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

< 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.