I3+ - Ir4+ charge disproportionation in spinel Cu Ir2 S4 investigated by synchrotron-radiation photoemission spectroscopy

We have studied the electronic structure of the spinel Cu Ir2 S4 using synchrotron-radiation photoemission spectroscopy. The phase transition from a high temperature paramagnetic metal to a low temperature diamagnetic insulator at ∼230 K is clearly observed through the significant line shape change of the Ir 4f photoemission spectra and the ∼0.09 eV gap opening of the valence band spectra. The photon energy dependence of the Ir 4f photoemission spectra enables us to characterize the satellite peaks in the metallic phase of Cu Ir2 S4, providing compelling experimental evidence for the Ir3+ - Ir4+ charge disproportionation in Cu Ir2 S4.