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 approximately 226 K. 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 approximately 0.15 eV. 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+:Ir4+=1:1. 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.