Crystal structure, electric and magnetic properties in NaxCoO2

The electron density distributions of γ-Na_xCoO₂, 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, the deviation of (008) and (108) peaks is very large relative to that of other reflections, and the change in x-ray diffraction data is clearer than that in neutron diffraction data at 10K and room temperature. This indicates that electron density distributions in γ-Na_xCoO₂ are slightly modulated with increasing x. In fact, there found to be the obvious overlapping of the charge density between O-O network and Co ions, but no significant overlapping with increasing x. This is the direct observation of decrease of covalency between O 2p and Co t_2g in the CoO₂ layer with increasing the sodium content.