A Spectroscopic and Computational Study of Cm Incorporation in Lanthanide Phosphate Rhabdophane (LnPO·0.67HO) and Monazite (LnPO).

This study investigates the incorporation of the minor actinide curium (Cm) in a series of synthetic LaGd PO ( x = 0, 0.24, 0.54, 0.83, 1) monazite and rhabdophane solid-solutions. To obtain information on the incorporation process on the molecular scale and to understand the distribution of the dopant in the synthetic phosphate phases, combined time-resolved laser fluorescence spectroscopy and X-ray absorption fine structure spectroscopy investigations were conducted and complemented with ab initio atomistic simulations. We found that Cm is incorporated in the monazite endmembers (LaPO and GdPO) on one specific, highly ordered lattice site. The intermediate solid-solutions, however, display increasing disorder around the Cm dopant as a result of random variations in nearest neighbor distances. In hydrated rhabdophane, and especially its La-rich solid-solutions, Cm is preferentially incorporated on nonhydrated lattice sites. This site occupancy is not in agreement with the hydrated rhabdophane structure, where two-thirds of the lattice sites are associated with water of hydration (LnPO·0.67HO), implying that structural substitution reactions cannot be predicted based on the structure of the host matrix only.