In Situ Observation of Radiation Damage in Rhabdophane- and Churchite-Type Rare-Earth Phosphate Nanocrystals

Rhabdophane (REPO₄·H₂O; RE = La to Dy) and churchite (RE'PO₄·2H₂O; RE' = Gd to Lu and Y) are secondary minerals formed via hydrothermal alteration of primary rare-earth minerals such as monazite (REPO₄; RE = La to Gd) and xenotime (RE'PO₄; RE' = Tb to Lu and Y). Monazite and xenotime are promising host matrices for actinide-rich nuclear wastes. However, the aqueous alteration of these materials could result in the precipitation of rhabdophane and churchite. In this scenario, the actinides could partition into the secondary phases, and the subsequent α-decay of actinides could compromise their structural stability. The effect of radiation on their structure is unknown, and this study determines the radiation stability of rhabdophane ((Sm/Gd/Dy)PO₄·H₂O), churchite ((Gd/Dy/Y)PO₄·2H₂O), monazite (SmPO₄), and xenotime (DyPO₄). The materials were irradiated with a 650 keV Xe²⁺ ion beam and the radiation response was monitored in situ using a TEM. Rhabdophane undergoes amorphization at a slightly higher ion fluence than churchite and, therefore, has a relatively higher radiation stability. However, both these materials were outperformed by monazite and xenotime, which demonstrate higher radiation tolerance. This study has demonstrated that rhabdophane and churchite are more susceptible to radiation damage and could have implications on their actinide retention capacity.