EFFECT OF GRAIN-BOUNDARIES ON THE LOW-TEMPERATURE IONIC-CONDUCTIVITY OF POLYCRYSTALLINE RBAG4I5 AND AG3SBR

The influence of grain boundaries on de and ac conductivities of polyc rystalline RbAg4I5 and Ag3SBr was studied in the temperature range 15 K <T <298 K using low-frequency EIS and high-frequency waveguide measu rements. A change of the ion conduction mechanism is found at a transi tion temperature in the range 60 K <T-t<90 K.The conductivity behaviou r shows that at T <T-t the ion transport occurs preferentially along p aths at the grain boundaries with very low activation energy, whereas the conventional conduction through the grains and across grain bounda ries dominates at T >T-t. A long-time relaxation effect is observed ar ound the transition temperature (T approximate to T-t). This effect is attributed to a subsurface superionic transition leading to a formati on of a subsurface superionic phase at the grain boundaries. The analy sis of experimental results shows that the temperature-independent hig h-frequency ac conductivity observed at T <100 K can be explained by a model involving a Debye relaxation of Ag+ ions in preferred sites wit h an appropriate distribution of relaxation times.