Elasticity and thermodynamic properties of RuB2 under pressure

First-principles calculations of the crystal structure and the elastic properties of RuB₂ have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The elastic constants c_ij, the aggregate elastic moduli (B, G, E), Poisson's ratio, and the elastic anisotropy with pressure have been investigated. Through the quasi-harmonic Debye model considering the phonon effects, the isothermal bulk modulus, the thermal expansions, Grüneisen parameters, and Debye temperatures depending on the temperature and pressure are obtained in the whole pressure range from 0 to 60 GPa and temperature range from 0 to 1100 K as well as compared to available data.     

EuS高压相变及其弹性和热力学性质

采用平面波赝势密度泛函理论,利用第一性原理的方法研究了EuS的晶体结构、高压相变以及弹性性质．计算结果和实验值以及前人利用不同计算模型得到的理论值相吻合．研究了EuS的弹性常数、弹性模量和弹性的各向异性等力学性质随压力变化的趋势．同时研究了泊松比、德拜温度及纵波和横波的弹性波速随压力的变化趋势．基于德拜模型,进而研究了EuS在0～800 K和0～60 GPa下相变前后的热膨胀系数、热熔、Grüneisen参数等热力学性质．     

The thermodynamic properties of lithium peroxide,Li2O2

Recently, Cota et al. gave an improved crystal structure of lithium peroxide. For the sake of investigation of the thermal properties for this structure of Li₂O₂, the Debye model is adopted in our work. This model combines with the ab initio calculations within local density approximation (LDA) using pseudopotentials and a planewave basis in the framework of density functional theory (DFT), and it takes into account the phononic effects within the quasi-harmonic approximation. We find that our calculated lattice constant using this model is in excellent agreement with the data from Cota et al. Based on the first principles study and the Debye model, the thermal properties including the equation of state, the Debye temperature, the heat capacity and the thermal expansion are obtained in the whole pressure range from 0 to 10 GPa and temperature range from 0 to 500 K.     

Structural and thermodynamic properties of AlB2 compound

We employ a first-principles plane wave method with the relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter (HGH) scheme in the frame of DFT to calculate the equilibrium lattice parameters and the thermodynamic properties of AlB₂ compound with hcp structure. The obtained lattice parameters are in good agreement with the available experimental data and those calculated by others. Through the quasi-harmonic Debye model, obtained successfully are the dependences of the normalized lattice parameters a/a₀ and c/c₀ on pressure P, the normalized primitive cell volume V/V₀ on pressure P, the variation of the thermal expansion α with pressure P and temperature T, as well as the Debye temperature \Theta_D and the heat capacity C_V on pressure P and temperature T.     

高压下TiC的弹性、电子结构及热力学性质的第一性原理计算

利用基于密度泛函理论的第一性原理平面波赝势方法 并结合准谐徳拜模型研究了NaCl结构的TiC在高压下的弹性性质、电子结构和热力学性质. 计算所得零温零压下的晶格常数、体弹模量及弹性常数与实验值符合得很好. 零温下弹性常数和弹性模量随压强增大而增大. 通过态密度和电荷密度的分析, Ti-C键随压强增大而增强. 运用准谐德拜模型, 成功计算了TiC在高温高压下的体弹模量、熵、热膨胀系数、徳拜温度、 Grüneisen参数和比热容. 结果表明压强对体弹模量、热膨胀系数和徳拜温度的影响大于温度对其的影响. 热容随着压强升高而减小, 在高温高压下, 热容接近Dulong-Petit极限.     

The structural, elastic and thermodynamic properties of thorium tetraboride

The structural, elastic and thermodynamic properties of thorium tetraboride (ThB₄) have been investigated by using first-principles plane-wave pseudopotential density functional theory with generalized gradient approximation. The behaviors of structural parameters under 0-70 GPa hydrostatic pressure are studied by means of Broyden, Fletcher, Goldfarb, and Shanno (BFGS) geometry optimization scheme. By using the stress-strain method, single crystal elastic constants are calculated to test the mechanical stability of the crystal structure and to determine mechanical properties such as bulk modulus at each pressure. However, in order to study the thermodynamic properties of ThB₄, the quasi-harmonic Debye model is used. Then, the dependencies of bulk modulus, heat capacities, thermal expansions, Grüneisen parameters and Debye temperatures on the temperature and pressure are obtained in the whole pressure range 0-70 GPa and temperature range 0-1500 K.     

First-principles study of the structural and thermodynamic properties of AsNMg3 antiperovskite

The structural and elastic properties of the antiperovskite semiconductor AsNMg₃ are investigated using the full-potential linearized augmented plane wave plus local orbital (FP-LAPW+lo) method within the generalized gradient in the frame of the density functional theory. The ground state properties such as lattice constant, bulk modulus, pressure derivative of the bulk modulus and elastic constants are in good agreement with numerous experimental and theoretical data. Through the quasi-harmonic Debye model, in which the phononic effects are considered, we have obtained successfully the thermodynamic properties such as the thermal expansion coefficient, Debye temperature and specific heats in the whole pressure range from 0 to 30 GPa and temperature range from 0 to 1200 K.     

Thermodynamic properties of OsB under high temperature and high pressure

The energy-volume curves of OsB have been obtained using the first-principles plane-wave ultrasoft-pseudopotential density functional theory (DFT) within the generalized gradient approximation (GGA) and local density approximation (LDA). Using the quasi-harmonic Debye model we first analyze the specific heat, the coefficients of thermal expansion as well as the thermodynamic Grüneisen parameter of OsB in a wide temperature range at high pressure. At temperature 300 K, the coefficients of thermal expansion α_V by LDA and GGA calculations are 1.67×10⁻⁵ 1/K and 2.01×10⁻⁵ 1/K, respectively. The specific heat of OsB at constant pressure (volume) is also calculated. Meanwhile, we find that the Debye temperature of OsB increases monotonically with increasing pressure. The present study leads to a better understanding of how the OsB materials respond to pressure and temperature.     

硅酸镁钙钛矿弹性及热力学特性的第一性原理计算

基于密度泛函理论的第一性原理计算,结合准谐德拜模型研究了高压下硅酸镁钙钛矿的弹性及热力学特性. 计算得到的物态方程数据、热容、热膨胀系数等在宽广的温度和压力范围与实验结果及其他理论计算结果吻合. 根据有限应变理论计算了硅酸镁钙钛矿的弹性常数,并讨论了杨氏模量、泊松比、德拜温度、晶体各向异性随压力的变化.     

ZrV2结构、弹性和热力学性质的第一性原理计算

采用基于密度泛函理论的第一性原理平面波赝势方法研究ZrV₂的晶体结构和弹性,利用准谐Debye模型计算在不同温度(T=0~1200 K)和不同压强(P=0~20 GPa)下ZrV₂的热力学性质,包括弹性模量与压强,热熔与温度,以及热膨胀系数与温度和压力的关系.结果表明:计算的ZrV₂晶格常数与实验值符合较好,晶体材料的弹性常数随着压力增加而增加;在一定温度下,相对体积、热熔随着压强的增加而减小,德拜温度、弹性模量随着压强的增加而增加,且高压下温度对ZrV₂热膨胀系数的影响小于压强的影响.     

First-principles calculations of structural and thermodynamic properties of BeB2 compound

The lattice parameter bulk modulus and pressure derivative of BeB2 are calculated by using the Cambridge Serial Total Energy Package （CASTEP） program in the frame of density function theory. The calculated results agree well with the average experimental data and other theoretical results. Through the quasi-harmonic Debye model, the dependences of the normalized lattice parameters a/ao, c/c0 and the normalized primitive cell volume V/Vo on pressure P, the variation of the thermal expansion coefficient ~ with pressure P and temperature T, as well as the dependences of the heat capacity Cv on pressure P and temperature T are obtained systematically.     

Elastic and thermodynamic properties of CaB6 under pressure from first principles

The elastic and thermodynamic properties of CsCl-type structure CaB₆ under high pressure are investigated by first-principles calculations based on plane-wave pseudopotential density functional theory method within the generalized gradient approximation (GGA). The calculated lattice parameters of CaB₆ under zero pressure and zero temperature are in good agreement with the existing experimental data and other theoretical data. The pressure dependences of the elastic constants, bulk modulus B (GPa), and its pressure derivative B′, shear modulus G, Young's modulus E, elastic Debye temperature Θ_B, Zener's anisotropy parameter A, Poisson ratios σ, and Kleinmann parameter ζ are also presented. An analysis for the calculated elastic constants has been made to reveal the mechanical stability of CaB₆ up to 100 GPa. The thermodynamic properties of the CsCl-type structure CaB₆ are predicted using the quasi-harmonic Debye model. The pressure-volume-temperature (P-V-T) relationship, the variations of the heat capacity C_V, Debye temperature Θ_D, and the thermal expansion α with pressure P and temperature T, as well as the Grüneisen parameters γ are obtained systematically in the ranges of 0-100 GPa and 0-2000 K.     

Thermodynamic properties of ZnSe under pressure and with variation in temperature

The thermodynamic properties of Zn Se are obtained by using quasi-harmonic Debye model embedded in Gibbscode for pressure range 0–10 GPa and for temperature range 0–1000 K. Helmholtz free energy, internal energy, entropy,Debye temperature, and specific heat are calculated. The thermal expansion coefficient along with Gruneisen parameter are also calculated at room temperature for the pressure range. It is found that internal energy is pressure dependent at low temperature, whereas entropy and Helmholtz free energy are pressure sensitive at high temperature. At ambient conditions,the obtained results are found to be in close agreement to available theoretical and experimental data.     

First-principles calculations of elasticity and thermodynamic properties of LaNi5 crystal under pressure

This paper investigates the equilibrium lattice parameters, heat capacity, thermal expansion coefficient, bulk modulus and its pressure derivative of LaNi 5 crystal by using the first-principles plane-wave pseudopotential method in the GGA-PBE generalized gradient approximation as well as the quasi-harmonic Debye model. The dependences of bulk modulus on temperature and on pressure are investigated. For the first time it analyses the relationships between bulk modulus B and temperature T up to 1000 K, the relationship between bulk modulus B and pressure at different temperatures are worked out. The pressure dependences of heat capacity C v and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperatures of LaNi 5 at different pressures are successfully obtained. The calculated results are in excellent agreement with the experimental data.     

First-principles study of structural and thermodynamic properties of osmium

We employ the first-principles plane wave pseudopotential density functional theory method to calculate the equilibrium lattice parameters of osmium and the thermodynamic properties of hcp structure osmium. The obtained lattice parameters are in good agreement with the experimental data investigated up to 58.2 GPa using radial X-ray diffraction (RXRD) together with lattice strain theory in a diamond-anvil cell and the available theoretical data of others. Through the quasi-harmonic Debye model, the dependencies of the normalized lattice parameters a/a₀ and c/c₀ on pressure P, the normalized primitive volume V/V₀ on pressure P, the Debye temperature Θ_D and the heat capacity C_V on pressure P and temperature T, as well as the variation of the thermal expansion α with temperature and pressure are obtained successfully.     

Effects of temperature and pressure on thermodynamic properties of Cd0.25Zn0.75 alloy

Thermodynamic properties of Cd_0.25Zn_0.75Se alloy are studied using quasi harmonic model for pressure range of 0 GPa-10 GPa and temperature range 0 K-1000 K. The structural optimization is obtained by self-consistent field calculations and full-potential linearized muffin-tin orbital method with GGA+U as an exchange correlation functional where U=2.3427 eV is Hubbard potential. The effects of temperature and pressure on bulk modulus, Helmholtz free energy, internal energy, entropy, Debye temperature, Grüneisen parameter, thermal expansion coefficient, and heat capacities of the material are observed and discussed. The bulk modulus, Helmholtz free energy, and Debye temperature are found to be decreased on increasing temperature while there is an increasing behavior with rise of the pressure. Whereas the internal energy has increasing trend with the rise in temperature and it almost remains insensitive to pressure. The entropy of the system increases (decreases) with rise of pressure (temperature).     

First-principles calculation of elastic and thermodynamic properties of Ni2MnGa Heusler alloy

The equilibrium lattice parameter, heat capacity, thermal expansion coefficient and bulk modulus of Ni 2 MnGa Heusler alloy are successfully obtained using the first-principles plane-wave pseudopotential (PW-PP) method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus B and temperature T up to 800 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure and decreases with increasing temperature. The pressure dependence of heat capacity C v and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperature of Ni 2 MnGa is determined from the non-equilibrium Gibbs function. Our calculated results are in excellent agreement with the experimental data.     

Theoretical studies of elastic and thermodynamic properties of cubic B20 CoSi

The full potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory as implemented in the WIEN2k package is applied successfully to the study of the equilibrium lattice parameter and the elastic constants of the cubic B20 structural CoSi. The quasi-harmonic Debye model, in which the phononic effects are considered, is used to investigate the thermodynamic properties of B20 CoSi. Young's modulus and Poisson ratio are obtained from the calculated elastic constants and compared with the available data. The pressure and temperature dependence of the volume, the bulk modulus, the thermal expansion coefficient, the heat capacity and the Debye temperature are successfully obtained in the whole pressure range from 0 to 40 GPa and temperature range from 0 to 1400 K .     

Mechanical,electronic and thermodynamic properties of TE-C36 under high pressure

The structural parameters, mechanical, electronic and thermodynamic properties of TE-C36 under high pressure were calculated via the density functional theory in combination with the quasi-harmonic Debye model. The results show that the pressure has significant effects on the equilibrium structure parameters, mechanical, electronic and thermodynamic properties of TE-C36. The obtained ground state structural parameters are in good agreement with previous theoretical results. The mechanically and dynamically stable under pressure were confirmed by the calculated elastic constants and phonon dispersion spectra. The elastic constants, elastic modulus, B/G ratio, Poisson’s ratio and Vicker’s hardness were determined in the pressure range of 0–100?GPa. The elastic anisotropy of TE-C36 under pressure are also determined in detail. The electronic structure calculations reveal that TE-C36 remains a direct band gap semiconductor when the pressure changes from 0 to 100?GPa, and the band gap decreases with increasing pressure. Furthermore, the pressure and temperature dependence of thermal expansion coefficient, heat capacity and Debye temperature are predicted in a wide pressure (0–90?GPa) and temperature (0–2500?K) ranges. The obtained results are expected to provide helpful guidance for the future synthesis and application of TE-C36.     

Structural, electronic, elastic and thermal properties of Mg2Si

First-principles calculations of the lattice parameter, electron density maps, density of states and elastic constants of Mg₂Si are reported. The lattice parameter is found to differ by less than 0.8% from the experimental data. Calculations of density of states and electron density maps are also performed to describe the orbital mixing and the nature of chemical bonding. Our results indicate that the bonding interactions in the Mg₂Si crystal are more covalent than ionic. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the plane-wave pseudopotential method, is applied to study the elastic, thermal and vibrational effects. The variations of bulk modulus, Grüneisen parameter, Debye temperature, heat capacity C_v, C_p and entropy with pressure P up to 7 GPa in the temperature interval 0-1300 K have been systemically investigated. Significant differences in properties are observed at high pressure and high temperature. When T<1300 K, the calculated entropy and heat capacity agree reasonably with available experimental data. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic Debye model is an efficient approach to simulate the behavior of Mg₂Si.