Electronic properties of single-crystalline Fe₄O₅

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 (σ_n ≈ σ_p), in line with the nominal average oxidation state of iron as Fe^2.5+. 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.