Reduction properties of phases in the system La0.5Sr0.5MO3 (M=Fe, Co)

Phases formed by the reduction of compounds of the type La0.5Sr0.5MO3 (M=Fe, Co) have been characterized by means of temperature programmed reduction, X-ray powder diffraction, 57Fe Mössbauer spectroscopy and Fe K-, Co K-, Sr K-, and La LIII-edge X-ray absorption spectroscopy. The results show that treatment of the material of composition La0.5Sr0.5FeO3 (which contains 50% Fe4+ and 50% Fe3+) at 650 °C in a flowing 90% hydrogen/10% nitrogen atmosphere results in the formation of an oxygen-deficient perovskite-related phase containing only trivalent iron. Further heating in the gaseous reducing environment at 1150 °C results in the formation of the Fe3+-containing phase SrLaFeO4, which has a K2NiF4-type structure, and metallic iron. The material of composition La0.5Sr0.5CoO3 is more susceptible to reduction than the compound La0.5Sr0.5FeO3 since, after heating at 520 °C in the hydrogen/nitrogen mixture, all the Co4+ and Co3+ are reduced to metallic cobalt with the concomitant formation of strontium- and lanthanum-oxides.