Mössbauer spectroscopy of pressure-induced phase transformation from maghemite to hematite

Using a diamond anvil cell, ⁵⁷Fe Mössbauer spectroscopy has been carried out on maghemite, γ-Fe₂O₃, under high pressure up to 30 GPa. Maghemite transforms to hematite, α-Fe ₂O₃, with a wide coexistent pressure range. The onset of phase transformation is 14 GPa and complete transformation to hematite is found at 26 GPa. The pressure-induced transformation is irreversible and hematite is preserved at decompression process. At decompression from 30 GPa the quadrupole splitting of hematite varies as a function of pressure, changing its sign at 11 GPa and room temperature, since Morin transition temperature is below room temperature at 11 GPa and the 90° spin rotation occurs. And furthermore, we clarified the orientation of a magnetization to exploit a characteristic in Mössbauer spectroscopy. The angle between the incident γ-ray and the magnetization direction changes from random to nearly parallel with the increase of pressure. The magnetization direction maintains nearly parallel after maghemite transforms to hematite. This strong orientation of magnetization does not appear in compressed original hematite. This orientation is characterized by the transformation.