Breakdown of the Verwey-Mott localization hypothesis in magnetite

Temperature-dependent⁵⁷Fe Mössbauer 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 ad-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. WhileT_V decreases with pressure the CC-transition temperature increases with pressure, reaching 300 K at 10 GPa. Thed electron localization mechanism proposed by Verwey and later by Mott forT<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 ∼T_V opens a small gap within the oxygenp-band, resulting in the observed insulating state atT>T_V.