PhD: Synthesis and investigation of structure and properties of some complex perovskites

Sandra Dimitrovska-Lazova
Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius University, Skopje, Macedonia
September, 2012
 

Abstract

In this doctoral dissertation, the results of the synthesis and investigation of four perovskite series of general formula: RCo1-xCrxO3 (R = Y, Pr, Gd); YCo1-xFexO3; Pr1-xAxCo0.5Cr0.5O3 (А = Bi, Pb) and Y0.8Са0.2Co0.5Fe0.5O3, are presented. In general, the solution combustion method was used for synthesis of the perovskites, but with different fuels and, in some cases, with previous gel formation. The obtained perovskites were identified with X-ray powder diffraction and their crystal structures were refined using the Rietveld method. The morphology of the perovskites was analyzed by SEM, and their electrochemical and catalytic properties were investigated by cyclic voltammetry.

It was shown that the solution combustion is an appropriate method for synthesis of these series of perovskites, but for some of them the use of glycine as a fuel (for example, the Pr-series), was more successful, for some other it was urea (the Gd and Y series) and YCo1-xFexO3 series was obtained via citrate precursor synthesis. The complex perovskites – those with two cations in B-position and those with substitution in A-position – are new synthesized, namely, no literature data for these compounds were found.

In order to find out the influence of the applied method of synthesis on the morphology and dimensions of the particles of the obtained perovskites, the SEM images were recorded. It could be concluded that the perovskites within these series are of the same morphology, consisted of nanoparticles joined in aggregates with porous structure. The additional heating of the particles leads to increasing of the dimension of the particles.

The X-ray diffraction patterns show that the obtained perovskites are orthorhombic, and crystallize within the space group Pnma. The crystal structure refinement (undertaken by the Rietveld method), as well as, the crystallochemical calculations, have shown that in these perovskites, there are two main reasons for deviation of the ideal perovskite structure. Namely, in all of the obtained perovskites octahedral tilting is evident, however in some of them (for example, the PrCo1-xCrxO3 series) in cases when the content of Со3+ exceeds that of Сr3+, besides the octahedral tilting, a more pronounced distortion of the octahedron is evident due to the deformation of the B-O distances. This could be due probably to some changes in the spin state of Co3+ ion, and also to the influence of A-cation on octahedral distortion.

The electrochemical and the catalytic properties of the obtained perovskites were investigated by cyclic voltammetry. The results of the analysis of these properties in different electrolytes showed that these perovskites (except the RCrO3 perovskites) exhibit pronounced catalytic activity toward chloride ions and towards basic solutions. In the first case, the perovskites catalysis the oxidation of the chloride ions and in the second case the catalytic oxidation of the hydroxide ions followed with evolution of oxygen. The electrochemical investigation of the systems of KOH and methanol showed that these materials exhibit catalytic influence to the oxidation of methanol in basic media.