Synthesis, Characterization, and Magnetic Properties of γ-NaxCoO2(0.70≤x≤0.84)

Powder NaₓCoO₂ (0.70≤ x≤ 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 γ-NaₓCoO₂ is formed only in the narrow range of 0.70≤ x≤ 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 Tₚ=16 K and an anomaly at Tₖ=9 K, as well as the transition at Tc=22 K and the broad maximum at Tₘ=50 K which had already been reported. A type of weak ferromagnetic transition seems to occur at Tₖ. The transition at Tc, which is believed to be caused by spin density wave formation, was observed clearly for x≥ 0.74 with constant Tc and Tₚ 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.