First-principles calculations of structural stability and mechanical properties of tungsten carbide under high pressure

The structural stability and mechanical properties of WC in WC-, MoC- and NaCl-type structures under high pressure are investigated systematically by first-principles calculations. The calculated equilibrium lattice constants at zero pressure agree well with available experimental and theoretical results. The formation enthalpy indicates that the most stable WC is in WC-type, then MoC-type finally NaCl-type. By the elastic stability criteria, it is predicted that the three structures are all mechanically stable. The elastic constants C_ij, bulk modulus B, shear modulus G, Young?s modulus E and Poisson?s ratio ν of the three structures are studied in the pressure range from 0 to 100 GPa. Furthermore, by analyzing the B/G ratio, the brittle/ductile behavior under high pressure is assessed. Moreover, the elastic anisotropy of the three structures up to 100 GPa is also discussed in detail.     

First-principles calculations for elastic and electronic properties of ZnSnO3 under pressure

The LiNbO₃ (LN)-type structure and the ilmenite (IL)-type structure of ZnSnO₃ are investigated with the ultrasoft pseudopotential scheme in the frame of the local density approximation (LDA). The calculated lattice parameters of ZnSnO₃ under zero pressure and zero temperature are in very good agreement with the existing experimental data. The pressure dependences of the elastic constants, Debye temperatures, Poisson's ratio, sound velocity, mechanical stability and mechanical anisotropy of the LN-type structure of ZnSnO₃ have also been investigated. We find that the LN-type structure of ZnSnO₃ is a mechanically stable phase under pressures up to 21 GPa; however, the mechanical anisotropy weakens with the increasing pressures. In addition, the calculated band structure indicates that LN-ZnSnO₃ has a direct band gap of 1.669 eV, and the total and partial densities of states, under diverse pressures of the LN-type structure ZnSnO₃ have also been obtained.     

Resistance anomaly and structural disorder in NiSi2 under high pressure

Results of X-ray diffraction, electrical resistance, thermoelectric power measurements and electronic band structure calculations on NiSi₂ under high pressure are reported. The thermoelectric power (TEP) changes sign near 0.5 GPa (from +30 to −20 μV/K). As the pressure is increased, the value of TEP increases further in magnitude and near 7 GPa it becomes −50 μV/K. The pressure vs. resistance curve measured up to 30 GPa using diamond anvil (DAC)-based technique exhibits a broad hump near 12 GPa and exhibits hysteresis on pressure release. The ADXRD patterns up to 42 GPa show a gradual irreversible loss of long-range order in NiSi₂ with the diffraction lines progressively broadening under pressure. The FWHM of the diffraction lines show a rapid increase in the half-widths close to 0.5 GPa and also near 12 GPa. The computed band structure at a compression (without any disorder) corresponding to 12 GPa, exhibits an electronic topological transition (ETT). The rapid increase in disorder above 12 GPa implies that the ETT may be facilitating the structural disorder. It is suggested that the pressure drives the material through a region of entropic and energetic barriers and induces disorder in the material.     

First-principles studies on Ti3Si0.5Ge0.5C2 under pressure

The structural, electronic and elastic properties of Ti₃Si_0.5Ge_0.5C₂ have been investigated by using the pseudopotential plane-wave method within the density-functional theory. Our calculated equation of state (EOS) is consistent with the experimental results. The density of states (DOS) indicates that Ti₃Si_xGe_1−xC₂ (x=0, 0.5, 1.0) are metallic, and these compounds have nearly the same electrical conductivity. The elastic constants for Ti₃Si_0.5Ge_0.5C₂ are obtained at zero pressure, which is compared to Ti₃SiC₂ and Ti₃GeC₂. We can conclude that Ti₃Si_0.5Ge_0.5C₂ is brittle in nature by analyzing the ratio between bulk and shear moduli. There appears to be little effect on the electronic and elastic properties with the Ge substitution to Si atoms in Ti₃SiC₂.     

Heat capacity of CeIrSi3 under pressure

We measured the heat capacity of CeIrSi₃ (100 mK<T<6 K) under high pressure up to P=1.38 GPa. The measurements have been used a quasiadiabatic method utilizing a CuBe piston-cylinder pressure cell in a dilution refrigerator. At 0 GPa, a sharp anomaly which indicates the antiferromagnetically transition is observed at T_N=5 K. T_N decreases monotonically with increasing pressure up to P=1.38 GPa. The magnetic entropy is released below T_N only 19% of R ln 2 at 0 GPa. And the magnetic entropy decreases with increasing pressure up to 1.38 GPa, 64% compared to that at 0 GPa.     

高压下不同结构的Zr3N4合金结构和弹性性质的第一性原理计算

用基于平面波赝势密度泛函理论的第一性原理方法研究了高压下空间群为I4-3D,Pna21和Pnam的三种结构的Zr3N4的弹性性质,获得了这三种物质在高压下的体弹模量B,剪切模量G,杨氏模量E,泊松比,B/G等力学性质。结果表明,在高压下这三种结构的Zr3N4都是保持力学稳定的,且这三种结构的Zr3N4在各个方向的可压缩性是各向异性的。三种结构的Zr3N4对应的体弹模量,剪切模量和杨氏模量都是随着压强的增大而增大。其中空间群为I4-3D的Zr3N4有最好的抵抗体积压缩,变形和线性压缩的能力。泊松比和B/G的数据表明三种结构在此压强范围内均有较好的延展性,高压下空间群为Pna21和Pnam的Zr3N4延展性更好。研究结论对理论研究和实验有一定参考价值。     

High pressure and temperature study of hydrogen storage material BH3NH3 from ab initio calculations

We report on BH₃NH₃, which is material considered promising to use as hydrogen storage, using density functional theory with generalized gradient approximation (GGA). We study the phase transition of BH₃NH₃ at high pressure and temperature. Our observed phase transition of BH₃NH₃ from body-centered tetragonal to orthorhombic at supports the recent and earlier studies. We observe the phase transformation of BH₃NH₃ at , which is in good agreement with experimental value. Specifically, we predict the phase transition at to be orthorhombic to body-centered tetragonal on the basis of our first principles calculations.     

Sulfur-catalyzed phase transition in MoS2 under high pressure and temperature

We report phase transition and stability of MoS₂ with and without the presence of sulfur melt under high-pressure and high-temperature conditions. Rhombohedral (3R) phase is found to be a high-temperature phase of MoS₂ at high pressures. Excess sulfur melt catalyzes the hexagonal (2H) to rhombohedral (3R) phase transformation and lowers the conversion temperature by more than 280 K. Boundary between 2H and 3R phases has been delineated with a negative slope. Based on experimental observations, sulfur-catalyzed 2H→3R transformation mechanisms are proposed involving atomic exchange between MoS₂ and sulfur, which is different from the case of without excess sulfur that proceeds through rotation and translation of the S–Mo–S sandwich layers.     

Ionic to electronic dominant conductivity in Al2(WO4)3 at high pressure and high temperature

Electrical conduction and crystal structure of Al₂(WO₄)₃ at 400 °C have been studied as a function of pressure up to 5.5 GPa using impedance methods and synchrotron radiation X-ray diffraction, respectively. AC impedance spectroscopy and DC polarization measurements reveal an ionic to electronic dominant transition in electrical conductivity at a pressure as low as 0.9 GPa. Conductivity increases with pressure and reaches a maximum at 4.0 GPa, where the conductivity value is 5 orders of magnitude greater than the 1 atm value. Upon decompression, the conductivity retains the maximum value until the sample is cooled at 0.5 GPa. The high pressure-temperature X-ray diffraction results show that the lattice parameters decrease as pressure increases and the crystal structure undergoes an orthorhombic to tetragonal-like transformation at a pressure ∼3.0 GPa. The change of conduction mechanism from ionic to electronic may be explained by means of pressure-induced valence change of W⁶⁺→W⁵⁺, which results in electron transfer between W⁵⁺-W⁶⁺ sites at high pressure.     

Thermoelectric properties of Bi0.9Sb0.1 prepared by high pressure

The Bi_0.9Sb_0.1 powders were prepared by mechanical alloying and then pressed under 6 GPa at different pressing temperatures. X-ray diffraction spectra showed that the single phase was formed. The nanostructure of grain was observed by bright-field imaging. Electrical conductivity, Seebeck coefficient, and thermal conductivity had been investigated in the temperature range of 80-300 K. The absolute Seebeck coefficient value of 120.3 μV/K was measured at 130 K. The figure-of-merit reached a maximum value of 0.90×10⁻³ K⁻¹ at 140 K.     

Structural, high pressure and elastic properties of transition metal monocarbides: A FP-LAPW study

The structural, elastic and thermal properties of four transition metal monocarbides ScC, YC (group III), VC and NbC (group V) have been investigated using full potential linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation (GGA) both at ambient and high pressure. We predict a B₁ to B₂ structural phase transition at 127.8 and 80.4 GPa for ScC and YC along with the volume collapse percentage of 7.6 and 8.4%, respectively. No phase transition is observed in case of VC and NbC up to pressure 400 and 360 GPa, respectively. The ground state properties such as equilibrium lattice constant (a₀), bulk modulus (B) and its pressure derivative (B′) are determined and compared with available data. We have computed the elastic moduli and Debye temperature and report their variation as a function of pressure.     

Techniques for measuring the elastic wave velocities of melts and partial molten systems under high pressure conditions

The Earth's deep interior is accessible only by indirect methods, first and foremost seismological studies. The interpretation of these seismic data and the corresponding numerical modelling require measurements of the elastic properties of representative Earth materials under experimental simulated in situ pressure-temperature conditions. Various experimental techniques for velocity measurements under crustal and mantle conditions and the results are described.     

First-principles calculations for elastic properties of OsB2 under pressure

The structure, elastic properties and elastic anisotropy of orthorhombic OsB₂ are investigated by density functional theory method with the ultrasoft pseudopotential scheme in the frame of the generalized gradient approximation (GGA) as well as local density approximation (LDA). The obtained structural parameters, elastic constants, elastic anisotropy and Debye temperature for OsB₂ under pressure are consistent with the available experimental data and other theoretical results. It is found that the elastic constants, bulk modulus and Debye temperature of OsB₂ tend to increase with increasing pressure. It is predicted that OsB₂ is not a superhard material from our calculations.     

High pressure phase transition and elastic properties of thorium chalcogenides

The structural and elastic properties of thorium chalcogenides at high pressure, have been investigated using a suitable inter-ionic potential. The calculated equation of state, phase transition pressures for B1-B2 transition and bulk moduli for ThX (X=S,Se,Te) compounds agree well with the experimental results. ThTe, which crystallizes in the CsCl structure, does not show any structural transition up to 48 GPa. The present analysis does not show any anomalous features in elastic properties arising from ‘f’ electrons.     

First-principles calculations for elastic properties of rutile TiO2 under pressure

This paper studies the equilibrium structure parameters and the dependences of the elastic properties on pressure for rutile TiO2 by using the Cambridge Serial Total Energy Package （CASTEP） program in the frame of density functional theory. The obtained equilibrium structure parameters, bulk modulus B0 and its pressure derivative B′0 are in good agreement with experiments and the theoretical results. The six independent elastic constants of rutile TiO2 under pressure are theoretically investigated for the first time. It is found that, as pressure increases, the elastic constants C11, C33, C66, C12 and C13 increase, The variation of elastic constant C44 is not obvious and the anisotropy will weaken.     

First-principles study on electronic structure and elastic properties of Fe16N2

We have performed first-principles study on structural stability, elastic properties and electronic structure of Fe₁₆N₂ by applying LSDA+U method. The calculated values of formation energy and reaction enthalpy for decomposition reaction indicate that Fe₁₆N₂ is a thermodynamically stable phase at the ground state. The six independent elastic constants are derived and the bulk modulus, Young's modulus, shear modulus, and Poisson's ratio are determined as 180 GPa, 199 GPa, 76 GPa and 0.32, respectively. The elastic constants meet all the mechanical stability criteria. The ductility of Fe₁₆N₂ is predicted by Pugh's criterion. The strong bonding between Fe and N atoms results in high values of elastic constants C₁₁ and C₃₃, and contributes to the strengthening of the Fe₁₆N₂ structural stability. The total and partial densities of states (DOS) suggest the existence of hybridization between N-p and Fe-d bands. The position of the Fermi level in DOS curve implies that Fe₁₆N₂ is a metastable phase.     

Enhancement of thermoelectric performance with pressure in Ce0.8Fe3CoSb12.1

Transport properties (resistivity, thermal conductivity, and Seebeck coefficient) and sound velocities have been determined for the skutterudite Ce_0.8Fe₃CoSb_12.1 with pressure up to 14 GPa. From these measurements, high pressure anomalous features were found in all transport properties. By correlating these with results from previous x-ray work, it has been determined that there is likely an electronic topological transition in this material induced by pressure. This is possibly due to the known pressure variation of valence in the void-filling Ce atom and has been found to induce an improved figure of merit at higher pressures, which shows a nearly two-fold increase with applied pressure. At higher pressures, it was determined that this anomalous behavior is suppressed and is possibly induced by insertion of Sb from the cage into the remaining central voids of the structure, similar to that seen in the CoSb₃ parent compound.     

First-principles calculations of electronic, optical and elastic properties of ZnAl2S4 and ZnGa2O4

The optimized crystal structures, band structures, partial and total densities of states (DOS), dielectric functions, refractive indexes and elastic constants for ZnAl₂S₄ and ZnGa₂O₄ were calculated using the CASTEP module of Materials Studio package. Pressure effects were modeled by performing these calculations for different values of external hydrostatic pressure up to 50 GPa. Obtained dependencies of the unit cell volume on pressure were fitted by the Murnaghan equation of state, and the relative changes of different chemical bond lengths were approximated by quadratic functions of pressure. Variations of applied pressure were shown to produce considerable re-distribution of the electron densities around ions in both crystals, which is evidenced in different trends for the effective Mulliken charges of the constituting ions and changes of contour plots of the charge densities. The longitudinal and transverse sound velocities and Debye temperatures for both compounds were also estimated using the calculated elastic constants.     

Phase transition of Ta2NiO6 with the trirutile-type structure under high pressure and high temperature

High-pressure phase transition of Ta₂NiO₆ with the trirutile-type structure was investigated from the viewpoint of crystal chemistry. A new quenchable high-pressure phase was found in the pressure range higher than 7 GPa and 900°C. The high-pressure phase has an orthorhombic cell (a=4.797(1) Å, b=5.153(2) Å and c=14.85(1) Å and space group; Abm2), and it is more dense by 9.6% than the trirutile-structured phase. Infrared spectra of the trirutile-type phase and the high-pressure phase show that Ni²⁺ ions in the high-pressure phase are still in octahedral sites. The crystal structure of the high-pressure phase is considered as a cation-ordering trifluorite-type structure, which can be stabilized by a crystal field effect of Ni²⁺ ions.