Volume of ionic sites in silicate glasses

Abstract

Molar volume data of alkali and alkaline earth silicate glasses have been used tocalculate the free volume associated with the bridging and nonbridging oxygen andmodifier ions. The free volume associated with the bridging oxygen is constant(15.39 × 10−24 cm3) for all modifier ions up to 33.3 mol% modifier oxide. It decreases (in alkali or alkaline earthsilicate glasses) with increasing number of nonbridging oxygen ions per structural unit and/orradius of the modifier ion. The nonbridging oxygen ion is associated with a constant free volume(6.50 × 10−24 cm3) in all cases. Modifier ions are associated with free volume that increases with increasingnumber of nonbridging oxygen ions per structural unit and/or radius of the modifier ion.The used model explores the change in the free volume due to changing the concentrationof alkali oxides in mixed alkali silicate glasses. The results show that, in such glasses, thefree volume related to a certain type of alkali oxide increases with increasing content.