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.

 

 

 

 

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