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$_{4}$

TL;DR

This study explores how Cd-doping affects the metal-insulator transition and bipolaron formation in Cu$_{1-x}$Cd$_{x}$Ir$_{2}$S$_{4}$, revealing changes in transition order, conductivity, and magnetic properties.

Contribution

It provides new insights into the doping-induced evolution of electronic phases and bipolaron formation in the thiospinel system, including the suppression of superconductivity.

Findings

Metal-insulator transition changes from first to second order with doping.

Bipolaron formation occurs below 185 K for certain compositions.

CdIr$_{2}$S$_{4}$ is an insulator with a 0.3 eV band gap.

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 $0.4< x \leqslant 0.6$. 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.