AC CONDUCTIVITY IN IONICALLY CONDUCTING CRYSTALS AND GLASSES

The ac conductivity behavior has been studied for crystalline oxygen-ion conductors: CeO2: Gd3+ and CaTiO3:Al3 as well several silicate and borate glasses over a wide range of temperature. In all cases, Jonscher's universal power law behavior: sigma(omega) = sigma(0) + Aomega(s) is observed, where sigma(0) is the dc conductivity and s the power law exponent. Furthermore, there are three regions of behavior. At the higher temperatures, s is a constant (usually almost-equal-to 0.6) independent of T. In this range a can be written in the form suggested by Almond and West: sigma(omega) =sigma(0) [1 + (omegatau)s]. We find that tau has the same activation energy as sigma(0) and a pre-exponential almost-equal-to 10(-14) s. However, the equality of the activation energies does not imply that all carriers are unassociated, as Almond and West have claimed. Rather, it suggests that the ac and dc parts of the conductivity are manifestations of a single hopping process involving ion-ion interactions. At lower temperatures, the exponent s increases as T decreases, finally leveling off at s = 1.