  The crystal structure of the iron oxide gamma-Fe2O3 is 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\approx 3. Using a supercell of the cubic structure, we obtain the spectrum
of energies of all the ordered configurations which contribute to the partially
disordered P4332 cubic structure. Our results show that the configuration with
space group P41212 is indeed much more stable than the others, and that this
stability arises from a favourable electrostatic contribution, as this
configuration exhibits the maximum possible homogeneity in the distribution of
iron cations and vacancies. Maghemite is 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. The confirmation of the ordered tetragonal structure
allows us to investigate the electronic structure of the material using density
functional theory (DFT) calculations. The inclusion of a Hubbard (DFT+U)
correction allows the calculation of a band gap in good agreement with
experiment. The value of the gap is dependent on the electron spin, which is
the basis for the spin-filtering properties of maghemite.
