Large thermopower in metallic misfit cobaltites

The possibility to improve the thermoelectric properties of the misfit layer cobaltite [Tl1-xSr2+x-yCOyO3] [CoO2](b1/b2), either by increasing the Tl content or by substituting Bi or Tl, has been investigated. Using a direct synthesis method, it is shown that the Tl content in this oxide, written for sake of clarity TlalphaSrCoO3+delta, can be increased significantly up to a maximum value alpha = (2)/(3), by simultaneously increasing the oxygen pressure during the synthesis. The so-prepared TlalphaSrCoO3+delta oxides are characterized by a decrease of resistivity as a increases up to (2)/(3), whereas the thermopower remains unchanged, suggesting a possible improvement of the figure of merit. The substitution of Bi for Tl leads to larger thermopower at room temperature but, in contrast, induces an upturn in the resistivity and negative magnetoresistance at low temperature.