Long-range modulation in uranium-bearing hematite superstructure constrained from STEM analysis and ab initio calculations

Natural hematite from the Mesoproterozoic Olympic Dam Cu-U-Au-Ag deposit, South Australia, contains pentavalent uranium, with average composition of Fe1.974U0.0138Pb0.0045O3. Scanning transmission electron microscopy (STEM) and ab initio calculations provide evidence for the superstructuring of hematite to accommodate uranium. High angle annular dark field STEM imaging reveals a 12-fold, long-range modulation along <1¯100>* directions as well as blocks of bright and dark contrast. This modulation was reproduced using an empirical model of a 4 × 4 × 1 hematite superstructure (Fe1.467O3) with variable Fe occupancy over 12 layers of Fe. An excellent match is observed between the model, electron diffraction patterns, and images, yet analytical data obtained on the [22¯01] zone axis require expansion of the superstructure to a 4 × 4 × 4 unit. Given that the large size of the 4 × 4 × 4 hematite superstructure prohibits ab initio calculations to constrain a model with variable Fe and O occupancies to achieve charge balance, a simplified Fe and O vacancy scheme is employed for incorporation of U5+. The model involves a double-corner Fe vacancy environment constrained for pentavalent U from prior ab initio simulations and localized O vacancies for charge balance. Our assessment of this relaxed superstructure (Fe1.9693+U0.0105+O2.9792−) with 12 Fe vacancies and 8 O vacancies shows a good match with analytical data. Calculated Bader charge and magnetic moment validate the inference of U5+ incorporation into the hematite superstructure. Moreover, formation energy calculations show how the superstructure is stabilized by the presence of U relative to the vacancy defect structure. The natural U-rich hematite coexists with nanoscale magnetite needles, and the formation of magnetite and local collapse of the hematite superstructure to disordered Ubearing hematite are interpreted as a response to radioactive decay of U over 1.6 Ga. Models for U incorporation in Fe-oxides that combine ab initio calculations and STEM analysis are further valuable for the long-term storage of radioactive waste as much as for optimized, low environmental impact exploitation of uranium-bearing ores.