Revealing a new electroneutral uranium oxide hydrate phase with a framework structure in triclinic space group

Abstract

Uranium hydrolysis processes and associated products are fundamentally important especially to uranium geochemistry and the nuclear fuel cycle. Herein, we report the formation and characterisation of a new electroneutral uranium oxide hydrate framework (UOHF), U(H₂O)₈[(UO₂)₁₀UO₁₃(OH)₄] (UOHF2), using both structural and spectroscopic techniques. As a minor phase from the hydrothermal uranium hydrolysis in the presence of Cu(II) ions, compound UOHF2 crystallises in the triclinic P$\overline{1}$ space group having a framework structure constructed with β-U₃O₈ type layers pillared by double pentagonal uranium bipyramids and tetravalent U(IV) ions sitting inside the framework channels. The crystal structure has been revealed using synchrotron single crystal X-ray diffraction and confirmed by electron diffraction with transmission electron microscopy. The characteristic vibrational modes due to the presence of various hexavalent uranium centres were revealed by Raman spectroscopy. The successful synthesis and characterisation of UOHF2, together with an earlier reported U(H₂O)₂[(UO₂)₁₀O₁₀(OH)₂(UO₄)(H₂O)₂] (UOHF1), highlights the complex nature of uranium hydrolysis processes and its related products, with direct implications to fundamental uranium chemistry, geochemistry and the current nuclear fuel cycle.