Direct current electrical conductivity of gamma ferric oxide

Abstract

The study of the direct current electrical conductivity, σ, of freshly prepared γ-Fe₂O₃ and that of a sample stored for seven days in static air suggests that γ-Fe₂O₃ adsorbs oxygen and water from the atmosphere. From infra-red spectra it is deduced that the absorbed water in γ-Fe₂O₃ is present as the physically adsorbed water and as lattice water. The adsorbed oxygen and physically adsorbed water are removed by heating to 100‡ C, while the lattice water remains in γ-Fe₂O₃ even up to 280‡ C. The removal of lattice water is associated with a decomposition during which some of the hydrogen formed occupies the vacancy sites. This suggested formation of the hydrogen ferrite phase is based on the kink in the log σ against T ⁻¹ curve observed at 177‡ C. This kink is very well resolved for a sample equilibrated at 100‡ C in normal atmosphere, and the measurements of σ above 100‡ C of this sample are done in an N₂ atmosphere. The suggestion that the hydrogen ferrite phase is formed has been substantiated by comparison of the X-ray diffraction patterns of γ-Fe₂O₃ heated under the different atmospheres. From the log σ against T ⁻¹ plot for a sample heated under a nitrogen atmosphere the activation energy is small (< 0.05 eV) up to 215‡ C, and it is comparatively large (0.95 eV) above 215‡ C. These results suggest a hopping mechanism for the direct current electrical conductivity of γ-Fe₂O₃. This suggestion has been substantiated by data of the temperature variation of Seebeck voltage.