  From 59Co and 23Na NMR, we demonstrate the impact of the Na+ vacancy ordering
on the cobalt electronic states in Na0.75CoO2: at long time scales, there is
neither a disproportionation into 75 % Co3+ and 25 % Co4+ states, nor a
mixed-valence metal with a uniform Co3.25+ state. Instead, the system adopts an
intermediate configuration in which 30 % of the lattice sites form an ordered
pattern of localized Co3+ states. Above 180 K, an anomalous mobility of
specific Na+ sites is found to coexist with this electronic texture, suggesting
that the formation of the latter may contribute to stabilizing the Na+
ordering. Control of the ion doping in these materials thus appears to be
crucial for fine-tuning of their thermoelectric properties.
