High pressure effect on structural and mechanical properties of some LnO (Ln=Sm, Eu, Yb) compounds

The structural and mechanical properties of LnO (Ln=Sm, Eu, Yb) compounds have been investigated using a modified interionic potential theory, which includes the effect of Coulomb screening. We predicted a structural phase transition from NaCl (B₁)- to CsCl (B₂)-type structure and elastic properties in LnO compounds at very high pressure. The anomalous properties of these compounds have been correlated in terms of the hybridisation of f-electrons of the rare earth ion with conduction band and strong mixing of f-states of lanthanides with the p-orbital of neighbouring chalcogen ion. For EuO, the calculated transition pressure, bulk modulus and lattice parameter are close to the experimental data. The nature of bonds between the ions is predicted by simulating the ion-ion (Ln-Ln and Ln-O) distances at high pressure. The second order elastic constants along with shear modulus and Young's modulus, elastic anisotropy and Poisson's ratio are also presented for these oxides.     

高压下RhB的相变、弹性性质、电子结构及硬度的第一性原理计算

采用密度泛函理论中的赝势平面波方法系统地研究了高压下RhB的结构相变、弹性性质、电子结构和硬度.分析表明,RhB在25.3 GPa时从anti-NiAs结构相变到FeB结构,这两种结构的弹性常数、体弹模量、剪切模量、杨氏模量和弹性各向异性因子的外压力效应明显.电子态密度的计算结果显示,这两种结构是金属性的,且费米能级附近的峰随着压强的增大向两侧移动,赝能隙变宽,轨道杂化增强,共价性增强,非局域化更加明显.此外,硬度计算结果显示,anti-NiAs-RhB的金属性比较弱,有着较高的硬度,属于硬质材料.     

Theoretical investigations on structural, elastic and electronic properties of thallium halides

Theoretical investigations on structural, elastic and electronic properties, viz. ground state lattice parameter, elastic moduli and density of states, of thallium halides (viz. TlCl and TlBr) have been made using the full potential linearized augmented plane wave method within the generalized gradient approximation (GGA). The ground state lattice parameter and bulk modulus and its pressure derivative have been obtained using optimization method. Young's modulus, shear modulus, Poisson ratio, sound velocities for longitudinal and shear waves, Debye average velocity, Debye temperature and Grüneisen parameter have also been calculated for these compounds. Calculated structural, elastic and other parameters are in good agreement with the available data.     

First-principles studies of structural,mechanical, electronic,optical properties and pressure-induced phase transition of CuInO2 polymorph

Using the first-principles density-functional theory within the generalized gradient approximation (GGA), we have investigated the structural, elastic, mechanical, electronic, and optical properties and phase transition of CuInO₂. Structural parameters including lattice constants and internal parameter, pressure effects and phase transition pressure were calculated. We have obtained the elastic coefficients, bulk modulus, shear modulus, Young's modulus and Poisson's ratio. We find that two phases of CuInO₂ are indirect band gap semiconductors (F–Γ and H–Γ for 3R and 2H, respectively). Optical properties, including the dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, loss function and optical conductivity have been obtained for radiations of up to 30 eV.     

High pressure behavior of alkaline earth tellurides

We have predicted high pressure structural behavior and elastic properties of alkaline earth tellurides (AETe; AE = Ca, Sr, Ba) by using two body interionic potential approach with modified ionic charge (Z _m e). This method has been found quite satisfactory in case of the rare earth compounds. The equation of state curve, structural phase transition pressure from NaCl (B1) to CsCl (B2) phase and associated volume collapse at transition pressure of alkaline earth tellurides (AETe) obtained from this approach, so have been compared with experimentally measured data reveal good agreement. We have also investigated bulk modulus, second and third order elastic constants and pressure derivatives of second order elastic constants at ambient pressure which shows predominantly ionic nature of these compounds. First time, we have calculated the Poisson ratio, Young and Shear modulus of these compounds.      

First-principles lattice dynamical study of ScAs and ScSb at zero and high pressure

A first-principles pseudopotential method is used to investigate the structural and elastic properties of ScAs and ScSb in their ambient B1(NaCl) and in high pressure B2 (CsCl) phases and phonon structures at zero and close to phase transition pressure. The calculated lattice constants, static bulk modulus, first order pressure derivative of the bulk modulus and the elastic constants are reported in B1 and B2 structures and compared with available experimental and other theoretical results. The phonon properties of these two compounds are compared among themselves which reveal that these compounds are predominantly metallic, due to degeneracy of optical frequencies at the zone centre. At high pressure, near the B1 to B2 transition, the LA mode at X-point softens leading to structural instability.     

Structural,mechanical and thermal properties of some Holmium Pnictides under pressure: A theoretical approach

The high pressure structural, elastic and thermal properties of holmium pnictides HoX (X=N, P, As and Bi) were investigated theoretically by using an inter-ionic potential theory with modified ionic charge parameter. We have predicted a structural phase transition from NaCl (B₁) to CsCl (B₂)-type structure at pressure of 139 GPa for HoN, 52 GPa for HoP, 44 GPa for HoAs and 26 GPa for HoBi. Other properties, such as lattice constant, bulk modulus, cohesive energy, second and third-order elastic constants were calculated and compared with the available experimental and theoretical data. In order to gain further information the brittle behaviour of these compounds was observed. Some other properties like Shear modulus (G), Young's modulus (E), Poisson's ratio (ν), anisotropy factor (A), sound velocities, Debye temperature (θ_D) were calculated. The variation of elastic constants (C₁₁ and C₄₄) and Debye temperature (θ_D) with pressure was also presented.     

Ab initio study of the structural,electronic and elastic properties of AgSbTe2, AgSbSe2, Pr3AlC,Ce3AlC,Ce3AlN,La3AlC and La3AlN compounds

In this paper, we study the structural, electronic and elastic properties of the ternary AgSbTe₂, AgSbSe₂, Pr₃AlC, Ce₃AlC, Ce₃AlN, La₃AlC and La₃AlN compounds using the full-potential linearized augmented plane wave (FP-LAPW) scheme and the pseudopotential plane wave (PP-PW) scheme in the frame of generalized gradient approximation (GGA). Results are given for the lattice parameters, bulk modulus, and its pressure derivative. The calculated lattice parameters are in good agreement with experimental results. We have determined the full set of first-order elastic constants, shear modulus, Young's modulus and Poisson's ratio of these compounds. Also, we have presented the results of the band structure, densities of states, it is found that this compounds metallic behavior, and a negative gap Г→R for Pr₃AlC. The analysis charge densities show that bonding is of covalent–ionic and ionic nature for AgSbSe₂ and AgSbTe₂ compounds.     

Electronic,elastic and dynamical properties of MgSe under pressure: rocksalt and iron silicide phase

The electronic, elastic and dynamical properties of MgSe in the rocksalt (B1) and iron silicide (B28) phase and the effects of pressure on these properties are investigated using first-principles method. The calculated electronic band structure indicates that the B1 phase of MgSe presents an indirect band-gap feature and the band gaps initially increase with pressure and subsequently decrease upon compression. Remarkably, an indirect-to-direct band-gap transition has been observed at the phase transition pressure. The elastic constants, bulk modulus, shear modulus, Young’s modulus, elastic anisotropy and B/G ratio of MgSe in the B1 and B28 phase at high pressure have also been investigated. The bulk modulus, shear modulus and Young’s modulus all increase monotonously with the increasing of pressure for the B1 and B28 phase of MgSe. The calculated phonon frequencies of the B1 phase at zero pressure agree well with available theoretical results. And the transverse acoustic phonon TA(X) mode of this phase completely softening to zero at 82 GPa. The phonon curves of the B28 phase under pressure have also been successfully investigated.     

Pseudo-potential calculations of structural and elastic properties of spinel oxides ZnX2O4 (X=Al,Ga, In) under pressure effect

First principle calculations of structural and elastic properties of ZnAl₂O₄, ZnGa₂O₄ and ZnIn₂O₄ compounds are presented, using the pseudo-potential plane-waves approach based on density functional theory, within the generalized gradient approximation GGA. The lattice constants and internal parameters are in good agreement with the available experimental results. Young's modulus, Poisson ratio, bulk modulus, elastic constants and their pressure dependence are also calculated. As the experimental elastic constants are not available hence our results were only compared with the available theoretical values obtained at equilibrium volume.     

A first-principle study of the structural, elastic, lattice dynamical and thermodynamic properties of PrX (X=P, As)

The structural, phase transition, elastic, lattice dynamic and thermodynamic properties of rare-earth compounds PrP and PrAs with NaCl (B1), CsCl (B2), ZB (B3), WC (B_h) and CuAu (L1₀) structures are investigated using the first principles calculations within the generalized gradient approximation (GGA). For the total-energy calculation, we have used the projected augmented plane-wave (PAW) implementation of the Vienna Ab-initio Simulation Package (VASP). Specifically, some basic physical parameters, e.g. lattice constants, bulk modulus, elastic constants, shear modulus, Young's modulus and Poison's ratio, are predicted. The obtained equilibrium structure parameters are in excellent agreement with the experimental and theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacity and Debye temperature are calculated in wide pressure and temperature ranges. The phonon dispersion curves and corresponding one-phonon density of states (DOS) for both compounds are also computed in the NaCl (B1) structure.     

Structural,mechanical, thermodynamics properties and phase transition of FeVSb

The structural, mechanical, thermodynamics properties and phase transition of FeVSb are investigated extensively using the first principle calculations and the quasi-harmonic Debye model. From the calculated elastic constants of cubic FeVSb, some other mechanical quantities, such as bulk modulus and Poisson's ratio, are drawn. Surprisingly, it is found that almost all these mechanical quantities are larger than those of CoVSb (Bo Kong et al., J. Alloys Compd. 509 (2011) 2611); the obtained corresponding transition pressure from fcc to hcp is also larger than that of CoVSb. For these distinctions, their complete different electronic and magnetic behaviors in their cubic structures may be responsible. However, in their hexagonal structures, atomic configurations are similar in terms of the analysis of both the ground-state structure and enthalpy–pressure curves. It is also shown that the elastic instability of cubic FeVSb does not appear with pressure up to 120 GPa and should not be a reason for the pressure-induced phase transition. In addition, heat capacity, Debye temperatures, and so on are obtained successfully for cubic FeVSb under the quasi-harmonic Debye model. Furthermore, we attempt to explore the phase diagram of FeVSb with the model.     

Structural and elastic properties of TiN under high pressure

We have investigated the structural and elastic properties of TiN at high pressures by the first-principles plane wave pseudopotential density functional theory method at applied pressures up to 45.4 GPa. The obtained normalized volume dependence of the resulting pressure is in excellent agreement with the experimental data investigated using synchrotron radial x-ray diffraction (RXRD) under nonhydrostatic compression up to 45.4 GPa in a diamond-anvil cell. Three independent elastic constants at zero pressure and high pressure are calculated. From the obtained elastic constants, the bulk modulus, Young's modulus, shear modulus, acoustic velocity and Debye temperature as a function of the applied pressure are also successfully obtained.     

First principles study of structural,elastic and electronic structural properties of strontium chalcogenides

Structural, elastic and electronic properties of strontium chalcogenides SrX (X = O, S and Se) in the B1 (NaCl) and B2 (CsCl) phases were investigated in the present work. The calculations were performed using density functional theory (DFT) within generalized gradient approximation (GGA) using scalar relativistic Vanderbilt-type ultrasoft pseudopotentials. Results for structural properties of both phases, the pressure at which transition from B1 to B2 phase occurs and the volume compression ratio for each compound were reported. Elastic properties of the B1 phase of these compounds, such as elastic constants C₁₁, C₁₂, and C₄₄, shear modulus (G), Young's modulus (E), Poisson's ratio (σ), Kleinman parameter (ξ), and anisotropy factor (A) were also calculated at ambient conditions. The band gaps and density of states were studied too for the B1 structure of these compounds. The present results were compared with the available experimental and other theoretical results, and found to be in satisfactory agreement with them.     

Structural phase transition and elastic properties of Curium and Uranium monobismuthides under pressure effect

A density functional (DFT) calculations of the structural, elastic and high pressure properties of the cubic XBi (X=U,Cm) compounds, has been reported using the full potential linear muffin-tin orbital (FP-LMTO) method. In this approach the local density approximation (LDA) is used for the exchange-correlation (XC) potential. Results are given for lattice constant, bulk modulus and its pressure derivatives. The pressure transitions at which these compounds undergo structural phase transition from NaCl-type (B1) to CsCl-type (B2) phase were found to be in good agreement with the available theoretical results. We have determined the elastic constants C₁₁, C₁₂, C₄₄ and their pressure dependence which have not been established experimentally or theoretically.     

Electronic structure,elastic and thermodynamic properties of α-phase Na3N under pressure from first principles

The structural, electronic, elastic and thermodynamic properties of α-phase Na₃N under pressure are investigated by performing first principles calculations within generalized gradient approximation. The elastic constants, bulk modulus, shear modulus, Young's modulus, and Poisson's ratio dependencies on pressure are also calculated. The thermodynamic properties of the α-phase Na₃N are calculated using the quasi-harmonic Debye model. The dependencies of the heat capacity and the thermal expansion coefficient, as well as the Grüneisen parameter on pressure and temperature are investigated systematically in the ranges of 0–1 GPa and 0–100 K.     

First-principles study of structural,elastic, lattice dynamical and thermodynamical properties of GdX (X = Bi,Sb)

The results are presented of first-principles calculations of the structural, elastic and lattice dynamical properties of GdX (X = Bi, Sb). In particular, the lattice parameters, bulk modulus, phonon dispersion curves, elastic constants and their related quantities, such as Young's modulus, Shear modulus, Zener anisotropy factor, Poisson's ratio, Kleinman parameter, and longitudinal, transverse and average sound velocities, were calculated and compared with available experimental and other theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacities, Grüneisen parameter and Debye temperatures were predicted in wide pressure (0?50 GPa) and temperature ranges (0–500 K). The plane-wave pseudopotential approach to the density-functional theory within the GGA approximation implemented in VASP (Vienna ab initio simulation package) was used in all computations.     

Ab initio investigation into the structural, electronic and elastic properties of AlCu2TM (TM = Ti, Zr and Hf) ternary compounds

The Al-Cu-TM (TM = transition metal) alloy system has attracted great attention for both excellent glass-forming ability and its interesting physical properties. In this work, an investigation into the crystal, electrical and elastic properties of the AlCu₂TM (TM = Ti, Zr, and Hf) compounds has been conducted by first-principles calculations based on density-functional theory. The fully relaxed structure parameters of the AlCu₂TM compounds are in good agreement with previous experimental and other theoretical results. Besides, the cohesive energies of all the AlCu₂TM compounds have been evaluated. The energy band and densities of state of these compounds are also obtained. According to the calculated single crystal elastic constants, all the compounds are mechanically stable. The polycrystalline bulk moduli, shear moduli, Young’s moduli and Poisson’s ratio have been deduced by using Voigt-Reuss-Hill (VRH) approximations. The calculated negative Cauchy pressure and ratio of bulk modulus to shear modulus indicated that the AlCu₂TM compounds are ductile materials. The Debye temperatures of the AlCu₂TM compounds decrease with increasing the TM (Ti, Zr, and Hf) atomic number.     

Ab initio investigation of the structural,elastic and electronic properties of the anti-perovskite TlNCa3

We have investigated the structural, elastic and electronic properties of the anti-perovskite TlNCa₃ using ab initio calculations within the generalized gradient approximation and the local density approximation for the exchange–correlation potential. The lattice constant, bulk modulus, elastic constants and their pressure dependence, energy band structures, density of states and charge density distribution are calculated and analyzed in comparison with the available experimental and theoretical data. The bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, Lamé’s coefficients, average sound velocity and Debye temperature are numerically estimated for ideal polycrystalline TlNCa₃ aggregates in the framework of the Voigt–Reuss–Hill approximation. This is the first theoretical prediction of the elastic constants and their related properties for TlNCa₃ that requires experimental confirmation.     

立方KCaF_3电子、弹性和光学性质的第一性原理研究

基于密度泛函理论和赝势平面波近似法计算研究了立方钙钛矿KCaF_3的弹性、电子和光学性质.基态时,KCaF_3平衡晶格常数、体积弹性模量和实验及其他计算值一致.根据Hooke定律和Christoffel方程,研究了KCaF_3弹性常数Cij、体积弹性模量B、各向同性波速和弹性各向性异性因子随压力的变化关系.从电子能带理论出发,计算得到了KCaF_3电子能带、态密度和Milliken电荷布居数,并对其电子性质进行了详细分析.结果显示:立方钙钛矿KCaF_3为直接带隙绝缘体材料,其禁带宽度为6.22 e V;电荷主要从Ca和K原子向F原子转移;立方钙钛矿KCaF_3属于纯粹的共价型化合物.同时,本文还计算研究了KCaF_3的光学介电函数、吸收系数、复折射率、能量损失谱和反射系数等光学性质.