X-ray photoemission study of CuIr2S4:Ir3+-Ir4+ charge ordering and the effect of light illumination

We have studied the electronic structure of the spinel-type compound CuIr2S4 using x-ray photoemission spectroscopy (XPS). CuIr2S4 undergoes a metal-insulator transition (MIT) at â226K. In going from the metallic to insulating states, the valence-band photoemission spectrum shows a gap opening at the Fermi level and a rigid-band shift of â0.15eV. In addition, the Ir 4f core-level spectrum is dramatically changed by the MIT. The Ir 4f line shape of the insulating state can be decomposed into two contributions, consistent with the charge disproportionation of Ir3+â¶Ir4+=1â¶1. XPS measurements under laser irradiation indicate that the charge disproportionation of CuIr2S4 is very robust against photo-excitation in contrast to Cs2Au2Br6 which shows photo-induced valence transition.