Flux crystal growth of uranium(<scp>v</scp>) containing oxyfluoride perovskites

The novel phases Rb4NaU3O12−xFx (1), K4NaU3O12−xFx (2), and Rb2.1K1.9KU3O12−xFx (3) were synthesized by molten flux methods using mixed alkali fluoride melts. The oxyfluorides crystallize in the cubic space group Imm with a lattice parameters of 8.7472(2) A, 8.6264(2) A, and 8.8390(3) A, respectively. All three structures crystallize in a cubic perovskite structure, ABO3 (A4BB′3O12), where the A site is fully occupied by an alkali cation, and the B site is shared by the remaining smaller alkali cation and uranium in an ordered fashion such that the alkali cation on the B site is surrounded by square uranyl bipyramids. The structures were characterized by single crystal X-ray diffraction, energy dispersive spectroscopy, X-ray absorption near edge structure spectroscopy, X-ray photoelectron spectroscopy, magnetic susceptibility measurements, DFT calculations, thermogravimetric analysis, and UV-vis spectroscopy, all of which support the presence of U(V) in the three new materials.