NaPO_(3)高压结构行为的第一性原理理论研究

First-Principles Theoretical Study on the Structure Behaviors of NaPO_(3) under Compression

探索PO_(6)配位八面体的高压晶体化学行为是理解磷的高压化学性质、了解磷在下地幔中可能的赋存方式及磷的地球化学循环的重要基础。在0~80 GPa压力范围内,对MgSiO_(3)等电子体的NaPO_(3)开展第一性原理密度泛函理论研究,通过对其常压β相(P2_(1)/n)、透辉石相(C2/c)、钛铁矿相(R3^(-))、斜方钙钛矿相(Pnma)和立方钙钛矿相(Pm3m)的几何优化和总能对比,获得了NaPO_(3)的结构相变序列及相变压力:P2_(1)/n→C2/c(2 GPa)→R3^(-)(20 GPa)→Pnma(50 GPa),相变导致的体积变化分别为7.1%、11.5%和9.0%。Pm3m-NaPO_(3)的声子色散曲线在R点和M点呈现出显著且相似的虚频,而Pnma-NaPO_(3)在整个布里渊区均表现为实频,表明Pnma-NaPO_(3)动力学稳定。Pnma-NaPO_(3)的晶格常数、P-O键长、P-O-P键角、NaO12和PO_(6)多面体体积比V_(NaO12)/VPO_(6)与压力的关系表明,PO_(6)八面体在计算的整个压力范围内都较规则,且NaO12多面体的压缩性比PO_(6)八面体的压缩性更大。电子结构计算表明,在Pnma-NaPO_(3)的PO_(6)八面体中,P的3p和3s轨道与O的2p轨道强烈混合,P-O键表现出的强共价性对稳定其斜方钙钛矿结构发挥了关键作用。

Exploring the high-pressure crystal chemical behaviors of the PO_(6) coordinated octahedron is an important basis for understanding the high-pressure chemistry,the possible occurrence in the lower mantle,and the geochemical cycle of the phosphorus element.In this study,NaPO_(3),which is isoelectronic with the major component of the lower mantle MgSiO_(3),was studied with the first-principle density functional theory in the pressure range of 0-80 GPa.By ways of geometric optimization and total energy comparison of its ambient pressureβphase(P2_(1)/n),diopside phase(C2/c),ilmenite phase(■),orthorhombic(Pnma)and cubic(Pm3m)perovskite phases,the structural phase transformation sequence and phase transformation pressures were obtained:P2_(1)/n C2/c(2 GPa)→■(20 GPa)→Pnma(50 GPa),with the unit-cell volume changes of 7.1%,11.5%and 9.0%,respectively.The phonon dispersion curves of Pm3m-NaPO_(3) show remarkable and similar imaginary frequencies at R and M points,while the orthorhombic perovskite structure shows real frequencies throughout the whole Brillouin zone reflecting its dynamic stability.The pressure dependence of lattice constants,P-O bond lengths,P-O-P bond angles and V_(NaO_(12))/V_(PO_(6))polyhedron volume ratio of Pnma-NaPO_(3) shows that the PO_(6) octahedron is regular in the whole calculated pressure range,and the compressibility of NaO_(12)polyhedron is greater than that of PO_(6) octahedron.The electronic structure calculation shows that the 3p and 3 s orbitals of P are strongly mixed with 2p orbitals of O in the PO_(6) octahedron of Pnma-NaPO_(3),and the P-O bond exhibits strong covalency,which plays a key role in stabilizing the orthorhombic perovskite structure.