Theoretical and experimental study of the structural stability of TbPO4 at high pressures

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 similar to 9.8 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 zircon -> monazite -> scheelite -> SrUO4-type sequence of stable structures with increasing pressure. In this sequence, sixfold P coordination is attained for the SrUO4-type structure above 64 GPa. The whole sequence of transitions is discussed in association with the high-pressure structural behavior of oxides isomorphic to TbPO4.