Ionic conduction and mixed cation effect in silicate glasses and liquids: 23Na and 7Li NMR spin-lattice relaxation and a multiple-barrier model of percolation

The mixed cation effect in glasses affects transport properties such as diffusion and ionic conductivity. The temperature dependence of 7Li and 23Na NMR spin-lattice relaxation times, T1, have been measured in a series of trisilicate (Na2Si3O7, NaRbSi3O7, NaCsSi3O7 and NaCa0.5Si3O7) and disilicate (Li2Si2O5, LiKSi2O5 and LiBa0.5Si2O5) glasses in order to understand the atomic scale mechanism of this effect. Diffusion-induced NMR spin-lattice relaxation of alkali cations is shown to be affected by a mixed cation effect which tends to shift the T1 minimum to a higher temperature and increases the slope of the T1 vs. 1000T 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 the high energy cutoff for percolation in a disordered barrier height landscape increases significantly on going from a single alkali to a mixed alkali (or, alkali-alkaline earth) glass. The dc conductivities calculated on the basis of a random walk model with different percolation thresholds for single and mixed alkali glasses are in reasonably good agreement with the experimental results. This interpretation of the NMR spin-lattice relaxation results, thus, successfully combines the two recent models — that of Svare et al. on the general relationship between ionic transport and spin-lattice relaxation and of Maass et al. on mixed alkali effect resulting from strong energetic preference for hopping associated with percolation transition. In addition, the mixed alkali effect can be generalized to mixed alkali-alkaline earth systems.