Electrical resistivity and thermopower of hole-doped delafossite CuCoO₂ polycrystals

We have synthesized the polycrystalline samples of the delafossite oxide CuCo₁−_yMg_yO₂ through the thermal decomposition of copper–cobalt–magnesium hydroxycarbonates. As is expected from the theoretical calculations, relatively large thermopower of S ∼ 700 μV K⁻¹ is observed at room temperature in the parent compound CuCoO₂, whereas the resistivity is too high even in the doped compounds compared with other thermoelectric oxides. The high-temperature transport is thermal-activation type characterized by two energy gaps, Δ_ρ and Δ_S, which are estimated from the resistivity and the thermopower, respectively. In the parent compound, we obtain Δ_ρ ; 0.47 eV and Δ_S ; 0.38 eV. We find that Δ_ρ is larger than Δ_S in all the samples, implying a mobility gap opening due to a grain-boundary scattering.