Clarifying a Competitive Crystallization Mechanism of Upconversion Luminescent Oxyfluoride Glass Ceramics by Solid-State NMR Spectroscopy

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.