Crystal Chemistry of the Potassium and Rubidium Uranyl Borate Families Derived from Boric Acid Fluxes

The reaction of uranyl nitrate with a large excess of molten boric acid in the presence of potassium or rubidium nitrate results in the formation of three new potassium uranyl borates, K2[(UO2)2B12O19(OH)4]·0.3H2O (KUBO-1), K[(UO2)2B10O15(OH)5] (KUBO-2), and K[(UO2)2B10O16(OH)3]·0.7H2O (KUBO-3) and two new rubidium uranyl borates Rb2[(UO2)2B13O20(OH)5] (RbUBO-1) and Rb[(UO2)2B10O16(OH)3]·0.7H2O (RbUBO-2). The latter is isotypic with KUBO-3. These compounds share a common structural motif consisting of a linear uranyl, UO22+, cation surrounded by BO3 triangles and BO4 tetrahedra to create an UO8 hexagonal bipyramidal environment around uranium. The borate anions bridge between uranyl units to create sheets. Additional BO3 triangles extend from the polyborate layers and are directed approximately perpendicular to the sheets. All of these compounds adopt layered structures. With the exception of KUBO-1, the structures are all centrosymmetric. All of these compounds fluoresce when irradiated with long-wavelength UV light. The fluorescence spectrum yields well-defined vibronically coupled charge-transfer features.