Structural evolution and effects of calcium doping on nanophasic LaCoO3 powders prepared by non-alkoxidic sol–gel technique

Abstract

The structural evolution and the effects of calcium doping on LaCoO₃ powders as a function of thermal annealing were investigated by means of X-ray absorption fine structure spectroscopy from both Co and La K-edges. The samples, synthesized by a sol–gel process starting from methanolic solutions of cobalt(II) and calcium(II) acetates, and lanthanum(III) nitrate, were analysed as xerogels (as-prepared) and after heat treatment. The investigations indicated that the lanthanum cobaltite skeleton was not yet formed in the as-prepared samples whereas significant structural changes ascribable to the formation of the LaCoO₃ network were detected after annealing at temperatures between 700 and 1000 °C. Doping with calcium induced remarkable influences in the local structure of the system, which are dependent on the treatment temperatures. These changes could be attributed to different effects: i) the formation of cobalt and calcium oxide-based compounds, ii) the distortion in the perovskite structure and iii) the partial substitution of La(III) sites by Ca(II) ions. The studies also indicated the change in the oxidation state of cobalt from +2 in the xerogels to +3 after annealing.