Atomic structure of hot compressed borosilicate glasses

Borosilicate glasses have been in widespread use for over a century; however, a detailed understanding of the structural response to densification is still lacking. In this work, two commercial borosilicate glasses, viz., SCHOTT N-BK7^® (N-BK7) and Borofloat33^® (Boro33), are hot compressed up to 2 GPa with nitrogen gas, and the structural response to this densification is explored via ¹¹B solid-state nuclear magnetic resonance (NMR) spectroscopy and classical molecular dynamics (MD) simulations. The molar volume (V_m) of N-BK7 and Boro33 decreases ~5% and ~10%, respectively, as a result of hot compression at 2 GPa. The NMR results demonstrate the presence of three different types of fourfold coordinated boron species (B^[4]), which are confirmed in MD simulations to be (Formula presented.) (0B,4Si^[4]), (Formula presented.) (1B^[3],3Si^[4]), and (Formula presented.) (1B^[4],3Si^[4]) (where subscripts represent different B^[4] types and brackets indicate the next nearest neighbors (NNN)). The NMR results also show that the fraction of B^[4] increases by ~13% in N-BK7 glass upon hot compression at 2 GPa via the trigonal boron to tetrahedral boron (B^[3] to B^[4]) conversion, while the fraction of B^[4] in Boro33 glass only increases by ~2% at the same pressure, despite the fact that the V_m decrease in N-BK7 is double that of Boro33. The MD simulations capture the experimental trends in B^[4] populations, despite an underestimation of the B^[4] increase of N-BK7 (only ~6%) at 2 GPa. Moreover, the MD simulations suggest that the V_m reduction is a linear function of bond angle change and the fraction of Si-O-Si and B^[4]-O-Si. The modifiers and boron coordination conversion also influence the volume densification of borosilicate glasses by increasing the difficulty of bond bending, decreasing the bond lengths, and increasing the population of B^[4]-O-Si linkages. Finally, the B^[4] to B^[4] conversion, that is, (Formula presented.) (1B^[3],3Si^[4]) and (Formula presented.) (1B^[4],3Si^[4]) to (Formula presented.) (0B,4Si^[4]), is observed in hot compressed N-BK7 and Boro33 from NMR and qualitatively confirmed in MD.