Pressure induced phase transformations in nanocrystalline maghemite (γ-Fe2O3)

Macro-crystalline γ-Fe2O3 transforms to the α-Fe2O3 structure at a pressure of 35 GPa. We performed a Raman spectroscopic study to explore the pressure induced phase transformation in nanocrystalline γ-Fe2O3 to 57.5 GPa. We found that at a pressure of 26.6 GPa, there was a significant transformation of the γ-phase to α-structure. All Raman modes of the α-Fe2O3 nanophase as extrapolated to 1 atm are higher than those observed in the bulk counterpart, and also exhibit lower pressure shifts, except one mode at 269.1 cm−1. The α-Fe2O3 phase transforms to a perovskite phase at ∼53.3 GPa, which is the same as that observed in the bulk material. Upon release of pressure to ambient conditions, the perovskite phase transforms to the α-Fe2O3 phase by a slow distortional mechanism. The recovered α-Fe2O3 phase exhibits the same Raman characteristics as those in the bulk sample. Particle size affects the transition pressure of the first phase transformation but does not affect the continuous post-phase transformations.