An extended coupling model description of the evolution of dynamics with time in supercooled liquids and ionic conductors

In the past the coupling model (CM) was focused on the dynamics at long times, when all relaxing units of an interacting system participate in the terminal, cooperative Kohlrausch relaxation. No attention was paid to the dynamics at short times when all the relaxing units are caged, nor to intermediate times when an increasing number of them are no longer caged. We now extend the CM to also address the dynamics in these earlier time regimes. The crux of the extended CM is the quantitatively determinable independent relaxation time, from which the characteristics of the dynamics in the short-time and intermediate-time regimes can be deduced. This description of the evolution of the dynamics by the extended CM is supported by broadband dielectric relaxation spectra of two archetypal systems, the glassy/molten ionic conductors and supercooled liquids. In supercooled liquids, the 'universal' Johari–Goldstein β-relaxation provides evidence for the physical reality of the independent relaxation of the extended CM.