Phase transformations in xenotime rare-earth orthophosphates

Xenotime rare-earth orthophosphates were made with compositions close to the xenotime–monazite phase boundary. Stress-driven phase transformations and their associated deformation mechanisms were characterized for these compositions by extensive conventional and high-resolution transmission electron microscopy. Three phase transformations were identified beneath indentations in polycrystalline TbPO4, in deformation bands in (Gd0.4Dy0.6)PO4 fiber coatings after fiber push-out, and in polycrystalline (Gd0.3Tb0.7)PO4: xenotime → monazite, xenotime → anhydrite, and anhydrite → monazite. A structure with many alternating monolayers of {0 1 0} xenotime and (0 1 0) anhydrite was common, along with structures with larger and variable lamellar thickness. The xenotime → anhydrite transformation occurs by b = ½〈1 0 0〉{0 1 0} slip of xenotime. However, the occasional presence of other habit planes suggests there are other transformation mechanisms. Atom shuffles were calculated for all observed transformations. Shuffles required for the anhydrite → monazite transformation are smaller than those for xenotime → monazite. This suggests that the preferred pathway for the xenotime → monazite transformation may have an anhydrite intermediate. Anhydrite also transformed to monazite in areas where high compatibility stress during (1 2 0) and (1 0 2) anhydrite deformation twinning was inferred. Monazite–anhydrite interphase boundaries and (1 2 0) anhydrite twin boundaries were mobile at room temperature under electron beam illumination. Xenotime rare-earth orthophosphates with compositions close to the monazite stability field are suggested also to be close to the monazite–xenotime–anhydrite phase-stability triple point. Transformation and deformation mechanisms are discussed.