Microscopic Dynamics of Li⁺ in Rutile TiO₂ Revealed by ⁸Li β-Detected Nuclear Magnetic Resonance

Abstract

We report measurements of the dynamics of isolated 8 Li + in single crystal rutile TiO2 using βdetected 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) × 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) × 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.

Ionic and electronic properties of the topological insulator Bi2Te2Se investigated via β -detected nuclear magnetic relaxation and resonance of McFadden1, Chatzichristos2, Chow3 et al. 2019Phys. Rev. BSelf Cite12060View full textAdd to dashboardCite

“…Interestingly, the mobility of Cu in isostructural Bi 2 Te 3 is also extremely high, as revealed by 64 Cu radiotracer 70 and electrochemical methods. 73 Lastly, we note that similarly large D values were reported recently for 8 Li + in the one dimensional ion conductor rutile TiO 2 68 and we speculate that the exceptional mobility may be generic for isolated Li + (i.e., at infinitely dilute concentrations) in ion conducting solids. It would be interesting to test this conjecture against detailed ab initio calculations.…”

“…This approach has the advantage that it does not rely on any particular form of J n , and we recently used it to quantify diffusion of isolated 8 Li + in rutile TiO 2 . 68 Finally, for each T max we assume τ −1 c matches the Larmor frequency ω 0 . The results are shown in the Arrhenius plot in Figure 8, where the linearity of the data, spanning three orders of magnitude, demonstrates the consistency of the approach.…”

Investigation of ionic and anomalous magnetic behavior in CrSe2 using 8Li β-NMR

“…When ν hop matches the NMR frequency, 1/T 1 is maximized at a Bloembergen-Purcell-Pound (BPP) peak [36]. From this, one can determine the coupling constant and measure ν hop as well as the associated activation energy barrier, E a , unambiguously [12,31]. However, absence of a clear 1/T 1 maximum at high temperatures prevents extracting quantitative information from this BPP model.…”

“…Furthermore, this state was found to be highly sensitive to S/Se substitution [11]. While the muon is a good probe of magnetic behavior, investigation of the dilute-limit Li + dynamics can also provide important details of the short-range, microscopic Li + motion [12]. This prompts the use of a similar, but complementary probe: the short-lived radioisotope 8 Li.…”

The Site and High Field βNMR Properties of 8Li+ Implanted in α-Al2O3

“…In Figure 5 the relaxation is seen to be considerably faster in rutile TiO 2 and deuterated polystyrene (PS-d8). In the former case 8 Li + is mobile [35] and the relaxation results from stochastic diffusion. In the latter, glassy dynamics of the host provides analogous fluctuations [36].…”