Multiple‐Quantum Magic‐Angle Spinning 17 O NMR Studies of Borate, Borosilicate, and Boroaluminate Glasses

Multiple‐quantum magic‐angle spinning (MQMAS) 17 O NMR spectroscopy has been applied to study boron oxide, sodium borate, sodium potassium borosilicate, and sodium boroaluminate glasses. Up to eight distinct oxygen sites are identified, with the chemical shift and quadrupolar coupling parameters determined for each. Previous assignment of the B‐O‐B and Si‐O‐B resonances has been found to be incorrect. In contrast to standard models of glass structure, three‐coordinated boron mixes with the silicon oxide network to a great extent. Sodium borosilicate glasses with low‐sodium content are likely to be phase‐separated on the nanoscale. Those with intermediate sodium content form homogeneous glasses with boron atoms distributed evenly in the SiO 2 network. A boron avoidance rule analogous to the aluminum avoidance rule may apply in this region, when BO 4 groups are abundant. Adding excess sodium cations to the system may again lead to compositional heterogeneity. The existence of diborate group as the basic building unit in glasses with appropriate sodium content is not well supported. The network‐forming cations in sodium boroaluminate glasses are well mixed, with no Al‐O‐Al resonance observed by MQMAS. The effect of cation type and thermal history on glass structures is also discussed.