Thermoelectric properties of NaxCoO2 and prospects for other oxide thermoelectrics

We discuss the thermoelectric properties of NaxCoO2 using the electronic structure, as determined in first principles calculations, and Boltzmann kinetic transport theory. The Fermi energy lies near the top of a manifold of Co t(2g) bands. These t(2g) bands are separated by a large gap from the higher-lying e(g) states. Although the large crystal-field splitting implies substantial Co-O hybridization, the bands are narrow. Application of standard Boltzmann transport theory to such a narrow band structure yields high thermopowers in accord with experimental observations, even for high metallic carrier densities. The high thermopowers observed for NaxCoO2 can therefore be explained by standard band theory and do not rely on low dimensionality or correlation effects specific to Co. We also present results for the cubic spinel structure ZnRh2O4. Like NaxCoO2, this compound has very narrow valence bands. We find that if it could be doped with mobile carriers, it would also have a high thermopower, comparable with that of NaxCoO2.