Evidence for the two-dimensional hybridization in Na0.79CoO2 and Na0.84CoO2

The electron density distributions of Na_0.79CoO₂ and Na_0.84CoO₂ have been obtained by the maximum entropy method and the Rietveld analysis using powder X-ray diffraction data at room temperature. In the Rietveld refinement, there are good agreement between x=0.79(4) and x=0.84(9), except for (008) and (108) peaks. The deviations of the two reflections are very large relative to those of other reflections, and the change in X-ray diffraction data is clearer than that in neutron diffraction data. This indicates that electron density distributions in Na_xCoO₂ are slightly modulated with increasing x. In fact, there are an obvious overlapping of the electron density between Co and O due to the Co–O hybridization in the CoO₂ layer, but the two-dimensional networks in the electron densities of x=0.84(9) are suppressed by the existence of the O–O network on (008) plane. This is direct evidence of decrease of two-dimensional hybridization in the CoO₂ layer with increasing a sodium content.