Investigation of defect structures formed by doping tetravalent ions into spinel-related iron oxides using atomistic simulation calculations

Results from atomistic simulation calculations and X-ray photoelectron spectroscopy are used to define the proposed defect structures of gamma-Fe2O3 and Fe3O4 doped with tetravalent ions. These structures are of interest because of the effect of doping on the properties of the oxides. Calculations confirm that occupation by Sn4+ and Ti4+ of octahedral sites in Fe3O4 and gamma-Fe2O3 is more favourable than occupation of tetrahedral asites and indicate that the M4+ ions substitute on iron sites rather than occupy interstitial sites. For gamma-Fe2O3, XPS shows no reduction of Fe3+ occurs. Of the possible remaining balancing defects, calculations indicate that vacancies on the iron sites with six nearest-neighbour iron atoms are favoured. In the case of Fe3O4 results suggest that reduction of Fe3+ to Fe2+ is preferred and that reduction occurs on a site adjacent to M4+. This small cluster localises the disruption to the crystal structure. These results suggest that the pattern of doping by tetravalent ions in spinel-related iron oxides differs from that in the corundum-related alpha-Fe2O3