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 F-19 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 differencewas 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+:S-4(3/ 2) lifetime was increased in GCs as compared to parent glass.