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

We showhowthe near constant loss (NCL) crosses over to cooperative ion hopping from extensive dielectric measurements of the glass-forming molten salt 0.4Ca(NO3)2–0.6KNO3 (CKN) of Lunkenheimer and co-workers at temperatures belowand above the glass transition temperature. The NCL ends w hen time exceeds tx1 and cooperative hopping becomes prominent beyond tx2. The crossover, occurring over the time region tx1 < t < tx2, is not sharp but tx1 and tx2 are not far from each other in order of magnitude. At most temperatures the primitive relaxation time s0 of an ion in the coupling model calculated from the experimental data lies inside the crossover region and is not much shorter than tx2 Since s0 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 slowdecay 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. 2002 Elsevier Science B.V. All rights reserved.