First-principles determination of the relative stability of the α and Cmcm structures of AlPO₄

Abstract

Berlinite, α-AlPO₄, belongs to a class of isostructural compounds MXO₄ (where MX refers to IV-IV or III-V elements), whose high-pressure behavior is of considerable interest due to their geophysical importance. We present here an ab initio study, based on density-functional theory, of the equation of state of AlPO₄ in two phases, α and Cmcm, and their relative stabilities as a function of pressure. Our total-energy calculations show that the α to Cmcm phase transformation is energetically favorable beyond ≈9.5GPa, consistent with recent synchrotron X-ray-diffraction studies which showed that the α-AlPO₄ transforms to another crystalline phase (identified as the Cmcm phase) at a pressure of ≈13GPa. For both α and Cmcm phases the fractional atomic coordinates, optimized through minimization of forces, are predicted and, whenever a comparison is possible, are found to be in good agreement with available experimental results.