The crystal structures of α- and β-CdUO4

The structural parameters of α- and β-CdUO4 crystals are determined by X-ray powder diffraction technique. α-CdUO4 is rhombohedral and cell parameters are a = 6.233(3) Å and α = 36.12(5)°. β-CdUO4 crystallizes in a C-centered orthorhombic cell with a = 7.023(4), b = 6.849(3), c = 3.514 (2) Å. The space groups are R3m for α-CdUO4 and Cmmm for β-CdUO4. α-CdUO4: 1U in (000), 1Cd in (12 12 12), 2O(1) in ±(uuu), 2O(2) in ±(vvv); u = 0.113, v = 0.350, Z = 1. β-CdUO4: 2U in (000; 12 12 0), 2Cd in (12 0 12; 0 12, 12), 4O(1) in (0, ±y, 0; 12, 12 ±y, 0), 4O(2) in (±x, 0, 12; 12 ±x, 12, 12); x = 0.159, y = 0.278, Z = 2. β-CdUO4 contains collinear uranyl UO2+2 groups with a UO(1) distance of 1.91 Å, located either along or parallel to the c axis whereas the UO(1) bond length in α-CdUO4 is 1.98 Å which is longer than the usual uranyl bond length.