Pressure-induced structural changes in a borosilicate glass-forming liquid: boron coordination, non-bridging oxygens, and network ordering

Two of the most important structural controls on the properties of borosilicate glasses and glass melts are the variation between three- and four-coordination of network-forming boron cations, and the extent of mixing of Si and B. The effects of composition on these key parameters are relatively well studied. However, proposed mechanisms could be better constrained by testing with another, independent parameter that can also strongly affect the network. Here we present some of the first quantitative structural data on the effects of high pressure on the network structure of a sodium borosilicate glass. Using high-resolution ¹¹B and ¹⁷O NMR on a sample melted at 5 GPa, we demonstrate that the formation of tetrahedral boron from trigonal boron is indeed closely coupled to the conversion of non-bridging to bridging oxygens. The increased fraction of tetrahedral boron at high pressure also causes increased mixing of boron and silicate structural units, as oxygens bridging between two BO₄ groups are energetically relatively unfavorable.