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

TL;DR

This study uses NMR and NQR techniques to reveal that in Na$_{2/3}$CoO$_{2}$, sodium ordering induces a kagome lattice structure on cobalt planes, influencing magnetic and electronic properties.

Contribution

It provides the first detailed atomic-level understanding of Na and Co ordering in Na$_{2/3}$CoO$_{2}$, linking structural order to magnetic behavior.

Findings

Na ordering stabilizes non-magnetic Co$^{3+}$ on 25% of sites

Remaining cobalt sites form a kagome lattice with delocalized holes

Results suggest magnetic properties arise from a narrow band in a frustrated lattice

Abstract

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 kagom\'{e} 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.