Structure of boroaluminosilicate glasses: Impact of [Al2O3]/[SiO2] ratio on the structural role of sodium

In order to explore the structural roles of sodium in boroaluminosilicate glasses, we have designed ten Na2O-B2O3-Al2O3-SiO2 glasses with varied [Al2O3]/[SiO2] ratio to access different regimes of sodium behavior. Multinuclear nuclear magnetic resonance (NMR) experiments on B-11, Al-27, Si-29, and Na-23 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 [Na2O] < [Al2O3], all available sodium is used to charge compensate fourfold coordinated aluminum (Al-IV), and deficiency in sodium concentration leads to fivefold coordinated aluminum (Al-V) groups. When [Na2O] > [Al2O3], sodium first charge compensates Al-IV, and thus all aluminum is fourfold coordinated and unaffected by other compositional changes. Hence, the preference in the formation of Al-IV over that of fourfold coordinated boron (B-IV) is confirmed. Excess sodium can be used to convert threefold coordinated boron (B-III) to B-IV or to create nonbridging oxygen (NBO) on Si and B, with a thermodynamic competition among these mechanisms. The NBOs on Si are quantified using Si-29 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.