[U(H₂O)₂]{[(UO₂)₁₀O₁₀(OH)₂][(UO₄)(H₂O)₂]}: A Mixed-Valence Uranium Oxide Hydrate Framework

A uranium oxide hydrate framework, [U(H₂O)₂]{[(UO₂)₁₀O₁₀(OH)₂][(UO₄)(H₂O)₂]} (UOF1), was synthesized hydrothermally using schoepite as a uranium precursor. The crystal strucutre of UOF1 was revealed with synchrotron single-crystal X-ray diffraction and confirmed with transmission electron miscroscopy. The typical uranyl oxide hydroxide layers similar to those in β-U₃O₈ are further connected via double-pentagonal-bipyramidal uranium polyhedra to form a three-dimensional (3D) framework structure with tetravalent uranium species inside the channels. The presence of mixed-valence uranium was investigated with a combination of X-ray absorption near-edge structure and diffuse reflectance spectroscopy. Apart from the major hexavalent uranium, evidence for tetravalent uranium was also found, consistent with the bond valence sum calculations. The successful preparation of UOF1 as the first pure uranium oxide hydrate framework sheds light on the structural understanding of the alteration of UO₂₊ as either a mineral or spent nuclear fuel.