METAL-INSULATOR TRANSITION IN CUIR2S4: XAS RESULTS ON THE ELECTRONIC STRUCTURE

S K and Ir ${L}_{3}$ x-ray absorption measurements across the temperature-induced metal (M) to insulator (I) transition in ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ 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.