Cation−Cation Interactions in Sr₅(UO₂)₂₀(UO₆)₂O₁₆(OH)₆(H₂O)₆ and Cs(UO₂)₉U₃O₁₆(OH)₅

Two novel U⁶⁺ compounds, Sr₅(UO₂)₂₀(UO₆)₂O₁₆(OH)₆(H₂O)₆ (SrFm) and Cs(UO₂)₉U₃O₁₆(OH)₅ (CsFm), have been synthesized by mild hydrothermal reactions. The structures of SrFm (orthorhombic, C222₁, a = 11.668(1), b = 21.065 (3), c = 13.273 Å, V = 3532.5(1) ų, Z = 2) and CsFm (trigonal, R3̄c, a = 11.395(2), c = 43.722(7) Å, V = 4916.7(1) ų, Z = 6) are rare examples of uranyl compounds that contain cation−cation interactions where an O atom of one uranyl ion is directly linked to another uranyl ion. Both structures are complex frameworks. SrFm contains sheets of polyhedra that are linked through cation−cation interactions with uranyl ions located between the sheets. CsFm possesses an unusually complex framework of vertex- and edge-sharing U⁶⁺ polyhedra that incorporates cation−cation interactions.