Revealing the structural role of MgO in aluminosilicate glasses

The addition of MgO to aluminosilicate glasses usually leads to significant changes for many macroscopic properties. The underlying microscopic structure origin of this effect, however, remains elusive. We herein conduct systematic studies to reveal the structural role of MgO in sodium aluminosilicate glasses over a wide composition range from peralkaline to peraluminous using molecular dynamics simulations. Our results provide pieces of evidence to show that MgO plays a dual role of glass network modifier/charge compensator and network former, with the balance of these two roles strongly depending on the composition of other oxides in the glass. Specifically, Mg2+ tends to play a network modifier/charge compensator role when the glass simultaneously contains large amount Al atoms that need to charge balanced and a low concentration of other modifier oxides. In contrast, Mg2+ exhibits a more pronounced network-forming role in Al-poor glasses. Additionally, by considering the structural role of MgO in the glass network, we further demonstrate that the non-linear evolution of the isokom annealing temperature upon increasing Al2O3 content in the peraluminous regime can be rationalized in terms of the combined effects of short-range connectivity and medium-range ring size distribution. The findings provide insights to further exploit MgO in making glasses with advanced properties.