Abstract
At ambient conditions the anhydrous rare-earth orthophosphates assume either the xenotime (zircon) or the monazite structure, with the latter favored for the heavier rare earths. Tb0.5Gd0.5PO4 assumes the xenotime structure at ambient conditions but is at the border between the xenotime and monazite structures. Here we show that, at high pressure, Tb0.5Gd0.5PO4 does not transform directly to monazite but through an intermediate anhydrite-type structure. We show softening of (c1133 + c1313) combined elastic moduli close to the transition from the anhydrite to the monazite structure. Stress response of rare-earth orthophosphate ceramics can be affected by both formation of the anhydrite-type phase and the elastic softening in the vicinity of the monazite-phase. We report the first structural data for an anhydrite-type rare earth orthophosphate.