In Situ Ion Irradiation of Rhabdophane- and Churchite-Type Hydrous Rare-Earth Phosphates

Rare-earth phosphates adopting the monazite (REPO₄, RE = La to Gd) and xenotime (REPO₄, RE = Tb to Lu and Y) structures are proposed as one of the candidate matrices for nuclear waste immobilization applications. Several studies focussing on the effect of radiation and corrosion on the structure of monazite and xenotime materials have been reported in the literature. To the best of our knowledge, the effect of radiation on the structure of metastable polymorphs (rhabdophane – REPO₄.H₂O (RE = La to Dy) and churchite-REPO₄.2H₂O (RE = Gd to Lu and Y)) of the rare-earth phosphate family haven’t been reported in the literature. Rhabdophane and churchite phases are proposed as an actinide-solubility controlling phase and is expected to precipitate on the surface of monazite and xenotime materials upon aqueous corrosion of the latter. In the current study, the effect of radiation on rhabdophane (SmPO₄.H₂O, DyPO₄.H₂O) and churchite (YPO₄.2H₂O, DyPO₄.2H₂O) materials was studied by irradiating these materials with 650 keV Xe²⁺ ions under in-situ conditions and the structural response were monitored using in-situ TEM (MIAMI facility, University of Huddersfield). Phase-pure rhabdophane and churchite materials were synthesized using wet-chemistry methods at ~25°C - 50°C. Upon irradiation, these materials transform to the amorphous state at much lower ion-fluences in comparison to their anhydrous counterparts. Amorphous rhabdophane and churchite materials were found to be very sensitive to the electron beam and quickly reverts back to their original crystalline state upon electron beam exposure.