镁铝尖晶石高压相变的理论计算

Theoretical Calculations for Phase Transition of MgAl_2O_4 Spinel Under High Pressure

运用密度泛函(DFT)平面波赝势方法(PWP),计算了镁铝尖晶石三种物相的状态方程和热力学生成焓以及在0～50GPa高压范围内的力学性质.研究结果表明:利用状态方程得到的镁铝尖晶石转变为CF相和CT相的相变压强分别为26.79GPa和30.19GPa,与实验值误差分别为+0.79GPa和-11.81GPa;而利用热力学生成焓,在GGA近似下得到的CF相的相变压强为24.52GPa;LDA近似下CT相的相变压强为39.85GPa,与实验值误差为-1.48GPa和-2.15GPa.在对高压下镁铝尖晶石三种相结构的力学稳定性的分析发现,尖晶石相在压力超过30GPa时力学结构变得不稳定,而两个高压相在所考虑的压强范围内力学结构是稳定的.

The equations of state, thermodynamically formed enthalpy and mechanic properties of the three phases of MgAl2O4 oxides under 50 GPa were presented via the pseudowaves plane （PWP） approach based on density functional theory （DFT）. The calculated results showed that according to the state equations the transition pressures under which the MgAl2O4 spinel was transformed into CaFe2O4-type MgAl2O4 CF-phase and then into CaTi2O4-type MgAl2O4 CT- phase are 26.79 GPa and 30.19 GPa, respectively, and the differences between the calculated and tested values are ＋ 0.79 GPa and 11.81 GPa, respectively. With the thermodynamically formed enthalpy in combination with GGA, the CF-phase transition pressure is 24.52 GPa, while the CT-phase transition pressure is 39.85 GPa if combined with LDA, i. e., the comparative errors are 1.48 GPa and -2.15 GPa, respectively. It was found that the mechanical structure of spinel are unstable under the pressure over 30 GPa. But, in case the spinel phase is under the other two high pressures as above, its mechanical properties are stable if the pressures are in the range that has already been taken into account.