Pressure-induced coordination crossover in magnetite; the breakdown of the Verwey-Mott localization hypothesis

Temperature-dependent 57Fe Moessbauer spectroscopy to 40 GPa shows that Fe3O4 magnetite undergoes a coordination crossover (CC), whereby charge density is shifted from octahedral to tetrahedral sites and the spinel structure thus changes from inverse to normal with increasing pressure and decreasing temperature. A precursor to the CC is a d-charge decoupling within the octahedral sites at the inverse-spinel phase. The CC transition takes place almost exactly at the Verwey transition temperature (TV=122 K) at ambient pressure. While TV decreases with pressure the CC-transition temperature increases with pressure, reaching 300 K at 10 GPa. The d electron localization mechanism proposed by Verwey and later by Mott for TTV