Observation of an Unusual Uranyl Cation-Cation Interaction in the Strongly Fluorescent Layered Uranyl Phosphates Rb-6[(UO2)(7)O-4(PO4)(4)] and Cs-6[(UO2)(7)O-4(PO4)(4)]

Single crystals of two new uranyl phosphates, A(6)[(UO2)(7)O-4(PO4)(4)] (A = Cs, Rb), featuring cation-cation interactions (CCIs) rarely observed in uranium(VI) compounds were synthesized by molten flux methods. This structure crystallizes in the triclinic space group P (1) over bar with lattice parameters, a = 9.2092(4) angstrom, b = 9.8405(4) angstrom, c = 10.1856(5) angstrom, alpha = 92.876(2)degrees, beta = 95.675(2)degrees, and gamma = 93.139(2)degrees for A = Cs and a = 9.2166(9) angstrom, b = 9.3771(10) angstrom, c = 10.1210(11) angstrom, alpha = 89.981(4)degrees, beta = 96.136(4)degrees, and gamma = 92.790(4)degrees for A = Rb. The optical properties are reported for both compounds and compared to a layered uranyl phosphate, K-4[(UO2)(3)O-2(PO4)(2)], having a similar phosphuranylite-based structure but no CCIs. Partial ion exchange of Cs and Rb cations into the Rb-6[(UO2)(7)O-4(PO4)(4)] and Cs-6[(UO2)(7)O-4(PO4)(4)] structures, respectively, was achieved.