6Li nuclear magnetic resonance chemical shifts, coordination number and relaxation in crystalline and glassy silicates

Abstract

Unlike 7Li magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra, 6Li MAS NMR spectra of silicates are dominated by chemical shift effects, often have a very high resolution and hence can provide significant structural information. In this study we demonstrate a good correlation between 6Li isotropic chemical shifts and oxygen coordination number, and use this result to describe the range of coordination environments for Li in silicate glasses. We also show that the second-order quadrupolar shift for 7Li can often be derived from 7Li and 6Li MAS spectra acquired at a single magnetic field. For a series of natural lepidolite samples with significant but varying contents of Mn and Fe, spin-lattice relaxation data show a power-law behavior and a three-dimensional distribution of paramagnetic centers, but homonuclear dipolar couplings can be important. The 6Li spectrum for lithium orthosilicate (which has three-, four-, five- and six-coordinated Li) is consistent with that predicted by the X-ray structure.