Crystal structure, morphology and physical properties of LaCo1−xTixO3±δ perovskites prepared by a citric acid assisted soft chemistry synthesis

The influence of the partial substitution of Co by Ti in the LaCoO3 perovskite system is studied by evaluating the electrical conductivity, the Seebeck coefficient and the thermal conductivity of the compounds up to T = 1273 K. The X-ray diffraction patterns of the LaCo1−xTixO3±δ (0.01 ⩽ x ⩽ 0.5) phases show two structural modifications depending on the Ti content. Compounds with x < 0.3 crystallize in the rhombohedrally distorted perovskite structure while samples with x ⩾ 0.3 possess an orthorhombic unit cell. The oxidation state of the Co ions is studied by X-ray absorption near edge structure (XANES) spectroscopy. A negative thermoelectric power is found in the LaCoO3 system for low level Ti substitution (x = 0.01). In contrast, samples with higher Ti content show a large positive Seebeck coefficient, indicating positive majority charge carriers in the system. The electrical resistivity of the studied materials reveals a semiconducting-like behaviour. The lattice thermal conductivity was found to be low and nearly temperature-independent. The samples exhibit very small crystallite sizes in the range of few nanometres. Therefore, the low thermal conductivity can be assigned to an enhanced phonon scattering on grain boundaries.