Influence of glass chemical composition on the Na-O bond distance: a Na-233Q-MAS NMR and molecular dynamics study

The sodium environment in oxide glasses was investigated by Na-23 multiple- quantum magic-angle spinning (MQ-MAS) NMR spectroscopy and compared with mo lecular dynamics simulations. In the experimental approach, a spectrum-inve rsion was employed taking into account the transfer efficiency involved in the MQ-MAS experiment. This allowed the reconstruction of the underlying tw o-dimensional distribution of the isotropic chemical shift correlated with the quadrupolar interaction. The isotropic chemical shift distributions wer e extracted from the MQ-MAS spectra to infer Na-O distance distributions. F irst, a Na2O-2SiO(2) glass and its crystal analogue were characterized by t his method to observe the disorder effect in the glass through the Na-O dis tance distribution. Thereafter, in order to study the influence of the chem ical composition on the Na-O distance and distribution, additional glasses were investigated with NMR and simulation: Na2O-5SiO(2), Na2O-2CaO-3SiO(2), Na2O-Al2O3-3SiO(2) and Na2O-B2O3-3SiO(2). The molecular dynamics results a re 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 simulati on 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 compensa tor. This can be explained through charge and geometric effects. (C) 2000 E lsevier Science B.V. All rights reserved.