Correlated ion diffusion in γ-Ag0.7Cu0.3I

Correlation effects in the ionic conductivity of γ-Ag0.7Cu0.3I were studied by impedance spectroscopy measurements in the frequency range 5 Hz-13 MHz and below the superionic transition of AgI at 147 °C. As the temperature decreases from 147 °C, the spectra of the real part of the conductivity (σ′) shows a transition from their dc values to a dispersive regime where the conductivity shows a power-law frequency dependence at higher frequencies, as a consequence of correlation effects among charge carriers. The dielectric permittivity (ε*) shows frequency dependence that may be related to dielectric relaxation of the material as a consequence of ionic hopping. Also, the modulus spectra shows asymmetric peaks that have been analyzed in terms of non-exponential decay function, with a Kohlrausch parameter β that increases from 0.8 at 22 °C to 1.0 at 122 °C In contrast to single cation AgI-systems which follow a temperature invariant shape, the observed behavior can be explained by a mixed-cation effect due to enhancement of Cu+ mobility with increasing temperature. Both the dc conductivity (σ0) and the crossover frequency (ωp) to the power-law dependent regime of σ′(ω) were found to be thermally activated with the same energy, Eσ, pointing to a common origin of them in ionic hopping.