An electron-diffraction study of the transformation α-Fe₂O₃ to γ-Fe₂O₃

Abstract

Outgrowths formed on haematite single crystals by heating in air have been examined by electron diffraction. At 500°C a change occurs in their crystal structure, apparently due to the migration of some cations to tetrahedral interstices. Otherwise, the unit cell and oxygen-ion framework of this 'β-phase' remain as in α-Fe₂O₃ (haematite). At 700°C the structure of the outgrowths changes to one characteristic of γ-Fe₂O₃ and persists as such, even after prolonged heating at 900°C. Thus the transformation γ-Fe₂O₃ to α-Fe₂O₃ is not, as has been supposed, strictly irreversible, since the α-γ transition has now been found to occur under suitable conditions. The presence of β-phase traces seems to be the cause of the anomalous magnetic behaviour of haematite observed by Neel and others. It is concluded that the α-γ transformation is a necessary step preceding the solid-state spinelization reaction involved in the formation of a ferrospinel.