Simulation models of defects in MgAl2O4:Fe2+, Fe3+ spinels

Abstract

A static computer simulation using the GULP code has been used to study dipole defects in iron doped spinels in normal and inverse structures. The occupation of tetrahedral (Mg2+) and octahedral (Al3+) sites in the normal structure was simulated and the results indicate a preferential replacement of Mg2- by an Al3+ justified by the observed stoichiometric deviation in synthetic spinels. Besides this, Al2O3 excess produces Mg2+ vacancies and favours the hole centre production. Antisite disorder of the form [Mg2+]Al 3+ and [Al3+]Mg 2+ is one of the possible mechanisms responsible for the existence of dipole defects in these materials. The obtained energy for the interchange between Al3+ and Mg2+ was 0.52eV. The calculated energies indicate that Fe2+ replaces Mg2+ and Fe3+ replaces Al3+. The presence of Fe3+ in octahedral sites takes into account the oxidation process of Fe2+ in these sites. Computer simulation has also been carried out on the inverse structure where the stability of the lattice was obtained for the Imma space group.