Far-infrared study on pressure-induced metal-insulator transition of CuIr2Se4 under high pressure

CuIr₂Se₄ is a metal at atmospheric pressure, but shows a stabilization of insulating phase from metal phase under high pressure. In order to study the electronic structure very near to the Fermi level (E_F) and its change due to a metal-insulator transition (MIT), we measured the temperature and pressure dependence of optical spectra on polycrystalline spinel compound CuIr₂Se₄ under high pressure. At ambient pressure, the growth of the peak structure at ∼0.2 eV corresponding to the formation of a pseudo-gap state was found with decrease in temperature but a Drude component still remained even at 8 K. Under pressure, the σ-spectra changed much from that at ambient pressure. At 0.5 GPa, two peaks at ∼0.2 eV and at ∼0.8 eV corresponding to interband transitions were found together with a Drude component. With pressure, the peak at ∼0.2 eV that were assigned to the electronic excitation across the hybridization bands of Ir5d and Se4p states lost its intensity as the Drude component did at 5.5 GPa. At 6.0 GPa, the spectrum showed the disappearance of the Drude component and the shift to high energy of two interband transition peaks. Our experiments indicated clearly the pressure-induced MIT due to the increase in the degree of hybridization.