Densification of Sodium Borosilicate Glasses at Ambient Temperature: Structural Investigations by Solid-State Nuclear...

Alkali-borosilicate glasses with composition (80-x)SiO₂-xB₂O₃-20Na₂O (10 ≤ x ≤ 30) were subjected to a 25 GPa compression and decompression at room temperature, resulting in density increases between 1.4% and 1.9%. The structural changes associated with this process have been investigated and compared with uncompressed glasses having the same thermal history. Systematic trends are identified, using Raman scattering and multinuclear solid-state Nuclear Magnetic Resonance (ssNMR). Perhaps counterintuitively, pressurization tends to increase the concentration of three-coordinated boron species (B(III) units) at the expense of four-coordinated boron (B(IV) units). ²³Na NMR spectra show a systematic shift toward higher frequencies in the pressurized glasses, consistent with shorter average Na-O distances. The results are consistently explained in terms of a breakage of Si-O-B⁴ linkages resulting in the formation of nonbridging oxygen species. Pressure effects on the spectra are reversed by annealing the glasses at their respective glass transition temperatures.