Electrical properties of anhydrous iron oxides

The 3 main binary iron oxides: Fe1-xO (Wustite), Fe3O4 (Magnetite) and alpha Fe2O3 (Hematite) are considered, in the present review, exclusive of any other binary (as gamma Fe2O3) or ternary (as MgFe2O4) iron oxide. Beside their technological importance, in metallurgy, electronics or mineralogy, these oxides provide an experimental test-bed for studying the evolution of charge-transfer processes from the localized-electron to the itinerant-electron regimes, as this appears in the electronic properties. However in order to deal with these electric properties, it was necessary to update the knowledge of these compounds by reviewing the basic properties, going from the defect structure to magnetic and other properties. In wustite the electric conductivity cannot be understood without percolation models which take into account the clusters population as a function of temperature and oxygen activity. Anyway it is now admitted that the conductivity data need to be combined with the thermopower data, just because the defect structure depends on the experimental conditions. As regards the p-n transition, known since a long time, it is clearly explained by the spin degeneracy factor which modulates the statistics. Finally the conduction mechanism is in agreement with the small polaron model and with an activated hopping on the cation sites. This leads to confirmation of the band structure proposed by Goodenough; however recent photoemission studies open the question to know whether wustite has a Mott-Hubbard or a charge-transfer regime.