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.
Local order in silicate glasses has been observed by many experimental techniques to be similar to that in crystalline materials. Details of the intermediate-range order are more elusive, but essential for understanding the lack of long-range symmetry in glasses and the effect of composition on glass structure. Two-dimensional 17O dynamic-angle-spinning nuclear magnetic resonance experiments reveal intermediate-range order in the distribution of inter-tetrahedral (Si-O-Si) bond angles and a high degree of order in the disposition of oxygen atoms around the network-modifying cations.
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