Phase transition temperature of γ-Fe2O3 ultrafine particle

The iron oxide ultrafine particles prepared by the gas evaporation method, in which commercial alpha-Fe2O3 powder is evaporated, are composed of alpha-Fe2O3 particles covered with a gamma-Fe2O3 mantle layer. To study the phase transition temperature of the gamma-Fe2O3 mantle layer, the particles were heated and observed in a transmission electron microscope. The present specimen of gamma-Fe2O3 transformed completely to alpha-Fe2O3 at above 700degreesC, and this phase transition was irreversible. The phase transition temperature was 1.4 times higher than the bulk value. The growth mechanism of the present specimen of alpha-Fe2O3 particles covered with the gamma-Fe2O3 mantle layer has been discussed in terms of the difference in the cooling rates between the inside and the surface of the particle at around the evaporation source.