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

Abstract

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.