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

Pressure Response of Vacancy Ordered Maghemite (γ-Fe2O3) and High Pressure Transformed Hematite (α-Fe2O3). G.Hearne, V.Pischedda: Journal of Solid State Chemistry, 2012, 187, 134–42