The Crystal Structure of Triuranyl Diphosphate Tetrahydrate

The hydrated neutral uranyl phosphate, (UO₂)₃(PO₄)₂(H₂O)₄, was synthesized by hydrothermal methods. Intensity data were collected using MoKα radiation and a CCD-based area detector. The crystal structure was solved by direct methods and refined by full-matrix least-squares techniques to agreement indices wR₂=0.116 for all data, and R1=0.040, calculated for the 2764 unique observed reflections (∣F_o∣≥4σ_F). The compound is orthorhombic, space group Pnma, Z=4, a=7.063(1) Å, b=17.022(3) Å, c=13.172(3) Å, V=1583.5(5) ų. The structure consists of sheets of phosphate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO₂)(PO₄)]⁻ and the uranophane sheet anion topology. The sheets are connected by a uranyl pentagonal bipyramid in the interlayer that shares corners with a phosphate tetrahedron on each of two adjacent sheets, resulting in an open framework with isolated H₂O groups in the larger cavities of the structure.