Analysis of thermodynamic identities for materials under extreme compression

Some basic relationships for materials under extreme compression are analyzed with the help of the calculus of indeterminates. The analysis presented here provides an understanding of the origin of identities and constraints at infinite pressure which are satisfied by all physically acceptable equations of state. These identities involve the bulk modulus and its pressure derivatives, the Grüneisen parameter and its volume derivatives, the thermal expansivity, and the Anderson-Grüneisen parameter. The identity for the third-order Grüneisen parameter in terms of the pressure derivatives of the bulk modulus at extreme compression is valid even if the free-volume parameter changes with pressure.     

Formulation of the third-order Grüneisen parameter at extreme compression

We present a direct method using the basic principles of calculus to derive the expression for the third-order Grüneisen parameter in terms of the pressure derivatives of bulk modulus at extreme compression. The derivation presented here does not depend on the assumptions regarding the values of free-volume parameter and its variation with pressure. The identities used in the present analysis are valid at extreme compression for all physically acceptable equations of state.     

Defects induced melting in alkali halides

In the present paper we study the pressure dependence of melting of NaCl and CsCl crystals. A formulation has been presented for the pressure dependence of melting temperature on the basis of the vacancy model using the expression for the pressure dependence of the volume of Schottky defects from the Roy-Roy equation of state. Values of pressure derivatives of melting temperature have been calculated at elevated pressures to determine the rate of change of melting temperature with increase in pressures using the data of vacancy formation energy and effective volume of Schottky defects. The vacancy model revised in the present study takes into account the variation of bulk modulus with pressure, whereas in the Ksiazek and Gorecki model, it was treated constant. Results for pressure derivative of melting temperature are calculated for the solids under study. The melting curves have also been obtained and found to compare well with results based on molecular dynamics simulation and experimental data reported in recent literature.     

High-pressure and high-temperature bulk modulus of cubic fluorite-type MgF2 from quasi-harmonic Debye model

A detailed theoretical study of the isothermal and adiabatic bulk moduli of MgF₂ with a fluorite structure under high pressure and temperature has been carried out by means of first-principles density functional theory calculations combined with the quasi-harmonic Debye model in which the phononic effects are considered. Particular attention is paid to the prediction of the isothermal bulk modulus and its first and second pressure derivatives for the first time. The calculated ground state properties agree well with other theoretical values. At extended pressure and temperature ranges, the variation of the bulk modulus which plays a central role in the formulation of approximate equations of state has also been predicted. The properties of MgF₂ with a fluorite structure are summarized in the pressure range of 0–135 GPa and the temperature up to melting temperature 1500 K.     

Static and thermophysical properties of chalcogenide crystals with NaCl structure

In the present communication an analysis of interionic potentials in fourteen chalcogenide crystals has been performed. This interionic potential has been used to predict the values of cohesive energy, isothermal bulk modulus and the pressure derivatives of bulk modulus in the solids under study. The many body interaction (MBI) effects have been taken into account within the framework of Hafemeister Zarht potential. Instead of using BM potential the Hafemeister-Zarht (HZ) type short range overlap potential has been considered between nearest as well as between next nearest neighbour ions. The short-range interactions, effective up to second neighbours are treated by considering the hardness parameter as an ionic property. The hardness parameter ρ_ij is evaluated using the data on overlap integrals. The results achieved in the present study are generally in good agreement with available experimental data. Values of cohesive energy, bulk modulus and its pressure derivatives calculated by previous investigators have also been shown for the sake of comparison.     

Temperature effects on a new equation of state of solids

Recently we have proposed an isothermal equation of state of solids, and applied it to a variety of substances to show that it agrees with the isothermal pressure-volume data quite accurately up to ultrahigh pressures, and that its agreement with data is superior to the existing equations of state. Further, it has been shown that the bulk modulus and its first pressure derivative, extracted by it, are in excellent agreement with experiment. In the present study, temperature effect is added on this new equation of state, following a widely used approach involving the input of zero-pressure bulk moduli parameters and thermal expansion, all evaluated at a single reference temperature. The resultant temperature-dependent equation of state is applied to predict the isotherms over a wide range of temperature, the thermal expansion as a function of temperature, and the temperature dependence of the isothermal bulk modulus and its pressure derivative. These predictions are tested using literature data for four solids: sodium chloride, gold, molybdenum and tungsten. Good agreement is obtained between theory and data. Furthermore, the predictions are compared with those obtained from a similar temperature-dependent equation of state recently proposed, and the superior prediction capability of our model, in the P-V-T space, is demonstrated.     

First-principles study of lattice dynamics and thermodynamics ofosmium under pressure

We have performed the first-principles linear response calculations of the lattice dynamics, thermal equation of state and thermodynamical properties of hcp Os metal by using the plane-wave pseudopotential method. The thermodynamical properties are deduced from the calculated Helmholtz free energy by taking into account the electronic contribution and lattice vibrational contribution. The phonon frequencies at Gamma point are consistent with experimental values and the dispersion curves at various pressures have been determined. The calculated volume, bulk modulus and their pressure derivatives as a function of temperature are in excellent agreement with the experimental results. The calculated specific heat indicates that the electronic contribution is important not only at very low temperatures but also at high temperatures due to the electronic thermal excitation. The calculated Debye temperature at a very low temperature is in good agreement with experimental values and drops to a constant until 100~K.     

Pressure-volume relations and bulk modulus under pressure of tetrahedral compounds

The pressure-volume relation and the compression effect on the bulk modulus of tetrahedral compounds such as GaP, InP, ZnS, ZnSe, ZnTe and CdTe are investigated from the electronic theory of solids by using our recently presented binding force, which includes mainly covalent interactions in the pseudopotential formalism and partially ionic interactions. The calculated results of the pressure-volume relations involving the pressure-induced phase transition are useful when comparing with the experimental data under high pressure. The calculated bulk modulus of these compounds increases as the crystal volume decreases. Further, the pressure derivative of bulk modulus is not constant and decreases with the reduction of the crystal volume.     

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.     

Pressure dependence of thermodynamic properties of NaBr,NaI and AgCl

Various state-equations, derived by expanding energy as a function of volume in Taylor series and using different order Pade’s approximants, have been combined with quasi-harmonic approximation for free energy to reproduce the pressure dependence of thermodynamic properties of NaBr, NaI and AgCl crystals. We have used these state-equations to compute the reduced volume, the isothermal bulk modulus and the pressure derivative of isothermal bulk modulus for the three crystals at various pressures (up to 80 kbar) and at room temperature (T=298 K). The results obtained are reasonably good lending support to the state-equations and the technique used to extend their applicability. The significant results obtained in the present study include the unified reduced equation of state for the three crystals which generates almost a single curve for theP – V behaviour at room temperature.     

γ-Ce中的高压纵波声子模软化和状态方程描述

利用同步辐射角散X射线衍射技术测量了室温条件下0---0.74 GPa 压力范围内Ce的等温压缩线.发现γ-Ce的室温等温压缩线呈外凸形, 这是由其纵波声子模软化所致.利用超声测量得到的体弹性模量随压力变化的规律, 对实验所得到的压力与体积数据, 用二阶和三阶Murnaghan 方程、 二阶和三阶Birch 方程、 三阶Xu方程以及二阶Vinet方程进行比较, 并且对这些状态方程得到的体弹性模量随压力的变化规律与超声实验的结果相对比, 发现三阶Murnaghan 方程和三阶Xu方程对γ-Ce最适用.     

First principle study of structural,electronic, mechanical,dynamic and optical properties of half-Heusler compound LiScSi under pressure

In this study, several physical properties of LiScSi compound with MgAgAs phase were investigated via the plane-wave pseudo-potential technique in density functional theory (DFT). The calculated total energy-atomic volume was fitted to the Murnaghan equation of state in order to obtain bulk modulus, their first derivatives and the lattice constant. These results were compared to findings of recent literature. Afterwards, the partial density of states (PDOS) and charge density differences were used to evaluate the electronic band structure of LiScSi under pressure. By analysing elastic properties (shear modulus, Poisson ratio, Young’s modulus, etc.) of the material, it has been shown that MgAgAs phase of the compound is mechanically stable under pressure. Moreover, the dynamical stability of this compound is calculated by means of the phonon dispersion curves and one-phonon DOS. Finally, the optical properties and related parameters (refractive index, dielectric function, and loss function) of LiScSi were examined with subject to different pressures.     

Pressure derivatives of isothermal bulk modulus of rare gas solids

In the present study we have derived the expression for the bulk modulus using the Hildebrand approximation and the lattice potential energy for rare gas solids (RGS). The derived relation is further used to derive the expression for first and second order pressure derivatives of the bulk modulus for RGS. The derived relations are also used to compute the values of these constants in the case of Ne, Ar, Kr and Xe which are compared with the available values.     

Equation of state and compression effect on the bulk modulus of AlP,AlAs and AlSb compounds

The equation of state and the pressure effect on the bulk modulus of AlP, AlAs and AlSb are studied from the electronic theory of solids by using our presented binding force, although not reported experimentally. The obtained results of the pressure-volume relations involving the pressure- induced phase transition are useful to estimate the experimental data of these compounds. The obtained bulk modulus increases with the crystal volume compressed, and the pressure derivative of the bulk modulus for AlP, AlAs and AlSb is estimated theoretically.     

Analysis of K-prime equations of state

We present an analysis of the $K$-prime equations of state (EOS) due to Keane and Stacey. It is found that the two EOS differ significantly from each other in some important respects. $K$-prime represents the pressure derivative of the bulk modulus. It is shown that the volume dependence of $K$-prime and higher derivative properties predicted from the Keane EOS are compatible with those predicted from Stacey’s reciprocal $K$-prime EOS only when the Murnaghan approximation is valid. It has been emphasized that the Stacey EOS is more appropriate for describing the relationship between pressure and the bulk modulus and its pressure derivative. The results based on the two EOS have been compared and discussed.     

Isothermal compressibility and bulk modulus for methanol under the effect of pressure and temperatures

It has been demonstrated that the spinodal model produces very well the isothermal compressibility of liquid methanol for a wide range of pressures and temperatures. We have used the pseudospinodal model further to determine pressure derivatives, first-order as well as second-order of isothermal compressibility and bulk modulus for liquid methanol in the range of pressures (0–100 MPa) and temperatures (208.17 K–298.16 K). The results have been found to present close agreement with the available experimental data. We have also calculated the values of densities as a function of pressure and temperature for methanol using the Stacey equation of state.     

Elastic Constants of Superconducting MgB2 from Molecular Dynamics Simulations with Shell Model

The elastic constants of superconducting MgB₂ are calculated using a molecular dynamics method (MD) with shell model. The lattice parameters, five independent elastic constants, equations of state (EOS), Debye temperature, and bulk modulus of MgB₂ are obtained. Meanwhile, the dependence of the bulk modulus B, the lattice parameters a and c, and the unit cell volume V on the applied pressure are presented. It is demonstrated that the method introduced here can well reproduce the experimental results with a reasonable accuracy.     

金属铍热力学性质的理论研究

使用第一性原理方法结合平均场模型研究了压力从0到150GPa、温度从0到1500K，金属铍六角密排结构(hcp)的热力学性质，包括铍的常态性质，等温高压物态方程，以及常压下平衡体积、体弹模量随温度的变化，Hugoniot曲线等.0K物态方程由广义梯度近似下的密度泛函理论计算，粒子热运动的贡献由平均场模型计算.由于铍的Debye温度比较高，计算自由能时考虑了零点振动能修正.计算结果与已有的静力学和冲击波实验数据符合得非常好. 关键词： 热力学性质 物态方程 第一原理计算     

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运用密度泛函理论方法,采用MS中CASTEP模块计算了铀晶体的能量随体积变化的关系,并绘制了E-V曲线。运用准谐振子德拜模型计算了压强从0~100GPa、温度从0~2000K下的晶体铀的相对体积,发现相对体积随着温度和压强的增加而减小。并成功得到了热膨胀系数、热容、以及体弹模量随温度和压强的变化关系。预测了铀的热力学性质。为铀晶体在高温高压下的实验研究提供了理论依据。     

高温高压下闪锌矿相GaN结构和热力学特性的分子动力学研究

利用分子动力学方法和Buckingham经验势模型对重要半导体材料GaN立方闪锌矿相的晶格常数、相变压力(从闪锌矿到岩盐结构)、热膨胀、等温体模量、定压热容等结构和热力学特性在300—3000K的温度范围和0—65GPa的压力范围内进行了研究.研究表明，闪锌矿相GaN常态下的结构和热力学参数的模拟结果与实验数据及其他理论结果相符.同时在所选作用势模型可靠性检验的基础上，对等温体模量、定压热容诸非谐性参量在高温高压下的热力学行为进行了预测.所得结果在材料科学等领域的研究中具有一定的应用背景和参考价值. 关键词： GaN Buckingham势 分子动力学模拟 高温高压