Observation of an Unusual Uranyl Cation-Cation Interaction in the Strongly Fluorescent Layered Uranyl Phosphates Rb6[(UO2)7O4(PO4)4] and Cs6[(UO2)7O4(PO4)4].

Single crystals of two new uranyl phosphates, A6[(UO2)7O4(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 P1̅ with lattice parameters, a = 9.2092(4) Å, b = 9.8405(4) Å, c = 10.1856(5) Å, α = 92.876(2)°, β = 95.675(2)°, and γ = 93.139(2)° for A = Cs and a = 9.2166(9) Å, b = 9.3771(10) Å, c = 10.1210(11) Å, α = 89.981(4)°, β = 96.136(4)°, and γ = 92.790(4)° for A = Rb. The optical properties are reported for both compounds and compared to a layered uranyl phosphate, K4[(UO2)3O2(PO4)2], having a similar phosphuranylite-based structure but no CCIs. Partial ion exchange of Cs and Rb cations into the Rb6[(UO2)7O4(PO4)4] and Cs6[(UO2)7O4(PO4)4] structures, respectively, was achieved.