Crystal structure, morphology and physical properties of LaCo_(1-x)Ti_xO_(3+-δ) perovskites prepared by a citric acidassisted soft chemistry synthesis

摘要

The influence of the partial substitution of Co by Ti in the LaCoO_3 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 LaCo_(1-x)Ti_xO_(3+-δ) (0.01 = 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 LaCoO_3 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 semicon-ducting-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 pho-non scattering on grain boundaries.