Theoretical and experimental study of the structural stability ofTbPO4at high pressures

Abstract

We have performed a theoretical and experimental study of the structural stability of terbium phosphate at high pressures. Theoretical ab initio total-energy and lattice-dynamics calculations together with x-ray diffraction experiments have allowed us to completely characterize a phase transition at $\ensuremath{\sim}9.8\text{ }\text{GPa}$ from the zircon to the monazite structure. Furthermore, total-energy calculations have been performed to check the relative stability of 17 candidate structures at different pressures and allow us to propose the $\text{zircon}\ensuremath{\rightarrow}\text{monazite}\ensuremath{\rightarrow}\text{scheelite}\ensuremath{\rightarrow}{\text{SrUO}}_{4}$-type sequence of stable structures with increasing pressure. In this sequence, sixfold P coordination is attained for the ${\text{SrUO}}_{4}$-type structure above 64 GPa. The whole sequence of transitions is discussed in association with the high-pressure structural behavior of oxides isomorphic to ${\text{TbPO}}_{4}$.