Experimental frequency-dependent conductivity relaxation spectra of a number of molten, glassy, and crystalline ionic conductors that showed both the presence of the near-constant loss and the cooperative ion hopping contribution were analyzed. On decreasing frequency, the near-constant loss appeared first but terminates at some frequency νx1. At a still lower frequency, νx2, cooperative ion hopping dispersion took over. The independent ion hopping frequency ν0 of the coupling model was calculated from the parameters characterizing the cooperative ion hopping dispersion. It was found for all ionic conductors that νx1 >> ν0 and ν0 always fall inside the frequency region νx1 > ν > νx2. The empirical results led to a qualitative theory for the origin of the near-constant loss, which gave physical meanings of the two crossover frequencies νx1 and νx2, as well as explaining the role of the independent hopping frequency ν0, in determining them. The weak temperature dependence of the near-constant loss was recaptured by the qualitative theory. An improved understanding was gained of the evolution of the ion dynamics from early times when the cages decayed very slowly with time, giving rise to a near-constant loss, to long times when ions moved cooperatively, leading finally to direct-current conductivity.

Cage Decay, Near Constant Loss and Crossover to Cooperative Ion Motion in Ionic Conductors - Insight from Experimental Data. K.L.Ngai, C.León: Physical Review B, 2002, 66[6], 064308 (11pp)