Crystal structure and thermal properties of La1-xCaxCoO3-δ (0 ≤ x ≤ 0.4)

The crystal structure and thermal properties of La1-xCaxCoO3-delta have been investigated by thermogravimetrical analysis and high-temperature X-ray diffraction in air and N-2. La1-xCaxCoO3-delta is oxygen deficient at ambient temperature, and the oxygen nonstoichiometry increases with Ca content, temperature, and reducing atmosphere in line with expectations. The crystal structure of La1-xCaxCoO3-delta at ambient temperature changed from rhombohedral (hexagonal) with space group R c for x < 0.2 to orthorhombic with space group Pnma for x > 0.3. Above 200 degrees C the structure was rhombohedral or cubic with space group Pm (3) over barm. The anisotropic thermal expansion of the lattice parameters was obtained by Rietveld refinement, and the phase transition temperature from the rhombohedral to cubic symmetry was deduced from the diffraction data. The phase transition temperature was strongly depressed by increasing the Ca content, and the findings are summarized in a phase diagram for the La1-xCaxCoO3-delta system. The second-order nature of the phase transition was discussed in terms of strain and rotation of the CoO6 octahedra. Finally, the temperature dependence of the CaO solubility in La1-xCaxCoO3-delta was established, and isothermal phase diagrams of the pseudo-ternary system La(2)A(3)-CaO-CoO are presented.