11B-MQMAS and 29Si–{11B} Double-Resonance NMR Studies on the Structure of Binary B2O3–SiO2 Glasses

Abstract

Glasses in the system x B2O3·(1−x)SiO2 (0.2≤x≤1.0) were studied using 11B multiple quantum magic angle spinning NMR spectroscopy (MQMAS), 29Si–\\{11B\\} rotational echo adiabatic passage double-resonance and 29Si–\\}11B\\{ CP heteronuclear correlation spectroscopy. The results can be quantitatively interpreted in terms of a phase separation of the borosilicate glasses into a virtually SiO2-free B2O3 phase and a mixed borosilicate phase. While the MQMAS spectra allowed the site speciation and resolution of at least two different 11B resonances, attributable to BO3/2 units consumed in boroxol rings, BO3/2 units connecting the boroxol rings and BO3/2 units involved in B–O–Si linkages, the analysis of the double-resonance data further elucidated the structure of the mixed borosilicate phase. The results indicate that only a fraction of 0.48 mol B2O3 can be accommodated per mole SiO2, building a mixed borosilicate network.