Structure and fragility of normal and invert lanthanum borate glasses: Results from 11B and 17O NMR spectroscopy and calorimetry

• Published in: Journal of non-crystalline solids 2023-03, Vol.603, p.122119, Article 122119
• Main Authors: Swanson, R.K., Stebbins, J.F., Yeo, T.M., Xu, Y., Hung, I., Gan, Z., McCormack, S.J., Sen, S.
• Format: Article
• Language: English
• Subjects: 11B NMR; 17O NMR; Calorimetry; Containerless melting; Fragility; Invert glass; Lanthanum borate glass; Structure
• ISSN: 0022-3093; 1873-4812
• DOI: 10.1016/j.jnoncrysol.2022.122119

•Structure of lanthanum borate glasses are studied using high-field 11B and 17O NMR.•The structure of glasses with ≤ 30% La2O3 consists of a La-modified borate network.•Structure of glasses with 55 ≤ La2O3≤ 60 consist of a network of La-O polyhedra.•The La-O-La polyhedral network in invert glasses is interspersed with [BO3]3− units.•Temperature dependent B and O speciation in these glasses are linked to fragility. The structure of select normal and invert (La2O3)x(B2O3)100-x glasses with 22 ≤ x ≤ 60, synthesized using conventional and containerless levitation melt-quenching, is studied using 11B and 17O NMR spectroscopy. The results indicate that the structure of normal glasses with 22 ≤ x ≤ 30 is characterized by a borate network of B-O-B bridging oxygen (BO) linkages between BO3 and BO4 units and non-bridging oxygen (NBO) with La acting as a charge balancing modifier cation. In contrast, the invert glasses with 55 ≤ x ≤ 60 consist of a network of La-O polyhedra connected via La-O-La linkages i.e. “free-oxide” ions and via La-NBO-B linkages with interspersed [BO3]3− units. This structural evolution in lanthanum borate glasses is consistent with the composition dependence of their glass transition temperature. The temperature dependent boron and oxygen speciation in these glasses are linked to their calorimetrically determined fragility index.