The medium range structure of sodium silicate glasses: a molecular dynamics simulation

The structure of xNa(2)O-(100 - x)SiO2 (x = 0-50) glasses have been studied by molecular dynamics simulation. Local structures around sodium and silicon agree well with neutron diffraction and EXAFS data. The medium range structures were characterized and compared with different structure models. The Q(n) distributions show a trend closer to, but more ordered than, random distribution and the compositional dependence agrees well with recent NMR results. A maximum in Q(n) species at the sodium disilicate composition is reproduced. The connectivities of Q(n) species are higher than those would be seen from a random distribution suggesting the existence of silica rich regions in these glasses. Ring size distributions show consistent decreases in the main peak at sixmembered ring and increases in the number of larger member rings. The second moments due to sodium-sodium dipole coupling were calculated and compared with those derived from different theoretical models, crystalline and experimental data. The distribution of sodium ions was found to be slightly more clustered than a random distribution would predict. These results support the modified random network model. (C) 2004 Elsevier B.V. All rights reserved.