Influence of glass chemical composition on the Na-O bond distance:: a 23Na 3Q-MAS NMR and molecular dynamics study

The sodium environment in oxide glasses was investigated by (23)Na multiple-quantum magic-angle spinning (MQ-MAS) NMR spectroscopy and compared with molecular dynamics simulations. In the experimental approach, a spectrum-inversion was employed taking into account the transfer efficiency involved in the MQ-MAS experiment. This allowed the reconstruction of the underlying two-dimensional distribution of the isotropic chemical shift correlated with the quadrupolar interaction. The isotropic chemical shift distributions were extracted from the MQ-MAS spectra to infer Na-O distance distributions. First, a Na(2)O-2SiO(2) glass and its crystal analogue were characterized by this method to observe the disorder effect in the glass through the Na-O distance distribution. Thereafter, in order to study the influence of the chemical composition on the Na-O distance and distribution, additional glasses were investigated with NMR and simulation: Na(2)O-5SiO(2), Na(2)O-2CaO-3SiO(2), Na(2)O-Al(2)O(3)-3SiO(2) and Na(2)O-B(2)O(3)-3SiO(2). The molecular dynamics results are in good agreement with the experimental findings. The mean Na-O distance is higher when network formers are added to the sodium silicate glass. The effects on the Na-O distance distribution are also discussed. The simulation relates these results to the existence of several types of Na: near the non-bridging oxygen of the silicon, or as aluminum or boron charge compensator. This can be explained through charge and geometric effects. (C) 2000 Elsevier Science B.V. All rights reserved.