STRUCTURAL EVOLUTION OF EXCEPTIONALLY IRON-DEFICIENT HEMATITE NANOCRYSTALS AS OBSERVED BY IN SITU SYNCHROTRON X-RAY DIFFRACTION

Stoichiometric hematite (Fe₂O₃) is used in energy storage devices, catalysts, and contaminant ion removers because of its low-cost, nontoxicity, natural abundance, and, most important, structural capacity. The hematite structure can embed high concentrations of foreign ions to enhance its chemical performance. A surprisingly Fe-deficient hematite, which we describe as hydrohematite, has been successfully synthesized in our lab, and potentially allows higher ion adsorption capacity than stoichiometric hematite. We have investigated the structural transformation of ferrihydrite to (hydro)hematite and goethite using time-resolved synchrotron X-ray diffraction (TR-XRD) at temperatures ranging from 70-180^oC and starting pH concentrations from 2-13. Our Rietveld structure refinements illustrate for the first time how the structure evolves as Fe occupancy increases during crystal growth.

The formation of hydrohematite always occurred in two stages. An initial rapid increase in Fe occupancy was followed by a stabilization with Fe_occ= 0.80-0.95, depending on temperature and pH. For example, the Fe occupancy at pH 11 and 90^oC increased from 0.69 to 0.84 within the first 30 min and stabilized to ~0.84 over the next 4 hours. As Fe occupancy increased during crystal growth, the lattice parameters and unit-cell volume at first increased and then decreased. We interpret the initial expansion in lattice parameters as resulting from the introduction of crystal boundary defects as nanocrystals aggregate. The contraction of lattice parameters can be explained by crystal coarsening and deprotonation. Larger and more mature crystals contain fewer vacancies and exhibit smaller lattice parameters. Moreover, deprotonation results in a shortening of the longer Fe-O octahedral bond lengths. When Fe occupancies were less than 0.80, the hydrohematite structure exhibited extremely distorted octahedra, which evolved to more symmetrical octahedra with time. Our structural study of hydrohematite microcrystals highlights their unexpectedly high tolerance for Fe deficiencies, suggesting a promising strategy to enhance the assimilation capacity of foreign ions.