Thermoelectric properties of perovskites:: Sign change of the Seebeck coefficient and high temperature properties

The possibility to change the Seebeck coefficient sign has been evidenced in the LaCoO3 perovskites. A small hole doping (Co3+/Co4+) will result in a large positive Seebeck coefficient, while a small electron doping (Co2+/Co3+) will give a large negative Seebeck coefficient at room temperature. This mechanism is shown to be efficient as well in 1D Ca3Co2O6 deriving from hexagonal perovskites. By doping Ca3Co2O6 with Ti4+, a mixed valency Co2+/Co3+ is introduced and the thermopower turns negative. At high temperature, the Seebeck coefficients of LaCoO3 and related compounds decrease to small values due to the spin state transition. The values converge towards a positive value, close to +20 mu V/K at 800 K. This suggests that at high T, the Seebeck coefficients in the case of localized charges do not depend on the doping, but only on the spin and orbital degeneracies. On the other hand, in the case of metallic-like samples as electron-doped manganites, the properties can be described up to high T in terms of a single-band metal. Due to the linear variation of S as a function of T and the almost constant value of rho, the ratio S-2/rho which is crucial for high temperature applications increases. (c) 2007 Elsevier Ltd. All rights reserved.