Uncovering the Effects of Non-Hydrostaticity on Pressure-Induced Phase Transformation in Xenotime-Structured TbPO4

The pressure-induced phase transformations of rare earth orthophosphates (REPO(4)s) have become increasingly relevant in ceramic matrix composite (CMC) research; however, understanding of the shear-dependence of these transformations remains limited. This study employs diamond anvil cell experiments with three pressure media (neon, KCl, sample itself/no medium) to systematically assess the effect of shear on the phase transformations of TbPO4. Results show a lowering of the TbPO4 transformation onset pressure (P-onset) as well as an extension of the xenotime-monazite phase coexistence range under non-hydrostatic conditions. The TbPO4 P-onset under no medium (4.4(3) GPa) is the lowest REPO4 P-onset reported to date and represents a similar to 50% drop from the hydrostatic P-onset. Enthalpic differences likely account for lower P-onset values in TbPO4 compared to DyPO4. Experiments also show scheelite may be the post-monazite phase of TbPO4; this phase is consistent with observed and predicted REPO4 transformation pathways.