U(v) in metal uranates: A combined experimental and theoretical study of MgUO₄, CrUO₄, and FeUO₄

Although pentavalent uranium can exist in aqueous solution, its presence in the solid state is uncommon. Metal monouranates, MgUO₄, CrUO₄ and FeUO₄ were synthesized for detailed structural and energetic investigations. Structural characteristics of these uranates used powder X-ray diffraction, synchrotron X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and ⁵⁷Fe-Mössbauer spectroscopy. Enthalpies of formation were measured by high temperature oxide melt solution calorimetry. Density functional theory (DFT) calculations provided both structural and energetic information. The measured structural and thermodynamic properties show good consistency with those predicted from DFT. The presence of U⁵⁺ has been solidly confirmed in CrUO₄ and FeUO₄, which are thermodynamically stable compounds, and the origin and stability of U⁵⁺ in the system was elaborated by DFT. The structural and thermodynamic behaviour of U⁵⁺ elucidated in this work is relevant to fundamental actinide redox chemistry and to applications in the nuclear industry and radioactive waste disposal.