Thermoelectric properties in the misfit-layered-cobalt oxides [Bi2A2O4][CoO2]b1/b2 (A=Ca, Sr, Ba, b1/b2=1.65, 1.82, 1.98) single crystals

Single crystals of different phases of misfit cobalt oxides have been synthesized. Using these materials, the intrinsic power factor has been determined. By combining thermopower S and Hall coefficient measurements, we will show how S can be tuned by doping in this family of materials, the doping being strongly dependent on the misfit ratio p=b₁/b₂ ([Bi₂A₂O₄][CoO₂]_b1/b2 with A=Ca, Sr, Ba, b₁/b₂=1.65, 1.82, 1.98). The in-plane resistivity also strongly depends on the doping : the more metallic samples are obtained for the largest carrier concentration, i.e. the smaller S value. The combination of these two factors leads to an important conclusion that the power factor is independent of the doping in these Bi-based materials converging to a unique value of 2.2plusmn0.3 muW/cmK² at room temperature, which is not so trivial. We will discuss the origin of the almost unique value of 2.2plusmn0.3 muW/cmK².