On the Local Structure and Lithium Dynamics of La_0.5(Li,Na)_0.5TiO₃ Ionic Conductors. A Raman Study

We present a Raman study of structural and dynamical properties of the La 0.5 (Li,Na) 0.5 TiO 3 perovskites, closely related to the La (2 - x )/ 3 Li x TiO 3 ionic conductors. Despite intense experimental effort, the structure, lithium location, and diffusion mechanism of these materials are still controversial. The structure of La 0.5 (Li,Na) 0.5 TiO 3 compounds, in particular, has been refined as rhombohedral by neutron diffraction, with La and Na occupying randomly the A-type site. However, the correlation of their Raman spectra with those of La (2 - x )/ 3 Li x TiO 3 compounds indicates that some degree of cationic order is preserved on the nanometric scale and that rhombohedral symmetry may result from an average of ordered domains forming randomly. With regard to dynamical properties, we show that all compounds of the series with [Li] ≠ 0 present lithium diffusion, even for compositions below the percolation limit. This result is attributed to lithium diffusion being restricted, in such cases, to finite clusters, not contributing to macroscopic conductivity but being detected by Raman scattering due to the sensitivity of this technique to local environments.