Silver dynamics in silver iodide/silver phosphate glasses studied by multi-dimensional 109Ag NMR
Abstract
Various 109Ag NMR techniques are used to study silver diffusion in silver iodide/silver phosphate glasses. In particular, for the first time, four-dimensional (4D) NMR is applied on solid ion conductors. All experiments consistently show that silver dynamics are governed by a very broad, continuous distribution of correlation times G(lg τ). First, over a wide temperature range, the 109Ag NMR spectra are well described by a weighted superposition of a rigid-limit spectrum and a motionally narrowed line resulting from the slow and the fast silver ions, respectively. Further, very stretched 109Ag NMR two-time correlation functions are observed where a lack of correlation for t → ∞ indicates that there are no truly immobile silver ions. When fitting the decays to a Kohlrausch function, exp(−(t/τ)β), the stretching parameter amounts to β ≈ 0.2 and the mean correlation times 〈τ〉 show activation energies consistent with those reported for the dc conductivity. Finally, in 4D 109Ag NMR echo experiments, the existence of dynamical heterogeneities is proven by the selection of a subensemble of silver ions with a dynamical behavior different from the ensemble average. In addition, the information content of multi-dimensional NMR experiments on solid ion conductors is visualized by model simulations.