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

Temperature-dependent Fe-57 Mossbauer 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 (T-V = 122 K) at ambient pressure. While T-V decreases with pressure the CC-transition temperature increases with pressure, reaching 300K at 10GPa. The d electron localization mechanism proposed by Verwey and later by Mott for T < T-V is shown to be unrelated to the actual mechanism of the metal-insulator transition attributed to the Verwey transition. It is proposed that a first-order phase transition taking place at similar to T-V at ambient pressure opens a small gap within the oxygen p-band, resulting in the observed insulating state at T > T-V. (C) 2003 Elsevier B.V. All rights reserved.