Iron Vacancies Accommodate Uranyl Incorporation into Hematite

Radiotoxic
uranium contamination in natural systems and nuclear
waste containment can be sequestered by incorporation into naturally
abundant iron (oxyhydr)­oxides such as hematite (α-Fe2O3) during mineral growth. The stability and properties
of the resulting uranium-doped material are impacted by the local
coordination environment of incorporated uranium. While measurements
of uranium coordination in hematite have been attempted using extended
X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell
EXAFS fitting yields ambiguous results. We used hybrid functional ab initio molecular dynamics (AIMD) simulations for various
defect configurations to generate synthetic EXAFS spectra which were
combined with adsorbed uranyl spectra to fit experimental U L3-edge EXAFS for U6+-doped hematite. We discovered
that the hematite crystal structure accommodates a trans-dioxo uranyl-like
configuration for U6+ that substitutes for structural Fe3+, which requires two partially protonated Fe vacancies situated
at opposing corner-sharing sites. Surprisingly, the best match to
experiment included significant proportions of vacancy configurations
other than the minimum-energy configuration, pointing to the importance
of incorporation mechanisms and kinetics in determining the state
of an impurity incorporated into a host phase under low temperature
hydrothermal conditions.