In situ measurements of the elastic properties of an alkali-free alumino-borosilicate glass under high pressure

The density and ultrasonic velocities of a commercial alumino-borosilicate glass were measured under hydrostatic compression; pressure-induced structural changes were investigated by in situ high-pressure Raman spectroscopy. Density increased with pressure, showing an increasing compressibility up to ∼5 GPa. At higher pressures, up to ∼12 GPa, the compression behavior became more nearly monotonic. Both the longitudinal and transverse acoustic velocities exhibited a clear negative pressure dependence up to 5–6 GPa—the so-called acoustic anomaly. Consistently, the bulk modulus showed a negative pressure dependence up to around 6 GPa, indicating an elastic anomaly. Raman spectra at low pressures suggest that densification proceeds primarily by simple contraction of network rings (e.g., six-membered rings), corresponding to elastic deformation of the glass network. Above ∼5 GPa, signatures of three-membered ring formation emerge, implying that larger rings collapse and reorganize into smaller ones once the elastic limit is exceeded. Indeed, glasses recovered from pressures ≥5 GPa retain enhanced Raman scattering attributable to three-membered rings.