LI-6 NUCLEAR-MAGNETIC-RESONANCE CHEMICAL-SHIFTS, COORDINATION-NUMBER AND RELAXATION IN CRYSTALLINE AND GLASSY SILICATES

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