7Li and 11B nuclear spin lattice relaxation in B 2O 3 + 0.7Li 2O + XLiCl glassy fast ionic conductors

Abstract

Nuclear spin-lattice relaxation rates, R 1 , are reported for lithium chloro-borate glassy fast ionic conductors for both the mobile 7 Li nuclei and the stationary 11 B nuclei, as functions of temperature and NMR resonance frequency. The 7 Li relaxation is driven by the lithium diffusion via quadrupole interactions. The data for all frequencies can be fitted by assuming a single correlation function of the form exp(−( t / τ c *) β ). In the fit a thermally activated correlation time, τ c *, is assumed with an effective activation energy E a * = 7400 K, higher than the one deduced from dc conductivity measurements. It is argued that the correlation function (CF) which determines the conductivity and the CF which determines the NMR relaxation may differ in the presence of collective effects in the lithium diffusion. The 11 B data indicate that the nuclear relaxation proceeds via a Raman two phonon process involving internal vibrational modes of the BO 4 units heavily damped by reorientational motion. The data are fitted by an expression for R 1 derived from an extension of the Van Kranendonk two phonon relaxation mechanism in insulators. The activation energy for the damping frequency is compared with E a = β E a * which should represent the ‘single particle’ energy barrier for the lithium motion.