Na ordering imprints a metallic kagomé lattice onto the Co planes of Na 2/3 CoO 2

We report 23 Na and 59 Co nuclear magnetic (NMR) and quadrupolar resonance (NQR) studies for the x =2/3 phase of the lamellar oxide Na x CoO 2 , which allowed us to establish reliably the atomic order of the Na layers and their stacking between the CoO 2 slabs. We evidence that the Na + order stabilizes filled non-magnetic Co 3+ ions on 25% of the cobalt sites arranged in a triangular sublattice. The transferred holes are delocalized on the 75% complementary cobalt sites which unexpectedly display a planar cobalt kagomé structure. These experimental results resolve a puzzling issue by precluding localized moments pictures for the magnetic properties. They establish that the quasi-ferromagnetic properties result from a narrow band connecting a frustrated arrangement of atomic orbitals, and open the route to unravel through similar studies the electronic properties of the diverse ordered phases of sodium cobaltates.