Thermoelectric power in misfit cobaltites ceramics: Optimization by chemical substitutions

The ceramics of misfit cobaltites form a large family of layer oxides which exhibit large values of Seebeck coefficient (S-300K >= 90 mu V.K-1). They can be divided in two categories depending on the number of the rocksalt-type (RS) layers, n = 3 or n = 4, separating the [CoO2] conducting layers. From the comparison of their structural features, it is found that the S values are governed by the incommensurability ratio and/or the charge of the RS block. This is clearly evidenced by making chemical substitutions at the level-of the RS block: isovalent but with cations of different ionic radius (r(Ba)(2+) > r(Sr)(2+) > r(Ca)(2+)) modifying the incommensurability ratio or, aliovalent substitutions for cobalt (Pb4+, Ti4+) which keep unchanged the structural incommensurability but make the RS charge hanging.