IONIC-CONDUCTION AND MIXED CATION EFFECT IN SILICATE-GLASSES AND LIQUIDS - NA-23 AND LI-7 NMR SPIN-LATTICE RELAXATION AND A MULTIPLE-BARRIER MODEL OF PERCOLATION

The mixed cation effect in glasses affects transport properties such a s diffusion and ionic conductivity. The temperature dependence of Li-7 and Na-23 NMR spin-lattice relaxation times, T-1, have been measured in a series of trisilicate (Na2Si3O7, NaRbSi3O7, NaCsSi3O7 and NaCa0.5 Si3O7) and disilicate (Li2Si2O5, LiKSi2O5 and LiBa0.5Si2O5) glasses in order to understand the atomic scale mechanism of this effect. Diffus ion-induced NMR spin-lattice relaxation of alkali cations is shown to be affected by a mixed cation effect which tends to shift the T, minim um to a higher temperature and increases the slope of the T-1 vs. 1000 /T curve on the low temperature side of the minimum. These changes are consistent with an effective reduction in the number of hopping sites available to the alkali cation in a mixed cation glass. As a result t he high energy cutoff for percolation in a disordered barrier height l andscape increases significantly on going from a single alkali to a mi xed alkali (or, alkali-alkaline earth) glass. The de conductivities ca lculated on the basis of a random walk model with different percolatio n thresholds for single and mixed alkali glasses are in reasonably goo d agreement with the experimental results. This interpretation of the NMR spin-lattice relaxation results, thus, successfully combines the t wo recent models that of Svare et al. on the general relationship betw een ionic transport and spin-lattice relaxation and of Maass et al. on mixed alkali effect resulting from strong energetic preference for ho pping associated with percolation transition. In addition, the mixed a lkali effect can be generalized to mixed alkali-alkaline earth systems .