Composition dependence of the dispersive nature of the ac conductivity in ionic conductors Gd2Ti2−Zr O7 and Li0.5−Na La0.5TiO3

We present Admittance Spectroscopy measurements on two different ionically conducting materials, Gd 2 Ti 2-y Zr y O 7 and Li 0.5-x Na x -La 0.5 TiO 3 . 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 Gd 2 Ti 2-y Zr y O 7 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 Li 0.5-x -Na x La 0.5 TiO 3 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.