Short- and long-range order in La_1-xSr_xCoO₃ and La_1-xBa_xCoO₃

The short- and long-range order correlations of the crystal structure in the distorted perovskites La_1-xSr_xCoO₃ and La_1-xBa_xCoO₃ (0.0≤x≤0.5) have been studied by the neutron powder diffraction (NPD) and the Co K-edge X-ray absorption spectroscopy (XAS) measurements. The results of XAS and NPD indicate a local distortion around the Co³⁺ ions in LaCoO₃ at room temperature. The substitution of the La³⁺ ions by the Sr²⁺(Ba²⁺) ions leads to a gradual increase of the Co-O-Co angle and is accompanied by an increase of the mean square relative displacement (MSRD) of the Co-O bond. These results correlate with an increase of the oxygen amplitude vibration in the direction perpendicular to the Co-O bond. The possible explanation of the observed changes of the crystal and electronic structures in the above-mentioned cobaltites is discussed.