Search for high temperature p-type thermoelectrics: cobalt oxides
J. Hejtmánek1, D. Sedmidubsky1, A. Maignan2, C. Martin2, H. Fujishiro3
1Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21, Prague, Czech Republic
2Laboratoire CRISMAT, CNRS/ISMRA/Universite de Caen, 6 boulevard du Maréchal Juin, 14050 CAEN cedex, France.
3Dept. of Materials Science and Technology , Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
Within a large family of conducting oxides the complex cobaltites are of the fundamental importance as candidates for the high temperature p-type thermoelectric applications. This is due to the unique concurrence of a low electrical resistivity and a high positive thermoelectric power. A plausible explanation link the high and weakly temperature dependent thermoelectric power to the configurational entropy of "quasi" itinerant charge carriers with the decisive contribution originating from the interplay of the number of charge carriers and the degeneracy of Co3+ and Co4+(2+) groundstates, respectively.
Structurally the studied cobaltites can be divided in two sub-groups – 3D perovskites, with corner shared octahedral or pseudo-octahedral (CoO6) structural network, and low dimensional (2D or 1D) compounds where the dominating Co-O-Co bond is mediated via edge/face shared (CoO6) building blocks. We discuss the eligibility of the use of cobaltites as the p-type thermoelectrics namely with respect to the relation between the thermoelectric power and spin (Co3+ as low, intermediate and high spin) and orbital (eg = dz2, dx2-y2 ) degeneracy of cobalt ions. The discussion of transport and thermal properties is complemented, both for perovskites and low dimensional cobaltites, by thermal conductivity, thermoelectric power, electrical resistivity and magnetic data measured up to 1000 K.