Structure of boroaluminosilicate glasses: Impact of [Al ₂O ₃]/[SiO ₂] ratio on the structural role of sodium

In order to explore the structural roles of sodium in boroaluminosilicate glasses, we have designed ten Na ₂O-B ₂O ₃-Al ₂O ₃-SiO ₂ glasses with varied [Al ₂O ₃]/[SiO ₂] ratio to access different regimes of sodium behavior. Multinuclear nuclear magnetic resonance (NMR) experiments on 11B, 27Al, 29Si, and 23Na were performed to determine the complicated network former speciation and modifier environments as a function of glass composition. The different roles of sodium in relation with the network-forming cations (Si, B, and Al) have been clarified and quantified. When [Na ₂O] < [Al ₂O ₃], all available sodium is used to charge compensate fourfold coordinated aluminum (AlIV), and deficiency in sodium concentration leads to fivefold coordinated aluminum (AlV) groups. When [Na ₂O] > [Al ₂O ₃], sodium first charge compensates AlIV, and thus all aluminum is fourfold coordinated and unaffected by other compositional changes. Hence, the preference in the formation of AlIV over that of fourfold coordinated boron (BIV) is confirmed. Excess sodium can be used to convert threefold coordinated boron (BIII) to BIV or to create nonbridging oxygen (NBO) on Si and B, with a thermodynamic competition among these mechanisms. The NBOs on Si are quantified using 29Si wide-line and magic angle spinning NMR. The fraction of silicon atoms associated with NBOs is calculated using a random model and compared with the NMR results. Finally, we have found that our previously proposed two-state statistical mechanical model of boron speciation accurately predicts the fraction of tetrahedrally coordinated boron atoms (N ₄) in these mixed network former glasses.