Effect of grain boundaries on the low-temperature ionic conductivity of polycrystalline RbAg_4I_5 and Ag_3SBr

摘要

The influence of grain boundaries on dc and ac conductivities of polycrystalline RbAg_4I_5 and Ag_3SBr was studied in the temperature range 15 K < T < 298 K using low-frequency EIS and high-frequency waveguide measurements. A change of the ion conduction mechanism is found at a transition temperature in the range 60 K < T_t < 90 K. The conductivity behaviour shows that at T < T_t the ion transport occurs preferentially along paths at the grain boundaries with very low activation energy, whereas the conventional conduction through the grains and across grain boundaries dominates at T > T_t. A long-time relaxation effect is observed around the transition temperature (T ≈ T_t). This effect is attributed to a subsurface superionic transition leading to a formation of a subsurface superionic phase at the grain boundaries. The analysis of experimental results shows that the temperature-independent high-frequency ac conductivity observed at T < 100 K can be explained by a model involving a Debye relaxation of Ag~+ ions in preferred sites with an appropriate distribution of relaxation times.