AlAs相变及热动力学性质的第一性原理研究

利用基于密度泛函理论的全势能线性糕模轨函法研究了闪锌矿（B3），NiAs（B8）和岩盐（B1）结构的AlAs的相变、结构性质以及热动力学性质.对B3-B8和B3-B1结构的能量体积曲线做公切线，得到了B3→B8相变压力为5.44 GPa，并预测到B3→B1相变压力为6.46 GPa.同时计算了高压下B8相的结构性质，结果显示当V/V₀≈0.7—1.05时，c/a基本保持恒定（仅有约 0.2%的波动）；当V/V₀≈0.4—0.7，c/a随着V/V₀的减小而近似线性地增大.通过状态方程拟合，得到了AlAs的相对体积V/V₀与压强P的函数关系，B8相的状态方程与实验结果符合很好.最后利用准谐德拜模型得到了AlAs的体弹模量B随压力P的变化关系以及不同压强下热容C_V与温度T的关系.关键词：相变热力学性质第一性原理

First-principles studies of the phase transition and thermodynamic properties of AlAs

We have investigated the phase transition, the structural and the thermodynamic properties of AlAs in three crystallographic structures, i.e. B3 （zinc blende）, B1 （rocksalt）, and B8 （nickel arsenide）, at high pressures by using the full-potential linearized muffin-tin orbital scheme in the framework of density functional theory. By taking the common tangent to the energy-volume curves of B3-B8 and B3-B1 structures, we obtain a pressure of about 5.44 GPa for the transition from B3 to B8 and forecast a transition pressure of about 6.46 GPa for the transition from B3 to B1. We have also investigated the structural properties of B8 under high pressures, the results show that c/a ratio keeps nearly constant （～0.2% fluctuation） when V/V₀≈0.7—1.05; c/a ratio increases linearly with the value of V/V₀ decreasing when V/V₀≈0.4—0.7. The relation between relative volume V/V₀ and pressure P is obtained by fitting equation of state （EOS）, and the EOS of B8 accords well with measurements. Finally according to quasi-harmonic Debye model the dependences of bulk module B on pressure P and heat capacity C_V on temperature T under different pressures are successfully obtained.