Structure and Composition of Nanoscopic Domains in Functional Perovskite-Type Materials

Abstract

A- and B-site substituted cobaltate perovskites (ABO3) were prepared by soft chemistry and Pulsed laser Deposition (PLD) processes. The formation of nano-and microdomains was examined by means of transmission electron microscopy to determine the influence on the materials properties. The growth of thin epitaxial film son oxide substrates by pulsed reactive cross beam laser ablation proceeds through island formation in stranski-Krastanov fashion; after the islands reach a certain height they develop laterally to form a dense epitaxial film. Orientation relationships, interfacial strain and surface roughness depend on the misfit with the respective substrate. Oxygen deficient La/Ca cobaltates exhibit Brownmillerite and Ruddleston-Popper defects, and increasing O-deficiency results in increasing resistivity of the films. Local defects are found as well in the microstructure of Mn-doped cobaltates obtained by soft chemistry and sintered for long times at high temperatures. This perovskite phase exists in the orthorhombic phase at higher Mn-concentration and presents an array of inter-grown twin domains. Thin films of cobaltates with nominal composition of La0.6Ca0.4CoO3 exhibit catalytic activity for oxygen reduction and evolution in alkaline electrolytes. Control over the crystallinity of the thin film, achieved by pulsed reactive crossed beam laser ablation, was used to show that the catalytic activity varies with crystallinity. Single crystalline films exhibit the highest activity, followed by polycrystalline film and amorphous films. Even the orientation of single crystalline films has an influence on the catalytic activity.