Abstract
ScPO4 and YPO4 with the tetragonal zircon-structure were studied at room temperature and pressures up to ~50 GPa. Pressure-induced phase transitions to the sheelite structure occur at 30 GPa for ScPO4 and 16.3 GPa for YPO4, respectively. In addition to the scheelite-type high-pressure phase, an intermediate phase with the monoclinic monazite-type structure formed during the phase transition process of YPO4. The high-pressure phases of ScPO4 and YPO4 are not quenchable on pressure release. The pressure dependence of the total energy of the different phases was calculated using density functional method, and the results confirm the experimentally observed phase relations under pressure. Structural parameters and compressibility of each phase were determined by refinement of the x-ray diffraction patterns. The high-pressure phase of ScPO4 has a very large bulk modulus (376(8) GPa).