Ab Initio Molecular Dynamics Study of Trivalent Rare Earth Rich Borate Glasses: Structural Insights and Formation Mechanisms

In this work, trivalent rare earth (RE)-rich borate glasses (30 La₂O₃-70 B₂O₃, 50 La₂O₃-50 B₂O₃, 60 La₂O₃-40 B₂O₃, and 50 Y₂O₃-50 B₂O₃) were modeled using ab initio molecular dynamics (AIMD) simulations through the melt-quenching route. It was found that the AIMD-derived structures reproduced the experimental structure factors and ¹¹B solid-state nuclear magnetic resonance data. Isolated borate units (monomers, dimers, and trimers) terminated with nonbridging oxygen were found in the structures. Polymer units containing four or more boron atoms were identified with and without three-membered boron rings (3-rings). Increasing the proportion of La₂O₃ in La₂O₃-B₂O₃ glasses resulted in an increased number of isolated units, indicating that La³⁺ acts as a network modifier, breaking the borate glass network. The formation of these units via the melt-quenching process was detected by labeling boron species at each AIMD step from 1500 to 300 K. Representation with transition matrices clarified the specific reaction routes, leading to the formation of isolated boron units in solid glass. A key finding is the stabilization of polymer units involving 3-ring formation. The formation of isolated units is achieved through the reaction of polymers without 3-rings. The RE coordination structure was thoroughly analyzed from the perspective of shape and symmetry. Reference structures derived from the solution of the Thomson problem were compared to the AIMD-derived coordination structures and crystalline LaBO₃ and YBO₃. The results highlight the specificity of the Y coordination structure with 3-rings in YBO₃, which is not observed in RE borate glasses. The analytical approaches and interpretations used in this study provide insights into the diverse coordination structures of glasses containing heavy elements other than REs.