LiTaO_3晶体高压结构相变的理论研究

Theoretical investigations of high-pressure phase transition of LiTaO_3

利用基于密度泛函理论的平面波赝势结合局域密度近似的从头算方法，计算了LiTaO3晶体在0～200GPa压力范围内的冷压曲线（P—V／V0）和零温焓，以研究它的高压结构相变．参照同构体UNTO3的高压相结构，对LiTaO3的菱形相（R3c对称群，室温大气压结构）和正交相（Pbnm对称群）进行计算．结果表明，菱形相压缩线与低压冲击实验数据和静压结果符合较好，而正交相压缩线与扣除热压贡献的高压冲击实验数据相符；正交相更难压缩且各轴向的压缩率不同，对应的常态密度比菱形相高约24％．理论预测的相变起始压力约为23GPa．由此可见LiTaO3的冲击高压相具有正交对称性，与LiNbO3的室温高压相类似．

In order to investigate the high-pressure phase transition behavior of LiTaO3 crystal, the OK isothermal compression curves （P-V/Vo） and enthalpies （H-P） were calculated up to 200 GPa with ab initio plane wave pseudopotential local density approximation method. Referring to the high-pressure phases of its isomorphous crystal, LiNbO3, rhombohedral phase （R3c space group, the room-temperature and pressure structure） and orthorhombic phase （Pbnm space group） of LiTaO3 have been computed. Results show that, the 0 K compression curve of rhombohedral phase was in good agreement with DAC and low-pressure shock compression data, while that of orthorhombic phase was in consistent with our high-pressure shock compression data deducting thermal pressure contribution. Orthorhombic phase was much incompressible, and the zero-pressure density of orthorhombic phase was 24 % higher than that of rhombohedral phase. Intersection of H-P curves suggested that onset pressure of the phase-transition be about 23 GPa. It is concluded that the shock high-pressure phase of LiTaO3 has an orthorhombic symmetry, which is similar to the room-temperature and high-pressure phase of LiNbO3 crystal.