NATURE OF THE AC CONDUCTIVITY OF IONICALLY CONDUCTING CRYSTALS AND GLASSES

Many disordered systems, both crystalline and glasses, show 'universal dynamic response', in which the ac conductivity as a function of frequency is given by sigma(omega) = sigma(0) + A omega(s). Here sigma(0) is the dc conductivity and the exponent s (less than or equal to 1) generally decreases with increasing temperature, T. For various ionically conducting crystals and glasses, we have obtained two ranges in which s = constant, independent of T: at high temperatures s = s(0) (a value near 0.6) called the 'Jonscher regime', and at low temperatures s = 1, called the constant-loss regime. For the Jonscher regime, the parameter A is activated with activation energy closely related to that of sigma(0). On the other hand, for the constant-loss regime (which extends down to cryogenic temperatures), the parameter A varies only slowly with rand is not activated. For the cases of berate and silicate glasses as well as heavily doped crystalline CaTiO3 and CeO2, careful analysis was carried out over the intermediate regime in which the effective s falls from 1.0 to s(0). The results show that this regime can simply be described as a superposition of the Jonscher and constant-loss behaviors. Work by others on silicate glass over a much wider frequency range than that of the present study falls into the same pattern. Finally, a brief review is given of possible mechanisms of both the Jonscher and constant-loss types of behavior.