Co $K$-edge x-ray absorption spectroscopy (XAS) measurements have been performed on anhydrous ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ $(x=0.3,0.74,1.0)$, $\mathrm{Li}\mathrm{Co}{\mathrm{O}}_2$, ${\mathrm{H}}_{\delta }\mathrm{Co}{\mathrm{O}}_2$, and hydrated ${\mathrm{Na}}_{0.3}\mathrm{Co}{\mathrm{O}}_2\bullet y{\mathrm{H}}_2\mathrm{O}$ $(y=0.6,1.2)$ layered cobaltates. Sealed environments were employed through the duration of the experiments to maintain phase purity. The XAS of anhydrous ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ shows systematic changes with $x$ indicative of a Co oxidation state increase with decreasing Na content. This trend is shown to correlate well with a detailed review of the structural parameters. In contrast, although the XAS results on the ${\mathrm{Na}}_{0.3}\mathrm{Co}{\mathrm{O}}_2\bullet y{\mathrm{H}}_2\mathrm{O}$ $(y=0,0.6,1.2)$ materials show detectable spectral changes, they indicate that there is no Co valence change upon hydration. Here pre and post spectroscopy material characterization allows a unique certainty in these exceedingly sensitive materials. The XAS spectral changes which accompany hydration are discussed in terms of a homogenization of the electronic structure in the $\mathrm{Co}{\mathrm{O}}_2$ planes and are correlated with structural properties and recent NMR results.

• Received 1 June 2006

DOI:https://doi.org/10.1103/PhysRevB.74.125103