Advancing the Probing of Li Dynamics in NMR: Isotope Dilution Effects on Spin-Alignment Echo Generation

Solid-state diffusion plays a critical role in materials science, particularly in the precise measurement of ion dynamics in materials with structural disorder. Spin-alignment echo (SAE) nuclear magnetic resonance (NMR), which is comparable to exchange spectroscopy, has proven to be a powerful technique for probing (ultra)slow Li dynamics, even in amorphous materials. However, 7Li NMR Jeener-Broekaert echoes can be influenced by the simultaneous generation of dipolar and quadrupolar order. In many cases, the dipolar interactions can be suppressed by selecting evolution times (tp) shorter than 20 μs. In this study, glassy Li2Si3O7 is used as a model system to investigate the positive impact of isotope dilution on 7Li SAE NMR. Specifically, we explore how the reduction of homonuclear dipole–dipole interactions through spatial separation of spin-3/2 probe nuclei influences the results. Two samples were analyzed: one with 100% 7Li and the other with 5% 7Li (95% 6Li). The samples were studied using mixing-time and preparation-time dependent 7Li SAE NMR. Jeener-Broekaert echoes and their Fourier transforms reveal that, for sufficiently short tp, dipolar interactions can be entirely suppressed in the diluted sample with reduced 7Li–7Li dipolar interactions. Fortunately, at sufficiently high temperatures, the activation energies and decay rates of the diluted sample closely match those of the 100% 7Li sample, demonstrating the robustness of SAE NMR in accurately determining dynamic parameters that characterize slow Li+ ion dynamics in condensed matter.