Solid-state NMR study of metastable immiscibility in alkali borosilicate glasses

In several series of lithium, sodium, and potassium borosilicate glasses whose compositions traverse known regions of liquid-liquid phase separation, we have applied triple-quantum magic-angle spinning (3QMAS) ¹¹B and ¹⁷O NMR to obtain high-resolution information about short-range structure and connections among various network structural units, and their variation with composition and thermal history. 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 B-O-B, Si-O-B and Si-O-Si, and of non-bridging oxygens. ^[3]B-O-Si and ^[4]B-O-Si as well as ^[3]B-O-^[3]B and ^[4]B-O-^[3]B linkages can be distinguished. ¹¹B 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 non-ring sites and ^[4]B with 3 versus 4 Si neighbors. By combining the ¹¹B and ¹⁷O NMR results, detailed information on Si/B mixing in sodium borosilicates can be derived, showing, for example, that ^[4]B and non-ring ^[3]B tend to mix with silicate units, while ring ^[3]B is mainly connected to borate groups. In a preliminary study of the effects of varying alkali cation, potassium-containing glasses are similar to those in the sodium borosilicate system, but a lithium borosilicate seems to exhibit considerably greater chemical heterogeneity. In annealing experiments that converted an optically clear to obviously phase-separated glasses, the ratio of ^[3]B to ^[4]B does not change significantly, but part of the non-ring ^[3]B converts to ring ^[3]B as the degree of unmixing increases.