Pressure response of vacancy ordered maghemite (γ-Fe2O3) and high pressure transformed hematite (α-Fe2O3)

Combined XRD and Mössbauer effect spectroscopy studies to high pressures of ∼30 GPa of vacancy ordered maghemite are presented. The vacancy ordered superstructure is robust and remains intact up to the pressure-induced onset transition to hematite at 13–16 GPa. The pressure transformed hematite is shown to be crystallographically textured, unlike the randomised low pressure maghemite phase. This arises out of a pressure or stress instigated topotactic transformation of the cubic-spinel to hexagonal-corundum structure. The textured sample permits us to obtain information on the spin reorientation behavior of the pressure transformed hematite in compression and decompression sequences. Spin reorientation is restricted to ∼15° over wide pressure ranges, attributable to the effect of entrapped vacancies in the high pressure structure. Thus there are structural and magnetic peculiarities specific to pressure transformed hematite not evident in pressurized hematite starting material. These are triggered by the maghemite→hematite transformation.

Pressure instigated topotactic transformation of vacancy ordered γ-Fe2O3→α-Fe2O3. There is restricted spin (Bhf) reorientation in the new pressure transformed hematite due to entrapped vacancies. The change in direction of Vzz signifies a distortion of the FeO6 octahedral local environment.Figure optionsDownload as PowerPoint slideHighlights
► Robust vacancy ordered superstructure in maghemite to high pressures.
► Pressure instigated topotactic transformation to hematite and subsequent texture.
► Defect trapping in the pressure transformed hematite.
► Entrapped defects restricts spin reorientation in pressure transformed hematite.
► Contrasting behavior with pressurized hematite starting material.