Large thermopower in metallic misfit cobalt oxides: improvement by cationic substitutions
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Large thermopower in metallic misfit cobalt oxides : improvement by cationic substitutions S. Hébert, L. B. Wang, A. Maignan, D. Pelloquin, M. Hervieu and B. Raveau Laboratoire CRISMAT, UMR CNRS ISMRA 6508, 6 Bd du Maréchal Juin, 14050 Caen Cedex, France. ABSTRACT The thermoelectric properties of misfit cobaltites have been investigated. Their crystallographic structure consists in the stacking of layers of tilted CoO6 edge-shared octahedra, separated by rock-salt type layers. The Tl based family was first investigated : it is shown that by increasing the Tl content, the resistivity ρ can be reduced while keeping a large thermopower S. Another way to improve the figure of merit Z=S2/(ρκ) is to partially substitute Bi for Tl to increase S and keep a small ρ. A new family of Pb-based misfits has also been investigated. Two different techniques have been attempted to decrease ρ and/or increase S in this family : the partial substitution of Cu for Co, and the partial substitution of Ca for Sr.
INTRODUCTION The discovery of large thermopower in the metallic oxide NaCo2O4 [1] has shown that transition-metal oxides are good candidates for thermoelectric applications, especially at high temperatures because of their good stability up to 800°C. The crystallographic structure of NaCo2O4 consists in layers of edge-shared CoO6 octahedra separated by randomly half-filled Na layers. The large thermopower S (Seebeck) at room temperature in this metallic oxide has been ascribed to the peculiar triangular symmetry of the cobalt layers, in which trivalent and tetravalent cobalt species are in the t2g5 and t2g6 low-spin state [2]. Moreover, the random filling of the Na layer was proposed to be responsible for the small thermal conductivity κ [3], necessary to get a large figure of merit Z=S2/ρκ. The ‘misfit’ cobaltites are also built from similar CoO6 edge-shared octahedra. The latter are stacked with rock-salt type separating layers, and the structural discordance of the two kinds of sublattices make them composite crystals [4]. Most interestingly, good metallicity can be found in the Tl based family [5] with ρ as small as 10-20 mΩ.cm at 300K. Because of the structural analogy of this family with NaCo2O4, we have investigated their thermoelectric properties to determine whether they are good candidates for thermoelectricity. In the following, the properties of the Tl-based family will be described. The different cationic substitutions investigated to enhance the Z factor will be presented. EXPERIMENTAL DETAILS The chemical preparation of the polycrystalline Tl-based misfit cobaltite has been reported in [4-6]. The different cationic substitutions will be described in each paragraph. To clarify the comparison, the formula similar to the one which had been proposed to describe the misfit cobaltite Ca3Co4O9 [7], namely [CoCa2O3]RS[CoO2]CdI21.62, will be given for all the misfits in the different paragraphs.The value 1.62 corresponds to the experimental ratio of the b parameters of the two different sublattices [7]. The magnetization da
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