Optical study of the metal-insulator transition in CuIr2S4 crystals

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 ∼2 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.