A computational study of high pressure polymorphic transformations in monazite-type LaPO₄

Abstract

Polymorphic transformations in LaPO₄ are investigated as a function of pressure using density functional theory (DFT) based calculations under the generalized gradient approximation. The monazite-type (P2₁/n) → barite-type (Pbnm) structural transformation is identified at 16.2 GPa and experimentally, no transformation is observed near this pressure. A discontinuity in the pressure-volume relation (of 4.16% volume discontinuity compared to the monazite structure at the same pressure) and unit-cell dimensions is observed around 28 GPa, which matches well with the previous experimental results. The pressure of discontinuity matches the DFT calculated monazite-type (P2₁/n) → post barite-type (P2₁2₁2₁) structural transformation pressure. The equation of state, single crystal elastic constants and phonon dispersion curves of the different polymorphs as a function of pressure are determined. Both the barite-type (Pbnm) and post barite-type (P2₁2₁2₁) structures are mechanically and dynamically stable at 27 GPa indicating that the monazite-type (P2₁/n) → barite-type (Pbnm) phase transformation may be hindered by a kinetic barrier. The phase transformation in monazite-type LaPO₄ is driven by a softening of the C₂₅ single crystal elastic constant. Moreover, a small displacement and tilting of PO₄ tetrahedra as a function of pressure leads to a change in the La chemical environment and creates space for the construction of LaO₁₂ polyhedra from LaO₉ due to a phase transformation.