Nuclear spin-lattice relaxation rates in glassy fast ionic conductors were determined for mobile 7Li nuclei and stationary 11B nuclei as functions of temperature and nuclear magnetic resonance resonance frequency. The 7Li relaxation was driven by Li diffusion via quadrupole interactions. The data for all frequencies could be fitted by assuming a single correlation function of the form, exp[-(t/tc)B]. In order to fit the data, a thermally activated correlation time (tc) was assumed which had an effective activation energy, Ea, of 7400K. This was higher than the one which was deduced from direct current conductivity measurements. It was proposed that the correlation function which determined the conductivity, and that which determined the nuclear magnetic resonance relaxation, could differ in the presence of collective Li diffusion effects. The 11B data indicated that the nuclear relaxation occurred via a Raman 2-phonon process that involved internal vibrational modes of the BO4 units; heavily damped by reorientational motion. The data were fitted, by an expression for the nuclear spin-lattice relaxation rate, which was derived from an extension of the Van Kranendonk 2-phonon relaxation mechanism in insulators. The activation energy for the damping frequency was compared with E = BEa, which was expected to represent the single-particle energy barrier to Li motion.

M.Trunnel, D.R.Torgeson, S.W.Martin, F.Borsa: Journal of Non-Crystalline Solids, 1992, 139[3], 257-67