Thermopower enhancement in misfit cobaltites

The isovalent substitution of Ca2+ for Sr2+ substitution in the recently discovered misfit cobaltite [Pb0.7Co0.3Sr2O3] [CoO2](1.79) has been investigated for its very attractive thermopower properties. Starting from the pure Sr cobaltite with a thermopower S value at 300 K in the range from 100 to 130 muV K-1, it is found that the calcium substitution enhances the thermopower value up to +165 muV K-1 for the pure calcium phase, [Pb0.4Co0.6Ca2O3] [CoO2](1.61). Since these thermoelectric materials are studied for the high T applications, it should be pointed out that this S improvement induced by chemical substitution is even more pronounced for T>>300 K with S-700 K=+210 muV K-1. This beneficial effect is interpreted in the light of the clear modification in the geometry of CoO2 layers which could play a role on the cobalt electronic configurations. (C) 2002 American Institute of Physics.