Crossover to nearly constant loss in ac conductivity of highly disordered pyrochlore-type ionic conductors

We report on ac conductivity measurements of oxide ion conductors with composition Gd_(2)(Zr_(y)Ti_(1−y)_(2)O_(7), at temperatures between 170 and 500 K and in the frequency range 1 Hz–3MHz, and show that a crossover from a sublinear power law to a linear frequency dependence (or nearly constant loss behavior) in the ac conductivity can be clearly observed in a wide temperature range. This crossover is found to be thermally activated, and its activation energy ENCL to be much lower than the activation energy Edc for the dc conductivity. We also found that the values of ENCL are almost independent of composition, and therefore of the concentration of mobile oxygen vacancies, unlike those of Edc. Moreover, for each composition, the values of ENCL=0.67 +/- 0.04 are very similar to those estimated for the energy barrier for the ions to leave their cages, Ea=0.69 +/- 0.05. These results support that the nearly constant loss behavior, ubiquitous in ionic conductors, is originated from caged ion dynamics