STRUCTURE OF LI,NA METAPHOSPHATE GLASSES BY P-31 AND NA-23 MAS-NMR CORRELATED WITH THE MIXED ALKALI EFFECT

P-31 and Na-23 NMR spectra for glasses along the Li,Na metaphosphate join have been obtained by MAS-NMR spectroscopy. The P-31 results are interpreted in terms of phosphate Q2 (two bridging oxygens) tetrahedra. The P-31 chemical shifts become less shielded (less negative) with increasing [Na]/[Na + Li] ratio. This observation is rationalized in terms of increased paramagnetic deshielding due to increased average electron density in the P-O bonds with increasing [Na]/[Na + Li] ratio. The Na-23 peak maxima also become deshielded with increased [Na]/[Na + Li], suggesting the effects of Li-(non-bridging oxygen)-Na linkages. Both P-31 and Na-23 peaks have greater full-widths at half-height (FWHH) for the intermediate (mixed Li,Na) compositions compared with the Li or Na end-member, reflecting a greater distribution of P and Na sites due to the presence of both Li and Na. The glass transition temperatures have a minimum at [Na]/[Na + Li] = 0.6, thus showing a mixed alkali effect, previously explained by mixed alkali pairs (Li-NBO-Na) detected by the NMR experiments.