Correlated ionic hopping processes in crystalline and glassy electrolytes resulting in MIGRATION-type and nearly-constant-loss-type conductivities

Solid electrolytes with disordered structures may be crystalline or glassy. Their complex ionic conductivity displays a characteristic frequency dependence. Modelling the dynamics of the mobile ions, we have developed the MIGRATION concept, the acronym standing for MIsmatch Generated Relaxation for the Accommodation and Transport of IONs. With the help of the MIGRATION concept it is possible to reproduce frequency-dependent experimental conductivities and permittivities including their scaling behaviour. Scaling is a property typically observed in and below the radio frequency regime. At sufficiently high frequencies and low temperatures, however, conductivity spectra of crystals and glasses are often found to contain a second component which displays the so-called nearly-constant-loss (NCL) behaviour. Suitably modifying the MIGRATION concept, we are able to explain this feature and to show that it is caused by a displacive or hopping ionic motion that stays completely localised. Here, as in the unmodified MIGRATION concept, interactions between the ions play an essential role. Experimentally, interesting differences are detected between the NCL-type dynamics in a crystalline and in a glassy ion conductor. In crystalline γ-RbAg4I5 we find the same elementary rates for the MIGRATION-type and NCL-type hopping movements of the ions, suggesting identical barrier heights for the respective processes. On the other hand, the two rates are found to differ markedly from each other in glassy AgI-AgPO3, not only with regard to their absolute value but also in their temperature dependence. We suggest that the NCL effect in the glass results from dynamic localised displacements involving both the silver ions and negatively charged entities such as iodide ions and/or non-bridging oxygen ions.