Metal-insulator transition inCuIr2S4: XAS results on the electronic structure

Abstract

S K and Ir L 3 x-ray absorption measurements across the temperature-induced metal ͑M͒ to insulator ͑I͒ transition in CuIr 2 S 4 are presented. Dramatic S K-edge changes reflect the Ir d-electronic state redistribution across this transition. These changes, along with a detailed consideration of the I-phase structure, motivate a model in which the I-phase stabilization involves an interplay of charge and d-orbital orientation ordering along Ir chains, a quadrupling of the Ir-chain repeat unit, and correlated dimer spin-singlet formation.Metal-insulator transitions involving transition metal ͑T͒ compounds have been of intense interest in recent years on both fundamental and technological grounds. 1,2 This field has been dominated by 3d-row T-oxide compounds, by virtue of the renaissance in these materials that followed the discovery of high-T c superconductivity. 3 The subclass of mixed valent metallic compounds that ''charge order'' into an insulating state has been the focus of special recent interest. 2 The compound CuIr 2 S 4 stands out as unique in this subclass for a number of reasons. 4 -9 The spinel structure CuIr 2 S 4 compound has a paramagnetic, high-temperature, homogeneously mixed Ir 3ϩ /Ir 4ϩ , metallic ͑M͒ phase, which undergoes a first-order transition ͑near 230 K͒ to a low-temperature, charge-ordered, diamagnetic, insulating ͑I͒ phase. 4 -9 Recent definitive x-ray and neutron scattering measurements showed this I phase to involve a complex ordering of Ir 4ϩ -spin-singlet dimers and undimerized Ir 3ϩ sites. 10 This finding is novel, since such dimerization transitions have previously occurred almost exclusively in compounds with clearly defined quasi-onedimensional ͑1D͒ chains, and certainly not in a complex three-dimensional ͑3D͒ type structure. 11 Moreover, the complex pairwise ordering of Ir 3ϩ -Ir 3ϩ nondimer and Ir 4ϩ -Ir 4ϩ singlet-dimer moieties appears not to have been previously observed. In addition, the broadness of the 5d orbitals makes such M-I transitions rare in 5d-row compounds, and still rarer in a nonoxide ͑i.e., S͒ 5d compound. These properties, along with the previous dearth of precise structural or electronic information, have hindered the development of the outlines of a theory for the M-I transition in CuIr 2 S 4 . In this paper, we present S and Ir XAS results spanning the M-I transition in CuIr 2 S 4 . The S K-edge results, along with a careful review of the dimerized/charge-ordered chain character of the I phase, motivate a proposal for a specific electronic/structural basis from which to approach understanding of this novel I-M transition.The sample preparation and characterization techniques were as discussed in Ref. 10. The S K and Ir L 3 XAS measurements were, respectively, performed on beamlines X19A and X18B at the Brookhaven National Synchrotron Light Source, using methods discussed in Refs. 12 and 13. The low-temperature XAS measurements on X19A utilizes a nitrogen cryostat ͑in the fluorescence mode͒, and those on X18B used a displex refrigerator ͑using the trans...

X-ray absorption spectroscopic analysis of CuIr2S4

“…This finding supports that Ir forms a mixed valence state of Ir 3+ and Ir 4+ which is responsible for metallic conduction in the cubic phase of CuIr 2 S 4 . Croft et al also measured the L III -edge XANES spectra of Ir, IrO 2 , and CuIr 2 S 4 at room temperature [10]. Their spectra are consistent with our spectra.…”

“…This observation would be reflected to the Ir electronic state redistribution across the MIT. In prior work [10], Croft et al reported the significant changes of S K-edge XANES spectra of CuIr 2 S 4 across the MIT. They propose that the dimerization due to charge and orbital ordering is responsible for the electronic structural change [10].…”

“…In the diamagnetic insulator phase, the dimerization of Ir 4+ -Ir 4+ having S = 0 is formed [1,5,10], and Ir 3+ is non-magnetic state with t 6 2g occupation. The dimerized Ir 4+ -Ir 4+ bonds are highlighted in Fig.…”

Electronic environments in carrollite, CuCo2S4, determined by soft X-ray photoelectron and absorption spectroscopy

“…Nevertheless, the leading absorption peak in both the S K-and L-edge spectra were at an energy above the Fermi level similar to that of the Co K pre-edge peak, but $2 eV lower than for the Cu K pre-edge peak. While there was no resolved structure in the principal absorption peak in the S K-edge spectrum for carrollite reported by Croft et al (2003), a shoulder was just discernible about 1 eV below the maximum of the peak that was interpreted as arising from the Co d-orbital splitting. The S K-and L 3 -edge spectra simulated on the basis of the WIEN2k calculations were in good agreement with the corresponding spectra observed by Li et al (1995) and Croft et al (2003), including the low-energy shoulder in the S K-edge spectrum.…”

Broadening of the orbitally-induced Peierls phase transition in Cu 1−x Na x Ir 2 S 4

“…The iridium ions form isomorphic octamers of Ir 3þ 8 S 24 and Ir 4þ 8 S 24 respectively after the phase transition, where chemical dimerization occurs to Ir 4+ along the h1 1 0i direction [25,26]. The motivation of the phase transition has been explained by an orbitally-induced Peierls phase transition mechanism, which suggested that the anisotropic coupling of 5d xy orbits of Ir 4+ along the h1 1 0i direction due to the crystal field and JahnTeller distortion results into the orbital Peierls instability [12,27].…”