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

Abstract

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. 5 d ε electrons do not participate in the crystallization bonds of d γ 2 s 1 p 3 for spinel structure, however these electrons are responsible for MIT and for conductivity. The cooperative Jahn–Teller effect with dimerization is proposed for MIT: The energy of an electronic system of 5 d ε decreases at the expense of crystallographic energy coming from the d γ 2 s 1 p 3 electron system due to tetragonal deformation as in normal Jahn–Teller effect. Further triclinic deformation occurs to minimize energy by dimerization of 5 d ε ( x y ) orbitals in the <110> direction, resulting in charge ordered Pierls state. A new type of conductivity named traveling dimer conduction is proposed.