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 Zn cations, the two differ significantly. The compound Zn(OH)(HO)[(UO)UO(HO)] (), forming a framework-type structure in the 1̅ space group, was composed of β-UO layers pillared by uranyl polyhedra, with the Zn cations incorporated within the framework channels. In contrast, the compound CsZn(HO)[(UO)O(OH)]·3HO () crystallized in the 2 space group with a schoepite-like uranyl oxide hydroxide layered topology and both Zn 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 Zn (U/Zn ratio of 5.5:1) in , whereas in , the larger Cs ions were preferentially incorporated at the expense of fewer Zn 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 UO alteration in the setting of deep geological repositories.