Non-Debye relaxations in disordered ionic solids

Motions of charged defects in ionic solids, including glassy ionic conductors, defective crystals and composite materials, imply slow relaxation processes, which are observable within a wide range of timescales larger than microscopic (vibrational) times. These processes manifest themselves in numerous dynamical probes, like ac-conductivity, nuclear spin-relaxation, quasi-elastic neutron scattering and mechanical relaxation. The present theoretical understanding of the corresponding response functions is reviewed. Stochastic models based on ion hopping are the most natural approach for systems with structural disorder on microscopic length scales, but more coarse-grained, phenomenological schemes are addressed as well. Macroscopically inhomogeneous systems and interfacial problems are modeled by random impedance networks. Generally, non-exponential relaxation gets enhanced when Coulomb interactions between ions are taken into account. This is demonstrated by large-scale Monte Carlo simulations of disordered lattice gases for ion diffusion and is supported further by new results on random dipolar systems in the context of the "nearly constant dielectric loss response". (C) 2002 Elsevier Science B.V. All rights reserved.