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 $1623\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 $170\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 $250--350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this temperature range, a plateau is detected in ${T}_{2}^{\ensuremath{-}1}$ 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 ($\ensuremath{\omega}\ensuremath{\tau}⪡1$ regime). ${T}_{1\ensuremath{\rho}}$ measurements indicate that there is a slower motion of lithium with a characteristic time of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ at room temperature, assigned to isotropic three-dimensional diffusion.