Thermoelectric Properties of Two NaxCoO2Crystallographic Phases

We measured and analyzed the electrical resistivity (ρ), Hall coefficient ( R H ), Seebeck coefficient ( S ) and magnetic susceptibility (χ) of γ-phase Na 0.6 CoO 2 and α-phase Na 0.9 CoO 2 crystals prepared by solution growth using NaCl-Na 2 CO 3 -B 2 O 3 fluxes. The ρ of α-phase Na 0.9 CoO 2 (ρ α ) is much higher than that of γ-phase Na 0.6 CoO 2 (ρ γ ), while the magnitude of S of α-phase Na 0.9 CoO 2 ( S α ) exceeds that of γ-phase Na 0.6 CoO 2 ( S γ ). The origin of these differences is primarily the difference in carrier density. In addition, ρ α has a minimum at around 20 K, while ρ γ exhibits a metallic temperature-dependence throughout the measured temperature range. Since α-phase Na 0.9 CoO 2 exhibits an antiferromagnetic transition at the same temperature 20 K, the increase in ρ α at low temperatures implies the carrier localization effect caused by forming a long-range order among magnetic moments in the conductive CoO 2 layer.