Anion-cation dynamic cooperation in a paradigmatic ionic conductor around its superionic transition

Abstract

Silver iodide (AgI) is one of the most extensively studied superionic conductors. In this work, we have applied the standard Buckingham pair potential and standard charge. For the first time, the microscopic dynamic events that allow the achievement of the superionic transition are revealed. A collaborative effect among cations and anions that makes possible the hopping mechanism within the appropriate timescale is revealed. One of the most accepted explanations states that the Ag+ moves easily through the lattice, described as a molten sub-lattice of Ag+ ions, and that the high conductivity of α-AgI is achieved owing to: a) the low charge cation and the large number of vacant sites available for the cations to move; b) the open framework; c) the low ion coordination which involves a low activation energy; and d) the polarizable anions that facilitate the displacement of cations. Though this explanation appears to be accurate the question of what removes those constrains on the dynamic of the silver cation allowing its ionic conductivity value to be incremented by several magnitude orders is still open. The present work is focused on applying the molecular dynamics formalism to reveal the structural features that cause this property change. The study of the movement of the silver iodide ions applying the Molecular Dynamics formalism, in terms of atom interactions through the most common pair potential Buckingham, allows us to reproduce the experimental behavior expected in its electrical conductivity. Empirical models such as the Universal Dielectric Response and the Jump Relaxation Model describe the charge carrier behavior in a solid matrix without a microscopic description of the transport phenomenon. However, even the dynamic heterogeneity model interpretation could be useful to analyze the AgI dynamic, the evidence given in this work of different kind of mobile cations and its collaborative mechanism with the anions (iodide) allows us to confirm the importance of those distinguishable cations jumps that in the context of the Anderson-Stuart’s activation energy calculation, a very traditional model applied to a solid ionic conductors, evidences the need for the iodide-silver mutual dynamic interaction. The purpose of this work is to shed light on the current knowledge related to the transport phenomena and the relaxation phenomena in the paradigmatic solid ionic conductor, the AgI, from a very detailed microscopic point of view. We were able to evidence the collaborative movement between the cations and the anions which is responsible of the superionic transition when the phase transition occurs. We also showed not only the hopping mechanism involved but also its mechanism at the appropriate timescale.