Flux versus Hydrothermal Growth: Polymorphism of A₂(UO₂)Si₂O₆ (A = Rb, Cs)

Single crystals of two new uranyl silicates were grown from mixed alkali fluoride/alkali chloride fluxes. Polymorphism is observed in Cs₂(UO₂)Si₂O₆, where hydrothermal conditions lead to the α polymorph and flux growth favors the β polymorph. Rb₂(UO₂)Si₂O₆ crystallizes in a monoclinic distortion of the α-Cs₂(UO₂)Si₂O₆ structure type with space group I2/a and lattice parameters a = 14.9932(5) Å, b = 14.8032(5) Å, c = 16.2377(9) Å, and β = 90.7220(10)°. β-Cs₂(UO₂)Si₂O₆ constitutes a new polymorph of Cs₂(UO₂)Si₂O₆ and crystallizes in the monoclinic space group C2 with lattice parameters a = 12.136(2) Å, b = 10.0426(17) Å, c = 7.7896(13) Å, and β = 95.849(4)°. The two structures are compared, the driving force behind the observed polymorphism is discussed, and the luminescence properties are reported for each compound.