Flux Method Growth and Structural Analysis of Two Tetravalent Uranium Orthophosphate Single Crystals, Cs 2 U IV (PO 4 ) 2 and Isostructural Cs 2 (U IV 0.75 Ce IV 0.25 )(PO 4 ) 2 .

Two novel uranium(IV) orthophosphate framework compounds were obtained by the high-temperature flux method in CsCl-CsF eutectic salt. Cs 2 U IV (PO 4 ) 2 ( 1 ) and isostructural Cs 2 (U IV 0.75 Ce IV 0.25 )(PO 4 ) 2 ( 2 ) are tetragonal structures bridged by (U/Ce) IV -O octacoordinated dodecahedra and PO 4 tetrahedra, with Cs + cations filling in the channels. The crystal structures exhibit good structural and thermal stability with a potential capacity to immobilize tetravalent radionuclides.