First-principles determination of the relative stability of the α and Cmcm structures of AlPO<SUB>4</SUB>

Berlinite, &#945;-AlPO<SUB>4</SUB>, belongs to a class of isostructural compounds MXO<SUB>4</SUB> (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 AlPO<SUB>4</SUB> in two phases, &#945; and Cmcm, and their relative stabilities as a function of pressure. Our total-energy calculations show that the &#945; to Cmcm phase transformation is energetically favorable beyond &#8776;9.5GPa, consistent with recent synchrotron X-ray-diffraction studies which showed that the &#945;-AlPO<SUB>4</SUB> transforms to another crystalline phase (identified as the Cmcm phase) at a pressure of &#8776;13GPa. For both &#945; 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