Hopping conductivity in CuIr2S4 spinel compound:: I.: Empirical model for electronic configuration and mechanism of metal-insulator transition

Two types of spinel compound CuIr2S4 samples were investigated: sample I 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 a 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 rho is proportional to the same function of sigma 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 CuIr2S4 is estimated to a first approximation from the experimental results. 5d epsilon electrons do not participate in the crystallization bonds of d gamma(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 5d epsilon decreases at the expense of crystallographic energy coming from the d gamma(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 5d epsilon(xy) orbitals in the < 110 > direction, resulting in charge ordered Pierls state. A new type of conductivity named traveling dimer conduction is proposed.