Exploring the influence of pH on the structural intricacies of uranium oxide hydrates containing both Cd(ii) and K(i) ions

Abstract

We report the synthesis of two new dual-cation uranium oxide hydrate (UOH) materials, containing both Cd²⁺ and K⁺ ions, along with their characterisation by means of single-crystal X-ray diffraction and a range of other structural and spectroscopic techniques. The materials were found to differ in structures, topology and uranium to cation ratios, with the layered UOH-Cd crystallising in a plate morphology and containing a U : Cd : K ratio of 3 : 1.5 : 1. Conversely, the framework-type UOF-Cd incorporates much less Cd, with a U : Cd : K ratio of 4.4 : 0.2 : 1 and is found as needle-like crystals. A common feature in both structures is the presence of β-U₃O₈ type layers with a distinct uranium centre which lacks the expected uranyl bonds, highlighting the importance of the β-U₃O₈ layer in the subsequent self-assembly and preferential formation of a variety of structural types. Most importantly, by exploiting the additional flexibility provided by monovalent cation species (i.e., K⁺) as secondary metal cations to synthesise these novel dual-cation materials, this work highlights the potential for broadening the scope of viable synthetic UOH phases towards furthering the understanding of these systems in their roles as alteration products in the surrounds of spent nuclear fuel in deep geological repositories.