NaH高压相变、弹性和热力学性质的第一性原理研究

First-principles calculations on phase transition and elastic and thermodynamic properties of NaH

摘要:

基于密度泛函理论，通过平面波赝势方法计算研究了NaH的高压结构相变、弹性和热力学性质。详细计算了NaH的平衡晶格常数a0，弹性常数Cij，体积弹性模量B0及其导数B0′。结果显示：本文计算值与实验值和其他理论值一致。利用吉布斯自由能等熵条件计算发现，NaH从B1结构向B2结构发生相变时的相变压为30.5GPa，体积塌缩率为4.5%，与实验值29.3±0.9GPa接近，但小于其他理论计算值(37.0GPa)。采用静水有限应变技术计算研究了弹性常数Cij、弹性波速、德拜温度ΘD、弹性各向异性因子随压力的变化关系。根据准谐德拜模型，计算研究了NaH的热容Cv和热吸收系数等热力学性质。所选择的压力范围为0到50GPa，温度范围为0到1500K。

Abstract:

The structural phase transition, elastic and thermodynamic properties of NaH were investigated by the plane-wave pseudopotential method within the density functional theory. The equilibrium lattice parameters a0 , elastic constants Cij, bulk modulus B0 and it’s derivative B0′ were calculated. The results are consistent with the experimental and theoretical data. From the usual condition of equal enthalpy, the phase transition of NaH from B1 to B2 phase occurs at 30.5 GPa with a volume collapse of about 4.5%, consistent with the experimental (29.3±0.9GPa) and calculated (37.0 GPa) results. The static finite strain technique was used to calculate elastic constants Cij at the different pressures. Furthermore, the elastic wave velocities, Debye temperature Θ and elastic anisotropic factors of NaH were also investigated in detail. Through the quasi-harmonic Debye model, in which the phononic effects were considered, the thermodynamic properties such as the specific heat capacity and thermal expansion coefficient of NaH were obtained in the whole pressure range from 0 to 50 GPa and temperature range from 0 to 1500 K.