Two-dimensional 109Ag NMR and random-walk simulation studies of silver dynamics in glassy silver ion conductors

By applying one- and two-dimensional 109Ag NMR, we demonstrate that silver diffusion in silver iodide/silver phosphate glasses is governed by a very broad, continuous distribution of correlation times G(lg tau). As a consequence, over a wide temperature range, the 109Ag NMR spectra can be described by a weighted superposition of a Gaussian and a Lorentzian where these line-shape components result from the slow and the fast silver ions in G(lg tau), respectively. For the 109Ag NMR two-time correlation functions F2(t), measured as a stimulated echo, a very stretched decay to F2SS(t(m)) = 0 is observed. When fitting to a Kohlrausch function, exp[-(t/tau)beta], a stretching parameter beta approximately = 0.2 is found. The temperature dependence of the mean correlation time of silver dynamics is described by an Arrhenius law where the activation energy is consistent with the one reported for the dc conductivity sigma(dc). In addition, it is shown that the effect of complex dynamical processes on NMR multi-time correlation functions can easily be calculated when performing random-walk simulations for schematic models such as the random-barrier model and the random-energy model. Based on these models it is possible to simulate various NMR observables and the mean square displacement, thus revealing the information content of multi-dimensional NMR experiments on solid ion conductors.