Field Strength of Network-Modifying Cation Dictates the Structure of (Na-Mg) Aluminosilicate Glasses

Aluminosilicate glasses are materials with a wide range of technological applications. The field strength of network-modifying cations strongly influences the structure of aluminosilicate glasses and their suitability for various applications. In this work, we study the influence of the field strength of network-modifying cations on the structure of [(Na₂O)_1–x(MgO)_x(Al₂O₃)₀.₂₅(SiO₂)₁.₂₅] glasses. Due to the higher cation field strength of magnesium than sodium, magnesium prefers the role of network modifier, while sodium preferentially acts as a charge compensator. When magnesium replaces sodium as network modifier, Q³ silicon species are converted into Q² species. The replacement of sodium with magnesium as charge compensator leads to the following changes: (1) the proportion of aluminum-rich Q⁴ species [Q⁴(4Al) and Q⁴(3Al)] decreases, while the proportion of aluminum-deficient Q⁴ species [Q⁴(2Al) and Q⁴(1Al)] increases; and (2) there is an increased tendency for phase separation between silica-rich and alumina-rich glasses.