Single
crystals of two new uranyl silicates were grown from mixed alkali
fluoride/alkali chloride fluxes. Polymorphism is observed in Cs<sub>2</sub>(UO<sub>2</sub>)Si<sub>2</sub>O<sub>6</sub>, where hydrothermal
conditions lead to the α polymorph and flux growth favors the
β polymorph. Rb<sub>2</sub>(UO<sub>2</sub>)Si<sub>2</sub>O<sub>6</sub> crystallizes in a monoclinic distortion of the α-Cs<sub>2</sub>(UO<sub>2</sub>)Si<sub>2</sub>O<sub>6</sub> structure type
with space group <i>I</i>2/<i>a</i> and lattice
parameters <i>a</i> = 14.9932(5) Å, <i>b</i> = 14.8032(5) Å, <i>c</i> = 16.2377(9) Å, and
β = 90.7220(10)°. β-Cs<sub>2</sub>(UO<sub>2</sub>)Si<sub>2</sub>O<sub>6</sub> constitutes a new polymorph of Cs<sub>2</sub>(UO<sub>2</sub>)Si<sub>2</sub>O<sub>6</sub> and crystallizes
in the monoclinic space group <i>C</i>2 with lattice parameters <i>a</i> = 12.136(2) Å, <i>b</i> = 10.0426(17) Å, <i>c</i> = 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
