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

Abstract

Powder Na${}_x$CoO${}_2$ ($0.70\le x\le 0.84$) samples were synthesized and characterized carefully by X-ray diffraction analysis, inductive-coupled plasma atomic emission spectroscopy, and redox titration. It was proved that $\gamma$-Na${}_x$CoO${}_2$ is formed only in the narrow range of $0.70\le x\le 0.78$. 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 $x\ge 0.74$ 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.

4 pages (2 figures included) and 2 extra figures (gif), to be published in J. Phys. Soc. Jpn. 73 (8) with possible minor revisions