Oxidation states of Co and Fe in Ba1−xSrxCo1−yFeyO3−δ (x, y = 0.2–0.8) and oxygen desorption in the temperature range 300–1273 K

Abstract

Four compositions of Ba_1−xSr_xCo_1−yFe_yO_3−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.