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

The electron density distributions of Na0.79CoO2 and Na0.84CoO2 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 NaxCoO2 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