Incorporating Transition Metal Ions into Uranium Oxide Hydrates: The Role of Zn(II) and the Effect of the Addition of Cs(I) Ions

The synthesis of two zinc-bearing uranium oxide hydrate (UOH) materials has been achieved, and their crystal structures, obtained via single-crystal X-ray diffraction using synchrotron radiation, and additional structural and spectroscopic properties are reported herein. Although both structures incorporate Zn²⁺ cations, the two differ significantly. The compound Zn₂(OH)₂(H₂O)₅[(UO₂)₁₀UO₁₄(H₂O)₃] (UOHF-Zn), forming a framework-type structure in the P1̅ space group, was composed of β-U₃O₈ layers pillared by uranyl polyhedra, with the Zn²⁺ cations incorporated within the framework channels. In contrast, the compound Cs₂Zn(H₂O)₄[(UO₂)₄O₃(OH)₄]₂·3H₂O (UOH-Zn) crystallized in the Cmc2₁ space group with a schoepite-like uranyl oxide hydroxide layered topology and both Zn²⁺ and Cs⁺ cations making up the interlayer species. The apparent driving force for the differences in the structures was the change from KOH to CsOH during synthesis, with the smaller K⁺ ions excluded in lieu of a higher proportion of Zn²⁺ (U/Zn ratio of 5.5:1) in UOHF-Zn, whereas in UOH-Zn, the larger Cs⁺ ions were preferentially incorporated at the expense of fewer Zn²⁺ cations (U/Cs/Zn ratio of 8:2:1). Highlighted in this work is the effect of the chemical species and, in particular, their ionic radius on UOH formation, further improving the understanding of UO₂ alteration in the setting of deep geological repositories.