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

High-precision powder x-ray diffraction and Fe-57 Mossbauer 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 (similar to 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: Fe-A(3+)-> Fe-A(2+) and Fe-B(2.5+)-> Fe-B(3+). There is an intermediate mixed configuration region at pressure-temperature (P-T) conditions lying between those at which magnetite is normal, T < T-norm(P), or inverse, T>T-inv(P).