Percolation-Limited Ionic Diffusion in Li_0.5-xNa_xLa_0.5TiO₃ Perovskites (0 ≤ x ≤ 0.5)

The dependence of ionic transport properties on the structure and composition of perovskites Li 0.5 - x Na x La 0.5 TiO 3 (0 ≤ x ≤ 0.5) has been analyzed by means of ND, XRD, NMR, and impedance spectroscopy. Local lithium mobility is shown to decrease progressively by 2 orders of magnitude along the series; however, long-range dc conductivity decreases sharply at x = 0.2 more than 6 orders of magnitude. The decrease of dc conductivity from values typical of fast ionic conductors, 10 - 3 S/cm at room temperature, to values of insulators, 10 - 10 S/cm, is discussed in terms of a three-dimensional percolation model for lithium diffusion. As deduced from XRD and ND data, the number of vacant sites in conduction pathways is controlled by the amount of Na and La in the perovskite.