurement of a solid-content ɛ p and an average diameter dp of solid particles (fats crystals) in them. Further, micro-structures of these systems were observed by a scanning electron microscope (SEM). As the result, it has been clarified that the spherical fats crystals of several μm in size appeared in the initial solid-fats products are agglomerates of fine particles of ca. 0.1 μm and that these fine particles are uniformly redispersed during an annealing treatment accompanying the reduction of ɛ p and dp. It is strongly suggested that this phenomenon is caused by a transition of fat crystals into a more stable polymorph.

  11. Dramatically Different Conductivity Properties of Metal-Organic Framework Polymorphs of Tl(TCNQ): An Unexpected Room-Temperature Crystal-to-Crystal Phase Transition

    SciTech Connect

    Avendano, Carolina; Zhang, Zhongyue; Ota, Akira; Zhao, Hanhua; Dunbar, Kim R

    2012-02-07

    Tl(TCNQ) polymorphs with very different charge-transport properties have been isolated, one of which undergoes a remarkable crystal-to-crystal phase transition to the second phase when exposed to ambient water vapor (see picture; TCNQ=tetracyanoquinodimethane).

  12. Pressure-induced superconductivity in europium metal

    SciTech Connect

    Debessai, M.; Matsuoka, T.; Hamlin, J.J.; Bi, W.; Meng, Y.; Shimizu, K.; Schilling, J.S.

    2010-05-24

    Of the 52 known elemental superconductors among the 92 naturally occurring elements in the periodic table, fully 22 only become superconducting under sufficiently high pressure. In the rare-earth metals, the strong local magnetic moments originating from the 4f shell suppress superconductivity. For Eu, however, Johansson and Rosengren have suggested that sufficiently high pressures should promote one of its 4f electrons into the conduction band, changing Eu from a strongly magnetic (J=7/2) 4f{sup 7}-state into a weak Van Vleck paramagnetic (J=0) 4f{sup 6}-state, thus opening the door for superconductivity, as in Am (5f{sup 6}). We report that Eu becomes superconducting above 1.8 K for pressures exceeding 80 GPa, T{sub c} increasing linearly with pressure to 142 GPa at the rate +15 mK/GPa. Eu thus becomes the 53rd elemental superconductor in the periodic table. Synchrotron x-ray diffraction studies to 92 GPa at ambient temperature reveal four structural phase transitions.

  13. Pressure-induced diffusion in natural garnets

    NASA Astrophysics Data System (ADS)

    Floess, David; Vrijmoed, Johannes; Baumgartner, Lukas; Podladchikov, Yuri

    2015-04-01

    Recent efforts in metamorphic petrology suggest that significant pressure gradients exist on the grain-scale and provide tools for its quantification [1,2]. Here we propose that pressure gradients around coesite inclusions induced diffusion of major elements within garnet crystals upon exhumation. This is based on the fact that the molar mass of garnet endmembers vary between 403 and 497 g/mol, thus up to 23 %. Whiteschists from the Dora Maira Massive in the Western Alps underwent eclogite facies metamorphism (3.3-4.3 GPa, 720-780 °C) during the Alpine event at 35 Ma [3]. Coesite included in garnet (py0.96gr0.02alm0.02) during the HP stage was partially transformed to quartz during the subsequent, rapid exhumation (from 3.5 to 1 GPa within 2 Ma [4]). Coesite is preserved by maintaining a high pressure on the inclusion wall due to the large volume change of the phase transition. The surface of the host garnet experiences a lower pressure controlled by the exhumation P-T path. This pressure difference should induce diffusion of major elements in the garnet surrounding the inclusion. Element distribution maps show well-defined Fe-rich, Ca-poor halos surrounding the coesite-inclusions. The observed diffusion profiles are in agreement with predictions, assuming a positive ΔP around the inclusions. The results are based on thermodynamic equilibrium calculations assuming heterogeneous pressure [5]. Hence, the observed profiles are interpreted as an equilibrium state reflecting the pressure (stress) distribution within the crystal and can be used as tool to constrain the exhumation path. Understanding the effect of pressure gradients on diffusion and, alternatively, the generation of pressure due to relaxation of chemical gradients by diffusion, is crucial for interpreting P-T-t paths of zoned minerals correctly. [1] Baumgartner et al. (2010), GSA meeting Denver. [2] Tajčmanová et al. (2014) CMP 32, 195-207. [3] Compagnoni & Rolfo (2003), UHP Metamorphism - EMU notes 5

  14. Pressure-Induced Metallization of the Mott Insulator MnO

    SciTech Connect

    Patterson, J R; Aracne, C M; Jackson, D D; Weir, S T; Malba, V; Baker, P A; Vohra, Y K

    2004-01-12

    High-pressure electrical conductivity experiments have been performed on the Mott insulator MnO to a maximum pressure of 106 GPa. We observe a steady decrease in resistivity to 90 GPa, followed by a large, rapid decrease by a factor of 10{sup 5} between 90 and 106 GPa. Temperature cycling the sample at 87 and 106 GPa shows insulating and metallic behavior at these pressures, respectively. Our observations provide strong evidence for a pressure-induced Mott insulator-to-metal transition with an accompanying magnetic collapse beginning at 90 GPa.

  15. Ab initio study of pressure induced structural and electronic properties in uranium monobismuthide

    NASA Astrophysics Data System (ADS)

    Pataiya, Jagdish; Aynyas, Mahendra; Makode, C.; Singh, A.; Sanyal, Sankar P.

    2014-04-01

    We have investigated the pressure induced structural and electronic properties of uranium monobismuthide. The total energy as a function of volume is obtained by means of self-consistent tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). We predict structural phase transition from NaCl to CsCl-type structure at a pressure of 4.6 GPa. From energy band diagram it is observed that UBi exhibits metallic behavior. The calculated equilibrium lattice parameter is in good agreement with the experimental and other theoretical work.

  16. Magnetotransport study of the pressure-induced antiferromagnetic phase in FeSe

    NASA Astrophysics Data System (ADS)

    Terashima, Taichi; Kikugawa, Naoki; Kasahara, Shigeru; Watashige, Tatsuya; Matsuda, Yuji; Shibauchi, Takasada; Uji, Shinya

    2016-05-01

    The resistivity ρ and Hall resistivity ρH are measured on FeSe at pressures up to P =28.3 kbar in magnetic fields up to B =14.5 T. The ρ (B ) and ρH(B ) curves are analyzed with multicarrier models to estimate the carrier density and mobility as a function of P and temperature (T ≤110 K). It is shown that the pressure-induced antiferromagnetic transition is accompanied by an abrupt reduction of the carrier density and scattering. This indicates that the electronic structure is reconstructed significantly by the antiferromagnetic order.

  17. On the measurement of pressure induced shift by diode lasers and harmonic detection

    NASA Astrophysics Data System (ADS)

    De Rosa, M.; Ciucci, A.; Pelliccia, D.; Gabbanini, C.; Gozzini, S.; Lucchesini, A.

    1998-02-01

    We present an analysis of the line shape of ro-vibrational molecular absorptions, in case of frequency modulation spectroscopy with diode lasers, and in the presence of a residual amplitude modulation. Subtle effects, such as pressure induced shift, can be measured with sufficient accuracy also for the weak transition lines of the overtone and the combination bands of the molecules, under the proviso of considering the correct fit function in order to avoid possible systematic errors. Some results are given for acetylene overtone absorptions in the derivative spectroscopy limit (small amplitude of modulation) and in the case of large amplitude of modulation.

  18. Ab initio study of pressure induced structural and electronic properties in uranium monobismuthide

    SciTech Connect

    Pataiya, Jagdish Makode, C.; Aynyas, Mahendra; Singh, A.; Sanyal, Sankar P.

    2014-04-24

    We have investigated the pressure induced structural and electronic properties of uranium monobismuthide. The total energy as a function of volume is obtained by means of self-consistent tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). We predict structural phase transition from NaCl to CsCl-type structure at a pressure of 4.6 GPa. From energy band diagram it is observed that UBi exhibits metallic behavior. The calculated equilibrium lattice parameter is in good agreement with the experimental and other theoretical work.

  19. Single crystal X-ray diffraction study of a mixed-valence gold compound, Cs{sub 2}Au{sup I}Au{sup III}Cl{sub 6} under high pressures up to 18 GPa: Pressure-induced phase transition coupled with gold valence transition

    SciTech Connect

    Matsushita, Nobuyuki Ahsbahs, Hans; Hafner, Stefan S.; Kojima, Norimichi

    2007-04-15

    We performed the single-crystal X-ray diffraction study of a perovskite-type gold mixed-valence compound, Cs{sub 2}Au{sup I}Au{sup III}Cl{sub 6}, under high pressures up to 18 GPa by using a diamond-anvil-cell with helium gas as an ideal hydrostatic pressure-transmitting medium. The lattice parameters and the variable atomic positional parameters were obtained with reasonable accuracy at various pressures. A structural phase transition at ca. 12.5 GPa from I4/mmm to Pm3m was found. The lattice parameters a {sub 0} and c {sub 0}, denoted in the tetragonal cell setting, result in the relationship 2{sup 1/2} a {sub 0}=c {sub 0}, and the superstructure reflections h k l (l is odd), caused by the shift of the Cl ions from the midpoint of the Au ions, disappeared at pressures above the phase transition. Both elongated [Au{sup III}Cl{sub 6}] and compressed [Au{sup I}Cl{sub 6}] octahedra in the low-pressure phase smoothly approach regular octahedra with increasing pressure. Above the structural phase transition at 12.5 GPa, all the [AuCl{sub 6}] octahedra are crystallographically equivalent, which shows that the tetragonal-to-cubic phase transition accompanies the valence transition from the Au{sup I}/Au{sup III} mixed-valence state to the Au{sup II} single-valence state. - Graphical abstract: Single-crystal X-ray diffraction study under high pressures up to 18 GPa by using a diamond-anvil-cell with helium gas as an ideal hydrostatic pressure medium has revealed that a perovskite-type gold mixed-valence compound, Cs{sub 2}Au{sup I}Au{sup III}Cl{sub 6}, exhibits the structural phase transition from tetragonal to cubic at 12.5 GPa accompanying gold valence transition.

  20. Pressure-induced disproportionation in CuBr

    NASA Astrophysics Data System (ADS)

    Skelton, E. F.; Webb, A. W.; Qadri, S. B.; Ingalls, R. G.; Traquada, J. M.

    1983-04-01

    CuBr has been pressurized to 8.3 GPa in four separate runs. Prima facie evidence of pressure-induced disproportionation of cuprous bromide into cupric bromide has been observed on release of pressure from 7 GPa. Evidence is also seen of a phase change in CuBr2 above 5.5 GPa.

  1. Polymorphism, phase transitions, and thermal expansion of K3Lu(PO4)2

    SciTech Connect

    Farmer, James Matthew; Boatner, Lynn A; Chakoumakos, Bryan C; Rawn, Claudia J.; Mandrus, D.; Bryan, Jeff C.

    2014-01-01

    Alkali rare-earth double phosphates have been studied for use as long-wavelength scintillators for -ray detection using Si photodiodes. Single-crystal and powder x-ray diffraction (XRD) and powder neutron diffraction have been used to study the structure as a function of temperature. K3Lu(PO4)2 crystallizes with a hexagonal unit cell at room temperature, space group P 3. The Lu ion is six-coordinated to the oxygen atoms of the phosphate groups. Two lower-temperature phases were characterized using single-crystal XRD and powder neutron diffraction. The first transition occurs at 230 K with a transformation to a monoclinic P21/m space group symmetry, and the Lu retains six coordination. The second phase transition occurs at 130 K, with a large change in the cell volume, keeping the same P21/m space group symmetry; however, one of the phosphate groups rotates to increase the coordination of the Lu ion to seven. This is an unusual example of an isosymmetric phase transition with a coordination change, driven by temperature. High-temperature powder neutron diffraction and high-temperature powder XRD have been used to study the thermal expansion of K3Lu(PO4)2 and indicate a large thermal expansion anisotropy. The crystallographic axes with largest changes account for the structural collapse, which rotates the phosphate group to increase the Lu coordination. The lowest temperature form of K3Lu(PO4)2 is the same as the room temperature form for all the lighter RE compounds of the same type, which is not surprising, given the lighter (larger) RE ions would prefer a higher coordination number.

  2. Polymorphism in glassy silicon: Inherited from liquid-liquid phase transition in supercooled liquid

    NASA Astrophysics Data System (ADS)

    Zhang, Shiliang; Wang, Li-Min; Zhang, Xinyu; Qi, Li; Zhang, Suhong; Ma, Mingzhen; Liu, Riping

    2015-02-01

    Combining molecular dynamics (MD) simulation and Voronoi polyhedral analyses, we discussed the microstructure evolution in liquid and glassy silicon during cooling by focusing on the fraction of various clusters. Liquid-liquid phase transition (LLPT) is detected in supercooled liquid silicon However, freezing the high-density liquid (HDL) to the glassy state is not achieved as the quenching rate goes up to 1014 K/s. The polyamorphism in glassy silicon is found to be mainly associated with low-density liquid (LDL).

  3. Polymorphism in glassy silicon: Inherited from liquid-liquid phase transition in supercooled liquid

    PubMed Central

    Zhang, Shiliang; Wang, Li-Min; Zhang, Xinyu; Qi, Li; Zhang, Suhong; Ma, Mingzhen; Liu, Riping

    2015-01-01

    Combining molecular dynamics (MD) simulation and Voronoi polyhedral analyses, we discussed the microstructure evolution in liquid and glassy silicon during cooling by focusing on the fraction of various clusters. Liquid-liquid phase transition (LLPT) is detected in supercooled liquid silicon However, freezing the high-density liquid (HDL) to the glassy state is not achieved as the quenching rate goes up to 1014 K/s. The polyamorphism in glassy silicon is found to be mainly associated with low-density liquid (LDL). PMID:25716054

  4. Polymorphism and phase transitions of K_3Lu(PO_4)_2

    NASA Astrophysics Data System (ADS)

    Farmer, J. Matt; Boatner, Lynn A.; Chakoumakos, Bryan C.; Mandrus, David; Jin, Rongying

    2001-03-01

    Alkali lanthanide double phosphates have been studied for uses as long-wavelength scintillators for γ-ray detection using Si photodiodes. This family of compounds exhibits layered crystal structures, in the sequence lanthanide, phosphate alkali, alkali, alkali - phosphate. Current research has focused on K_3Lu(PO_4)_2. At room temperature, this compound is hexagonal, P 3 space group symmetry. The Lu ion is six-coordinated to the oxygen atoms of the phosphate groups. Our group has recently characterized two lower-temperature phases of K_3Lu(PO_4)2 using single-crystal XRD and powder neutron diffraction. The first transition occurs at 230 K, with a transformation to monoclinic P 2_1/m space group symmetry, and the Lu still retains six coordination. Another transition occurs at 130 K, with a small change in the cell volume, keeping the same P 2_1/m space group symmetry; however, one of the phosphate groups rotates to increase the coordination of the Lu ion to seven. This new structure is isostructural with the room-temperature form of K_3Yb(PO_4)_2. A heat capacity versus temperature study of K_3Lu(PO_4)2 confirms the transformations and indicates a large thermal hysteresis as the crystals are thermally cycled between 15 and 295 K. Research sponsored by the U.S. Department of Energy under contract DE-AC05-00OR22725 with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC.

  5. Mechanism of pressure-induced gelation of milk.

    PubMed

    Keenan, R D; Young, D J; Tier, C M; Jones, A D; Underdown, J

    2001-07-01

    The pressure-induced gelation of concentrated skimmed milk and milk-sugar mixtures was studied to discover the main components responsible for gelation. The major protein component responsible for gelation is micellar casein. Gelation occurs at similar pressures to casein micelle disintegration in dilute milk, and both can be prevented by inclusion of excess calcium chloride. Transmission electron micrographs show that the protein network is formed from particles with diameters approximately an order of magnitude smaller than those of intact casein micelles. Gelation occurs on decompression and is found to be baroreversible. Concentrations of sugar up to 30% reduce the critical concentration of casein required for gelation, but higher sugar concentrations inhibit gelation. A mechanism of gelation based on the aggregation of casein submicelles formed by pressure-induced disintegration of casein micelles is proposed. Observations on the effect of sucrose on gelation are discussed in terms of the influence of sugars on the solvent quality in aqueous casein systems. PMID:11453781

  6. Pressure-induced depolarization and resonance in Raman scattering of single-crystalline boron carbide

    SciTech Connect

    Guo Junjie; Zhang Ling; Fujita, Takeshi; Chen Mingwei; Goto, Takashi

    2010-02-01

    We report polarized and resonant Raman scattering of single-crystal boron carbide (B{sub 4}C) at high pressures. Significant intensity enhancements of 270 and 1086 cm{sup -1} Raman bands of B{sub 4}C have been observed at quasihydrostatic pressures higher than approx20 GPa. The pressure-induced intensity change of the 1086 cm{sup -1} band is mainly due to the resonance between excitation energy and electronic transition, whereas the intensity change of 270 cm{sup -1} band is caused by the depolarization effect. Importantly, the first-order phase transition has not been found at high quasihydrostatic pressures and all the Raman intensity changes along with the corresponding high-pressure lattice distortion can be recovered during unloading.

  7. Pressure-induced phase transition in La1xSmxO0.5F0.5BiS2

    SciTech Connect

    Fang, Y.; Yazici, D.; White, B. D.; Maple, M. B.

    2015-09-15

    Electrical resistivity measurements on La1–xSmxO0.5F0.5BiS2 (x = 0.1, 0.3, 0.6, 0.8) have been performed under applied pressures up to 2.6 GPa from 2 K to room temperature. The superconducting transition temperature Tc of each sample significantly increases at a Sm-concentration dependent pressure Pt, indicating a pressure-induced phase transition from a low-Tc to a high-Tc phase. At ambient pressure, Tc increases dramatically from 2.8 K at x = 0.1 to 5.4 K at x = 0.8; however, the Tc values at P > Pt decrease slightly with x and Pt shifts to higher pressures with Sm substitution. In the normal state, semiconducting-like behavior is suppressed and metallic conduction is induced with increasing pressure in all of the samples. Furthermore, these results suggest that the pressure dependence of Tc for the BiS2-based superconductors is related to the lattice parameters at ambient pressure and enable us to estimate the evolution of Tc for SmO0.5F0.5BiS2 under pressure.

  8. The origins of pressure-induced phase transformations during the surface texturing of silicon using femtosecond laser irradiation

    NASA Astrophysics Data System (ADS)

    Smith, Matthew J.; Sher, Meng-Ju; Franta, Benjamin; Lin, Yu-Ting; Mazur, Eric; Gradečak, Silvija

    2012-10-01

    Surface texturing of silicon using femtosecond (fs) laser irradiation is an attractive method for enhancing light trapping, but the laser-induced damage that occurs in parallel with surface texturing can inhibit device performance. In this work, we investigate the light-material interaction during the texturing of silicon by directly correlating the formation of pressure-induced silicon polymorphs, fs-laser irradiation conditions, and the resulting morphology and microstructure using scanning electron microscopy, micro-Raman spectroscopy, and transmission electron microscopy. We show that raster scanning a pulsed laser beam with a Gaussian profile enhances the formation of crystalline pressure-induced silicon polymorphs by an order of magnitude compared with stationary pulsed fs-laser irradiation. Based on these observations, we identify resolidification-induced stresses as the mechanism responsible for driving sub-surface phase transformations during the surface texturing of silicon, the understanding of which is an important first step towards reducing laser-induced damage during the texturing of silicon with fs-laser irradiation.

  9. Pressure-induced phase and chemical transformations of lithium peroxide (Li2O2).

    PubMed

    Dunuwille, Mihindra; Kim, Minseob; Yoo, Choong-Shik

    2016-08-28

    We present the pressure-induced phase/chemical changes of lithium peroxide (Li2O2) to 63 GPa using diamond anvil cells, confocal micro-Raman spectroscopy, and synchrotron x-ray diffraction. The Raman data show the emergence of the major vibrational peaks associated with O2 above 30 GPa, indicating the subsequent pressure-induced reversible chemical decomposition (disassociation) in dense Li2O2. The x-ray diffraction data of Li2O2, on the other hand, show no dramatic structural change but remain well within a P63/mmc structure to 63 GPa. Nevertheless, the Rietveld refinement indicates a subtle change in the structural order parameter z of the oxygen position O (13, 23, z) at around 35 GPa, which can be considered as a second-order, isostructural phase transition. The nearest oxygen-oxygen distance collapses from 1.56 Å at ambient condition to 1.48 Å at 63 GPa, resulting in a more ionic character of this layered crystal lattice, 3Li(+)+(LiO2)3 (3-). This structural change in turn advocates that Li2O2 decomposes to 2Li and O2, further augmented by the densification in specific molar volumes. PMID:27586935

  10. Barocaloric effect and the pressure induced solid state refrigerator

    SciTech Connect

    Oliveira, N. A. de

    2011-03-01

    The current refrigerators are based on the heating and cooling of fluids under external pressure variation. The great inconvenience of this refrigeration technology is the damage caused to the environment by the refrigerant fluids. In this paper, we discuss the magnetic barocaloric effect, i.e., the heating or cooling of magnetic materials under pressure variation and its application in the construction of refrigerators using solid magnetic compounds as refrigerant materials and pressure as the external agent. The discussion presented in this paper points out that such a pressure induced solid state refrigerator can be very interesting because it is not harmful to the environment and can exhibit a good performance.