Low-temperature thermoelectric properties of the composite crystal [Ca₂CoO_3.34]_0.614[CoO₂]

Electric resistivity, thermoelectric power and thermal conductivity of a polycrystalline sample of the composite crystal [Ca₂CoO_3.34]_0.614[CoO₂], also known as Ca₃Co₄O₉, have been measured below 300 K. Metallic conductivity accompanied by large thermoelectric power has been observed down to 50 K. At 300 K, the sample exhibits a thermoelectric power of S = 133 μV·K^-1, resistivity of ρ = 15 mΩ·cm and thermal conductivity of κ = 9.8 mW·K^-1·cm^-1. The resulting dimensionless figure of merit becomes ZT₃₀₀ = 3.5×10^-2, which is comparable to the value reported for a polycrystalline sample of NaCo₂O₄, indicating that the title compound is a potential candidate for a thermoelectric material.