Exploring the High-Temperature Electrical Performance of Ca 3−x La x Co 4 O 9 Thermoelectric Ceramics for Moderate and Low Substitution Levels.

Aliovalent substitutions in Ca₃Co₄O₉ often result in complex effects on the electrical properties and the solubility, and impact of the substituting cation also depends largely on the preparation and processing method. It is also well-known that the monoclinic symmetry of this material's composite crystal structure allows for a significant hole transfer from the rock salt-type Ca₂CoO₃ buffer layers to the hexagonal CoO₂ ones, increasing the concentration of holes and breaking the electron–hole symmetry from the latter layers. This work explored the relevant effects of relatively low La-for-Ca substitutions, for samples prepared and processed through a conventional ceramic route, chosen for its simplicity. The obtained results show that the actual substitution level does not exceed 0.03 (x < 0.03) in Ca_3−xLa_xCo₄O₉ samples with x = 0.01, 0.03, 0.05 and 0.07 and that further introduction of lanthanum results in simultaneous Ca₃Co₄O₉ phase decomposition and secondary Ca₃Co₂O₆ and (La,Ca)CoO₃ phase formation. The microstructural effects promoted by this phase evolution have a moderate influence on the electronic transport. The electrical measurements and determined average oxidation state of cobalt at room temperature suggest that the present La substitutions might only have a minor effect on the concentration of charge carriers and/or their mobility. The electrical resistivity values of the Ca_3−xLa_xCo₄O₉ samples with x = 0.01, 0.03 and 0.05 were found to be ~1.3 times (or 24%) lower (considering mean values) than those measured for the pristine Ca₃Co₄O₉ samples, while the changes in Seebeck coefficient values were only moderate. The highest power factor value calculated for Ca₂.₉₉La₀.₀₁Co₄O₉ (~0.28 mW/K²m at 800 °C) is among the best found in the literature for similar materials. The obtained results suggest that low rare-earth substitutions in the rock salt-type layers can be a promising pathway in designing and improving these p-type thermoelectric oxides, provided by the strong interplay between the mobility of charge carriers and their concentration, capable of breaking the electron–hole symmetry from the conductive layers. [ABSTRACT FROM AUTHOR]