title: Local-field approximation of homonuclear dipolar interactions in ⁷Li-NMR: density-matrix calculations and random-walk simulations tested by echo experiments on borate glasses.
abstract: NMR echo techniques have proven to be important to study dynamics in ion conductors and other solid materials. Using the spin-3/2 nucleus (7)Li 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.
authors.affiliation: Fakultät Physik, Technische Universität Dortmund, 44221 Dortmund, Germany. Electronic address: michael.storek@tu-dortmund.de.
authors.forename: Michael
authors.initials: M
authors.lastname: Storek
authors.affiliation: Department of Physics, Guelph-Waterloo Physics Institute, University of Guelph, MacNaughton Building, Gordon Street, Guelph, Ontario, Canada N1G 2W1.
authors.forename: Kenneth R
authors.initials: KR
authors.lastname: Jeffrey
authors.affiliation: Fakultät Physik, Technische Universität Dortmund, 44221 Dortmund, Germany.
authors.forename: Roland
authors.initials: R
authors.lastname: Böhmer
keywords: Dipolar interaction; Lithium ion motion; Quadrupolar interaction; Random-walk simulations; Solid-state NMR; Spin echo