Network Connectivity, Bond Lengths, and Modifier Coordination in Alkali Aluminoborosilicate Glasses

The structural role of alkali modifiers (Li, Na, and K) in aluminoborosilicate glasses remains incompletely understood despite their widespread industrial use. Here, we investigate a series of glasses with compositions 20<i>M</i>₂O-<i>x</i>B₂O₃-(20-<i>x</i>)Al₂O₃-60SiO₂ (<i>M</i> = Li, Na, K; <i>x</i> = 5, 10, 15, with additional <i>x</i> = 0 and 20 for Na). Lithium-containing samples are prepared in duplicate using isotope-enriched compositions to enable isotopic-difference neutron diffraction. Solid-state NMR shows that ∼98% of Al is tetrahedrally coordinated, independent of alkali type, while the fraction of tetrahedral B increases with decreasing alkali-field strength and increasing B/Al ratio. Neutron total scattering data are analyzed by peak fitting of real-space correlation functions, using NMR-derived coordination numbers as constraints. Subtle variations in B-O, Si-O, and Al-O bond lengths are observed, and by the isotopic difference method, we find that the Li-O bonds exhibit an asymmetric bond length distribution and an increasing coordination number with increasing B/Al ratio. A similar trend is observed for the Na-O bonds, exploiting the improper difference method. These results provide new insight into how alkali modifiers influence the structure of aluminoborosilicate glasses, which will be helpful for establishing composition-structure-property relations.