The crystal structure of the iron oxide γ-Fe2O3 was usually reported in either the cubic system (space group P4332) with partial Fe vacancy disorder or in the tetragonal system (space group P41212) with full site ordering and c/a ≈ 3. Using a super-cell of the cubic structure, the spectrum of energies was obtained for all of the ordered configurations which contributed to the partially disordered P4332 cubic structure. The results showed that the configuration with space group P41212 was indeed much more stable than the others, and that this stability arose from a favourable electrostatic contribution, as this configuration exhibited the maximum possible homogeneity in the distribution of iron cations and vacancies. Maghemite was therefore expected to be fully ordered in equilibrium, and deviations from this behaviour should be associated with metastable growth, extended anti-site defects and surface effects in the case of small nanoparticles. This confirmation of the ordered tetragonal structure permitted the investigaton of the electronic structure of the material using density functional theory calculations. The inclusion of a Hubbard (DFT + U) correction permitted the calculation of a band gap in good agreement with experiment. The value of the gap was dependent upon the electron spin, which was the basis for the spin-filtering properties of maghemite.

Vacancy Ordering and Electronic Structure of γ-Fe2O3 (Maghemite): a Theoretical Investigation. R.Grau-Crespo, A.Y.Al-Baitai, I.Saadoune, N.H.De Leeuw: Journal of Physics - Condensed Matter, 2010, 22[25], 255401