We report resonant x-ray scattering measurements on a single crystal of the orbitally degenerate triangular metallic antiferromagnet $2H\mathrm{\text{−}}{\mathrm{AgNiO}}_2$ to probe the spontaneous transition to a triple-cell superstructure at temperatures below $T_S=365\mathrm{K}$. We observe a strong resonant enhancement of the supercell reflections through the Ni $K$ edge. The empirically extracted $K$-edge shift between the crystallographically distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of $\sim 1\mathrm{eV}$, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.

• Received 10 September 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.157206