Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering

TL;DR

This paper presents direct evidence of charge order in the triangular metallic compound AgNiO2 using resonant X-ray scattering, revealing a spontaneous honeycomb charge order below 365 K and supporting it with band-structure calculations.

Contribution

It provides the first direct observation of charge order in AgNiO2 through resonant X-ray scattering and explains the electronic structure changes with theoretical calculations.

Findings

Detection of a superstructure below 365 K indicating charge order.

A significant Ni K-edge shift evidencing charge disproportionation.

Band-structure calculations confirming charge order effects.

Abstract

We report resonant X-ray scattering measurements on the orbitally-degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below 365 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 ~1 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.