Two opposite effects of sodium on elastic constants of silicate binary glasses

(1 - x)SiO₂-xNa₂O glasses have been studied by Brillouin scattering when sodium oxide molar concentration x, varies in the range 0.05 < x < 0.44. Non-linear dependence of elastic constants on sodium oxide concentration is shown unambiguously for the first time and the compressive elastic constant C₁₁ experienced a non-monotonic behaviour with a minimum value around x = 0.15. These results are compared with those obtained previously in the case of other alkali silicate glasses (Li, K). The variations of elastic constants of these glasses with the alkali metal nature and concentration are discussed on the basis of relations with the molar density of silicon atoms and the force constants between modifiers and non-bridging oxygens.