A microscopic model for ionic transport above the vitreous

7 A microscopic model for ionic transport above the vitreous transition temperature 

Above the vitreous transition temperature Tg, ionic conductivity increases steeply as represented in Fig. 4.6 from data obtained in the Agl-AgMoQ mixture. Above Tg, ionic conductivity is no longer represented by an Arrhenius law (4.1) and experimental results are better represented by an empirical relationship 

Such a relationship is also commonly observed for salt-polymer complexes (Armand, Chabagno and Duclot, 1979). As mentioned in the context of the structural relaxation time (Section 4.3) it is referred to as VTF behaviour. 

Generally, the VTF behaviour of all transport properties may be understood from the free volume concept introduced by Doolittle (1951) and further developed by Cohen and Turnbull (1959). Essentially, any diffusing species is depicted as encaged by the nearest atoms in a cell 

Finally, the structural relaxation time for any diffusing species in the supercooled liquid is proportional to the probability of this species having access to a free volume over the minimum value Vf required for an elementary displacement. 

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