Grain boundary analysis and secondary phases in LaCoO3-based perovskites

Secondary phases and grain boundaries in LaCoO3 and La0.7Sr0.3CoO3–d ceramics have been studied by transmission electron microscopy. Both materials contained small amounts of grains of a secondary phase; the face centered cubic spinel structure Co3O4, located at triple junctions. These grains were agglomerates of several smaller grains. High resolution electron microscopy combined with annular dark field scanning transmission electron microscopy and electron energy loss spectroscopy were used to analyze the grain boundaries. In LaCoO3, the grain boundaries were sharp with stable compositions of La, Co and O across the boundaries. In La0.7Sr0.3CoO3–d, a 1–2 nm thick intergranular layer between the grains was observed. This layer was rich in O and Co and deficient in Sr and La, compared to the nominal composition of the material.