Clarifying a Competitive Crystallization Mechanism of Upconversion Luminescent Oxyfluoride Glass Ceramics by...

LaF₃ and NaLaF₄ crystals were selectively precipitated in the SiO₂-Al₂O₃-AlF₃-Na₂O-NaF-LaF₃-ErF₃ glass system by adjusting their compositions. The structural evolution at the atomic level driven by heat treatment and glass compositions was studied using the state-of-the-art magic-angle spinning nuclear magnetic resonance technique. From a comprehensive local structural study, we found that LaF₃ and NaLaF₄ crystals compete in crystallization in these glasses. The crystallization ability of NaLaF₄ increases with the increase of the content of Na⁺ ions within the F-enriched phase, but for LaF₃ crystals, it is reverse. These two crystals can be selectively precipitated in the glasses by adjusting the content of these Na⁺ ions within the F-enriched phase. When the crystallization ability of these two crystals becomes similar, none of them can be precipitated due to their mutual interference in crystallization. Intense single green upconversion luminescence occurs in glasses precipitating LaF₃ or NaLaF₄ crystals. The underlying relationship between compositions, structures, crystallization, and upconversion luminescence properties is unearthed based on the structural evolution, crystallization mechanism, and luminescence properties. This relationship will facilitate the compositional design of these kinds of glasses. It is inferred that it will be better to precipitate LaF₃ rather than NaLaF₄ crystals for achieving highly efficient upconversion luminescence.