Structural and Thermal Properties of La_1-xSr_xCoO_3-δ

The crystal structure and the thermal properties of La 1- x Sr x CoO 3- δ (0 ≤ x ≤ 0.5) perovskites have been investigated by high-temperature X-ray diffraction. At ambient temperature, the crystal structure changes from a rhombohedral structure with the space group R 3̄ c to a cubic structure with space group Pm 3̄ m at x ≥ 0.55. The thermal evolution of the lattice parameters was determined by Rietveld refinement of the diffraction data. The rhombohedral distortion from cubic symmetry decreased nearly linearly with increasing temperature up to the phase transition to the cubic perovskite structure. The linear thermal expansion coefficients of the lattice parameters were found and the rhombohedral to cubic phase transition temperature was determined. The phase transition temperature decreases rapidly with increasing Sr content in La 1- x Sr x CoO 3- δ . The phase transition is discussed with relation to the angle of rotation and the strain parameter of the CoO 6 octahedra. Finally, the structural and thermal properties of Sr- and Ca-substituted LaCoO 3 are compared, demonstrating the significant difference in the effect of the two alkaline earth cations.