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 +/- delta, and Sr3.6La0.4Fe6Oz (nominal composition) have been studied by electron microscopy. The lanthanum free material shows a microstructure containing mainly Sr4Fe6O13 +/- delta, with some 5 vol% Sr1-yLayFe12O19 embedded as micron-size inclusions. The lanthanum-containing material revealed a significantly different microstructure consisting of similar to 20 vol% micron-size Sr1-yLayFe12O19 embedded in similar to 40 vol% (Sr0.85La0.15)FeO3-delta tetragonal perovskite, and similar to 40 vol% of plate-shaped nano-scale intergrowths between (Sr1-xLax)FeO3-delta and Sr4Fe6O13 +/- delta phases. 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 +/- delta phase. The observed microstructure is important input in explaining the significant effect of La addition on the transport properties of Sr4Fe6O13 +/- delta materials. (C) 2001 Kluwer Academic Publishers.