Ion hopping in crystalline and glassy spodumene LiAlSi₂O₆ : ⁷Li spin-lattice relaxation and ⁷Li echo NMR spectroscopy

Nuclear magnetic resonance spectroscopy was used to study polycrystalline β -spodumene (β-LiAlSi₂O₆) as well as glassy specimens with the same chemical composition. ⁷Li spin-lattice relaxation measurements were carried out in a broad temperature range and for several Larmor frequencies. In addition to a pronounced rate maximum at high temperatures, stemming from the long-range Li motion in these aluminosilicates, we found a weak maximum in the crystalline modification near 120  K . The latter result confirms the existence of a local double-well structure in which the Li ions reside. The ionic motion was also monitored by solid- and stimulated-echo spectra as well as by the decay of the Jeener-Broekaert echo. Under conditions which are discussed in detail, the latter is a direct measure of the hopping correlation function. For the glass this function was found to decay faster and more stretched than that of the crystal at a given temperature. Furthermore, the relevant barriers against the high-temperature long-range Li motion are larger in the crystal as compared to the glass.