Structure of uranium (VI) oxide dihydrate, UO₃·2H₂O; synthetic meta-schoepite (UO₂)₄O(OH)₆·5H₂O

The structure of uranium oxide dihydrate, also known as meta-schoepite (UO2)4O(OH)6·5H2O, has been determined from a synthetic single crystal. The structure, at 150 K, space group Pbcn, lattice constants a = 14.6861 (4), b = 13.9799 (3) and c = 16.7063 (5) Å, consists of layers of stoichiometry (UO2)4O(OH)6, formed from edge-sharing UO7 pentagonal bipyramids, interleaved with hydrogen-bonded water molecules. Three of the layer hydroxyl groups are linked through hydrogen bonding to single water molecules and the three remaining OH units form interactions with water molecules that each act as acceptors in two hydrogen bonds. One of the water molecules in the inter-layer region is disordered over two symmetry-related sites and forms hydrogen-bonded interactions only within the layer and with the uranyl O atoms. The relationship of the structure of meta-schoepite to that of schoepite is discussed in detail.