Network Glasses under Pressure: Permanent Densification in Modifier-Free Al2 O3- B2 O3- P2 O5-SiO2 Systems

SiO2, P2O5, B2O3, and Al2O3 are all well-known network formers in glasses, but the structure and properties of mixed Al2O3-B2O3-P2O5-SiO2 glasses without the presence of network modifiers are poorly understood. The relatively low atomic packing density of these glasses should favor network densification when subjected to high local stress (e.g., indentation) at room temperature, and it is therefore interesting to examine their structural response to high-pressure treatment. In the present study, we investigate the pressure-induced changes in volume, structure, and mechanical properties (hardness and crack resistance) of five Al2O3-B2O3-P2O5-SiO2 glasses with varying SiP ratio. The glasses are isostatically compressed at 1 GPa at the glass transition temperature, enabling permanent densification of large (approximately cm2) sample specimens. In the as-prepared glasses, boron atoms become partially converted from the threefold- to the fourfold-coordinated state when [P2O5]>[Al2O3], with all Al2O3 maintained in tetrahedral groups. For [P2O5]>([Al2O3]+[B2O3]), boron is exclusively found in fourfold coordination, while the aluminum coordination number increases, and all aluminum atoms are preferentially associated with phosphorus as next-nearest-neighbor cations compared to silicon. Upon isostatic compression, the glasses permanently densify up to approximately 6%, leading to an increase in hardness and a change in the indentation cracking pattern. We discuss these pressure-induced changes in glass properties in relation to the structural changes quantified through Raman and B11, Al27, and P31 NMR spectroscopy.