Variation of the metal-insulator transition and formation of bipolarons by Cd doping in the thiospinel system Cu 1 − x Cd x Ir 2 S 4

The pseudobinary thiospinel system ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}{\mathrm{Ir}}_{2}{\mathrm{S}}_{4}$ 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 were obtained by quenching at 1373 K, except for $0.4lxl~0.6.$ 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 ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ is changed into a second-order transition around 185 K when $x\ensuremath{\sim}0.25,$ whereafter the second-order transition disappears at $x\ensuremath{\sim}0.8.$ No superconductivity was observed down to 1.8 K. The end-member compound ${\mathrm{CdIr}}_{2}{\mathrm{S}}_{4}$ 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 $0.25lxl0.8,$ which accounts for the absence of superconductivity in terms of the transition from the BCS Cooper pairs to small bipolarons.