High-pressure phase transitions of ScPO₄ and YPO₄

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].