Structural changes in calcium aluminoborosilicate glasses recovered from pressures of 1.5 to 3Â GPa: Interactions of two network species with coordination number increases

We report here the effects of compression on the network cation coordination and overall densification of 15CaO·15Al2O3·xB2O3·(70 − x)SiO2 glasses quenched from near glass transition temperatures at pressures of 1.5 to 3 GPa. Al and B coordinations are measured using 11B at 14.1 T and 27Al MAS NMR at 18.8 T fields; 29Si spectra were recorded as well. Compositions were designed to have low nominal concentrations of non-bridging oxygens, and thus to complement previous studies that highlighted the role of that species in pressure-induced network structural changes. For glasses quenched at atmospheric pressure, most of the aluminum cations are present in tetrahedral coordination and most boron in trigonal form, with minor concentrations of high coordinated Al ([5]Al, [6]Al) and [4]B. With increasing pressure, both aluminum and boron coordination increase significantly in all compositions, reaching 28% [4]B and 44% [5]Al + [6]Al in the most boron-rich composition (x ≈ 25); recovered density increases reach over 14%. Glasses containing higher concentration of boron showed systematically larger increases both in densification and average aluminum coordination, revealing important interactions between B and Al in this mixed network former glass.