压力及杂质氢对金属锂弹性特性的影响

针对六角密堆金属锂16个原子超晶胞(supercell)、填隙一个氢原子的周期单元，采用基于密度泛函理论的平面波-赝势方法，研究了零温条件下压力及填隙氢掺杂对体系弹性性质的影响.结果表明氢掺杂导致体系的体模量增加.常压下掺杂体系的弹性常数C₁₁，C₃₃，C₆₆和C₁₂高于单质体系，剪切模量C₄₄有所下降，而C₁₃则与单质体系持平.压力作用下C₁₁，C₃₃和C₆₆一直大于单质体系，但C₁₂的值低于单质体系.在2GPa—4GPa压力区间内，弹性常数C₁₃呈反常变化，小于单质体系；在高压区掺杂体系的C₄₄和C₁₃则高于单质体系的相应量值，压力导致掺杂体系和单质体系之间剪切模的偏离加剧.掺杂体系在压力作用下依然保持压缩模的各向同性，具有和单质体系相似的特性. 关键词： 第一性原理 压力效应 弹性常数 金属锂     

First principle calculation of elastic and thermodynamic properties of stishovite

Using ab initio plane-wave pseudo-potential density functional theory method,the elastic constants and band structures of stishovite were calculated.The calculated elastic constants under ambient conditions agree well with previous experimental and theoretical data.C13,C33,C44,and C66 increase nearly linearly with pressure while C 11 and C 12 show irregularly changes with pressure over 20 GPa.The shear modulus(C11-C12)/2 was observed to decrease drastically between 40 GPa and 50 GPa,indicating acoustic mode softening in consistency with the phase transition to CaCl 2-type structure around 50 GPa.The calculated band structures show no obvious difference at 0 and 80 GPa,being consistent with the high incompressibility of stishovite.With a quasi-harmonic Debye model,thermodynamic properties of stishovite were also calculated and the results are in good agreement with available experimental data.     

Compression curves of the cesium halides using murnaghan logarithmic equation

The pressure derivatives of the adiabatic bulk moduli of the cesium halides are evaluated from the pressure derivatives of their elastic constants C₁₁ and C₁₂. Murnaghan's logarithmic equation is then used to determine the compression curves of these solids. The agreement between the calculated and experimental compression-ratio values for these halides of cesium is good.     

Pressure dependence of elastic constants in zinc-blende GaN and InN and their influence on the pressure coefficients of the light emission in cubic InGaN/GaN quantum wells

We have studied the influence of nonlinear elastic effects on the pressure coefficients of light emission, dE_E/dP, in cubic InGaN/GaN quantum wells. By means of ab-initio calculations, we have determined the pressure dependences of the elastic constants, C₁₁, C₁₂ and C₄₄ in zinc-blende InN and GaN. Further, we show that the pressure dependence of the elastic constants results in significant reduction of dE_E/dP in cubic InGaN/GaN quantum wells and essentially improves the agreement between experimental and theoretical values.     

Pressure dependence of the ordering temperature of the quasi two-dimensional ferromagnet (CH3NH3)2CuCℓ4

From magnetic susceptibility measurements we have determined the ordering temperature T_C of ferromagnetic (CH₃NH₃)₂CuC?₄ as a function of hydrostatic pressure up to P ～ 11 kbar. The increase of T_C, ～ 30% for ～ 11 kbar, is roughly quadratic with P. The data are discussed in terms of the P-dependence of the effective interlayer exchange field H_É.     

First-principles study of structural and elastic properties of CrO2 at high pressure

The structural and elastic properties of CrO₂ in the rutile phase under high pressures have been investigated using pseudopotential plane-wave method based on density functional theory. The optimized lattice parameters and the bulk modulus at zero pressure agree well with available experimental and theoretical data. The elastic constants C ₁₁, C ₁₂, C ₄₄, C ₃₃, C ₁₃, and C ₆₆ at zero pressure are calculated to be 359.91, 264.69, 143.28, 309.45, 218.45, and 260.74 GPa, respectively. Elastic constants, bulk modulus, shear modulus, Young's modulus, and Poisson's ratio under pressures are obtained. Our results indicate that the rutile phase is mechanically stable below 11.99 GPa. The elastic anisotropy of rutile phase under pressures has also been predicted.     

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.     

Buffer gas pressure shift in the mF = 0 → mF = 0 ground state hyperfine line in Cs

The pressure shift of the m_F:0 → 0 microwave transition of the cesium ground state has been measured for seven buffer gases (He, Ne, N₂, CH₄, C₂H₆, C₃H₈, C₄H₁₀). The high resolution of the experimental apparatus (about one part in 10¹¹ also allows an accurate measurement of the temperature dependence of the shift.     

Single-crystal elastic constants of magnesium difluoride (MgF2) to 7.4 GPa

The six independent elastic constants (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, and C₆₆) of single-crystal MgF₂ in the rutile structure have been measured by Brillouin spectroscopy at room temperature from ambient conditions to 7.4 GPa. Measurements were performed on two monocrystals with perpendicular faces, (001) and (100). A quasi-linear fit from finite strain theory was applied to the experimental data revealing the pressure dependence of the six elastic constants of MgF₂. The shear modulus C_S=1/2(C₁₁−C₁₂), and the aggregate shear (Voigt–Reuss–Hill) modulus G show a softening with increasing pressure, indicating the approach of the rutile-to-CaCl₂-type structural phase transition at P~9 GPa. The adiabatic bulk modulus (Reuss average) and its pressure derivative have been determined: K_0S=105.1±0.3 GPa, (∂K_0S/∂P)_T=4.14±0.05. The pressure–volume equation of state of MgF₂ was computed self-consistently from the Brillouin data. Our results are in good agreement with X-ray diffraction data. As the phase transition is approached, MgF₂ becomes strongly anisotropic and develops partially auxetic behavior (a negative Poisson's ratio in certain directions).     

Characterisation of the high-pressure structural transition and elastic properties in boron arsenic

This paper carries out the First principles calculation of the crystal structures (zinc blende (B3) and rocksalt (B1)) and phase transition of boron arsenic (BAs) based on the density-functional theory. Using the relation between enthalpy and pressure, it finds that the transition phase from the B3 structural to the B1 structural occurs at the pressure of 113.42GPa. Then the elastic constants C₁₁, C₁₂, C₄₄, bulk modulus, shear modulus, Young modulus, anisotropy factor, Kleinman parameter and Poisson ratio are discussed in detail for two polymorphs of BAs. The results of the structural parameters and elastic properties in B3 structure are in good agreement with the available theoretical and experimental values.     

Study of elastic properties of CeO2 by statistical moment method

The purpose of the present paper is to investigate the temperature and pressure dependences of the elastic properties of cerium dioxide using the statistical moment method (SMM). The equation of states of bulk CeO₂ is derived from the Helmholtz free energy, and the pressure dependences of the elastic moduli like the bulk modulus, B_T, shear modulus, G, Young’s modulus, E, and elastic constants (C11, C12, and C44) are presented taking into account the anharmonicity effects of the thermal lattice vibrations. In the present study, the influence of temperature and pressure on the elastic moduli and elastic constants of CeO₂ has also been studied, using three different interatomic potentials. We compare the results of the present calculations with those of the previous theoretical calculations as well as with the available experiments.     

高压下闪锌矿InN光电性质的密度泛函研究

 采用密度泛函理论,计算了闪锌矿型InN在压力下的结构、力学性质和光学性质,结果显示,随着压强的增大晶格常数减小。给出了零压下C₁₁、C₁₂、B、C_s、C₄₄的值及至70 GPa压力下弹性常数随压强的变化关系。结果表明,C₁₁、C₁₂、B随压强增大而增大,C_s、C₄₄随压强增大而减小,计算结果与现有实验和理论结果符合较好。在价带区,InN的分态密度（PDOS）有两个带,且在费米面附近密度很小,显示其倾向于形成稳定结构并且导电性较差。对闪锌矿型InN在高压下的光学性质研究发现,导带电子向高能方向偏移,而价带电子向低能方向偏移,结果导致能带间隙增大,光吸收谱在压力的作用发生了“蓝移”。研究结果对认识高压下闪锌矿型InN的结构、电学及光学性质具有重要意义。     

Study of first principles on anisotropy and elastic constants of Y3Al2 compound

In this work primarily structural parameters of the Y₃Al₂ was optimized. In the calculations, PBE type Exchange-Correlation function was selected in the GGA approximation and ultra-soft pseudopotential was used. Crystalline lattice parameters a = 15.581 Bohr, c / a = 0.923. Total energy was calculated by small deformations in volume. These values were fitted to the equation of state and the bulk modulus was calculated as 58.4 GPa. The working structure has elastic constants as C₁₁, C₁₂, C₁₃, C₃₃, C₄₄ and C₆₆. These constants have been calculated under ambient pressure. Using the elastic constants, bulk, shear, Young modulus and Poisson ratio (in Voigt approach) were obtained as 58.12, 34.31, 86.01 GPa and 0.25, respectively. Melting temperature of the material was guessed from the elastic constant. The results were compared with the current experimental and theoretical data.     

Structural and elastic properties of antiperovskites XNBa3 (X=As, Sb) under pressure effect

First-principle calculations of structural, elastic and high pressure properties of antiperovskites XNBa₃ (X=As, Sb) are performed, using the full-potential linear muffin-tin orbital (FP-LMTO) method. 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. We have determined the elastic constants C₁₁, C₁₂ and C₄₄ and their pressure dependence. We derived shear moduli, Young's modulus, Poisson's ratio and Lamé's constants for ideal polycrystalline XNBa₃ aggregates. By analyzing the ratio of the bulk to shear moduli, we conclude that XNBa₃ compounds are brittle in nature. We estimated the Debye temperature of XNBa₃ from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of AsNBa₃ and SbNBa₃ compounds, and it still awaits experimental confirmation.     

Elastic and electronic properties of YNi2B2C under pressure from first principles

The elastic and electronic structure properties of YNi₂B₂C under pressure are investigated by performing the generalized gradient approximation (GGA) and local density approximation (LDA) correction scheme in the frame of density functional theory (DFT). The pressure dependences of the normalized lattice parameters a/a₀ and c/c₀, the ratio c/a, and the normalized primitive volume V/V₀ of YNi₂B₂C are also obtained. The lattice constants and bulk modulus obtained are in agreement with the available experimental and other theoretical data. We have also studied the pressure dependences of elastic properties. It is found that, as pressure increases, the elastic constants C₁₁, C₃₃, C₆₆, C₁₂, and C₁₃ increase, the variation of elastic constant C₄₄ is not obvious. Moreover, our compressional and shear wave velocities V_L=6.99 km/s and V_S=3.67 km/s as well as the Debye temperature Θ=549.7 K at 0 GPa compare favorably with the available experimental data. The pressure dependences of band structures, energy gap and density of states are also investigated.     

High pressure structural phase transition in uranium monochalcogenides

The pressure induced phase transition in uranium monochalcogenides, UX (X = S, Se, and Te) is studied by two-body potential approach. It is found that US, USe and UTe undergo a structural phase transition from NaCl (B1) type to CsCl (B2) type at 78.5, 21 and 9.5 GPa, respectively, which is in good agreement with the recent experimental data. In addition, second-order elastic constants (SOECs) (C ₁₁, C ₁₂ and C ₁₄) have been calculated which can be used to establish the nature of the forces in these materials. The present study shows that the considered two-body potential model can be used to predict the phase transition pressure in UX compounds provided the strength and hardness parameters in B1 and B2 phases are different.     

Complete pentagon orientational ordering in C60 fullerite charged with NO

Solid C₆₀ was stored in NO under high pressure, and the gas molecules NO were found to diffuse into the octahedral interstitial sites in its fcc crystal lattice. Its ¹³C NMR MAS spectra are composed of a primary resonance at 143.7 ppm accompanied by two minor peaks shifted 0.4 and 0.8 ppm downfield, respectively. The dopant was found to depress its phase transition temperature at 260 K in pure C₆₀ and to substantially reduce the drop Δ?′ at the phase transition temperature. Furthermore, the spectral features associated with relaxation during glass transition at lower temperature, as observed in impedance spectra, were smeared. The fraction of P-orientation below T _c was calculated to be larger than 11/12. These results show that a completely P-oriented phase occurred in (NO)_0.1C₆₀ and that this phase is favored by a negative pressure on the C₆₀ lattice exerted by NO, as well as by the electrostatic interaction between the two.     

Ab initio study of structural,electronic, and elastic properties of M<Subscript>2</Subscript>SbP (M = Ti,Zr, and Hf)

This study predicts the structural behaviour of selected M₂SbP compounds with the same structure as MAX phases. Zero pressure results of the lattice parameters, equilibrium volume, and the internal parameter Z_M are calculated with an error less than 3%. Band structure, total and partial density of states were calculated and show the metallic character of these phases. Moreover we observed strong hybridising states; M d–P p, and M d–Sb p. The pressure dependence of the volume, and the lattice parameters were studied. The stiffness of M–P, and M–Sb bonds was discussed in term of relative length change under hydrostatic pressure. Hf₂SbP present the highest bulk modulus and the unidirectional elastic modulus C₃₃ is slightly greater than C₁₁.     

Single-crystal elasticity of the rhodochrosite at high pressure by Brillouin scattering spectroscopy

ABSTRACT

The sound velocity properties of single-crystal rhodochrosite (MnCO₃) were determined up to 9.7?GPa at ambient temperature by Brillouin scattering spectroscopy. Six elastic constants were calculated by a genetic algorithm method using the Christoffel's equations at each pressure. The elastic constants increased linearly as a function of pressure and its pressure derivatives ?Cij/?P for C₁₁, C₃₃, C₄₄, C₁₂, C₁₃, C₁₄ were 5.86 (±0.36), 3.82 (±0.44), 2.06 (±0.39), 5.07 (±0.27), 5.34 (±0.44), 1.52 (±0.24), respectively. Based on the derived elastic constants of rhodochrosite, the aggregate adiabatic bulk and shear moduli (K_s and G) were calculated using the Voigt-Reuss-Hill averages and the linear fitting coefficients (?K_s/?P)_T and (?G/?P)_T were 5.05(±0.26) and 0.73(±0.05), respectively. The aggregate V_p of rhodochrosite increased clearly as a function of pressure and its pressure derivative ?V_p/?P was 7.99(±0.53)?×?10^?2?km/(s?GPa), while the aggregate V_s increased slowly and ?V_s/?P was only 1.19(±0.12)?×?10^?2?km/(s?GPa). The anisotropy factor for As of rhodochrosite increased from ～40% at 0.8?GPa to ～48% at 9.7?GPa, while A_p decreased from ～19% to ～16% at the corresponding pressure.     

The second-order elastic constants of AgBr from 77 to 300 K

The three independent second-order elastic constants of AgBr have been measured from room temperature down to liquid nitrogen temperature A single crystal with a (110) axis was used for the measurements The measured longitudinal elastic constant C'₁₁ = (C₁₁ + C₁₂ + 2C₄₄)/2 increases by 16% over this temperature range The elastic shear constant C₄₄) increases by almost 11%, while the elastic shear constant C' = (C₁₁ ? C₁₂)/2 increases by 50% over this range The measured bulk modulus B = (C₁₁) + 2C₁₂)/3 increases by almost 15% as the lattice becomes stiffer The initial temperature derivatives of the elastic constants are similar to those previously measured The changes in the elastic constants are basically linear down to approximately 150 K, where the temperature derivatives begin to decrease in magnitude These results are similar to those previously obtained for AgCl