Redox behavior and transport properties of La0.5−2xCexSr0.5+xFeO3−δ and La0.5−2ySr0.5+2yFe1−yNbyO3−δ perovskites

The effects of doping the mixed-conducting (La,Sr)FeO3−δ system with Ce and Nb have been examined for the solid-solution series, La0.5−2xCexSr0.5+xFeO3−δ (x = 0–0.20) and La0.5−2ySr0.5+2yFe1−yNbyO3−δ (y = 0.05–0.10). Mössbauer spectroscopy at 4.1 and 297 K showed that Ce4+ and Nb5+ incorporation suppresses delocalization of p-type electronic charge carriers, whilst oxygen nonstoichiometry of the Ce-containing materials increases. Similar behavior was observed for La0.3Sr0.7Fe0.90Nb0.10O3−δ at 923–1223 K by coulometric titration and thermogravimetry. High-temperature transport properties were studied with Faradaic efficiency (FE), oxygen-permeation, thermopower and total-conductivity measurements in the oxygen partial pressure range 10−5–0.5 atm. The hole conductivity is lower for the Ce- and Nb-containing perovskites, primarily as a result of the lower Fe4+ concentration. Both dopants decrease oxide-ion conductivity but the effect of Nb-doping on ionic transport is moderate and ion-transference numbers are higher with respect to the Nb-free parent phase, 2.2 × 10−3 for La0.3Sr0.7Fe0.9Nb0.1O3−δ cf. 1.3 × 10−3 for La0.5Sr0.5FeO3−δ at 1223 K and atmospheric oxygen pressure. The average thermal expansion coefficients calculated from dilatometric data decrease on doping, varying in the range (19.0–21.2) × 10−6 K−1 at 780–1080 K.

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