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 doublephosphates is presented. High-quality single crystals of Rb3Ln-(PO4)2(Ln = Y, La, Pr, Nd, and Sm-Lu) were grown via high-temperatureflux growth methods. The Rb3Ln(PO4)2(Ln = La, Pr,Nd, and Sm-Tb) phases crystallize in space groupP21/m, and theRb3Ln(PO4)2(Ln = Y and Dy-Lu) phases crystallize in spacegroupP31m. A thermally induced and reversible structuraltransition, due to a change in the denticity of a rare earth-ligatedphosphate group, is observed between the two structures at atemperature that depends on the incorporated rare earth. High-entropy versions, Rb3[Eu0.2Gd0.2Tb0.2Dy0.2Ho0.2](PO4)2andRb3[Gd0.2Tb0.2Dy0.2Ho0.2Er0.2](PO4)2, of the double phosphatewere prepared to assess how readily they could be obtained insingle-crystal form. A high observed radiation damage tolerance and favorable density functional theory-calculated formationenthalpies for the trivalent actinide analogues of Rb3M(PO4)2suggest likely successful actinide analogue syntheses in the future.