Modifier cation (Ba, Ca, La, Y) field strength effects on aluminum and boron coordination in aluminoborosilicate glasses: the roles of fictive temperature and boron content

The field strength of modifier cations has long been known to have important effects on oxide glass properties, but effects on network structure can be complex. For two series of barium, calcium, lanthanum and yttrium aluminoborosilicates with two different B/Si ratios, we report systematic variations in boron and aluminum coordination determined by NMR, and glass transition and heat capacities from differential scanning calorimetry. Data on glasses with different fictive temperatures allow B and Al speciation to be compared on an isothermal basis, rather than as conventionally done for as-quenched structures. Temperature and compositional effects can thus be isolated. These data and comparison to previous studies on glasses with lower B/Si ratios clearly show that higher modifier cation field strength increases the fraction of five- and six-coordinated Al in all compositions. In contrast, the previously documented trend towards more three-coordinated boron (and hence more non-bridging oxygens, NBO) in low B/Si glasses with higher field strength cations reverses in high B/Si and in high NBO compositions. Al and B coordination numbers both decrease with higher fictive temperature in the glasses studied here, suggesting a simple mechanism of coupled structural change.