Variation of metal-insulator transition and formation of bipolarons by the Cd-doping in the thiospinel system of Cu${}_{1-x}$Cd${}_x$Ir${}_2$S${}_4$

Abstract

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 $0.4<x⩽0.6$. 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.

7 pages, 9 figures