THE NGAI COUPLING MODEL OF RELAXATION - GENERALIZATIONS, ALTERNATIVES, AND THEIR USE IN THE ANALYSIS OF NON-ARRHENIUS CONDUCTIVITY IN GLASSY, FAST-IONIC MATERIALS

The ionic conductivity of glassy, fast-ion-conducting materials can sh ow non-Arrhenius behavior and approach saturation at sufficiently high temperatures [J. Kincs and S. W. Martin, Phys. Rev. Lett. 76, 20 (199 6)]. The Ngai coupling model was soon applied to explain some of these observations [K. L. Ngai and A. K. Rites, Phys. Rev. Lett. 76, 1296 ( 1996)], but detailed examination and generalization of the coupling mo del suggested the consideration of a related, yet different, approach, the cutoff model. Although both the coupling and cutoff models involv e a shortest nonzero response time, tau(c), and lead to single-relaxat ion-time Debye response at limiting short times and high frequencies, they involve different physical interpretations of their low- and high -frequency response functions. These differences are discussed; the pr edictions of both models in the frequency and time domains are compare d; and the utility of both models is evaluated for explaining the non- Arrhenius conductivity behavior associated with the dispersed frequenc y response of zAgI+(1-z)[0.525Ag(2)S+0.475B(2)S(3):SiS2] glass for z=0 and 0.4. The cutoff approach, using simulation rather than direct dat a fitting, yielded semiquantitative agreement with the data, but simil ar analysis using the coupling model led to poor results. The coupling model leads to an appreciable slope discontinuity at the tau(c) trans ition point between its two separate response parts, while the cutoff model shows no such discontinuity because it involves only a single re sponse equation with a smooth transition at tau(c) to limiting single- relaxation-time response. The greater simplicity, utility, and general ity of the cutoff model suggest that it should be the favored choice f or analyzing high-conductivity data exhibiting non-Arrhenius behavior. (C) 1998 American Institute of Physics. [S0021-8979(98)06714-0]