Ultra-slow Li ion jump diffusion in Li2SnO3 studied by two-time 7Li spin-alignment echo NMR and 7Li NMR relaxometry

A deeper understanding of atomic-scale Li+ hopping in solid Li ion conductors requires the study of ion dynamics over a broad time scale and length scale. Here, we report a complementary study, applying Li-7 spin-lattice relaxation (SLR) nuclear magnetic resonance (NMR) techniques together with spin-alignment echo (SAE) NMR to study slow Li+ self-diffusion in monoclinic Li2SnO3. The stannate serves as a model substance to quantify both local barriers of the elementary steps of ion diffusion as well as long-range ion transport. From spin-alignment echoes, if recorded at short preparation time of 20 mu s, activation energies (0.46 eV) and Li jump rates were deduced. The same diffusion process is seen via SLR NMR (0.45 eV) performed in the rotating frame of reference at a locking frequency of 20 kHz. By comparing jump rates extracted from SAE NMR with those recently measured by high-resolution 1D Li-6 exchange NMR we attribute the SLR and SAE NMR response to the Li(1,2)-Li(3) exchange process (5 s(-1)) taking place perpendicular to the ab-plane in Li2SnO3. SAE NMR measurements, dedicated to elaborate the influence of dipolar correlations on echo formation at t(p)> 20 mu s, reveal a strong dependence of the SAE rate constant on preparation time at low temperatures. This finding might points towards heterogeneous dynamics including localized jump processes which are likely to be caused by a slightly disordered structure of the oxide. (C) 2016 Elsevier B.V. All rights reserved.