The crossover from the near constant loss to ion hopping ac conductivity in ionic conductors:: the crossover times

We show how the near constant loss (NCL) crosses over to cooperative ion hopping from extensive dielectric measurements of the glass-forming molten salt 0.4Ca(NO3)(2)-0.6KNO(3) (CKN) of Lunkenheimer and co-workers at temperatures below and above the glass transition temperature. The NCL ends when time exceeds t(x1) and cooperative hopping becomes prominent beyond t(x2). The crossover, occurring over the time region t(x1) < t < t(x2), is not sharp but t(x1) and t(x2) are not far from each other in order of magnitude. At most temperatures the primitive relaxation time tau(0) of an ion in the coupling model calculated from the experimental data lies inside the crossover region and is not much shorter than t(x2) Since tau(0) is the relaxation time of a caged ion in a potential well to overcome the energy barrier by thermal activation, the findings here support the proposal that the origin of the NCL comes from the very slow decay with time of the cage confining the ion in its potential well. A simple theoretical model of a caged particle that has its cage decaying very slowly with time is solved numerically and a NCL over many decades of frequency is reproduced. (C) 2002 Elsevier Science B.V. All rights reserved.