Crystal growth and transport properties of α- and γ-phase NaxCoO2

The NaxCoO2 single crystals with γ- and α-phase structure were grown by an electro-crystallization of high temperature solution (EHTS) method. X-ray diffraction studies showed that γ-phase NaxCoO2 crystals were grown from a flux of pure NaOH, and α-phase NaxCoO2 crystals were grown from a mixed flux of NaOH with a small amount of SrCO3. The in-plane electrical resistivity of γ-phase NaxCoO2 decreases with decreasing temperature (T), exhibiting the metal-like properties, while that of α-phase NaxCoO2 increases with decreasing temperature, exhibiting the semiconductor-like properties. The thermoelectric power S(T) at 300 K is 80 μV/K and 30 μV/K for γ- and α-phase NaxCoO2, respectively, and both decrease with decreasing temperature. RH of γ-phase NaxCoO2 shows anomalous temperature dependence.