Electronic behavior of three oxygen non-stoichiometric Fe4+/Fe3+ oxoperovskites

For comparison with the Mn4+/Mn3+ oxoperovskites at the crossover from localized to itinerant behavior of the σ-bonding e electrons, the electronic properties of three oxygen non-stoichiometric, mixed-valent Fe4+/Fe3+ oxoperovskites were explored by measuring their resistivity ρ(T), thermoelectric power α(T), and magnetic susceptibility χ(T). Oxidation of Ca2Fe2O5 by annealing in ozone progresses by oxygen insertion to give conductive CaFeO3 perovskite clusters in a localized-electron, weakly oxidized brownmillerite Ca2Fe2O5+δ matrix. Removal of 0.12 oxygen per formula unit from La1/3Sr2/3FeO3 lowers somewhat its cooperative disproportionation reaction, and fivefold-coordinated ions neighboring oxygen vacancies in the more ionically bonded slabs act as donors to the covalently bonded Fe(V)O6 planes. Single-crystal SrFeO2.83 exhibited bad-metal behavior with superparamagnetic, electron-rich fluctuations below 240 K that, on cooling below 190 K, become progressively trapped by the oxide-ion vacancies as an immobile second phase; long-range antiferromagnetic order is stabilized below a TN≈60 K.