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 {010} xenotime and (010) anhydrite was common, along with structures with larger and variable lamellar thickness. The xenotime 4 anhydrite transformation occurs by b = 1/2(100){010} 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 -.4 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 (120) and (102) anhydrite deformation twinning was inferred. Monazite anhydrite interphase boundaries and (120) anhydrite twin boundaries were mobile at room temperature under electron beam illumination. Xenotime rareearth 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. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.