In situ XRD study of the phase transition of nanocrystalline maghemite (γ-Fe2O3) to hematite (α-Fe2O3)

Nanocrystalline iron oxide was produced by inert gas condensation (IGC). The phase and crystal size analysis were done by X-ray powder diffraction (XRD), X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM). All three methods show that the only oxide that was formed was maghemite. The crystal sizes vary for different samples from 9 nm to 16 nm. The kinetics of the phase transition from maghemite to hematite was observed by in situ XRD measurements at temperatures around 300°C. It is possible to describe the transition by first order kinetics with an activation energy which increases with increasing crystal size. The sizes of the produced hematite crystals were above 35 nm, but growing of the maghemite crystals was not observed. These results can be explained qualitatively under the aspect of the Gibbs energies.