High-pressure phase of magnetite.

Abstract

M\"ossbauer spectroscopy and x-ray diffraction were employed to investigate the magnetic and structural properties of the high-pressure monoclinic phase of magnetite. Measurements were performed to 66 GPa at 300 K using diamond anvil cells. Based primarily upon the M\"ossbauer parameters, the following features of the high-pressure phase were deduced. With increasing pressure the high-pressure phase evolves at P\ensuremath{\ge}25 GPa and its abundance increases monotonically at the expense of the low-pressure cubic phase. The high-pressure phase is not magnetic at 300 K and its monoclinic structural features resemble that of the low-pressure phase. The tetrahedral and octahedral sites characteristic of the inverse spinel structure, albeit distorted, remain the building blocks of the high-pressure phase. The fast electron hopping between ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$ at the octahedral sites prevails to the highest pressure. It is suggested that the cubic\ensuremath{\rightarrow}monoclinic\ensuremath{\rightarrow}cubic hysteretic cycle involves a mild displacive phase transition not affecting the coordination number of any of the iron cations.