Structure and topology of soda-lime silicate glasses: implications for window glass

The structural and topological properties of soda-lime silicate glasses of the form (1-2x)SiO2-xNa2O-xCaO are studied from classical molecular dynamics using a Buckingham type potential. Focus is made on three compositions (x = 6%, 12%, and 18%) which are either silica-rich or modifier-rich. We compare the results to available experimental measurements on structural properties and find that the simulated pair correlation function and total structure factor agree very well with available experimental measurements from neutron diffraction. The detail of the structural analysis shows that the Na and Ca coordination numbers tend to evolve with composition, and with increasing modifier content, changing from 5.0 to 5.6 and from 4.0 to 5.0 for Ca and Na, respectively. The analysis from topological constraints shows that the picture derived on a heuristic basis using classical valence rules remains partially valid. Ultimately, typical elastic phases are identified from the application of rigidity theory, and results indicate that the 6% system is stressed-rigid, whereas the modifier-rich composition (18%) is flexible. These results receive support from a full analysis of the vibrational density of states showing the low-energy bands at E < 20 meV increase as the system becomes flexible, providing another indirect signature of the presence of rigid to flexible transitions in this archetypal glass. Consequences for window glass are discussed under this perspective.