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We report a systematic study of the $c$ lattice parameter in theNa$_{x}$CoO$_{2}$ phases versus Na content $x>0.5$, in which sodium alwaysdisplays ordered arrangements. This allows us to single out the first phasewhich exhibits an AF magnetic order at a N\'eel temperature $T_{N}=$22 K whichis found to occur for $x\approx 0.77(1)$. Pure samples of this phase have beenstudied both as aligned powders and single crystals. They exhibit identical$^{23}$Na NMR spectra in which three sets of Na sites could be fully resolved,and are found to display $T$ dependencies of their NMR shifts which scale witheach other. This allows us to establish that the $T$ variation of the shifts isdue to the paramagnetism of the Co sites with formal charge state larger than3$^{+}$. The existence of a sodium site with axial charge symmetry and theintensity ratio between the sets of $^{23}$Na lines permits us to reveal thatthe 2D structure of the Na order corresponds to 10 Na sites on top of a 13 Cosites unit cell, that is with $x=10/13\approx 0.77$. This structure fits withthat determined from local density calculations and involves triangles of 3 Nasites located on top of Co sites (so called Na1 sites). The associated orderingof the Na vacancies is quite distinct from that found for $x<0.75$.

23Na NMR study of sodium order in NaxCoO2 with 22K Neel temperature