Four compositions of Ba1−xSrxCo1−yFeyO3−δ were studied for phase, oxygen uptake–release, and transition metal (TM) oxidation states after solid state processing and with in situ heating from 300 to 1273 K in air. X-Ray diffraction showed that all compositions except one had the cubic perovskite structure at all temperatures; that with x, y = 0.2 was a mixture as prepared, becoming predominantly cubic at high temperature. Thermogravimetry showed a reversible oxygen absorption–desorption of approximately ±1% from 700 to 1273 K. X-Ray absorption and Mössbauer spectroscopy showed a majority TM3+ valence, with at most 40% TM4+. Up to a temperature of 1073 K, the TM4+ was reduced to TM3+. Further heating of the composition with x, y = 0.2 to 1233 K resulted in the reduction of Co3+ to Co2+. Results from room temperature measurements confirm the thermally activated carrier hopping mechanism with charge fluctuations, while the high temperature delocalized carrier conductivity occurs with a small amount of TM reduction and without phase change for the initially cubic samples.
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