Microscopic dynamics of lithium diffusion in single crystal of the solid-state electrolyte La 2 / 3 − x Li 3 x TiO 3 ( x = 0.13 ) studied by quasielastic neutron scattering

Quasielastic neutron scattering (QENS) measurements combined with first principles based molecular-dynamics calculations were conducted to study the dynamics of ${\mathrm{Li}}^{+}$ ions in a solid-state electrolyte ${\mathrm{La}}_{2/3\ensuremath{-}x}{\mathrm{Li}}_{3x}\mathrm{Ti}{\mathrm{O}}_{3}$ (LLTO) with $x=0.13$. By using a large $^{7}\mathrm{Li}$-enriched single crystal sample, a QENS signal was clearly observed along the three principal axes [110], [111], and [001] at a temperature ($T$) of 600 K. Wave vector dependence of the linewidth of the QENS signal along each direction was explained well using the Chudley-Elliot model for jumps between the $A$ sites of the perovskite lattice through the bottleneck square, which was also supported by molecular dynamics calculations. At $T=600$ K, the estimated self-diffusion coefficient of ${\mathrm{Li}}^{+}$ (${D}_{\mathrm{Li}}$) in the $ab$ plane [${D}_{\mathrm{Li}}^{ab}=(6.8\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$/s] was slightly larger than that along the $c$ axis [${D}_{\mathrm{Li}}^{c}=(4.4\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$/s], suggesting quasi-isotropic diffusion, that is, the three-dimensional diffusion of ${\mathrm{Li}}^{+}$ ions. The decrease in ${D}_{\mathrm{Li}}$ with decreasing $T$ was reasonably explained by a thermal activation process with the activation energy determined from ionic-conductivity measurements. Furthermore, the estimated values of the self-diffusion coefficient of ${\mathrm{Li}}^{+}$ ions are comparable to those in the sulfide-based ${\mathrm{Li}}^{+}$ ion conductor, ${\mathrm{Li}}_{7}{\mathrm{P}}_{3}{\mathrm{S}}_{11}$, although its ionic conductivity is 10 times larger than that for LLTO. The obtained microscopic information on ${\mathrm{Li}}^{+}$ diffusion in LLTO clarifies how to understand the Li conduction mechanism in LLTO and ${\mathrm{Li}}_{7}{\mathrm{P}}_{3}{\mathrm{S}}_{11}$ in a unified manner and can provide a way to increase the ${\mathrm{Li}}^{+}$ ionic conductivity in oxide-based solid electrolytes.