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

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Abstract
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 20M2O-xB2O3-(20-x)Al2O3-60SiO2 (M = Li, Na, K; x = 5, 10, 15, with additional x = 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.
Funding Information
  • HORIZON EUROPE European Research Council (101044664)
  • ISIS Neutron and Muon Source (RB2410034)

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