90° Domain Structure of (La,Li)TiO3 and Its Influence on Li Conduction Properties

The influence of 90° domain boundaries in (La,Li)TiO₃ (LLTO) on the Li conduction mechanism has been examined by a combination of state-of-the-art experiments and first-principles calculations. The atomistic structure of 90° domain boundaries in LLTO has been determined using HAADF-STEM observations. Using HAADF-STEM observations, we can obtain Z-contrast image.  As shown in Fig.1, bright spots correspond to heavier La column. Important finding is that at 90° domain boundary, each cells connected by La layer. Based on this structural information, first-principles calculations were carried out. In this study, we use projector augmented wave (PAW) method based on the density functional theory (DFT).  Li conduction path and its activation energy were determined using nudged elastic band (NEB) method. Phonon dispersion relations were determined by frozen phonon method. Firstly we performed bulk crystal calculations using tetragonal (SG: P4/mmm) (La_0.5,Li_0.5)TiO₃ assuming alternative stacking La layer and Li layer along c-direction.  The calculated phonon dispersion for bulk crystal shows soft-mode in the entire wave vector space. This indicate lattice instability mainly TiO₆-octahedoron rotation. Because of this TiO₆-octahedoron rotation instability, calculated Li conduction activation energy for LLTO bulk shows exceptionally small value (E_a = 0.2 eV). However, this theoretical E_a was quite small to compare with experimentally reported value (E_a = 0.4 eV). To reveal the origin of this E_a discrepancy between theory and experiments, secondary, we examined Li-conduction energies at 90° domain boundary. We assumed two type of Li conduction: a) Li conduction through La layer, b)Li conduction through La vacancy at La layer. a) Li conduction through La layer shows very high E_a of 3 eV. This indicate La layer acting blocking layer for Li conduction. However, if we assume the La vacancy at La layer, conduction energy significantly decreased to 0.6 eV. This conduction energy well corresponds to experimental value. Our calculations reveal that Li conduction in LLTO should be strongly influenced by 90° domain boundaries in this system.