Elucidating the formation of Al-NBO bonds, Al-O-Al linkages and clusters in alkaline-earth aluminosilicate glasses based on molecular dynamics simulations.

Exploring the reasons for the initiation of Al-O-Al bond formation in alkali-earth alumino silicate glasses is a key topic in the glass-science community. Evidence for the formation of Al-O-Al and Al-NBO bonds in the glass composition 38.7CaO-9.7MgO-12.9Al2O3-38.7SiO2 (CMAS, mol%) has been provided based on Molecular Dynamics (MD) simulations. Analyses in the short-range order confirm that silicon and the majority of aluminium cations form regular tetrahedra. Well-separated homonuclear (Si-O-Si) and heteronuclear (Si-O-Al) cluster regions have been identified. In addition, a channel region (C-Region), separated from the network region, enriched with both NBO and non-framework modifier cations, has also been identified. These findings are in support of the previously proposed extended modified random network (EMRN) model for aluminosilicate glasses. A detailed analysis of the structural distributions revealed that a majority of Al, 51.6%, is found in Si-O-Al links. Although the formation of Al-O-Al and Al-NBO bonds is energetically less favourable, a significant amount of Al is found in Al-O-Al links (33.5%), violating Lowenstein's rule, and the remainder is bonded with non-bridging oxygen (NBO) in the form of Al-NBO (Al-O-(Ca, Mg)). The conditions necessary for the formation of less favourable bonds are attributed to the presence of a high amount of modifier cations in current CMAS glass and their preferable coordination.