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 (Vm) 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 B14(0B,4Si[4]), B24(1B[3],3Si[4]), and B34(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 Vm 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 Vm 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, B24(1B[3],3Si[4]) and B34(1B[4],3Si[4]) to B14(0B,4Si[4]), is observed in hot compressed N‐BK7 and Boro33 from NMR and qualitatively confirmed in MD.