Sodium ion ordering inNaxCoO2: Electron diffraction study

Abstract

The layered sodium cobalt oxide ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ is studied by electron diffraction for a wide range of sodium contents, $0.15<x<0.75$. An extensive series of ordered $\mathrm{Na}$ ion-$\mathrm{Na}$ vacancy superlattices is found beyond the simple hexagonal average structure. The most strongly developed superlattice is found for the composition ${\mathrm{Na}}_{0.5}{\mathrm{CoO}}_2$, which displays ${\mathrm{Co}}^{3+}∕{\mathrm{Co}}^{4+}$ charge ordering at low temperatures. The structural principle for some of the observed ordering schemes, particularly near $x=0.5$, is, surprisingly, the presence of lines of $\mathrm{Na}$ ions and vacancies rather than simply maximized $\mathrm{Na}$-$\mathrm{Na}$ separations.