Composition dependence of the dispersive nature of the ac conductivity in ionic conductors Gd2Ti2−yZryO7 and Li0.5−xNaxLa0.5TiO3

We present Admittance Spectroscopy measurements on two different ionically conducting materials, Gd2Ti2−yZryO7 and Li0.5−xNaxLa0.5TiO3. Electrical relaxation data have been found to be well described by stretched exponential functions of the form Φ(t) = exp(−(t/τ)1−n). In Gd2Ti2−yZryO7 the concentration of mobile ions in the whole series is very low, from 1% to 0.02 %, and we find that by strongly decreasing the density of mobile ions the value of the exponent n shows a significant decrease from 0.44 ± 0.01 to 0.16 ± 0.01. In Li0.5−xNaxLa0.5TiO3 we have investigated changes in ion dynamics when approaching the percolation threshold for lithium diffusion by increasing the number of immoble sodium ions, and found that the exponent n increases towards a value of 0.81 ± 0.01 as the mobility of lithium ions becomes more and more constrained close to the percolation threshold. Both results are discussed in terms of the importance of ion–ion correlations in the dispersive behavior of the electrical conductivity.