Metal-insulator transition in CuIr2S4 : XAS results on the electronic structure

S K and Ir L3 x-ray absorption measurements across the temperature-induced metal ~M! to insulator ~I! transition in CuIr2S4 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.