Optical response of CuIr 2X 4 (X=S,Se) due to metal–insulator transition

Abstract

CuIr 2X 4 (X=S,Se) are ones of ternary metal chalcogen compounds with a thiospinel structure. CuIr 2S 4 undergoes a temperature-induced metal–insulator transition around 226 K. We have measured the temperature dependence of the optical reflection spectra of both samples in the energy regions of 0.005–6 eV in order to study the electronic structure very close to the Fermi level ( E F) and its change across the metal–insulator transition (X=S). Obtained optical conductivity spectrum, σ( ω), of CuIr 2S 4 below 221 K showed a growth of a new band around 0.4 eV instead of a band at 0.7 eV in the metallic phase. The growth of a new band which compensates the decrease in the spectral intensity of the Drude component suggests the appearance of the localized band below the E F level.