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

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.