LI-6 MAGIC ANGLE SPINNING NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY - A POWERFUL PROBE FOR THE STUDY OF LITHIUM-CONTAINING MATERIALS

A narrow chemical-shift range has been established for a variety of lithium compounds via study of their Li-6 magic angle spinning nuclear magnetic resonance (MAS NMR) spectra. Comparative studies of Li-6 and Li-7 spectra (MAS, and 'off-angle' spinning) establish that, for solid-state (and even solution) analytical purposes, Li-6 is the preferred nucleus, since the advantage of narrow absorption lines outweighs the poorer sensitivity of Li-6 realtive to Li-7. Li-6 MAS NMR spectra have been obtained for laponite clay, which had been thermally treated at 200, 400, 600, 800 and 1300-degrees-C; at the lower temperatures (< 800-degrees-C) both dehydrated and rehydrated specimens were considered. The data are consistent with mobility of lithium ions from the trioctahedral clay sites at 600-degrees-C. Both conventional and microwave methods were used to prepare lithium-exchanged laponite. The superior resolution achievable in Li-6 MAS NMR in comparison with Li-7 MAS NMR is demonstrated with the microwave specimen where use of Li-6 spectroscopy revealed two lithium sites. On storage of the sample for 3 months, the two sites give way to a single lithium environment. Possible causes are discussed.