Extended X-ray absorption fine structure spectroscopy (EXAFS) is a powerful method for determining the local coordination environment of impurities in a host phase. Nonetheless, and in particular for seemingly incompatible dopants, a lack of appropriate standard structures and a priori knowledge concerning thermal disorder, coordination numbers, and potential defect associations can render traditional shell by shell fitting unreliable. In particular, studies with nearly identical EXAFS data for U(VI) incorporated in hematite have yielded different interpretations ranging from all U assuming a uranyl configuration with two short U-O bonds in regular Fe(III) structural sites to all U incorporating in octahedral sites, with no uranyl whatsoever. In order to resolve this uncertainty, we used ab initio molecular dynamics (AIMD) to derive synthetic EXAFS spectra as a function of different charge compensation schemes, namely the creation of Fe vacancies near U. The synthetic spectra were used in linear combination fits along with adsorbed uranyl to best model the experimental spectra. The results showed that uranium(VI) substitutes for Fe(III) in regular structural sites by adopting a uranyl-like configuration with bent trans-dioxo bonds with U-O distances at about 1.95 Å. This configuration is accommodated by two partially protonated Fe(III) vacancies at opposite nearest-neighbor corner-sharing sites. However, a large proportion of adsorbed uranyl was also present. Atomic resolution scanning transmission electron microscopy was consistent with incorporation of U in structural sites, and also suggests that remaining adsorbed uranyl is occluded in nano pores. The universal need for considering the coupling of defects, charge balance, and the incorporation of seemingly incompatible impurities in Fe (oxyhydr)oxides is discussed.

Coupling of defects and incorporation of uranium in hematite

256th National Meeting (2018)

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