Abstract
On the basis of the crystal chemistry and electronic structure of γ-Na x CoO 2, we explored many cobalt-based oxide systems and encountered the compound “Ca 3Co 4O 9”. 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 [CoO 2] has a CdI 2-type triangular lattice while the second subsystem [Ca 2CoO 3] is built up from an ordered three-layered NaCl-type (RS) block. The structure formula can be expressed as [Ca 2CoO 3] p CoO 2, where p stands for the b-axis ratio, b CoO 2 / b RS∼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 ( S 2/ 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.
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