Synthesis, Characterization, and Magnetic Properties of gamma-NaxCoO2 (0.70 < x <0.84)

Powder Na x CoO 2 (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-Na x CoO 2 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 Na 0.78 CoO 2 , 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 ≥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 sepa...