  The pseudo-binary thiospinel system Cu$_{1-x}$Cd$_{x}$Ir$_{2}$S$_{4}$ was
investigated by the x-ray diffraction, electrical resistivity, magnetic
susceptibility and specific heat measurements. It was shown that the system
exhibits a miscibility-gap behavior for the Cd substitution, however, nearly
monophasic samples was obtained by quenching at 1373 K, except for $0.4< x
\leqslant 0.6$. With increasing the Cd concentration, the room-temperature
electrical conductivity and Pauli susceptibility decrease monotonically,
consistent with the hole-filling picture. The first-order metal-insulator
transition at about 230 K in the parent compound CuIr$_{2}$S$_{4}$ is changed
into a second-order transition around 185 K when $x\sim$ 0.25, whereafter the
second-order transition disappears at $x\sim 0.8$. No superconductivity was
observed down to 1.8 K. The end-member compound CdIr$_{2}$S$_{4}$ is shown as
an insulator with a band-gap of 0.3 eV. Analysis for the data of magnetic
susceptibility and electrical resistivity suggests the formation of bipolarons
below 185 K for $0.25 <x<$ 0.8, which accounts for the absence of
superconductivity in terms of the transition from the BCS Cooper pairs to small
bipolarons.
