Structural characterization of temperature- and pressure-induced inverse↔normal spinel transformation in magnetite

High-precision powder x-ray diffraction and 57Fe Moessbauer studies up to 20 GPa show that magnetite (Fe3O4) undergoes a reversible normal↔inverse transition with increasing pressure or decreasing temperature. There is no resolvable change in the spinel-type crystal structure or unit-cell volume at the phase transition. However, the volume of the tetrahedral (A) site increases (17%) and that of the octahedral (B) site decreases (∼7%) as electron charge density is transferred from the B to the A site. The corresponding valence changes cause the inverse → normal transition with increasing pressure: FeA3+→FeA2+ and FeB2.5+→FeB3+. There is an intermediate mixed configuration region at pressure-temperature (P-T) conditions lying between those at which magnetite is normal, TTinv(P)