Direct Experimental Evidence for Abundant BO4-BO4 Motifs in Borosilicate Glasses From Double-Quantum 11B NMR Spectroscopy.

By using double-quantum-single-quantum correlation 11B nuclear magnetic resonance (NMR) experiments and atomistic molecular dynamics (MD) simulations, we resolve the long-standing controversy of whether directly interlinked BO4-BO4 groups exist in the technologically ubiquitous class of alkali/alkaline-earth based borosilicate (BS) glasses. Most structural models of Na2O-B2O3-SiO2 glasses assume the absence of B[4]-O-B[4] linkages, whereas they have been suggested to exist in Ca-bearing BS analogs. Our results demonstrate that while B[4]-O-B[4] linkages are disfavored relative to their B[3]-O-B[3]/B[4] counterparts, they are nevertheless abundant motifs in Na2O-B2O3-SiO2 glasses over a large composition space, while the B[4]-O-B[4] contents are indeed elevated in Na2O-CaO-B2O3-SiO2 glasses. We discuss the compositional and structural parameters that control the degree of B[4]-O-B[4] bonding.