Anionic structure and elasticity of Na2O–MgO–SiO2 glasses

The structure and elastic properties of a series of xNa2O · MgO · 4SiO2 glasses have been studied using both Raman and Brillouin spectroscopy. Relative to Na2O–SiO2 glasses, the maximum abundance for phyllosilicate structural units in the present glasses shows a lag of 0.5 units in the number of non-bridging oxygen per silicon atom (NBO/Si). This phenomenon has been attributed to the decrease in the average coordination number of modifying cations due to the presence of Mg2+. It has also been found that the decomposition of both metasilicate and disilicate (dimerized SiO4) anionic structural units in Na2O–SiO2 glasses are enhanced by the addition of MgO. However, the presence of Mg2+ does not cause a considerable effect on the decomposition of phyllosilicate structural unit. The acoustic data have revealed that both shear and Young’s moduli of the present glasses decrease with increasing NBO/Si (the variation in bulk modulus is reversed, however). The resistance to shear deformation for the anionic structural units in silicate glasses has been found to decrease in the following order: tectosilicate > phyllosilicate > metasilicate > disilicate > orthosilicate. The relative contribution of the various anionic structural units to the bulk modulus of a glass remains to be determined. The ideal mixing model using Makishima–Mackenzie’s relationship for predicting Young’s modulus is not applicable to the present glasses.