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

S Ganisetti and A Gaddam and R Kumar and S Balaji and GC Mather and MJ Pascual and M Fabian and R Siegel and J Senker and VV Kharton and J Guenole and NMA Krishnan and JMF Ferreira and AR Allu, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 21, 23966-23977 (2019).

DOI: 10.1039/c9cp04332b

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-12.9Al(2)O(3)-38.7SiO(2) (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.

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