Na ordering imprints a metallic kagome lattice onto the Co planes of Na2/3CoO2

We report Na and Co nuclear magnetic (NMR) and quadrupolar resonance (NQR) studies for the x = 2/3 phase of the lamellar oxide NaxCoO2, which allowed us to establish reliably the atomic order of the Na layers and their stacking between the CoO2 slabs. We evidence that the Na order stabilizes filled non magnetic Co 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. Introduction. – Peculiar atomic structures such as chains, ladders or the graphene honeycomb often exhibit remarkable singular physical properties, as does the triangular cobalt network in Na cobaltates [1] which displays high thermopower [2] and superconductivity [3]. There, the control of carrier content of the CoO2 planes by varying Na concentration between the planes yields a totally counter-intuitive sequence of magnetic properties including anomalous paramagnetism [4], charge disproportionation [5], metallic antiferromagnetism [6, 7]. An original aspect of these systems is that the Co atoms are stacked between two triangular oxygen layers with distorted CoO6 octahedrons, so that a large crystal field stabilizes the Co ions in a low spin state by lifting the degeneracy of the cobalt 3d levels. In a Co configuration the six lower energy levels (t2g) are filled, with a total spin S = 0, while Co should only retain one hole in the t2g multiplet, with S = 1/2, so that original magnetic properties have been anticipated to result from these local spins associated with charge ordering [8]. This charge order intrinsic to the Co planes would depend of x and would yield specific metallic and magnetic properties. For (a)E-mail: alloul@lps.u-psud.fr instance, for x = 2/3, the 2D charge ordered state would be a honeycomb network of Co ions intermixed with a triangular lattice of Co. We formerly established experimentally by NMR [5] that such a charge order does not occur, but that the charges disproportionates between Co and cobalt sites with an average formal valence of about 3.5. The differentiation of three Na sites [11], in a phase with x ∼ 0.7 (redefined as x = 0.67 or H67 in ref. [9]) implied an associated atomic Na ordering with a unit cell larger than that of the honeycomb lattice, in view of the large number of cobalt sites detected by NMR [5]. This further differentiates the Na cobaltates from the cuprates for which copper has a uniform charge while dopants are usually disordered, which influences the physical properties [10]. Indeed the Na atomic orderings stabilized for specific x values are found to play a role in determining the ground state metallic and magnetic properties [9]. But, while Na ordering has been evidenced by various experiments [11–13], the actual atomic order and its incidence on the local scale electronic properties is still unclear [9], except for two limiting cases, the x = 1 filled band insulator and the x = 1/2 ordered ”chain-like” compound [14–16]. Let us point out that the cobalt plane physical prop-