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)42·3H2O (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.