Universal dielectric response of variously doped CeO2 ionically conducting ceramics

The Jonscher power law, or "universal dielectric response" (UDR) behavior was studied for a range of CeO2 solid solutions with Y3+ and Gd3+ dopants, with particular emphasis on dilute systems which possess relatively simple defect structures. The results show power-law frequency dependence of the ac conductivity, with exponent s = 0.61+/-0.03, independent of temperature and concentration. The conductivity data also show scaling behavior in terms of a time constant tau whose activation energy is very close to that of the de conductivity. For 1% Y and 1% Gd samples, an additional Debye-type relaxation is observed due to dopant-oxygen-vacancy pairs. Such samples are clearly in the association range (stage III). These results contradict the assumption by Almond and West that tau(-1) is the hopping frequency of the carrier defects. At very low concentrations (similar to 0.01%), UDR behavior virtually disappears. The present results are then compared to the principal theories that describe UDR behavior. It is found that, while each theory suffers from some drawbacks, the more phenomenological theories fare better.