The nano-crystallization and luminescence in the visible spectral range of thulium doped Na2O/K2O/CaO/Al2O3/CaF2/SiO2 glasses

An oxyfluoride glass with the molar composition 59.3 SiO2·12.3 CaO·9.0 Na2O·5.3 K2O·3.7 Al2O3·10.4 CaF2 was doped with Tm3+ concentrations of 2.5 × 1018 and 1 × 1019 cm−3, which corresponds to about 0.005 and 0.02 mol% Tm2O3 in the batch. The glasses were thermally treated in the temperature range from 530 to 600 °C, which resulted in the crystallization of CaF2. The XRD patterns showed extremely broadened lines attributed to mean crystallite sizes in the range of 6-11 nm. Scanning electron microscopy shows phase separation in the glass samples and crystallization within these phase separated droplets. Static luminescence spectra and luminescence decay curves were recorded for samples with different Tm2O3 concentrations and after thermal treatment at different temperatures indicating that the Tm3+ ions are located in a fluoride enriched environment.