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...
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