Thermoelectric power in misfit cobalties 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 ≥ 90μV.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 [CoO 2 ] 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 (Pb 4+ , Ti 4+ ) which keep unchanged the structural incommensurability but make the RS charge changing.