Crystal structure and thermoelectric properties of the misfit-layered cobalt oxides

On the basis of the crystal chemistry and electronic structure of γ-NaxCoO2, we explored many cobalt-based oxide systems and encountered the compound “Ca3Co4O9”. Through the detailed structure analyses, the cobalt oxide has been revealed to be a misfit-layered compound, which consists of two interpenetrating monoclinic subsystems. The first subsystem [CoO2] has a CdI2-type triangular lattice while the second subsystem [Ca2CoO3] is built up from an ordered three-layered NaCl-type (RS) block. The structure formula can be expressed as [Ca2CoO3]pCoO2, where p stands for the b-axis ratio, bCoO2/bRS∼0.62. A typical polycrystalline sample of the compound exhibits reasonable thermoelectric (TE) performance with thermopower S=130 μV K−1, resistivity=15 mΩ cm and thermal conductivity κ=1 W m−1 K−1 at 300 K. The resulting figure-of-merit Z (S2/ρκ)=1.1×10−4 K−1 is comparable to that of polycrystalline samples of the γ-phase, indicating that the present misfit cobalt oxide is a potential candidate for a thermoelectric material.