Composition‐dependent structural role of lead in aluminoborate and galloborate glasses: A solid‐state NMR study

Investigations on binary lead‐bearing glass systems have shown that Pb 2+ functions as a charge compensator at low concentrations and can participate in network formation at higher loadings. We use multinuclear magnetic resonance spectroscopy to investigate the mechanism underlying this dual‐role nature of Pb 2+ in ternary lead‐borate glasses containing aluminum or gallium. 207 Pb nuclear magnetic resonance (NMR) spectra—in conjunction with charge‐balance calculations, bond‐valence principles, and precedents from crystalline lead‐borates—are interpreted in terms of high‐ and low‐coordinate Pb 2+ species, representing charge‐compensating “network modifier” sites and covalently‐bound “network forming” sites, respectively. Both high‐field 27 Al and 71 Ga magic‐angle spinning NMR spectra indicate the presence of substantial fractions of four, five, and six‐fold coordinated Al/Ga, reinforcing the isostructural nature of these elements. However, systematic differences in the boron speciation in Al‐ and Ga‐bearing glasses reveal that their size difference has a measurable effect on the short‐range structure in these glasses.