Synthesis, structure and thermoelectric properties of La1-xNaxCoO3 perovskite oxides

Monovalent ion doped lanthanum cobaltate La1-xNaxCoO3 (0 <= x <= 0.25) compositions were synthesized by the nitrate-citrate gel combustion method. All the heat treatments were limited to below 1123 K, in order to retain the Na stoichiometry. Structural parameters for all the compounds were confirmed by the Rietveld refinement method using powder X-ray diffraction (XRD) data and exhibit the rhombhohedral crystal structure with space group R-3c (No. 167). The scanning electron microscopy study reveals that the particles are spherical in shape and sizes, in the range of 0.2-0.5 mu m. High temperature electrical resistivity, Seebeck coefficient and thermal conductivity measurements were performed on the high density hot pressed pellets in the temperature range of 300-800 K, which exhibit p-type conductivity of pristine and doped compositions. The X-ray photoelectron spectroscopy (XPS) studies confirm the monotonous increase in Co4+ with doping concentration up to x = 0.15, which is correlated with the electrical resistivity and Seebeck coefficient values of the samples. The highest power factor of 10 mu W mK(-2) is achieved for 10 at% Na content at 600 K. Thermoelectric figure of merit is estimated to be similar to 1 x 10(-2) at 780 K for 15 at% Na-doped samples