Site discrimination in mixed-alkali glasses studied by cross-polarization NMR

Cation- cation interactions are thought to play a significant role in shaping the nonlinear compositional dependence of ionic conductivity, known as the mixed- alkali effect ( MAE) in glassy solid electrolytes. For providing a structural rationale of this effect, the discrimination of various cation sites in mixed- alkali glasses is of interest. In the present study, cross-polarization ( CP) experiments have been applied to glasses in the system [( Li2O) x( Na2O)(1-x)](0.3)[B(2)O3](0.7) to discriminate between alkali ions by virtue of different heteronuclear Li-7-Na-23 dipole- dipole coupling strengths. Cross-polarization studies involving two types of quadrupolar nuclei ( both Li-7 and Na-23 have a spin- quantum number I = 3/ 2) are complicated by spin state mixing under radio frequency irradiation and magic-angle spinning ( MAS). Therefore careful validation and optimization protocols are reported for the model compound LiNaSO4 prior to conducting the measurements on the glassy samples. Na-23 -> Li-7 CP/ MAS NMR spectra have been obtained on glasses containing the Na+ ions as the dilute species. They reveal that those lithium species interacting particularly strongly with sodium ions have the same average Li-7 chemical shift as the entire lithium population; the symmetrical situation applies to the Na-23 nuclei at the sodium rich end of the composition range. On the other hand, a clear site discrimination is afforded by temperature- dependent static Na-23 -> Li-7 CP experiments, indicating that the Li ions that are most strongly interacting with sodium ions are strongly immobilized. This finding provides the first direct experimental evidence for the proposed secondary mismatch concept invoked for explaining the strong MAE in the dilute foreign ion limit.