Synthesis, characterization, and magnetic properties of γ-NaxCoO2 (0.70≤x≤0.84)

Powder NaxCoO2 (0.70 less than or equal to x less than or equal to 0.84) samples were synthesized and characterized carefully by X-ray diffraction analysis, inductive-coupled plasma atomic emission spectroscopy, and redox titration. It was proved that gamma-NaxCoO2 is formed only in the narrow range of 0.70 less than or equal to x less than or equal to 0.78. Nevertheless, the magnetic properties depend strongly on x. We found, for the first time, two characteristic features in the magnetic susceptibility of Na0.78CoO2, a sharp peak at T-p = 16 K and an anomaly at T-k = 9 K, as well as the transition at T-c = 22 K and the broad maximum at T-m = 50 K which had already been reported. A type of weak ferromagnetic transition seems to occur at T-k. The transition at T-c, which is believed to be caused by spin density wave formation, was observed clearly for x greater than or equal to 0.74 with constant T-c and T-p independent of x. On the other hand, ferromagnetic moment varies systematically depending on x. These facts suggest the occurrence of a phase separation at the microscopic level, such as the separation into Na-rich and Na-poor domains due to the segregation of Na ions. The magnetic phase diagram and transition mechanism proposed previously should be reconsidered.