From Ostwald′s Times to Solid State Ionics: Migration and Localised Hopping of Silver Ions in Crystalline Rubidium Silver Iodide

Abstract

Abstract Since Ostwald′s times, the concepts of ionic motion in condensed matter have been extended to comprise systems with increasingly complex dynamic properties. Today, one of the central problems in the field of SOLID STATE IONICS consists in finding simple, yet relevant rules for the correlated hopping motion of the mobile ions in structurally disordered ionic materials. Rubidium silver iodide, RbAg4I5, belongs to this class of materials. To analyse the hopping dynamics of the mobile silver ions in the three phases of RbAg4I5, we have taken conductivity spectra at frequencies up to the far infrared. Also, a simple set of rules for the development of the ion dynamics with time has been provided by the MIGRATION concept, the acronym standing for MIsmatch Generated Relaxation for the Accommodation and Transport of IONs. In the phases α and β, an increasing tendency for correlated forward-backward hopping is observed as the temperature is decreased from 298K to 129K. At the first-order β-γ phase transition at 121.8K, the number of translationally mobile silver ions is found to be markedly reduced. The conductivity spectra of the different phases of RbAg4I5 are well explained by the MIGRATION concept as long as the angular frequency does not exceed the elementary hopping rate of the mobile silver ions. At higher frequencies, an additional dynamic feature is encountered, which is superimposed onto the MIGRATION-type conductivity. This feature, which shows a nearly constant loss (NCL) behaviour, becomes increasingly pronounced as the temperature is lowered. It is caused by a strictly localised motion of interacting silver ions. In our model treatment for the MIGRATION and NCL parts of the dynamic conductivity, the potentially translational and the strictly localised ionic hopping motion are best described with a single elementary hopping rate. Consequently, this rate marks the beginning of the NCL behaviour on the angular frequency scale. This observation is in agreement with earlier results obtained by Leon et al. on solid lithium-ion conductors.