Cage decay, near constant loss, and crossover to cooperative ion motion in ionic conductors:: Insight from experimental data -: art. no. 064308

Experimental frequency-dependent conductivity relaxation spectra of a number of molten, glassy, and crystalline ionic conductors that show both the presence of the near constant loss (NCL) and the cooperative ion hopping contribution are analyzed. On decreasing frequency, the NCL appears first but terminates at some frequency nu(x1). At still a lower frequency nu(x2) the cooperative ion hopping dispersion takes over. The independent ion hopping frequency nu(0) of the coupling model is calculated from the parameters characterizing the cooperative ion hopping dispersion. It is found for all ionic conductors that nu(x1)>nu(0), and nu(0) always fall inside the frequency region nu(x1)>nu>nu(x2). The empirical results leads to a qualitative theory for the origin of the NCL, which gives physical meanings of the two crossover frequencies nu(x1) and nu(x2), as well as explaining the role of the independent hopping frequency nu(0), in determining them. The weak temperature dependence of the NCL has been recaptured by the qualitative theory. An improved understanding is gained of the evolution of the ion dynamics from early times when the cages decay very slowly with time, giving rise to the near constant loss, to long times when ions move cooperatively, leading finally to dc conductivity.