Conductivity of RbCu4Cl3+xI2−x and copper electrode reaction computerized complex analysis

Attempts to measure the conductivity of solid electrolytes having the composition RbCu₄Cl_3+xI_2?x where x=0 or 0.25 were carried out using copper and Chevrel phase electrodes Cu₂Mo₆S₈. Complex impedance diagrams showed severe polarization effects at copper electrodes. Special programs were used with a computer coupled to the impedance meter allowing measurements at constant overpotential or constant current. In contrast to usual situations, the polarization at the copper electrode is a decreasing function of the ac signal voltage and a constant quantity of electrolysing current of ≈10^?2 C cm^?2 is required to disrupt the blocking effect at the interface. Results obtained with the Chevrel phases unambiguously show that the exchange of ions between two solid phases can take place at room temperature without any interfacial overpotential. The best conductivity value of RbCu₄Cl₃I₂ was 0.475 (Ω cm)^?1 at room temperature but all the measurements revealed a marked instability towards lower values. The conductivity of RbCu₄Cl_3.25I_1.75 at room temperature is 0.36 (Ω cm)^?1 and is stable. The activation energy is 0.045 eV.