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 (30La2O3-70B2O3, 50La2O3-50B2O3, 60La2O3-40B2O3, and 50Y2O3-50B2O3) 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 non-bridging oxygen were found in the structures. Polymer units containing four or more boron atoms were identified with and without three-membered boron rings (Bring). 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 ring formation. The formation of isolated units is achieved through the reaction of polymers without 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 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.