Understanding Enhanced Upconversion Luminescence in Oxyfluoride Glass-Ceramics Based on Local Structure Characterizations and Molecular Dynamics Simulations

In this Article, large enhancement in upconversion (UC) luminescence was verified in a transparent aluminosilicate glass-ceramics (GCs) containing CaF₂ nanocrystals (NCs) codoped with Er³⁺ and Yb³⁺ ions. On the basis of the joint spectroscopic and structural characterizations, we suggest that the precipitation of fluoride NCs is correlated with the pre-existence of the fluoride-rich domains in the as-melt glass, which is supported by scanning transmission electron microscopy (STEM) and reproduced by molecular dynamics (MD) simulation. The precipitation of the fluoride NCs starts from a phase-separated as-melt glass consisting of fluorine-rich and oxygen-rich domains, while the spatial distribution of rare earth (RE) ions and the vibration energies of the bonds connecting RE ions remain almost unchanged after crystallization. In the GCs, both the fluoride domain and the oxygen-containing polyhedrons surrounding RE ions experience significant ordering, which may affect the UC emission for both glasses and GCs. We therefore attribute the enhanced UC emissions of the GCs to the long-range structural ordering and the change of site symmetry surrounding RE ions, rather than the preference of RE ions in migrating from fluoride-rich phase to the fluoride NCs. Our results may have strong implications for a better understanding of the enhanced UC emission in similar oxyfluoride GCs