Influence of Vacancy Ordering on the Percolative Behavior of (Li1-xNax)3yLa2/3-yTiO3 Perovskites

Abstract

Influence of the vacancy concentration on the Li conductivity of the (Li_1-xNa_x)_0.2La_0.6TiO₃ and (Li_1-xNa_x)_0.5La_0.5TiO₃ perovskite series, with 0 ≤ x < 1, has been investigated by neutron diffraction (ND), impedance spectroscopy (IS), nuclear magnetic resonance (NMR), and Monte Carlo simulations. In both series, Li⁺ ions occupy unit cell faces, but Na⁺ ions are located at A sites of the perovskite. From this fact, the amount of vacant A sites that participate in Li conductivity is given by the expression n_v = [Li] + □, where □ is the nominal vacancy concentration. Substitution of Li by Na decreases the amount of vacancies, reducing drastically the Li conductivity when n_v approaches the percolation threshold of the perovskite conduction network. In disordered (Li_1-xNa_x)_0.5La_0.5TiO₃ perovskites, the percolation threshold is 0.31; however, in ordered (Li_1-xNa_x)_0.2La_0.6TiO₃ perovskites, this parameter changes to 0.26. Near the percolation threshold, the amount of mobile Li species deduced by ⁷Li NMR spectroscopy is lower than that derived from structural formulas but higher than deduced from dc conductivity measurements. Conductivity values have been explained by Monte Carlo simulations, which assume a random walk for Li ions in the conduction network of the perovskite. In these simulations, distribution of vacancies conforms to structural models deduced from ND experiments.