Structure and transport properties of La0.5Sr0.5−xCaxFeO3−δ

Abstract

Effects of calcium doping on the structure, dimensional stability, and mixed oxygen-ion and electron conductivity of perovskite-like La 0.5 Sr 0.5 − x Ca x FeO 3 − δ ( x = 0–0.3) were studied in light of potential membrane applications. The incorporation of relatively small Ca 2 + cations into the lanthanum-strontium ferrite lattice decreases unit cell volume, oxygen nonstoichiometry variations and chemical contribution to the thermal expansion in air. These changes correlate with rising tendency to local oxygen-vacancy ordering and the formation of nano-sized domains with the brownmillerite and LaCa 2 Fe 3 O 8 -type lattices, as revealed by electron diffraction. The resultant vacancy trapping, changing domain structure and enlargement of the interfacial boundary area lead to non-linear relationships between the partial ion conductivity and cation composition, while the apparent activation energy for ion transport at temperatures below 900 °С remains almost constant, 0.6–0.7 eV. The n-type electron contribution to the total conductivity, measured in the oxygen pressure range 10 − 20 –0.5 atm at 700–950 °С, is also essentially independent of the calcium concentration. • The electron diffraction reveals nano-sized domain structure in La 0.5 Sr 0.5 − x Ca x FeO 3 − δ . • The brownmillerite and LaSr 2 Fe 3 O 8 -type domains are detected. • The oxygen vacancy trapping in the ordered nanodomains decreases ion conduction. • The development of the interfacial boundary area promotes oxygen diffusion.