METAL-INSULATOR-TRANSITION IN THE SPINEL-TYPE CUIR2(S1-XSEX)(4) SYSTEM

The thiospinel CuIr2S4 exhibits a temperature-induced metal-insulator (M-l) transition around 226 K, showing hysteresis on heating and cooli ng, that manifests itself as a gap in the electronic density of state with increasing electrical resistivity at low temperatures. Conversely , CuIr2Se4 remains metallic down to 0.5 K. We have successfully synthe sized the spinel-type compound CuIr2(S1-xSex)(4) system. In order to s ee the effect of substitutions of Se at the S sites, we have carried o ut a systematic experimental study of structural, electrical, and magn etic properties of CuIr2(S1-xSex)(4). Mossbauer spectroscopy measureme nts of Ir-193 have been performed for CuIr2S4 and CuIr2Se4. The M-I tr ansition of CuIr2(S1-xSex)(4) for x less than or equal to 0.15 is acco mpanied by a structural transformation from tetragonal (low-temperatur e insulating phase) to cubic (high-temperature metallic phase) symmetr y. With increasing Se concentration x, the sharp M-l transition shifts to lower temperature. The resistivity shows a monotonous increase wit h decreasing temperature for 0.17 less than or equal to x less than or equal to 0.78 between 4.2 and 300 K, and the metallic state is recove red for x greater than or equal to 0.80. Magnetic susceptibility measu rements show the jump at the M-I transition temperature with hysteresi s on heating and cooling. The high-temperature metallic phase of CuIr2 S4 shows Pauli paramagnetism, having a density of states at the Fermi level, D(epsilon(F))= 0.67 states/eV atom. The insulating phase at low temperatures exhibits diamagnetism, and there is no localized magneti c moment. The Arrhenius regime is observed for the conductivity with a thermally activated process for 0 less than or equal to x less than o r equal to 0.70 in the insulating phase. There is a general trend towa rd increasing metallicity with increasing x, which is consistent with the magnetic susceptibility results. A possibility of a two-site model of different valence states for Ir ions in the insulating phase of Cu Ir2S4 will be discussed on the basis of the Mossbauer data. A phase di agram of temperature versus Se concentration x will be proposed for th e CuIr2(S1-xSex)(4) system. The mechanism of the M-l transition remain s enigmatic and is far from a complete picture.