Dušan Galusek

The influence of Al 2 O 3 addition on the precipitation of NaYF 4 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 Al 2 O 3 shows the presence of phase‐separated droplets about 69 nm in size containing several NaYF 4 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 Al 2 O 3 . The 19 F MAS‐NMR analysis indicated that fluorine atoms are present in different chemical environments (Na–F, Na–Si–F, Na–Al–F, and NaYF 4 ). The increasing amount of Al 2 O 3 reduces the crystallization of NaYF 4 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 Er 3+ ions emission transitions in the GCs was higher than that of the parent glass. This difference was attributed to the presence of Er 3+ ions bound in NaYF 4 crystalline phase. Further evidence that the Er 3+ ions were present in NaYF 4 phase was provided by the fact that the excited level Er 3+ : 4 S 3/2 lifetime was increased in GCs as compared to parent glass.