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 (TV=122 K) at ambient pressure. WhileTV 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<TV 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 ∼TV opens a small gap within the oxygenp-band, resulting in the observed insulating state atT>TV.
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|Published - 28 Dec 2003