The influence of Al2O3 concentration on the NaYF4 crystallization in oxyfluoride glass–ceramics

The influence of Al₂O₃ addition on the precipitation of NaYF₄ crystals in oxyfluoride glasses has been investigated through the thermal, structural, and optical characterization of the parent glasses and corresponding glass–ceramics (GCs). The high-resolution transmission electron microscopy analysis of the GC with 5 mol% of Al₂O₃ shows the presence of phase-separated droplets about 69 nm in size containing several NaYF₄ nanocrystals with the diameter of about 10–15 nm. Raman and magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy were used to examine the structural changes attributed to the addition of Al₂O₃. The ¹⁹F MAS-NMR analysis indicated that fluorine atoms are present in different chemical environments (Na–F, Na–Si–F, Na–Al–F, and NaYF₄). The increasing amount of Al₂O₃ reduces the crystallization of NaYF₄ phase due to the increased fraction of fluorine bound in Na–Al–F environments. The visible luminescence investigation of the glasses and GCs demonstrated that the intensity of Er³⁺ ions emission transitions in the GCs was higher than that of the parent glass. This difference was attributed to the presence of Er³⁺ ions bound in NaYF₄ crystalline phase. Further evidence that the Er³⁺ ions were present in NaYF₄ phase was provided by the fact that the excited level Er³⁺:⁴S_3/2 lifetime was increased in GCs as compared to parent glass.