Frequency-dependent conductivity and dielectric function of superionic conductors

The diffusive part and the oscillatory part (optically active lattice vibrations) of the conductivity in an ionic solid are related by an oscillator strength sum rule. In a superionic conductor a non negligible fraction of the total oscillator strength is in the diffusive part. A simple model is introduced which includes both diffusive and oscillatory part, takes account of the sum rule and is able to describe the full range from an insulating ionic solid to a free ion system. The model is applied to the frequency dependent conductivity σ(ω) of AgI and β-alumina at various temperatures. σ(ω) is determined from far infrared (FIR) reflectivity experiments. The importance is shown of such measurements for screening of new potential superionic conductors. FIR experiments are not hampered by contact, polarization, and grain boundary problems. A high conductivity at frequencies below the reststrahlen bands is thus a necessary condition for a high dc conductivity.