Likewise, with increasing temperature (to 333 K) the Li NMR linewidths decrease as the increased Li ion motion reduces the residual homonuclear Li- Li dipolar coupling. The spinlattice relaxation for Li is influenced by mobile Li ions, thus measuring the T1 relaxation parameters as a function of temperature allows the associated activation energy for Li dynamics for these materials to be determined using the Arrhenius equation. In the temperature range studied here, all Sb-containing materials display a decrease in their spin-lattice relaxation time with increasing temperature, with all T1 values located on the low-temperature side of the T1 curve, as shown in Figure . Using these data, the Sb-containing materials were found to range from 32 to 75 meV, as shown in Table . In contrast to the Sb-containing phases, the T1 minimum for Li3Bi is found at 233 K, revealing that Li ions are moving rapidly within this phase where the correlation time for Li (T1 minimum) can be obtained and an activation energy of 75 ± 5 meV is determined from the more accurate high-temperature side of the T1 curve. The interplay between the fast and slow-motion limits reveals that at 233 K, Li ion diffusion is occurring at approximately the Li nuclear Larmor frequency (ns timescale, ~0.8 ns), increasing at higher temperatures approaching the value reported for fast solid ionic conductors, consistent with the narrow Lorentzian lineshape observed above. The associated Li3Bi activation energy agrees well with prior conductance measurements (Ea < 100 meV), whereas the NMR results appear to underestimate Li3Sb and associated mixtures which is attributed to the inability to reach high temperature side of the T1 curve (due to hardware limitations) resulting in an underestimation of activation energy. The discrepancy for Li3Bi may be attributed to how the techniques respond to Li ion diffusion: NMR spectroscopy is sensitive to medium-and short-range structure, while conductivity measurement methods respond to bulk solid properties, probing long-range macroscopic dynamics whereby grain boundaries, defects or compositional/structural variations, for example that can influence the measurements. For example, an early NMR study provided evidence of lithium ion dynamics within Li3Sb, while lithium diffusion and conductivities were reported on bulk Li3Sb and Li3Bi materials using the galvanostatic intermittent titration technique (GITT). Using GITT, the authors demonstrated that variations in activation energy (200-300 meV for Li3Sb, and 130 meV for Li3Bi) were due to small deviations from the 3:1 stoichiometric ratio for the materials that impacted the measured activation energy. Therefore, the new synthetic approach (mechanochemical synthesis) used to provide single-phase crystalline materials that exhibit varying degrees of local order, may contribute to the variations in activation energies. Recently, this phenomenon has also been observed in other ionic conducting materials. We also caution that values determined from the GITT method date back nearly four decades and it has since been suggested that the approach may provide spurious results. 59