Electronic properties of single-crystalline Fe₄O₅

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Ovsyannikov, Sergey V. ; Karkin, Alexander E. ; Korobeinikov, Igor V. ; Morozova, Natalia V. ; Bykov, Maxim ; Bykova, Elena ; Dubrovinsky, Leonid:
Electronic properties of single-crystalline Fe₄O₅.
In: Dalton Transactions. Bd. 52 (2023) . - S. 5563-5574.
ISSN 1477-9234
DOI: https://doi.org/10.1039/D3DT00381G

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Abstract

We synthesized single and polycrystals of iron oxide with an unconventional Fe4O5 stoichiometry under high-pressure high-temperature (HP–HT) conditions. The crystals of Fe4O5 had a CaFe3O5-type structure composed of linear chains of iron with octahedral and trigonal-prismatic oxygen coordinations. We investigated the electronic properties of this mixed-valence oxide using several experimental techniques, including measurements of electrical resistivity, the Hall effect, magnetoresistance, and thermoelectric power (Seebeck coefficient), X-ray absorption near edge spectroscopy (XANES), reflectance and absorption spectroscopy, and single-crystal X-ray diffraction. Under ambient conditions, the single crystals of Fe4O5 demonstrated a semimetal electrical conductivity with nearly equal partial contributions of electrons and holes (σn ≈ σp), in line with the nominal average oxidation state of iron as Fe2.5+. This finding suggests that both the octahedral and trigonal-prismatic iron cations contribute to the electrical conductivity of Fe4O5via an Fe2+/Fe3+ polaron hopping mechanism. A moderate deterioration of crystal quality shifted the dominant electrical conductivity to n-type and considerably worsened the conductivity. Thus, alike magnetite, Fe4O5 with equal numbers of Fe2+ and Fe3+ ions can serve as a prospective model for other mixed-valence transition-metal oxides. In particular, it could help in the understanding of the electronic properties of other recently discovered mixed-valence iron oxides with unconventional stoichiometries, many of which are not recoverable to ambient conditions; it can also help in designing novel more complex mixed-valence iron oxides.