Suppression Of The Metal-Insulator Transition In The Spinel Cu1-Xinxir2s4 System

The thiospinel Cu1-xInxIr2S4(0 <= x <= 0.25) system was studied by the measurements of crystal structure, electrical resistivity, and magnetic susceptibility. The parent compound was known to exhibit an intriguing first-order metal-insulator (MI) transition with a simultaneous spin-dimerization and charge ordering at similar to 230 K with decreasing temperature. Upon indium doping on the copper site, the conduction holes of the metallic phase are depleted, or the doped electrons occupy the antibonding state of the insulating phase, suppressing the MI transition. Moreover, the first-order transition is changed into a higher-order one for x >= 0.2. Our experimental data suggest that the higher-order phase transition is associated with an electronic transformation from small polarons to small bipolarons. Comparing the doping effects of Zn, Cd, and In, we found that the variations of the electrical and magnetic properties depend on the lattice size, i.e., the suppression of the MI transition becomes weaker for an enlarged lattice. This lattice size effect is mainly explained in terms of the electron-phonon interactions, which is enhanced by the band narrowing due to the larger ionic size of the dopants.