Structure of Li,Na Metaphosphate Glasses by ³¹P and ²³Na MAS-NMR Correlated with the Mixed Alkali Effect

31P and 23Na NMR spectra for glasses along the Li,Na metaphosphate join have been obtained by MAS-NMR spectroscopy. The 31P results are interpreted in terms of phosphate Q2 (two bridging oxygens) tetrahedra. The 31P 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 Psingle bondO bonds with increasing [Na]/[Na + Li] ratio. The 23Na peak maxima also become deshielded with increased [Na]/[Na + Li], suggesting the effects of Lisingle bond(non-bridging oxygen)single bondNa linkages. Both 31P and 23Na 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 (Lisingle bondNBOsingle bondNa) detected by the NMR experiments. © 1992 Elsevier Science Publishers B.V. All rights reserved