  We have synthesized and characterized different stable phases of sodium
cobaltates Na$_{x}$CoO$_{2}$ with sodium content $0.65<x<0.80$. We demonstrate
that $^{23}$Na NMR allows to determine the difference in the susceptibility of
the phases and reveals the presence of Na order in each phase. $^{59}$Co NMR
experiments give clear evidence that Co charge disproportionation is a dominant
feature of Na cobaltates. Only a small fraction ($\approx$ 25%) of cobalts are
in a non-magnetic Co$^{3+}$ charge state whereas electrons delocalize on the
other cobalts. The magnetic and charge properties of the different Co sites are
highly correlated with each other as their magnetic shift $K_{ZZ}$ scales
linearly with their quadrupolar frequency $nu_Q$. This reflects the fact that
the hole content on the Co orbitals varies from site to site. The unusual
charge differentiation found in this system calls for better theoretical
understanding of the incidence of the Na atomic order on the electronic
structures of these compounds.
