Investigation of Rare Earth-Containing Double Phosphates of the Type A₃Ln(PO₄)₂ (Ln = Y, La, Pr, Nd, and Sm–Lu) as Potential Nuclear Waste Forms

A new series of rubidium rare earth double phosphates is presented. High-quality single crystals of Rb3Ln(PO4)2 (Ln = Y, La, Pr, Nd, and Sm–Lu) were grown via high-temperature flux growth methods. The Rb3Ln(PO4)2 (Ln = La, Pr, Nd, and Sm–Tb) phases crystallize in space group P21/m, and the Rb3Ln(PO4)2 (Ln = Y and Dy–Lu) phases crystallize in space group P31m. A thermally induced and reversible structural transition, due to a change in the denticity of a rare earth-ligated phosphate group, is observed between the two structures at a temperature that depends on the incorporated rare earth. High-entropy versions, Rb3[Eu0.2Gd0.2Tb0.2Dy0.2Ho0.2](PO4)2 and Rb3[Gd0.2Tb0.2Dy0.2Ho0.2Er0.2](PO4)2, of the double phosphate were prepared to assess how readily they could be obtained in single-crystal form. A high observed radiation damage tolerance and favorable density functional theory-calculated formation enthalpies for the trivalent actinide analogues of Rb3M(PO4)2 suggest likely successful actinide analogue syntheses in the future.