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 Cs 2 U Ⅳ Ln(PO 4 ) 3 (Ln = Ce – Nd and Sm – Lu), the first reported langbeinite-type structure incorporating actinides (U IV ) which crystallizes in the cubic space group P 2 1 3, 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 N 2 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.