11B-MQMAS and 29Si-{11B} double-resonance NMR studies on the structure of binary B2O3-SiO2 glasses

Glasses in the system xB(2)O(3) . (1-x)SiO2 (0.2 less than or equal to x less than or equal to 1.0) were Studied using B-11 multiple quantum magic angle spinning NMR spectroscopy (MQMAS). Si-29-{B-11} rotational echo adiabatic passage double-resonance and Si-29-{B-11} 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 B-11 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 call be accommodated per mole SiO2, building a mixed borosilicate network. (C) 2002 Elsevier Science (USA)