Time degradation of electronic and ionic transport in perovskite-like La0.5Ca0.5FeO3−δ

Abstract

The oxygen nonstoichiometry, local states of iron cations, thermomechanical behavior, total electrical conductivity, Seebeck coefficient and specific oxygen permeability of perovskite-like La0.5Ca0.5FeO3−δ ceramics were studied as function of the oxygen partial pressure, temperature and time. The p-type electronic conductivity and oxygen permeation both exhibit a slow and irreversible degradation on thermal and redox cycling, as for the long-term isothermal tests. These changes correlate with decreasing oxygen content measured by coulometric titration and thermogravimetry, and with the volume contraction demonstrated by the controlled-atmosphere dilatometry. The Mössbauer spectroscopy results indicate that the degradation is associated with progressive ordering processes. The reductive decomposition into LaFeO3-based perovskite solid solution and Ca2LaFe3O8±δ phases occurs via the formation of oxygen vacancy-ordered La0.5Ca0.5FeO2.75, where the ratio between octahedrally- and tetrahedrally-coordinated Fe3+ states is approximately 3:1. Under oxidizing conditions, microdomain texture comprising the ordered and disordered phases is formed at temperatures above 1023–1073 K.