In situ Raman spectroscopy of pressure-induced phase transformations in polycrystalline TbPO₄, DyPO₄, and Gd_xDy_(1−x)PO₄

Xenotime DyPO₄ and Gd_xDy_(1−x)PO₄ (x = 0.4, 0.5, 0.6) (tetragonal I4₁amd zircon structure) have been studied at ambient temperature under high pressures inside a diamond anvil cell with in situ Raman spectroscopy. The typical Raman-active modes of the xenotime structure were observed at low pressures and the appearance of new Raman peaks at higher pressures indicated a phase transformation to a lower symmetry structure—likely monoclinic. Raman mode softening was observed, resulting in a line crossing at approximately 7-8 GPa for each material and preceding the phase transformation. The onset of phase transformation for DyPO₄ occurred at a pressure of 15.3 GPa. DyPO₄ underwent a reversible phase transformation and returned to the xenotime phase after decompression. The transformation pressures of the solid solutions (Gd_xDy_(1−x)PO₄) were in the range 10-12 GPa. The Gd_xDy_(1−x)PO₄ solid solutions yielded partially reversible phase transformations, retaining some of the high-pressure phase spectrum while reforming xenotime peaks during decompression. The substitution of Gd into DyPO₄ decreased the transformation pressure relative to pure DyPO₄. The ability to modify the phase transformation pressures of xenotime rare-earth orthophosphates by chemical variations of solid solutions may provide additional methods to improve the performance of ceramic matrix composites.