Site Preference and Si/B Mixing in Mixed-Alkali Borosilicate Glasses: A High-Resolution 11B and 17O NMR Study

In a series of mono and mixed-alkali borosilicate glasses (∼0.7M 2 O·B 2 O 3 ·2SiO 2, M = Li, Na, K, ( 1 / 2 Li 2 O + ∼ 1 / 2 Na 2 O), ( 1 / 2 Li 2 O + 1 / 2 K 2 O), and (∼ 1 / 2 Na 2 O + 1 / 2 K 2 O)), we have applied triple-quantum magic-angle spinning (3QMAS) and 11 B and 17 O NMR to obtain high-resolution information about short-range structure and the connections among various network structural units, to study the effect of mixed cations on the glass structure, and to explore the distribution and mixing of these modifier cations. For a Li-containing glass, a lower fraction of [4] B, a lower fraction of Si−O−B, and a higher fraction of nonbridging oxygen than those found in Na- and K-containing glasses suggests that there is a significant heterogeneity in terms of Si/B mixing in the former. For Li−Na and Li−K mixed cation glasses, anomalously high fractions of nonbridging oxygen were also observed along with smaller fractions of [4] B and Si−O−B, again indicating significant chemical heterogeneity. For Na−K mixed cation glasses, the populations of boron and oxygen species are similar to the average population of Na- and K-containing glasses, indicating that these glasses are less heterogeneous. Oxygen-17 and boron-11 chemical shift data indicate cation preferences in coordination by nonbridging oxygen, Si−O−Si, [3] B(ring), and [4] B species for Li−Na and Li−K glasses but not for Na−K glass. The result suggests that in the Na−K glass, modifier cations are nearly randomly mixed and the degree of Si/B mixing is similar to that of the Na and K glasses. However, Li−Na and Li−K glasses are likely to contain nanoscale lithium-silicate rich domains embedded in Na/K borosilicate matrixes.