Phase Transition Temperature of γ-Fe₂O₃ Ultrafine Particle

Abstract

The iron oxide ultrafine particles prepared by the gas evaporation method, in which commercial α-Fe₂O₃ powder is evaporated, are composed of α-Fe₂O₃ particles covered with a γ-Fe₂O₃ mantle layer. To study the phase transition temperature of the γ-Fe₂O₃ mantle layer, the particles were heated and observed in a transmission electron microscope. The present specimen of γ-Fe₂O₃ transformed completely to α-Fe₂O₃ at above 700°C, 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 α-Fe₂O₃ particles covered with the γ-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.