The effect of boundaries upon the direct- and alternating-current conductivities of polycrystalline samples was studied at temperatures ranging from 15 to 298K. A change in the ionic conduction mechanism was found at a transition temperature of between 60 and 90K. At temperatures below the transition, ion transport occurred preferentially along paths at the grain boundaries, with a very low activation energy, whereas conventional conduction through grains and across boundaries predominated at other temperatures. A long-term relaxation effect was observed around the transition temperature. These results were attributed to a sub-surface super-ionic transition which led to the formation of a sub-surface super-ionic phase at the boundaries. An analysis of the results showed that the temperature-independent high-frequency alternating-current conductivity which was observed at temperatures below 100K could be explained in terms of a model which involved the Debye relaxation of Ag+ ions in preferred sites, with a suitable distribution of relaxation times.

G.Staikov, M.Nold, W.J.Lorenz, A.Froese, R.Speck, W.Wiesbeck, M.W.Breiter: Solid State Ionics, 1996, 93[1-2], 85-93