Synthesis and characterization of a uranium oxide hydrate framework with Sr(ii) ions: structural insights and mixed uranium valences

Abstract

A mixed-valence uranium oxide hydrate framework with Sr²⁺ ions (UOF-Sr2) was synthesized hydrothermally and characterized with multiple structural and spectroscopic techniques. Compound UOF-Sr2 crystallizes in the monoclinic space group C2/c, having a complicated three-dimensional structure with Sr²⁺ ions in the narrow channels. Both SEM and TEM characterization revealed a thin-plate crystal morphology, with the TEM-SAED pattern indexed to the crystal structure determined by synchrotron single-crystal XRD. Bond valence sum analysis indicated the possible presence of pentavalent uranium in octahedral coordination environments, subsequently confirmed with diffuse reflectance spectroscopy. Raman spectroscopy revealed extensive splitting of uranyl vibrations due to the presence of various coordination environments for uranyl moieties. This work highlights the complexity and flexibility of the uranium oxide hydrate system in the natural weathering of uraninite, and sheds light on the possible alteration pathways for spent nuclear fuels under geological disposal conditions.