Ab initio predicted elastic and thermodynamic properties of Imm2-BN under high pressure

The structural parameters, elastic constants, thermodynamic properties of Imm2-BN under high pressure were calculated via the density functional theory in combination with quasi-harmonic Debye approach. The results showed that the pressure has the significant effect on the equilibrium lattice parameters, elastic and thermodynamic properties of Imm2-BN. The obtained ground state structural parameters are in good agreement with previous theoretical results. The elastic constants, elastic modulus, and elastic anisotropy were determined in the pressure range of 0–90?GPa. Furthermore, by analyzing the B/G ratio, the brittle/ductile behavior under high pressure is evaluated and the elastic anisotropy of the Imm2-BN up to 90?GPa is studied in detail. Moreover, the pressure and temperature dependence of thermal expansion coefficient, heat capacity, Debye temperature, and Grüneisen parameter are predicted in a wide pressure (0–90?GPa) and temperature (0–1600?K) ranges. The obtained results are expected to provide helpful guidance for the future synthesis and application of Imm2-BN.     

Pressure dependence of single crystal elastic constants and anharmonic properties of wurtzite

The single crystal, elastic constants of wurtzite (ZnS) have been measured by means of the ultrasonic pulse superposition technique at 25°C as a function of pressure, and at 1 bar as a function of temperature between 25 and 100°C. Within experimental accuracy the pressure dependence in the range up to 10 kbar was found to be linear for the longitudinal modes and quadratic for the shear modes. The elastic constants and their first pressure derivatives agree approximately with theoretical values for the hcp structure with first nearest neighbor central force interaction. The experimental data are used for the discussion of the Born stability limit in relation to the high pressure transformations of wurtzite into the sphalerite and rock salt structures, for the calculation of the Grüneisen parameter in the anisotropic elastic continuum approximation, and for the calculation of the isothermal equation of state at high pressure. The elastic properties of sphalerite (ZnS) are calculated from the data for wurtzite in the ‘equivalent sphalerite approximation’ of Sullivan and used for discussing the dependence on ionic radii of the pressure coefficients of the elastic constants in sphalerite-type II–VI compounds. The Born stability limit, the Grüneisen parameter and the isothermal equation of state of sphalerite are also calculated and compared with the corresponding quantities for wurtzite.     

Grüneisen系数与铝的高温高压状态方程

基于Gray金属三相状态方程模型,分别采用ργ=常数,Gray,GRIZZLY的Grüneisen系数模型和从头算给出的Grüneisen系数,系统计算了铝的熔化曲线、等熵压缩线、等温压缩线和等熵卸载线,计算结果与实验数据比较表明;在冲击压力约为500GPa的宽广压力范围,GRIZZLY Grüneisen系数模型是最适合描述铝的热力学特性的形式,ργ=常数模型次之,在高压区,Gray和从头算的Grüneisen系数的计算结果与实验值差距较大. 关键词： Grneisen系数 状态方程 铝     

BaCuO2.5在高压下的状态方程与电学性质

 在活塞-圆筒式高压装置上研究了BaCuO_2.5在4.5 GPa内的p-V关系,给出了其状态方程、Grüneisen参数γ₀、零压体弹模量B₀以及B₀的压力导数B′₀。并在金刚石压砧装置（DAC）上测量了样品的电阻、电容随压力变化的关系,结果表明在0~20 GPa内没有发生相变。     

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.     

Raman spectroscopy of NdFeO3 at pressures up to 11 GPa

We report the theoretical and experimental study of evolution of the first-order Raman-active phonons in NdFeO₃ with pressure up to 11?GPa at room temperature. With non-polarized light, we have observed 10 Raman-active modes. Our study confirmed no structural phase transition in the studied pressure range. We have calculated that weighted average Grüneisen parameter is ?γ? =?1.19.     

First-principles calculation of the lattice compressibility, elastic anisotropy and thermodynamic stability of V2GeC

We investigate the elastic and the thermodynamic properties of nanolaminate V₂GeC by using the ab initio pseudopotential total energy method. The axial compressibility shows that the c axis is always stiffer than the a axis. The elastic constant calculations demonstrate that the structural stability is within 0-800 GPa. The calculations of Young's and shear moduli reveal the softening behaviour at about 300 GPa. The Possion ratio makes a higher ionic or a weaker covalent contribution to intra-atomic bonding and the degree of ionicity increases with pressure. The relationship between brittleness and ductility shows that V₂GeC is brittle in ambient conditions and the brittleness decreases and ductility increases with pressure. Moveover, we find that V₂GeC is largely isotropic in compression and in shear, and the degree of isotropy decreases with pressure. The Gr黱eisen parameter, the Debye temperature and the thermal expansion coefficient are also successfully obtained for the first time.     

KBrO3在高压下拉曼光谱和结构的研究

利用原位高压拉曼散射和X射线衍射技术,研究了KBrO3在高压下晶格振动和晶体结构演化行为.最高压力达30.9 GPa.通过拉曼光谱发现,在高压下拉曼峰位除了单调移动,没有其它变化,表明KBrO3在研究的压力范围没有发生相变.原位高压X射线衍射实验数据显示,其在高压下依然保持常压的六方结构.通过进一步分析,分别得到了体弹模量B0=25.9(2) GPa(B′0=5.68(0.38))和部分拉曼峰的Grüneisen参数.     

高压下铝的Grüneisen参数的实验测量

 采取突然加压的方法，测量在施压过程中产生的温度变化，从而直接决定物质的格留乃森（Grüneisen）参数。第一次在固体传压介质中进行该种实验，并在高达3.5 GPa的压力范围内，直接测量了铝的格留乃森参数随压力的变化。     

Elastic properties of Se and As2Se3 glasses under pressure and temperature

The sound velocities and their pressure and temperature variations of Se and As₂Se₃ glasses have been determined by means of a pulse superpositions method, and other elastic constants and their pressure and temperature derivatives were calculated from these data. The bulk modulus was found to be 94·6 kbar for Se glass and 143·7 kbar As₂Se₃ glass, both of which are higher than the values calculated from the previous compression data. No anomaly was observed in any of the pressure and temperature dependence of elastic behavior of these glassses. Furthermore, the comparison of the pressure and temperature derivatives of the bulk modulus indicates that the thermodynamic self-consistency is satisfied on these materials. The bulk moduli of these glasses and crystalline As and Se were used to obtain an empirical bulk modulus-volume relationship for compounds in the As?Se system. The acoustic Grüneisen parameter was calculated and compared with the thermal Grüneisen parameter.     

高压下CaPo弹性性质和热力学性质的第一性原理研究

利用密度泛函理论的平面波赝势方法预测研究了CaPo从岩盐结构(B1结构)到氯化铯结构(B2结构)的相变以及B1结构CaPo高压下的弹性性质以及热力学性质等.通过等焓原理发现B1→B2的相变压力为22.8GPa. 同时计算了B1结构CaPo高压下的弹性常数以及剪切模量、杨氏模量等相关弹性参数,结果发现当压力超过20GPa时,B1结构CaPo开始不稳定了,这和等焓原理所得结果相符合. 最后通过Debye模型成功获取了B1结构C 关键词： 相变 弹性性质 热力学性质 CaPo     

Elastic properties of aluminum nitride

The elastic moduli and their temperature and pressure derivatives for sintered, isotropic, polycrystalline aluminum nitride ceramic have been determined. The temperature derivatives of the elastic moduli are somewhat smaller than those of other wurtzite structure semiconductors, and the pressure derivative of the shear modulus is very small. The mode Grüneisen gamma of the shear mode is essentially zero, while the elastic Grüneisen constant agrees well with the thermodynamic Grüneisen constant. Both the bulk and shear modulus conform to the Keyes relationship.     

The effect of hydrostatic and uniaxial pressure on the elastic constants of cadmium

The third order elastic constants of the hep metal cadmium have been determined from measurements of the hydrostatic and uniaxial pressure dependences of the velocities of ultrasonic waves propagated through single crystals. The hydrostatic pressure dependences of the second order elastic constants have also been obtained. The compression of cadmium has been estimated by extrapolation of the data up to high pressures by using the Murnaghan equation of state. Using the generalised Grüneisen theory in the quasiharmonic approximation, the long wavelength acoustic mode Grüneisen parameters have been obtained; the mean acoustic mode parameter is compared with the thermal Grüneisen parameter. In general the anisotropy of the vibrational anharmonicity of the acoustic modes is found to be consistent with the deviation of the c/a ratio of cadmium from that expected for an ideally close-packed metal.     

高温高压下碳化钨晶体的结构、力学、电子、光学以及热力学性能的第一性原理计算

本文利用密度泛函理论中的广义梯度近似对碳化钨晶体的三种结构(碳化钨相、闪锌矿相以及纤锌矿相)进行了优化,得到能量最低的稳定构型,并在此基础上计算了它的力学、电子、光学和高温高压下的热力学性质.研究表明:在0～300 GPa压力范围内,碳化钨相具有最高的稳定性.同时,高压下碳化钨相的弹性常数满足Born-Huang准则,且0 GPa和300 GPa下的声子色散没有虚频,证明了高压下碳化钨相的静力学稳定性和动力学稳定性.电子性质表明了碳化钨的金属性.光学性质表明碳化钨在高能区很难吸收光.热力学性质的研究表明:体积比V/V_0对压强的变化更敏感;高温时C_V曲线近似一条直线;给定压强下热膨胀系数α在600 K温度以上增长非常缓慢;压强对德拜温度Θ_D的影响较大;在低压下格林艾森系数γ的变化较大.     

The hydrostatic pressure derivatives of the elastic constants of indium and an indium cadmium alloy

The hydrostatic pressure derivatives of the elastic stiffness constants of indium and indium-3.4 at.% cadmium alloy single crystals have been obtained from pulse echo overlap measurements of the dependence of ultrasonic wave velocities upon pressure. The softest zone centre acoustic phonon mode in indium is a shear mode propagating k along the [101] direction rather than that (k[110], e[11?0]) which drives the ferroelastic phase transition in the indium-cadmium alloys. The derivative δ((C₁₁ – C₁₂)/2)/δP is positive, accounting for the stability of the fct structure of indium under high pressure. Using the quasiharmonic, anisotropic continuum model the acoustic mode Grüneisen parameters have been calculated and are discussed in terms of mode softening. The high temperature limiting value$\text{\$}\text{?}$γ_H (= 2.56) of the mean acoustic mode Grüneisen parameter is found to be close to the thermodynamic Grüneisen parameter γ^th (=2.5).     

Elastic and thermophysical properties of BAs under high pressure and temperature

The pseudopotential plane-wave approach in the framework of the density functional theory, and the density functional perturbation theory with the generalized gradient approach for the exchange-correlation functional has been used to calculate the structural phase stability, elastic constants and thermodynamic properties of boron-arsenide (BAs) compound. The BAs compound transforms from the zincblende phase to rock-salt structure; the phase transition pressure was found to be 141.2 GPa with a volume contraction of around 8.2%. The thermodynamic properties under high pressure and temperature up to 125 GPa and 1200 K respectively were also determined, analyzed and discussed in comparison with other data of the literature. The systematic errors in the static energy were corrected using the bpscal EEC method. Our results agree well with those reported in the literature, where for example, our calculated melting temperature (2116 K) deviates from the theoretical one (2132.83 K) with only 0.8%, and the deviation between our result (1.86) of the Grüneisen parameter and the theoretical one (1.921) is only around 3.2%.     

Phase stability,mechanical and thermodynamic properties of orthorhombic and trigonal MgSiN2: an ab initio study

Structural stability and mechanical and thermodynamic properties of the orthorhombic and trigonal MgSiN₂ polymorphs (or-MgSiN₂ and tr-MgSiN₂) were investigated through density functional theory and quasi-harmonic Debye model (QHDM). Our calculations show that or-MgSiN₂ is energetically the stable polymorph at low pressure, in agreement with previous experimental and theoretical study. Under pressure, a crystallographic transition from the orthorhombic structure to the trigonal one occurs around 25, 17.45 and 19.05 GPa as obtained from the generalized gradient approximation of Perdew-Wang (GGA-PW91), the generalized gradient approximation parameterized recently by Perdew et al (GGA-PBEsol) and the local density approximation developed by Ceperley and Alder and parameterized by Perdew and Zunger (LDA-CAPZ), respectively. Single-crystalline and polycrystalline elastic constants and related properties, namely Vickers hardness, acoustic Grüneisen parameter, minimum thermal conductivity, isotropic sound velocities and Debye temperature, were numerically estimated for both or-MgSiN₂ and tr-MgSiN₂. We have showed that the hardness of tr-MgSiN₂ is comparable to that of the harder materials like c-BN and B₆O. Temperature and pressure dependencies of volume, bulk modulus, thermal expansion, Grüneisen parameter, heat capacities and Debye temperature were investigated using QHDM.     

The thermal expansion of GaP below room temperature

The thermal expansion of GaP is estimated theoretically as a function of temperature using the experimental pressure dependence of elastic stiffness constants and phonon frequencies. The numerical values of the linear expansion coefficient and the Grüneisen constant do not become negative at low temperatures, contrary to other tetrahedrally bonded covalent solids. This positive thermal expansion of GaP at low temperatures is closely connected with the small values of the transverse acoustical mode Grüneisen parameter and with the large value of the metallic phase transition pressure from the linear correlation obtained by Weinstein.     

The pressure and temperature derivatives of the elastic moduli of lithium hydride

The pressure and temperature derivatives of the elastic moduli of single crystal LiH have been determined at room temperature, by the ultrasonic pulse echo method. From the pressure data, the mode Grüneisen parameters as well as the low and high temperature limiting values of the Grüneisen constant were computed; the latter were only slightly different. An equation of state has been constructed for LiH, and found to be in agreement with the experimental data. Based on a non-central force model, the values of the elastic moduli and their pressure derivatives have been calculated theoretically, and the latter values compared with the experimental data.     

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.