高压下Pca21相TiO_2直接相变的第一性原理研究

First-principles Study on Direct Phase Transition of Pca21 TiO_2 under High Pressure

利用第一性原理的GGA+U方法对TiO_2的金红石相(P42/mnm)、铌铁矿相(Pbcn)和Pca21相(Pca21)进行了计算。首先,优化了这几个相的晶体结构及相关参数,分析了焓曲线图,发现在减压至8.4 GPa时,Pca21相直接相变为铌铁矿相。其次,通过拟合分析了三阶Birch-Murnaghan状态方程得到Pca21相的体弹模量B0与类萤石结构实验值(202 GPa)非常符合。最后,通过讨论Pca21相的相变压强和体弹模量,进一步证明Pca21相结构更符合类萤石实验结果。

The GGA ＋ U method based on the first principles is used to calculate the rutile,columbite and Pca21 TiO2 phases.Firstly,the crystal structure of these phases are optimized and the related parameters are obtained.The enthalpy curves are analyzed.It is found that the Pca21 can directly change to columbite structure when the decompression to 8.4 GPa.Secondly,it is obtained that bulk modulus B0 of Pca21 very accord with the fluoritelike TiO2 experimental values（202 GPa） by fitting the third-order Birch-Murnaghan equation of state.Finally, it is further proved that the Pca21 structure is more consistent with the fluoritelike TiO2 experimental results by discussing the phase transition pressure and the bulk modulus of Pca21 TiO2.