First-Principle Calculations for Thermodynamic Properties of LiBC Under High Temperature and High Pressure

The thermodynamic properties of LiBC are investigated by using the full-potential linearlzed muffin-tin orbital method （FP-LMTO） within the frame of density functional theory （DFT） and using the quasi-harmonic Debye model. The dependencies of the normalized lattice parameters a/a0 and c/c0, the ratio （c/a）/2, the normalized primitive volume V/V0 on pressure and temperature are successfully obtained. It is found that the interlayer covalent interactions （Li-B bonds or Li-C bonds） are more sensitive to temperature and pressure than intralayer ones （B-C bonds）, as gives rise to the extreme lattice anisotropy in the bulk hop LiBC.     

Electronic structure and thermodynamic properties of LiBC under high pressure

This paper investigates the electronic structure and thermodynamic properties of LiBC in the hexagonal structure by using the generalized gradient approximation （GGA） and local density approximation correction scheme in the frame of density functional theory. The geometric structure of LiBC under zero pressure, and the dependences of the normalized lattice parameters a/ao and c/co, the ratio e/a, the normalized primitive volume V/Vo on pressure are given. The thermodynamic quantity （including the heat capacity Cv, Debye temperature 6~D, thermal expansion a and Grfineisen parameter -y） dependences on temperature and pressure are obtained through the GGA method and the quasi-harmonic Debye model. The band structures and density of state of LiBC under different pressures have also been analysed.     

Structural and thermodynamic properties of wurtzite-type aluminium nitride from first-principles calculations

The lattice parameter, bulk modulus and its pressure derivative of the wurtzite-type aluminium nitride (w-AlN) are investigated by using the Cambridge Serial Total Energy Package (CASTEP) program in the framework of Density Functional Theory (DFT). The calculated results are in good agreement with the available experimental data and other theoretical results. Through the quasi-harmonic Debye model, the dependences of the normalized lattice parameters $a/a_{0}$ and $ c/c_{0}$, axial ratio $c/a$, normalized primitive-cell volume $V/V_{0}$, Debye temperature ${\it\Theta} _{\rm D} $ and heat capacity $C_{\rm V}$ on pressure $P $ and temperature $ T$ are obtained. It is found that the interlayer covalent interactions (Al-N bonds) are more (even a little) sensitive to temperature and pressure than intralayer ones (N--N bonds), which gives rise to a little lattice anisotropy in the w-AlN.     

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.     

Structural and Thermodynamic Properties of Wurtzitic Boron Nitride from First-Principle Calculations

Using ab initio plane-wave pseudopotential density functional theory method, we have studied the structural and thermodynamic properties of the wurtzite boron nitride （w-BN）. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependencies of the normalized lattice parameters a/ao and c/co, the axial ratio c/a, and the normalized primitive cell volume V/Vo on pressure P and temperature T are investigated. Furthermore, the Debye temperature θ, the variation of thermal expansion α, as well as the heat capacity Cv as functions of P and T are studied systematically.     

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.     

Structural and elastic properties of Ce2O3 under pressure from LDA+U method

We investigate the structural and elastic properties of hexagonal Ce₂O₃ under pressure using LDA+U scheme in the frame of density functional theory (DFT). The obtained lattice constants and bulk modulus agree well with the available experimental and other theoretical data. The pressure dependences of normalized lattice parameters a/a ₀ and c/c ₀, ratio c/a, and normalized primitive volume V/V ₀ of Ce₂O₃ are obtained. Moreover, the pressure dependences of elastic properties and three anisotropies of elastic waves of Ce₂O₃ are investigated for the first time. We find that the negative value of C ₄₄ is indicative of the structural instability of the hexagonal structure Ce₂O₃ at zero temperature and 30 GPa. Finally, the density of states (DOS) of Ce₂O₃ under pressure is investigated.     

Theoretical Study of Compressibility and Thermoelasticity of LaNi5-xAlx （x= 0.3, 0.5 and 1.0）

The compressibility, the temperature dependence of bulk modulus, the pressure dependence of normalized volume V/V0, thermal expansion coefficient and Debye temperature of LaNi5-xAlx compounds are successfully obtained using the first-principles plane-wave pseudopotential （PW-PP） method, the EOSFIT6.0 software and the quasiharmonic Debye model. The rapid decrease of relative lattice constant a/a0 shows that the deformation is easier in directions normal to the c-axis than that along it. The relationships between bulk modulus B and pressure at different temperatures are also analysed. It is found that the bulk modulus B increases monotonically with increasing pressure. Moreover, the pressure dependences of thermal expansion and Debye temperature are also successfully obtained. The calculated results are in agreement with the experimental data.     

The compressibility of high purity Y bB(2)

The compressibility and phase stability of Y bB(2) are investigated under high pressure using high-resolution synchrotron x-ray diffraction in a diamond anvil cell. The bulk modules of high purity Y bB(2) is obtained as ～182?GPa using the Birch-Murnaghan equation of state. The patterns measured up to 20?GPa and the pressure dependence of normalized lattice parameters, a/a(0) and c/c(0), reveal that the compressibility of Y bB(2) is low and fairly isotropic, and this material can be classified as a hard material. X-ray photoemission studies demonstrate that Yb in Y bB(2) has a mostly trivalent valence state at room temperature. Moreover, sample preparation details provide a new insight into the high purity synthesis of Y bB(2) at ambient pressure and moderate temperatures. The presented structural and compressibility results are in agreement with the available theoretical and experimental data on binary rare-earth borides and can serve as a reliable reference for future studies.     

压强下TaB和TaB2力学和超导性质的第一性原理研究

采用基于密度泛函理论的第一性原理赝势平面波方法, 在广义梯度近似下研究了TaB和TaB2在不同压强下的弹性常数、原子结构、电子结构以及超导性质及其对两者物理性质不同进行了比较. 计算结果表明随着压强的增加, 弹性常数和体弹模量随之增加, 而相对晶格常数a/a0, b/b0, c/c0和相对体积V/V0随压强增加而减小. 在高压下, TaB沿着a轴方向的压缩性要比b轴方向大, 而b轴方向的压缩性要比c轴方向大; TaB2是沿着c轴方向的压缩性要比a轴方向的压缩性大. 电子结构分析表明TaB2的原子态杂化程度比TaB的原子态杂化程度要高, 这与TaB2的体弹模量比TaB的体弹模量高的结果相一致. 依据Bardeen-Cooper-Schrieffer超导理论, TaB和TaB2费米能级处态密度的值随着压强的增加而降低, 说明它们的超导转变温度Tc随着压强的增加而降低.     

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.     

First-Principles Calculations of Elastic Properties of LaNi5 Compound

The equilibrium lattice constants, temperature dependence of bulk modulus, the pressure dependence of the normalized volume V/V0, elastic constants Cij and bulk modulus of LaNi5 crystal are obtained using the firstprincipies piane-wave pseudopotential method in the GGA-PBE generalized gradient approximation as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus and temperature up to 2000 K and obtain the relationship between bulk modulus B and pressure at diFFerent temperatures. It is found that the bulk modulus B increases monotonously with increasing pressure. Moreover, the pressure dependences of Debye temperatures and the pressure derivatives of lattice constants are also successfully obtained. The calculated results are in agreement with the experimental data and the other theoretical results.     

超高压下CsBr的结构与相变

 采用金刚石对顶压砧高压装置（DAC）、同步辐射X光源和能散法，对CsBr粉末样品进行了原位高压X光衍射实验，最高压力达115 GPa。观测到在53 GPa左右压力下，CsBr的最强衍射峰(110)劈裂成两个峰，标志了简单立方结构向四方结构的转变；在0至最高压力范围内（相应于V/V₀为1至0.463）测量了晶轴比c/a；在115 GPa内未观测到样品的金属化现象。     

Synthesis and physical properties of LiBC intermetallics

Polycrystalline samples of LiBC compounds, which were predicted as possible candidate for high-T_c superconductivity, have been synthesised by a flux method for a wide range of flux compositions Li_xBC (x=0.5-2.4). Scanning electron microscopy and X-ray diffraction patterns showed a plate-like morphology and a single-phase nature of LiBC samples for the starting flux composition of Li_1.25BC. The lattice constants a, c display a systematic variation with x and a maximum volume of the hexagonal unit cell at x=1.25. Electrical resistivity measurements revealed an extrinsic semiconducting behaviour of the single-phase LiBC with an activation energy of 18 meV and a maximum specific resistivity of 2.5 Ω cm at 300 K. In contrast to the theoretical prediction of high T_c, no superconducting features were detected down to 2 K both, by measurements of electrical resistivity and magnetic susceptibility. However, the results do not allow to refute the theoretical prediction because of the lack of accurate chemical analysis methods for the LiBC compounds synthesised.     

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.     

Structural and Thermodynamic Properties of TiAl intermetallics under High Pressure

The structural and thermodynamic properties of TiAl intermetallics under high pressure have been investigated by ab initio plane-wave pseudopotential density functional theory method.It is found that the ratio of lattice parameter c to a keeps almost constant with a value of 1.02 under the pressure from 0 to 20 GPa,which agrees well with the experimental results.With the pressure increasing from 20 to 45 GPa the values of c/a decrease almost linearly from 1.02 to 0.99.These calculated results indicate under low pressure the variation rate for a-axis is almost the same to that for c-axis,but under higher pressure the variation for a-axis is smaller than along c-axis.Through the quasi-harmonic Debye model,the equation of state(EOS) of TiAl intermetallics,as well as the thermal expansion and heat capacity at various pressures and temperatures are also studied.     

Anomalous behavior and phase transformation of α-GaOOH nanocrystals under static compression

The structural compression mechanism and compressibility of gallium oxyhydroxide, α-GaOOH, are investigated by in situ synchrotron radiation x-ray diffraction at pressures up to 31.0 GPa by using the diamond anvil cell technique. Theα-GaOOH sustains its orthorhombic structure when the pressure is lower than 23.8 GPa. The compression is anisotropic under hydrostatic conditions, with the a-axis being most compressible. The compression proceeds mainly by shrinkage of the void channels formed by the coordination GaO_3(OH)_3 octahedra of the crystal structure. Anomaly is found in the compression behavior to occur at 14.6GPa, which is concomitant with the equatorial distortion of the GaO_3(OH)_3 octahedra. A kink occurs at 14.6 GPa in the plot of finite strain f versus normalized stress F, indicating the change in the bulk compression behavior. The fittings of a second order Birch–Murnaghan equation of state to the P–V data in different pressure ranges result in the bulk moduli B_0= 199(1) GPa for P 14.6 GPa and B_0= 167(2) GPa for P 14.6 GPa. As the pressure is increased to about 25.8 GPa, a first-order phase transformation takes place, which is evidenced by the abrupt decrease in the unit cell volume and b and c lattice parameters.     

High field magnetoresistance of CeAl2 at low temperatures

We have measured the magnetoresistance ρ(H,T₀) of the magnetic Kondo lattice CeAl₂ at temperatures T₀ in the range 0.035 – 1.3 K, well below the Néel temperature T_N=3.8 K, in magnetic fields H up to 145 k0e. The ρ vs H curve exhibits a rapid decrease between H=45–65 kOe corresponding to the metamagnetic transition at H=H_M. The resistivity then levels out to a value which depends only weakly on field and temperature. For HM a small positive magnetoresistance was observed with the derivative dρ/dH increasing as the temperature is lowered. The present results are compared with the ρ(H,T₀) data obtained for the CePb₃ magnetic Kondo lattice, where the decrease in the resistivity at HH_M was considered earlier to be evidence of field induced superconductivity.     

Inverted V-shaped evolution of superconducting temperature in SrBC under pressure

Based on density functional first-principles calculations and anisotropic Eliashberg equations, we have investigated the electronic structure, lattice dynamics, and phonon-mediated superconductivity in newly synthesized layered compound Sr BC under pressure. Different from Li BC and Mg B2, our calculations surprisingly reveal that Sr BC is isotropic in compressibility, due to the accumulation of substantial electrons in the interstitial region. We find that the Sr phonons strongly couple with B-2 pz orbital and the interstitial states, giving rise to a two-gap superconductivity in Sr BC, whose transition temperature shows an inverted V-shaped dependence on pressure. The maximal transition temperature is about 22 K at50 GPa. On both sides of 50 GPa, the transition temperature exhibits quasi-linear variation with positive and negative slopes, respectively. Such a variation of transition temperature is infrequent among phonon-mediated superconductors. The competition between enhanced electron–phonon matrix element and hardened phonons plays an essential role in governing the behavior of the critical temperature.     

First-principles calculations of structure and high pressure phase transition in gallium nitride

The phase transitions of semiconductor GaN from the Wurtzite (WZ) structure and the zinc-blende (ZB) structure to the rocksalt (RS) structure are investigated by using the first-principles plane-wave pseudopotential density functional method combined with the ultrasoft pseudopotential scheme in the generalized gradient approximation (GGA) correction. It is found that the phase transitions from the WZ structure and the ZB structure to the RS structure occur at pressures of 46.1 GPa and 45.2 GPa, respectively. The lattice parameters, bulk moduli and their pressure derivatives of these structures of GaN are also calculated. Our results are consistent with available experimental and other theoretical results. The dependence of the normalized formula-unit volume $V/V_{0 }$ on pressure $P$ is also successfully obtained.