The density of alkali silicate glasses in relation to the microstructure

Densities of R(2)O-SiO2 glasses with R(2)O/SiO2 < 1 have been analyzed to determine the volumes of the structural units. This study presents a model to obtain the volumes as a function of the R(2)O content. The volume of the Q(4) unit is independent of the type or the concentration of R(2)O and has the same value as in the vitreous SiO2. The volume of both the Q(3) and Q(2) units depends only on the type of alkali ion and not on its concentration. The volume of Q(3) and Q(2) increases in the succession Li+ --> Cs+. The volumes obtained enable one to use the given model to determine the densities of alkali- and mixed-alkali silicate glasses with a satisfactory degree of accuracy. Relations are also derived to calculate the packing fraction for both the Q(3) and Q(2) units as a function of the alkali-ion volume. For the two units, the packing fraction increases from Li2O-SiO2 to Cs2O-SiO2 glasses.