Thermal and electrical properties of γ-NaxCoO2 (0.70≤x≤0.78)

We have performed specific heat and electric resistivity measurements of NaxCoO2 (x = 0.70-0.78). Two anomalies have been observed in the specific heat data for x = 0.78, corresponding to magnetic transitions at T-c = 22 K and T-k similar or equal to 9 K reported previously. In the electrical resistivity, a steep decrease at T-c and a bending-like variation at T-b (= 120 K for x = 0.78) have been observed. Moreover, we have investigated the x-dependence of these parameters in detail. The physical properties of this system are very sensitive to x, and the inconsistent results of previous reports can be explained by a small difference in x. Furthermore, for a higher x value, a phase separation into Na-rich and Na-poor domains occurs as we previously proposed, while for a lower x value, from characteristic behaviors of the specific heat and the electrical resistivity at the low-temperature region, the system is expected to be in the vicinity of the magnetic instability which virtually exists below x = 0.70.