A Structural Investigation of Hydrous and Anhydrous Rare-Earth Phosphates

Rhabdophane- (REPO₄·nH₂O; RE = La to Dy), monazite- (REPO₄; RE = La to Gd), and xenotime-type (REPO₄ and RE'PO₄·nH₂O; RE = Tb to Lu and Y; RE' = Ho to Lu and Y) rare-earth phosphate materials are being considered for a number of applications including as photonic materials, for biolabeling studies, and as potential nuclear wasteforms. Structural studies of hydrous rare-earth phosphates are rather limited when compared to anhydrous rare-earth phosphates. In this study, rhabdophane- (REPO₄·nH₂O; RE = La, Nd, Sm, Gd, and Dy) and xenotime-type (REPO₄·nH₂O; RE = Y and Yb) materials were synthesized by a precipitation-based method and investigated using X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES). Examination of the powder XRD data from rhabdophane-type materials has confirmed that the rhabdophane structure crystallizes in the monoclinic crystal system rather than the hexagonal structure that has most often been reported. Materials adopting the rhabdophane- or xenotime-type structure were studied as a function of temperature to understand how the structure varies with increasing annealing temperature. Local structural information was obtained by collecting P K- and RE L₁-edge XANES spectra. Examination of P K-edge XANES spectra from rhabdophane- and xenotime-type materials revealed changes in the local environment around P as a function of temperature. These changes were attributed to the removal of water from these structures as a result of high temperature annealing.