Metal-insulator transition in the spinel-type<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuIr</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mi>−</mml:mi><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Se</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>system (1998)

The thiospinel ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ exhibits a temperature-induced metal-insulator $(M\ensuremath{-}I)$ transition around 226 K, showing hysteresis on heating and cooling, that manifests itself as a gap in the electronic density of state with increasing electrical resistivity at low temperatures. Conversely, ${\mathrm{CuIr}}_{2}{\mathrm{Se}}_{4}$ remains metallic down to 0.5 K. We have successfully synthesized the spinel-type compound ${\mathrm{CuIr}}_{2}({\mathrm{S}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}{)}_{4}$ system. In order to see the effect of substitutions of Se at the S sites, we have carried out a systematic experimental study of structural, electrical, and magnetic properties of ${\mathrm{CuIr}}_{2}({\mathrm{S}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}{)}_{4}.$ M\"ossbauer spectroscopy measurements of ${}^{193}\mathrm{Ir}$ have been performed for ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ and ${\mathrm{CuIr}}_{2}{\mathrm{Se}}_{4}.$ The $M\ensuremath{-}I$ transition of ${\mathrm{CuIr}}_{2}({\mathrm{S}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}{)}_{4}$ for $x<~0.15$ is accompanied by a structural transformation from tetragonal (low-temperature insulating phase) to cubic (high-temperature metallic phase) symmetry. With increasing Se concentration $x,$ the sharp $M\ensuremath{-}I$ transition shifts to lower temperature. The resistivity shows a monotonous increase with decreasing temperature for $0.17<~x<~0.78$ between 4.2 and 300 K, and the metallic state is recovered for $x>~0.80.$ Magnetic susceptibility measurements show the jump at the $M\ensuremath{-}I$ transition temperature with hysteresis on heating and cooling. The high-temperature metallic phase of ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ shows Pauli paramagnetism, having a density of states at the Fermi level, $D({\ensuremath{\epsilon}}_{\mathrm{F}})=0.67$ states/eV atom. The insulating phase at low temperatures exhibits diamagnetism, and there is no localized magnetic moment. The Arrhenius regime is observed for the conductivity with a thermally activated process for $0<~x<~0.70$ in the insulating phase. There is a general trend toward 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 ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ will be discussed on the basis of the M\"ossbauer data. A phase diagram of temperature versus Se concentration $x$ will be proposed for the ${\mathrm{CuIr}}_{2}({\mathrm{S}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}{)}_{4}$ system. The mechanism of the $M\ensuremath{-}I$ transition remains enigmatic and is far from a complete picture.