Thermoelectric properties of misfit cobalt oxides: the crucial role of the t2g orbitals

HEBERT Résumé: The large Seebeck coefficient observed in the metallic oxide NaxCoO2 [I. Terasaki et al., PRB56, R12685 (1997)] has shown that oxides are potentially interesting as thermoelectric materials. Apart from these thermoelectric properties, NaxCoO2 also exhibits superconductivity for x ~ 0.3 in the hydrated phase [K. Takada et al., Nature 422, 53 (2003)], and this family actually possesses a complex and fascinating phase diagram [G. Lang et al., PRB78, 155116 (2008)]. The crucial parameter to consider is the presence of CoO2 layers, made of edge shared octahedra located on a triangular lattice, and separated by a randomly filled layer of Na+. Another interesting family with CoO2 layers is the misfit family with these layers separated by 2, 3 or 4 rock-salt layers, with the b axis of the two different sublattices being incommensurate. In this seminar, the thermoelectric properties of these misfit compounds will be presented. The origin of the large thermopower in these metallic oxides will be discussed emphasizing the importance of doping, and spin and orbital degeneracy of low spin states Co3+ and Co4+. The peculiar properties (metallicity and large thermopower) generated by these CdI2 type layers will be compared to other oxides, with a more classical behaviour.