A computational study of high pressure polymorphic transformations in monazite-type LaPO4. Issue 11 (2nd March 2018) - Ghosh, P. S.
Abstract : Polymorphic transformations in LaPO4 are investigated as a function of pressure using density functional theory (DFT) based calculations under the generalized gradient approximation. 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 ( P 21 / 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 ( P 21 / n ) → post barite-type ( P 21 21 21 ) 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 ( P 21 21 21 ) structures are mechanically and dynamically stable at 27 GPa indicating that the monazite-type ( P 21 / 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 C 25 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.