The thiospinel

{CuIr}_{2} 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,

{CuIr}_{2} {Se}_{4}

remains metallic down to 0.5 K. We have successfully synthesized the spinel-type compound

{CuIr}_{2} (S_{1 - x} {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

{CuIr}_{2} (S_{1 - x} {Se}_{x})_{4} .

Mössbauer spectroscopy measurements of

^{193} Ir

have been performed for

{CuIr}_{2} S_{4}

and

{CuIr}_{2} {Se}_{4} .

The

M - I

transition of

{CuIr}_{2} (S_{1 - x} {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 >~ 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

{CuIr}_{2} S_{4}

shows Pauli paramagnetism, having a density of states at the Fermi level,

D (ε_{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

{CuIr}_{2} 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

{CuIr}_{2} (S_{1 - x} {Se}_{x})_{4}

system. The mechanism of the

M - I

transition remains enigmatic and is far from a complete picture.

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