Growth and characterization of NaxCoO2-δ crystals

Na_xCoO_2-δ (0.5≤x≤1) crystals were grown by a flux method. The largest Na_xCoO_2-δ crystal size was 10×10×0.5 mm, and the crystal phase depended on the composition of the flux. Crystals grown from a flux of NaCl mixed with Na₂CO₃ and B₂O₃ and crystals grown from a flux of Na₂CO₃ mixed with B₂O₃ were identified as Na_0.6CoO_2-δ (γ phase) and Na_0.9CoO_2-γ (α phase), respectively. The in-plane (ab-axis) thermoelectric properties of each phase were measured. The electrical resistivity (ρ) of the γ phase and the a phase showed the almost same metallic temperature dependence except for an unusual increase of ρ_α at 5-20 K and the different magnitude. The value of ρ_γ and ρ_α were 1.3 mΩcm and 30 mΩcm at 300 K, respectively. The Seebeck coefficient (S) of each phase exhibited the different S(T) behavior and the different magnitude, and the values were 100 μV/K and 150 μV/K at 300 K, respectively.