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

[EN] 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-12.9AlO-38.7SiO (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.A. R. A. and S. B. would like to thank Dr Muraleedharan K., Director, CSIR-CGCRI for his strong support and encouragement. A. R. A. gratefully acknowledges the financial support of the Budapest Neutron Centre, Hungary, for allotting beam time and financial support (BRR_407) under the NMI3-II program. Part of this work was developed in the scope of the CICECO-Aveiro Institute of Materials (UID/CTM/50011/2013) project, and funded by FEDER funds through the Operational Programme Competitiveness Factors (COMPETE 2020) and the Portuguese Foundation for Science and Technology (FCT). S. G. would like to thank the German Research Foundation (DFG) for financial support through the priority program SPP 1594 ‘‘Topological Engineering of Ultra-Strong Glasses’’ (project BI1453/1-2). Simulations were performed with computing resources granted by RWTH Aachen University under project rwth0297.Peer reviewe