Anionic structure and elasticity of Na2O–MgO–SiO2 glasses

Abstract The structure and elastic properties of a series of x Na 2 O · MgO · 4SiO 2 glasses have been studied using both Raman and Brillouin spectroscopy. Relative to Na 2 O–SiO 2 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 Mg 2+ . It has also been found that the decomposition of both metasilicate and disilicate (dimerized SiO 4 ) anionic structural units in Na 2 O–SiO 2 glasses are enhanced by the addition of MgO. However, the presence of Mg 2+ 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.