A new crossover in Fe3O4magnetite under pressure near 6 GPa: modification to ‘ideal’ inverse cubic spinel?

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The results of the first study on the thermoelectric power (Seebeck effect) of single crystals of Fe3O4 magnetite at high pressure P up to 20 GPa and room temperature are reported. The electrical resistance and a sample's contraction (relative compressibility) were also investigated over a P range of 0–20 GPa. A smooth bend (crossover) in the pressure dependence of the thermopower was firmly established near 6 GPa. This feature was attributed to a modification of Fe3O4 to an 'ideal' inverse configuration, the case where equivalent amounts of charges (the Fe2+ ions) and vacancies (the Fe3+ ions) at the octahedral sites provide nearly metallic polaron hopping conductivity via the octahedral network. The origin of this transformation might lie in (i) a P-tuning 'perfection' of the electronic transport in the inverse spinel, and/or in (ii) a transfer of a minor group of the charges from the tetrahedral sites to the octahedral ones, i.e. a configuration transition. Opportunities for the opposite valence transition in magnetite, from the inverse spinel to the normal (direct) one, are also discussed. At ambient pressure the samples of Fe3O4 were probed by Raman spectroscopy and using the electrical resistivity across the Verwey transition.