On the nano-size intergrowth of Sr4Fe6O13±δ and (Sr1−xLax)FeO3−δ phases in the mixed conductor Sr3.6La0.4Fe6Oz

The microstructures of mixed-conducting Sr4Fe6O13±δ, and Sr3.6La0.4Fe6Oz(nominal composition) have been studied by electron microscopy. The lanthanum free material shows a microstructure containing mainly Sr4Fe6O13±δ, with some 5 vol% Sr1−yLayFe12O19embedded as micron-size inclusions. The lanthanum-containing material revealed a significantly different microstructure consisting of ∼20 vol% micron-size Sr1−yLayFe12O19embedded in ∼40 vol% (Sr0.85La0.15)FeO3−δtetragonal perovskite, and ∼40 vol% of plate-shaped nano-scale intergrowths between (Sr1−xLax)FeO3−δand Sr4Fe6O13±δphases. Domains with dimensions of 20–50 nm are observed in the tetragonal perovskite when viewed along its fourfold axis. From compositional analysis it is concluded that there is little or no solubility of La in the Sr4Fe6O13±δphase. The observed microstructure is important input in explaining the significant effect of La addition on the transport properties of Sr4Fe6O13±δmaterials.