Flux crystal growth of uranium(v) containing oxyfluoride perovskites

Abstract

The novel phases Rb₄NaU₃O_12−xF_x (1), K₄NaU₃O_12−xF_x (2), and Rb_2.1K_1.9KU₃O_12−xF_x (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) Å, 8.6264(2) Å, and 8.8390(3) Å, 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.