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

Results from atomistic simulation calculations and X-ray photoelectron spec troscopy are used to refine the proposed defect structures of gamma -Fe2O3 and Fe3O4 doped with tetravalent ions. These structures are of interest bec ause 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 g amma -Fe2O3 is more favourable than occupation of tetrahedral sites and ind icate that the M4+ ions substitute on iron sites rather than occupy interst itial sites. For gamma -Fe2O3, XPS results show that no reduction of Fe3+ o ccurs. Of the possible remaining balncing defects, calculations indicate th at vacancies on the iron sites with six nearest-neighbour iron atoms are fa voured. In the case of Fe3O4 the 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 res ults suggest that the pattern of doping by tetravalent ions in spinel relat ed iron oxides differs from that in the corundum-related alpha -Fe2O3. (C) 2001 Elsevier Science Ltd. Ail rights reserved.