High-precision powder x-ray diffraction and Fe57 Mössbauer studies up to 20 GPa show that magnetite (Fe3 O4) 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: FeA 3+ → FeA 2+ and FeB 2.5+ → FeB 3+. There is an intermediate mixed configuration region at pressure-temperature (P-T) conditions lying between those at which magnetite is normal, T< Tnorm (P), or inverse, T> Tinv (P).
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - 2007|