Hopping Conductivity in CuIr2S4 Spinel Compound: I. Empirical Model for Electronic Configuration and Mechanism of Metal–Insulator Transition

Two types of spinel compound CuIr 2 S 4 samples were investigated: sample 1 was synthesized under protective Ar atmosphere to avoid contamination by oxygen, whereas sample 2 was exposed to oxygen during preparation procedure. According to the results of scanning electron microscopy, the oxygen concentration in sample 2 is 5 times higher than in sample 1. However, the metal–insulator transition (MIT) temperature is the same for both samples irrespective of oxygen concentration. The conductivity σ in the insulating phase (LIP) is proportional to exp [-( T * / T ) 1/2 ] at T ≤120 K for sample 1 and at T ≤50 K for sample 2, while in the metallic phase the resistivity ρ is proportional to the same function of σ in LIP for both samples. Temperature independent diamagnetic susceptibility and temperature dependence of thermopower suggest that this compound is not a normal activation-type semiconductor. The electronic configuration of CuIr 2 S 4 is estimated to a first approximation from the experimental results. ...