Nature of Silicon−Boron Mixing in Sodium Borosilicate Glasses:  A High-Resolution ¹¹B and ¹⁷O NMR Study

In several series of sodium borosilicate glasses, we have applied triple-quantum magic-angle spinning (3QMAS) 11B and 17O NMR to obtain high-resolution information about short-range structure and the connections among various network structural units and their variations with composition. Oxygen-17 3QMAS spectra reveal changes in connectivities between silicate and BO3 ([3]B) and BO4 ([4]B) units, by quantifying populations of bridging oxygens such as Si−O−Si, Si−O−[3]B, Si−O−[4]B, [3]B−O−[3]B, and [4]B−O−[3]B and of nonbridging oxygens. Boron-11 MAS and 3QMAS at a magnetic field of 14.1 T allow proportions of several borate units to be determined, including [3]B in boroxol ring and nonring sites and [4]B with two, three, and four Si neighbors. By combining the 11B and 17O NMR results, detailed information on Si/B mixing in sodium borosilicates can be derived. Our results shows that [4]B is strongly associated with silicate units, nonring [3]B undergoes random mixing and ring [3]B tends to connect to borate groups. The behavior of Si/B mixing for each boron species can be quantitatively represented in terms of a parameter, θ, indicating the degree of random mixing, which is potentially useful for the thermodynamic modeling of such glass structures.