Rare-Earth-Induced Fluoride-Rich Local Structures Governing BaF₂ Nanocrystallization in Oxyfluoride Glasses

This study investigated the structural origin of BaF2 nanocrystallization in BaF2–ZnO–B2O3 oxyfluoride glasses by combining synchrotron high-energy x-ray scattering measurements with machine learning molecular dynamics simulations (MLMD). Particular attention was paid to the effect of rare-earth ion addition on the local structure and nucleation behavior. Time-dependent x-ray diffraction (XRD) measurements indicate that nucleation is almost completed within a short time at the early stage of heat treatment. Initially, BaF2 nanocrystals containing relatively large amounts of rare-earth ions are formed, after which crystal growth gradually proceeds. Synchrotron scattering measurements show that the Ba–Ba correlation peak shifts from about 4.07 to 4.32 Å with increasing temperature, approaching the Ba–Ba distance in BaF2 crystals. This result suggests that BaF2-like medium-range order develops in the glass prior to crystallization. MLMD simulations reproduce the shift of the Ba–Ba correlation toward longer distances and show that the Ba–F–Ba bond angle increases with temperature, geometrically supporting the expansion of the Ba–Ba distance. Coordination-number analysis further reveals that rare-earth fluoride addition increases the Ba–F coordination around Ba, making the local environment more fluoride-rich. The coordination environment around rare-earth ions is also closer to that of Ba in BaF2 crystals, suggesting that rare-earth ions stabilize BaF2-like local structures. These results indicate that BaF2 nanocrystallization proceeds from fluorine-rich local structures formed by preferential Ba–F coordination based on the hard and soft acids and bases (HSAB) principle and that rare-earth ions promote nucleation by stabilizing BaF2-like local coordination environments.