Metal-insulator transition in the spinel-type ${\mathrm{CuIr}}_2({\mathrm{S}}_{1-x}{\mathrm{Se}}_x{)}_4$ system

The thiospinel ${\mathrm{CuIr}}_2{\mathrm{S}}_4$ exhibits a temperature-induced metal-insulator $(M-I)$ 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 ${\mathrm{CuIr}}_2({\mathrm{S}}_{1-x}{\mathrm{Se}}_x{)}_4$ 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 ${\mathrm{CuIr}}_2({\mathrm{S}}_{1-x}{\mathrm{Se}}_x{)}_4.$ Mössbauer spectroscopy measurements of ${}^{193}\mathrm{Ir}$ have been performed for ${\mathrm{CuIr}}_2{\mathrm{S}}_4$ and ${\mathrm{CuIr}}_2{\mathrm{Se}}_4.$ The $M-I$ transition of ${\mathrm{CuIr}}_2({\mathrm{S}}_{1-x}{\mathrm{Se}}_x{)}_4$ 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 $M-I$ 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 $x>~\mathrm{0.80.}$ Magnetic susceptibility measurements show the jump at the $M-I$ 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, $D\left({\epsilon }_{\mathrm{F}}\right)=0.67\mathrm{}$ 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 Mössbauer data. A phase diagram of temperature versus Se concentration $x$ will be proposed for the ${\mathrm{CuIr}}_2({\mathrm{S}}_{1-x}{\mathrm{Se}}_x{)}_4$ system. The mechanism of the $M-I$ transition remains enigmatic and is far from a complete picture.