Conductivity in iron oxides

The conductivity and thermal e.m.f. of wüstite and magnetite have been measured as a function of the ratio O/Fe at T ⩾ 1000°C. The conductivity, which shows only a very small temperature dependence, changes slightly on crossing the wüstite-to-magnetite phase boundary but decreases rapidly in both phases as the composition approaches stoichiometric FeO and Fe2O3, respectively. The thermal e.m.f. is p-type in wüstite of low oxygen content, changes to n-type at high oxygen content and stays n-type in magnetite. The conductivity behavior of wüstite measured between room temperature and −170°C is similar to that of magnetite. The results fit an electron-transfer model. Such a model predicts a conductivity proportional to the concentration of divalent iron times that of trivalent iron, as shown by the isotherms at T ⩾ 1000°C. The electron-transfer model also explains the similarity in temperature behavior of wüstite and magnetite. A band model, on the other hand, needs a number of rather arbitrary assumptions to explain the high-temperature isotherms and cannot account for the low-temperature results. Some measurements on hematite show that this phase is very narrow and that electron transfer between iron ions of different valency plays only a minor role.