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) B-11 and O-17 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 (B-[3]) and BO4 (B-[4]) units, by quantifying populations of bridging oxygens such as Si-O-Si, Si-O-B-[3], Si-O-B-[4], B-[3]-O-B-[3], and B-[4]-O-B-[3] 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 B-[3] in boroxol ring and nonring sites and B-[4] with two, three, and four Si neighbors. By combining the B-11 and O-17 NMR results, detailed information on SUB mixing in sodium borosilicates can be derived. Our results shows that B-[4] is strongly associated with silicate units, nonring B-[3] undergoes random mixing and ring B-[3] tends to connect to borate groups. The behavior of SUB mixing for each boron species can be quantitatively represented in terms of a parameter, theta, indicating the degree of random mixing, which is potentially useful for the thermodynamic modeling of such glass structures.