Incorporation of uranium to the langbeinite family in Cs2UⅣLn(PO4)3 actinide waste forms

Langbeinite is proposed as a promising host matrix for radioactive wastes, but no structures incorporating actinides have been reported yet. In this work, a series of single crystals of Cs₂U^ⅣLn(PO₄)₃ (Ln = Ce - Nd and Sm - Lu), the first reported langbeinite-type structure incorporating actinides (U^IV) which crystallizes in the cubic space group P2₁3, have been successfully prepared by optimizing the high-temperature molten salt method. Their 3D frameworks are characterized by a typical langbeinite structure with U/Ln co-occupying six-coordinated octahedra and Cs cations filling closed large-sized cavities. The compounds exhibit high thermal stability up to 1200 °C under N₂ and show outstanding leaching resistance, with the leaching rates of U and Nd at 10^-8 g·m^-2·d^-1 and Cs 10^-3 g·m^-2·d^-1. This study highlights the potential of langbeinite-type structures for effectively immobilizing both tetravalent and trivalent actinides as promising waste forms.