We recently demonstrated evidence of a novel non-radiolytic, non-photochemical formation mechanism for solid-state uranyl peroxide from hydrated uranyl fluoride. We have now clarified the temperature and humidity conditions under which this transformation of uranyl fluoride to uranyl hydroxide and then uranyl peroxide occurs. X-ray diffraction of the hydration products suggests that the observed uranyl hydroxide intermediate is structurally similar to the layered uranyl hydroxide hydrate minerals schoepite and metaschoepite, but with a significantly expanded interlayer spacing. We further characterize this uranyl hydroxide intermediate by comparison to synthetic uranyl hydroxide hydrates, presenting a structural and spectroscopic study of synthetic metaschoepite using X-ray and neutron diffraction as well as Raman spectroscopy and inelastic neutron scattering. In addition to providing insight into the uranyl hydroxide intermediate in our peroxide formation reaction, these experiments clarify the vibrational spectra of synthetic metaschoepite and inform previous inconsistencies in the literature. In-situ experiments with humidity control reveal that the interlayer spacing of synthetic metaschoepite is highly dependent on water vapor pressure, with a subset of the interlayer water molecules easily removed and reinserted. A similar reversible expansion and contraction of the interlayer spacing is observed for the uranyl hydroxide intermediate. Further elucidation of the peroxide formation mechanism will be discussed.

Characterization of uranyl hydroxide hydrates and the formation of uranyl peroxide

2019 Spring National Meeting

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