Structural Origins of RF/NaRF Nanocrystal Precipitation from Phase-Separated SiO-AlO-RF-NaF Glasses: A Molecular Dynamics Simulation Study.

Oxyfluoride glass-ceramics with RF or NaRF (R: rare earth elements) nanocrystals are considered as favorable hosts for luminescence applications. In this work, we utilized large-scale molecular dynamics (MD) simulations with effective partial charge potentials to study a series of oxyfluoride glasses that are of interest to the precipitation of RF or NaRF nanocrystals as previous experiment results suggested. The results show that phase separation exists in all glass compositions with fluoride-rich regions made up of R, Na, and F and oxide-rich regions consisting of aluminosilicate networks. These fluoride-enriched regions can serve as the precursor for RF, cubic and hexagonal NaRF, and NaF crystal precipitation. The results also confirm that the concentration of Na in the fluoride phase plays a key role in determining the crystal phases (RF, NaRF, or NaF) and crystal structure (cubic vs hexagonal NaRF) to be precipitated. Consequently, this study shows that MD simulations with effective potentials can fill the gap in the structural understanding of oxyfluoride glass and provide insights into atomic scale information of the phase separation behavior that is useful in predicting the potential crystal types in oxyfluoride glass. When coupled with experimental validations, these simulations can expedite the exploration of novel luminescent oxyfluoride glass ceramics.