Ab initio prediction of pressure-induced structural phase transitions of CrVO4-type orthophosphates

In this work we present a theoretical study of structural stability of CrVO4-type orthophosphates InPO4 and TiPO4 at ambient and high pressures. Total energy calculations and lattice dynamics were used to obtain structural and vibrational properties of these compounds. Also, we have studied at ambient pressure the orthophosphates TlPO4 and VPO4 in order to compare their structural and vibrational properties with InPO4 and TiPO4. Here we analyze the variation of the Raman and IR frequencies as functions of the reduced mass. Using the phonon dispersion relations we have calculated the Gibbs free energy and evaluated the phase transitions at 300 K, at which most experimental measurements are performed. By taking into consideration the Bastide's diagram and previous theoretical and experimental studies in APO4 compounds, we have considered 12 candidate structures for the high-pressure regime. We found the following sequence for pressure-driven structural transition: CrVO4 type ? zircon ? scheelite ? wolframite, for InPO4 and TiPO4. The equations of state, phonon frequencies, and their behavior with pressure of the most stable polymorphs are also reported. We also included the phonon spectrum and the projected phonon density of states of each phase for both compounds. Finally, calculation of the evolution of magnetic moment is reported for TiPO4.