Flux Crystal Growth and Optical Properties of Two Uranium-Containing Silicates: A₂USiO₆ (A = Cs, Rb)

Single crystals of two uranium silicates, Cs₂USiO₆ and Rb₂USiO₆, have been grown from molten fluoride fluxes and structurally characterized by single-crystal X-ray diffraction. Cs₂USiO₆ crystallizes in the body-centered orthorhombic space group, Immm, with a = 8.5812(4) Å, b = 13.0011(6) Å, and c = 13.8811(7) Å. The size of Rb is slightly too small to fit into this structural framework without effecting slight structural changes that result in a 6-fold superstructure. Sharp satellite peaks were observed in the single-crystal X-ray diffraction data, indicating the existing of a superstructure. The crystals were examined by electron diffraction, the results of which suggest that the structure can be thought of as the Immm isotype (a = 8.4916(6) Å, b = 12.6678(9) Å, and c = 13.5077(9) Å) on average, with an approximately 6-fold superstructure along the c axis. The materials were further characterized by UV–vis reflectance spectroscopy.