Size effects of modifying cations on the structure and elastic properties of Na2O–MO–SiO2 glasses (M = Mg, Ca, Sr, Ba)

The structure and elastic properties of the ternary Na2O·MO·4SiO2 glasses (M = Mg, Ca, Sr, Ba) have been investigated using both Raman and Brillouin spectroscopy at ambient conditions. Unlike what observed in binary alkali silicate glasses, both the symmetric stretching frequencies of Q2 and Q3 species in the Na2O·MO·4SiO2 glasses decrease with increasing the size of M2+ cation. When the size of alkaline earth cation decreases, the Na2O·MO·4SiO2 glass becomes more polymerized with an increase in the equilibrium constant for the disproportional reaction Q3 ⇌ Q4 + Q2 (without balance) if the activity coefficients of all Q species are independent of M2+ cations. At the given MO concentration, except for M = Mg, the elastic moduli of the Na2O·MO·4SiO2 glasses increase with decreasing the size of M2+. That is, among the same group of cations in the period table, the higher-field modifying cation (e.g., Mg2+) in a glass does not always cause higher elastic moduli at the same apparent concentration of cation. The causes for these phenomena have been correlated to local environment of cations and the degree of dissociation of MO oxides in Na2O·2SiO2 glass.