Molecular dynamics simulations of magnesium aluminosilicate glass structure: High-coordinated alumina and oxygen tricluster formation

Previous research has shown a consistent discrepancy in the reported structure of alkaline earth aluminosilicate glasses using molecular dynamics (MD) simulations versus nuclear magnetic resonance (NMR) experiments. Past MD results have consistently shown less than 5% five-coordinated Al units (Al^[5]) in peraluminous glass compositions, but with high fractions of triple-bonded oxygens (TBO, i.e., triclusters). Experimental results have shown a high fraction of Al^[5] with no direct evidence for TBO. One of the main criticisms associated with high TBO content found in MD-generated glass structures is the use of classical interatomic potentials. To investigate this issue, we analyze the formation of both TBO and Al^[5] using three independently developed potentials with varying silica content and [Al₂O₃]/[MgO] ratios for the magnesium aluminosilicate (MAS) system. We specifically choose compositions with high ratios of alumina to magnesium oxide as this region is not as commonly explored. Results indicate that Al^[5] charge compensates the Al network in metaluminous compositions (compositions with more Mg than Al) while both TBO and Al^[5] are prevalent in peraluminous ranges (high Al content compositions) to charge balance Al units. From the literature, NMR experiments report MAS glasses with varying Al^[5] fractions and show significant differences for the same reported compositions. When comparing MD results from this work, the fraction of calculated Al^[5] is within the experimental variation found in the literature. This indicates that classical potentials can accurately capture alumina environments and that both Al^[5] and TBO can coexist in relatively high fractions. From the consistency in our results, we conclude that TBOs are inherent to the aluminosilicate glass system and are not simulation artifacts.