Direct Observation of Charge Order in Triangular MetallicAgNiO2by Single-Crystal Resonant X-Ray Scattering

Abstract

We report resonant X-ray scattering measurements on a single crystal of the orbitally-degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below TS = 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 bandstructure 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.PACS numbers: 75.25. Dk, 78.70.Ck, 71.45.Lr Since the discovery of the Verwey transition in 1939 [1], electronic ordering in oxides has been extensively studied, as more evidence progressively emerged of its importance in underpinning phenomena such as metalinsulator transitions, colossal magnetoresistance and possibly high-temperature superconductivity. Orbital ordering (OO) in particular, whereby orbital degeneracy is lifted either by a spontaneous lattice distortion driven by the (cooperative) Jahn-Teller (JT) effect [2], or by similar orbital physics [3], has become a cornerstone of our understanding of oxides. This was hitherto considered ubiquitous in both band and Mott insulators containing "JT active" ions. For this reason, the recent proposal [4] of a radically different type of electronic ordering in the weakly metallic 2H-AgNiO 2 came initially as a surprise. In this scenario, orbital degeneracy at JT-active low-spin Ni 3+ would be lifted through charge disproportionation and charge ordering (CO) rather than OO, in sharp contrast with the closely related (but insulating) nickelate NaNiO 2 [5], a rather conventional JT system. Although CO was observed before, e.g. at the simultaneous metal-insulator and magnetic ordering transition in rare-earth perovskite nickelates [6,7], elucidating the physics of 2H-AgNiO 2 is crucial in establishing a pure CO (decoupled from magnetic ordering [8]) as an alternative paradigm to the Jahn-Teller mechanism in "weakly" metallic systems close to the Mott transition [4,9], which remain metallic in the CO phase. Although the original clue for CO was from the magnetic structure below T N = 19.7 K, the primary indication was structural, and relied on an accurate neutron refinement of an oxygen breathing mode around the different Ni sites (Fig. 1b) in the √ 3a 0 × √ 3a 0 × c supercell structure at T < T S = 365 K. The pattern of charges ("honeycomb" CO -see Fig. 1a) and the amount of charge (formal valence) on each site can be estimated by analysing the Ni-O bond lengths through the so-called Bond Valence Sum (BVS) method [10], which yields Ni1 2.42+ and Ni2,3 3.07+ , see Fig. 1b), values that are fairly...

Geometrical frustration effects on charge-driven quantum phase transitions

“…Other materials such as the rare-earth nickelates, AgNiO 2 , do show threefold CO transitions similar to the one discussed here 72 although the origin of the CO transition may be nonCoulomb in origin since AgNiO 2 has a complex multiorbital structure 73 and other effects such as Hunds coupling and crystal fields can play a relevant role. Thus, the organic materials of the θ -(ET) 2 X type appear to be ideal candidates to single out the effects on electronic properties of metals close to a charge-driven quantum phase transition mediated solely by the off-site Coulomb repulsion between electrons at different sites.…”

Multiorbital kinetic effects on charge ordering of frustrated electrons on the triangular lattice

“…2(a)], and the existence of a "negative charge transfer" from the oxygen atoms is indeed compatible with the estimates of Ref. [16], which indicate a reduced average valence of 2.85+ per Ni. Studying the present model away from the perfectly quarter-filled configuration will certainly be of interest in view of these considerations.…”

Pinball liquid phase from Hund's coupling in frustrated transition-metal oxides

“…This system presents a robust threefold chargeordered phase, which is stable above room temperature (T CO = 365 K) and undergoes magnetic ordering only at much lower temperatures, T N = 20 K [9][10][11]. Contrary to the common behavior observed in oxides with Jahn-Teller active ions, the charge ordering in this material is not associated with any structural distortion, indicative of a purely electronic driving mechanism.…”