Crystal Structure Evolution of Nanohematite during Dissolution: Evidence for Partitioning of Iron Vacancies in Nanoparticle Shells

Changes in the crystal structure of hematite (Fe2O3) during dissolution were analyzed using in situ, time-resolved X-ray diffraction (TRXRD) of synthetic hematite powders in 3 M HCl solutions. The refined Fe occupancy in hematite increased from 0.899(4) to 0.98(1) during dissolution, and lattice parameters (a, c) and unit-cell volume also increased. Because of the shared octahedral faces in the crystal structure of hematite, increasing Fe3+ occupancy enhanced repulsion between Fe3+ ions, promoting structural expansion. Previous TRXRD studies of nanohematite growth revealed that Fe occupancy also increases as crystals enlarge (Chen et al. 2023). The increase in Fe occupancy during both growth and dissolution suggests that structural changes induced by nanoscale sizing are not always reversible as a function of particle size. We attribute the increase in Fe occupancy as hematite dissolved to the sequential removal of defective surface layers. A higher concentration of Fe vacancies in the outer shells relative to the cores of the particles may account for the increase in Fe occupancy during both particle growth and dissolution.