Effect of Relaxations on the Conductivity of La_{1/2+1/2 x} Li_{1/2-1/2 x} Ti_{1- x} Al _x O₃ Fast Ion Conductors

Perovskite-type solid-state electrolytes, Li_3x La_2/3-x TiO₃ (LLTO), are considered among the most promising candidates for the development of all-solid-state batteries based on lithium metal. Their high bulk ionic conductivity can be modulated by substituting part of the atoms hosted in the A- or B-site of the LLTO structure. In this work, we investigate the crystal structure and the long-range charge migration processes characterizing a family of perovskites with the general formula La_1/2+1/2x Li_1/2-1/2x Ti_1-x Al _x O₃ (0 ≤ x ≤ 0.6), in which the charge balance and the nominal A-site vacancies (n _A = 0) are preserved. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) investigations reveal the presence of a very complex nanostructure constituted by a mixture of two different ordered nanoregions of tetragonal P4/mmm and rhombohedral R3̅c symmetries. Broadband electrical spectroscopy studies confirm the presence of different crystalline domains and demonstrate that the structural fluctuations of the BO₆ octahedra require to be intra- and intercell coupled, to enable the long-range diffusion of the lithium cation, in a similar way to the segmental mode that takes place in polymer-ion conductors. These hypotheses are corroborated by density functional theory (DFT) calculations and molecular dynamic simulations.