Lithium dynamics in the fast ionic conductorLi0.18La0.61TiO3probed byLi7NMR spectroscopy

Abstract

Lithium dynamics in the ${\mathrm{Li}}_{0.18}{\mathrm{La}}_{0.61}{\mathrm{TiO}}_3$ perovskite quenched from $1623\mathrm{K}$ has been analyzed by means of ${}^7\mathrm{Li}$ Nuclear magnetic resonance (NMR) spectroscopy and neutron powder diffraction. The Rietveld analysis of the diffraction pattern shows rhombohedral symmetry with lithium ions occupying square windows that connect contiguous $A$ sites of the perovskite. The hopping of lithium ions through these windows produces the line narrowing detected above $170\mathrm{K}$ in ${}^7\mathrm{Li}$ NMR spectra. Deconvolution of NMR spectra shows the existence of two lithium species that exchange their positions along the temperature range $250–350\mathrm{K}$. In this temperature range, a plateau is detected in $T_2^{-1}$ plots, which has been ascribed to the existence of two-dimensional motions of lithium in ordered domains of the perovskite. Evidence of this limited motion comes from the frequency dependence of the spin-lattice relaxation rates measured at the high temperature side of the asymmetric $1∕T_1$ maximum ($\omega \tau ⪡1$ regime). $T_{1\rho }$ measurements indicate that there is a slower motion of lithium with a characteristic time of $3\times {10}^{-6}\mathrm{s}$ at room temperature, assigned to isotropic three-dimensional diffusion.