Resistivity and thermoelectric power of NaxCoO2 (x=1.0, 0.7, and 0.6) system

Abstract

Results of thermoelectric power (S) and electrical resistivity (ρ) measurements are reported on NaxCoO2 compounds with x=1.0, 0.7, and 0.6. These are single-phase compounds crystallizing in the hexagonal structure (space-group P63∕mmc) at room temperature. Thermoelectric power values at 300K (S300K) are ≅80, 39, and 37μV∕K for the x=1.0, 0.7, and 0.6 samples, respectively. The samples with x=0.7 and 1.0 are metallic down to 5K, while the x=0.6 sample is semiconducting. The value of ρ300K for the x=1.0 sample is ∼0.895mΩcm and the power factor (S2∕ρ) is ∼7.04×10−3W∕mK2 which qualifies it as a good thermoelectric material. In the x=1.0 sample, S(T) is positive throughout the 300–5-K temperature range and decreases monotonically to zero as temperature T→0. In contrast, S(T) of the x=0.7 and 0.6 samples changes sign and shows negative values between 90 and 16K before approaching zero as T→0. The anomalous S(T) behavior of the x=0.6 and 0.7 samples, which are coincidentally the precursor materials to the reported superconductivity in this class of materials, indicates a dramatic change in the electronic structure of these compounds on lowering the Na content.