Two-dimensional Li+ ionic hopping in Li3InCl6 as revealed by diffusion-induced nuclear spin relaxation

Ternary Li halides, such as ${\mathrm{Li}}_{3}{\text{Me}X}_{6}$ with, e.g., Me = In, Sc, Y and $X$ = Cl, Br, are the center of attention for battery applications, as these materials might serve as ionic electrolytes. To fulfill their function, such electrolytes must have an extraordinarily high ionic ${\mathrm{Li}}^{+}$ conductivity. Layer-structured ${\mathrm{Li}}_{3}{\mathrm{InCl}}_{6}$ represents such a candidate; however, understanding the origin of the rapid ${\mathrm{Li}}^{+}$ exchange processes needs further investigation. Spatially restricted, that is, low-dimensional particle diffusion, might offer an explanation for fast ion dynamics. It is, however, challenging to provide evidence for 2D diffusion at the atomic scale when dealing with polycrystalline powder samples. Here, we use purely diffusion-induced $^{7}\mathrm{Li}$ nuclear magnetic spin relaxation to detect anomalies that unambiguously show that 2D Li diffusion is chiefly responsible for the dynamic processes in a ${\mathrm{Li}}_{3}{\mathrm{InCl}}_{6}$ powder sample the present paper focusses on. The change of the spin-lattice relaxation rate $1/{T}_{1}$ as a function of inverse temperature $1/T$ passes through a rate peak that strictly follows asymmetric behavior. This feature is in excellent agreement with the model of P. M. Richards [Solid State Commun. 25, 1019 (1978)], suggesting a logarithmic spectral density function $J$ to fully describe 2D diffusion. Hence, ${\mathrm{Li}}_{3}{\mathrm{InCl}}_{6}$ belongs to the very rare examples for which 2D ${\mathrm{Li}}^{+}$ diffusion has been immaculately verified.