In SiO2 glass, oxygen has a very stable coordi nation number of 4 in regard to silicon. The corresponding coordination polyhedra are the SiO2 tetrahedra, which are interconnected through the tips.
By introducing alkali oxides in the SiO2 glass, some of the Si – O – Si bridges are broken, thus appearing a series of structural units having different geometries and structural compositions. These consist of structural entities of type Qn, having the formula Si(Ob)n(Onb)4-n, in which Ob represents a bridged oxygen atom and Onb a non-bridged oxygen. The size of the Qn structural units is below 1 nm. Herein is presented the distribution of these structural entities for glasses having the composition of disilicate from the alkali-silica systems studied.
The distribution of the considered structural entities was computed using a thermodynamic model, in correlation with some chemical and structural particularities of alkali oxides. The results were compared to experimental data obtained using the 29Si MAS-NMR method.