Impact of ZnO on the structure and properties of sodium aluminosilicate glasses: Comparison with alkaline earth oxides

• We study the role of ZnO in sodium aluminosilicate glasses with varying Si/Al ratios.
• We compare impact of ZnO on structure–properties with that of alkaline earth oxides.
• Physical and rheological properties exhibit inflection point around [Al2O3] = [Na2O].
• Fraction of five-fold coordinated Al increases with decreasing divalent cation size.
• Zn2 + is found to behave according to its field strength, i.e., similar to Mg2 +.

The structural role of ZnO in aluminosilicate glasses is not yet well understood. However, this understanding is requisite for explaining the impact of ZnO on the macroscopic properties of these glasses. In this work, we present a quantitative analysis of the impact of ZnO on the structure and properties of sodium aluminosilicate glass. The properties under study include density, molar volume, refractive index, coefficient of thermal expansion, elastic moduli, isokom temperatures, and liquid fragility. The impact of ZnO is compared with that of four alkaline earth oxides (MgO, CaO, SrO, and BaO) in the same family of aluminosilicate glasses. Our study is performed on a series of glasses with varying Si/Al ratios to quantify the impact of each divalent cation oxide on aluminum speciation and the resulting impact on macroscopic properties. Our results reveal pronounced changes in the scaling of most measured properties around the compositions with [Al2O3] = [Na2O], particularly for the glass series with MgO or ZnO. Based on 27Al MAS and 3QMAS NMR experiments at high field, the structural origin of this change is ascribed to the change in aluminum speciation. We thus demonstrate that the impact of Zn2 + on aluminosilicate glass structure and properties is similar to that of Mg2 +, following their similarity in ionic radius, despite their differences in electronic structure.