First-Principles Theoretical Study on the Structure Behaviors of NaPO₃ under Compression

Exploring the high-pressure crystal chemical behaviors of the PO₆ 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, NaPO₃, which is isoelectronic with the major component of the lower mantle MgSiO₃, 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 pressure β phase (P2₁/n), diopside phase (C2/c), ilmenite phase (R$\overline 3$), orthorhombic (Pnma) and cubic (Pm3m) perovskite phases, the structural phase transformation sequence and phase transformation pressures were obtained: P2₁/n→C2/c (2 GPa)→R$\overline 3$ (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-NaPO₃ 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 $V_{{\mathrm{NaO}}_{12}}$/$V_{{\mathrm{PO}}_6}$ polyhedron volume ratio of Pnma-NaPO₃ shows that the PO₆ octahedron is regular in the whole calculated pressure range, and the compressibility of NaO₁₂ polyhedron is greater than that of PO₆ octahedron. The electronic structure calculation shows that the 3p and 3s orbitals of P are strongly mixed with 2p orbitals of O in the PO₆ octahedron of Pnma-NaPO₃, and the P―O bond exhibits strong covalency, which plays a key role in stabilizing the orthorhombic perovskite structure.