Optical study of the metal-insulator transition in CuIr2S4 crystals -: art. no. 153104

We present measurements of the optical spectra on single crystals of spinel-type compound CuIr2S4. This material undergoes a sharp metal-insulator transition at 230 K. Upon entering the insulating state, the optical conductivity shows an abrupt spectral weight transfer and an optical excitation gap opens. In the metallic phase, Drude components in low frequencies and an interband transition peak at similar to2 eV are observed. In the insulating phase, a peak emerges around 0.5 eV. This peak is attributed to the transition of electrons from the occupied Ir3+ t(2g) state to upper Ir4+ t(2g) subband resulting from the dimerization of Ir4+ ions in association with the simultaneous formations of Ir3+ and Ir4+ octamers as recently revealed by the x-ray-diffraction- experiment. Our experiments indicate that the band structure is reconstructed in the insulating phase due to the sudden structural transition.