Local-field approximation of homonuclear dipolar interactions in 7Li-NMR: Density-matrix calculations and random-walk simulations tested by echo experiments on borate glasses

NMR echo techniques have proven to be important to study dynamics in ion conductors and other solid materials. Using the spin-3/2 nucleus 7Li as a probe, both the quadrupolar and the often neglected homonuclear dipolar interactions modulate the NMR frequency as the ion performs jump processes. Retai...

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Veröffentlicht in:Solid state nuclear magnetic resonance 2014-05, Vol.59-60, p.8-19
Hauptverfasser: Storek, Michael, Jeffrey, Kenneth R., Böhmer, Roland
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Sprache:eng
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Zusammenfassung:NMR echo techniques have proven to be important to study dynamics in ion conductors and other solid materials. Using the spin-3/2 nucleus 7Li as a probe, both the quadrupolar and the often neglected homonuclear dipolar interactions modulate the NMR frequency as the ion performs jump processes. Retaining only the local-field term of the many-body Hamiltonian, the impact of the dipolar interaction on various echo experiments was studied using spin dynamics calculations yielding products of dipolar and quadrupolar correlation functions. Using a simple stochastic model these functions were simulated with particular emphasis on the impact of ionic motions and on the conditions under which the dipolar and quadrupolar contributions factorize. The results of the computations and of the random-walk simulations are compared with experimental data obtained for various lithium borate and lithium borophosphate glasses. It is concluded that the local-field approximation is a useful means of treating the Li–Li dipole interactions and that the simple model that we introduce is capable of describing many experimentally observed features. Furthermore, because the dipolar and quadrupolar contributions essentially factorize, a selective determination of the corresponding correlation functions becomes possible. Illustration of the local dipolar field model. Due to the translational motion of adjacent spins βi on the coordination shell a trajectory of the NMR frequency ωD(α) at the centered spin site α is generated. The resulting distribution function G(ωD(α)) is shown. [Display omitted] •Truncation of the dipolar Hamiltonian to the local-field term is proposed.•Dipolar and quadrupolar correlation functions are calculated for I=3/2.•Stochastic model is developed to simulate spin motion.•Results are compared to experimental data.•Local-field approximation proves suitable to treat 7Li–7Li interactions.
ISSN:0926-2040
1527-3326
DOI:10.1016/j.ssnmr.2014.01.002