Incorporation of Trivalent Lanthanide (Ln) in Powellite by Ca _1– <i> _x </i> Na _{0.5 <i>x</i>} Ln _{0.5 <i>x</i>} MoO ₄ Solid Solution Formation

Powellite (CaMoO₄) is a secondary phase frequently detected in nuclear waste glass corrosion experiments. The coupled substitution in powellite of Ca by a trivalent lanthanide, used as a surrogate for trivalent actinides, and Na⁺ through Ca_1-<i>x</i>Na_0.5<i>x</i>Ln_0.5<i>x</i>MoO₄ solid solution formation was investigated. Results suggest homogeneous and complete (0 < <i>x</i> < 1) solid solution formation for Ln = La, Eu, and Yb. X-ray diffraction provides evidence of a progressive evolution of unit cell parameters with the mean ionic radius of cations substituting for Ca and a slight excess molar volume for the La- and Yb-based solid solutions. X-ray absorption spectroscopy revealed that interatomic distances depend on the size of atoms involved in bonding, independent of the extent of Ca substitution, and in some cases on the geometrical arrangement of connected polyhedra. A topological analysis suggests that the observed excess molar volume may originate from local structural disorder due to the difference in size between substituting and substituted dodecahedral cations. Outcomes collectively suggest that the formation of powellite as a glass corrosion product may represent a sink capable of efficiently immobilizing trivalent actinides.