Microscopic Dynamics of Li$^{+}$ in Rutile TiO$_{2}$ Revealed by $^{8}$Li $\beta$-detected NMR

TL;DR

This study uses $eta$-detected NMR to investigate the microscopic dynamics of isolated Li$^{+}$ ions in rutile TiO$_{2}$, revealing thermally activated processes and diffusion behavior consistent with macroscopic measurements.

Contribution

First microscopic measurement of Li$^{+}$ dynamics in rutile TiO$_{2}$ using $eta$-detected NMR, identifying distinct low and high temperature activation processes.

Findings

Low-temperature activation barrier of 26.8 meV likely related to electron polarons.

High-temperature Li$^{+}$ diffusion characterized by 0.32 eV activation energy.

Li$^{+}$ diffusivity is not limited by concentration up to high levels.

Abstract

We report measurements of the dynamics of isolated $^{8}$Li$^{+}$ in single crystal rutile TiO$_{2}$ using $\beta$-detected NMR. From spin-lattice relaxation and motional narrowing, we find two sets of thermally activated dynamics: one below 100 K; and one at higher temperatures. At low temperature, the activation barrier is $26.8(6)$ meV with prefactor $1.23(5) \times 10^{10}$ s$^{-1}$. We suggest this is unrelated to Li$^{+}$ motion, and rather is a consequence of electron polarons in the vicinity of the implanted $^{8}$Li$^{+}$ that are known to become mobile in this temperature range. Above 100 K, Li$^{+}$ undergoes long-range diffusion as an isolated uncomplexed cation, characterized by an activation energy and prefactor of $0.32(2)$ eV and $1.0(5) \times 10^{16}$ s$^{-1}$, in agreement with macroscopic diffusion measurements. These results in the dilute limit from a microscopic probe indicate that Li$^{+}$ concentration does not limit the diffusivity even up to high concentrations, but that some key ingredient is missing in the calculations of the migration barrier. The anomalous prefactors provide further insight into both Li$^{+}$ and polaron motion.