Temperature Effects on Structure and Dynamics in Borate and Borosilicate Liquids: High‐Resolution and High‐Temperature NMR Results

The fictive temperature dependence of the relative abundances of three‐ and four‐coordinated boron was investigated in several sodium borate and borosilicate glasses using high‐resolution 11 B nuclear magnetic resonance (NMR). In the compositions with low sodium/boron ratios, no effect was observed, but in the borosilicates, the fraction of the tetrahedral species decreased significantly as the fictive temperature increased because of the higher content of nonbridging oxygens. In situ , high‐temperature magic‐angle spinning NMR demonstrated the exchange of the two species at a rate comparable to the shear relaxation rate, indicating a close link between B‐O bond breaking and viscous flow.