Abstract
Densities of R2OSiO2 glasses with R2O/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 R2O content. The volume of the Q4 unit is independent of the type or the concentration of R2O and has the same value as in the vitreous SiO2. The volume of both the Q3 and Q2 units depends only on the type of alkali ion and not on its concentration. The volume of Q3 and Q2 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 Q3 and Q2 units as a function of the alkali-ion volume. For the two units, the packing fraction increases from Li2OSiO2 to Cs2OSiO2 glasses.
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