Comparison of the universal dynamic response power-law fitting model for conducting systems with superior alternative models

For at least 5 years there has been considerable controversy concerning the relative value of power-law and electric modulus formalism models for fitt ing and interpreting dispersed frequency-response data for ionically conduc ting glasses, melts, and other disordered solids. Conclusions of various au thors have ranged from preferring one or the other to neither. Here, detail ed complex-nonlinear-least-squares fitting of data for a trisilicate glass with several different dispersion models leads to the conclusion that 'neit her' of the above is the correct conclusion for an adequate analysis of bul k-material behavior in this and other materials. The power-law model is non physical, and the usual modulus formalism approach is faulty in two differe nt ways. For the near-room-temperature data set analyzed here, it was found that when electrode effects were included in a composite fitting model, th ey contributed significantly to high-, but not low-frequency response. Thei r presence may explain the increasing log-log slope of the real part of the conductivity with increasing frequency found for many materials. The corre cted modulus formalism approach, involving a Kohlrausch-Williams-Watts mode l, the KWW1, was found to be the best of those used to represent bulk respo nse. Contrary to common expectation, the original modulus formalism and KWW 1 models do not lead to stretched-exponential response in the time domain. Best fitting required not only a model for bulk response but one for electr ode response as well and necessarily also involved a separate fitting param eter to account for high-frequency-limiting dipolar dielectric effects. (C) 2000 Elsevier Science B.V. All rights reserved.