A spectroscopic and computational study of Cm3+ incorporation in lanthanide phosphate rhabdophane (LnPO4·0.67H2O) and monazite (LnPO4)

This study investigates the incorporation of the minor actinide curium (Cm3+) in a series of synthetic La1-xGdxPO4 (x = 0, 0.24, 0.54, 0.83, 1) monazite and rhabdophane solid solutions. To obtain information of 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 (TRLFS) and x-ray absorption fine structure (XAFS) spectroscopy investigations have been conducted and complemented with ab initio atomistic simulations. We found that Cm3+ is incorporated in the monazite endmembers (LaPO4 and GdPO4) on one specific, highly ordered lattice site. The intermediate solid solutions, however, display increasing disorder around the Cm3+ dopant as a result of random variations in nearest neighbor distances. In hydrated rhabdophane, and especially its La-rich solid solutions, Cm3+ is preferentially incorporated on non-hydrated 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 (LnPO4·0.67H2O), implying that structural substitution reactions cannot be predicted based on the structure of the host matrix only.