The contribution of configurational entropy to the effective hopping frequency of ionic transport in amorphous systems was considered. The effective rate of ion hopping was extracted from the onset frequency of the ac conductivity measured in ionically conducting silicate glasses. Both the onset frequency and the dc conductivity exhibited Arrhenius-type thermal activation with similar values for the activation energy, of 0.65eV. The pre-factor of the onset frequency resulted in ν0' = 1.05 x 1011Hz, which was much lower than the characteristic vibrational frequencies of 1013Hz. Following standard hopping percolation theory, the long-range motion was dominated by a fraction of high-energy barriers that connected clusters of faster sites. The multiplicity of equivalent sites for ion hopping involved a retardation of the effective jumping time with respect to the elementary hop. This effect could be assimilated into a negative activation entropy term in the frequency pre-factor of the ion hopping rate, which depended upon the features of energy clustering and accounted for the wide dispersion of ν0' reported for many conducting glasses. The model implied an effective percolation length of approximately 7nm; in good agreement with previous work.

Entropy Factor in the Hopping Frequency for Ionic Conduction in Oxide Glasses Induced by Energetic Clustering. G.Garcia-Belmonte, J.Bisquert: Journal of Chemical Physics, 2005, 123[7], 074504