Cubic and Hexagonal NaGdF₄ Crystals Precipitated from an Aluminosilicate Glass: Preparation and Luminescence Properties

Depending on the composition of the glass and the annealing procedure supplied, thermal annealing of silicate glasses in the system SiO 2 /Al 2 O 3 /AlF 3 /Na 2 O/Gd 2 O 3 /SmF 3 led to the precipitation of cubic or hexagonal NaGdF 4 nanocrystals. The glasses were already phase separated after casting and formed a droplet phase supposedly enriched in fluorides and rare earths. The droplets had sizes mainly in the range from 80 to 120 nm. During annealing at temperatures ≥600 °C, multicore particles of cubic or hexagonal NaGdF 4 were precipitated inside the amorphous droplet phase. Completely transparent hexagonal NaGdF 4 containing nano-glass-ceramics could be derived from this system. Fluorescence spectra and fluorescence decay curves of Sm 3+ show crystal phase dependent effects. Sm 3+ doped hexagonal NaGdF 4 exhibits notably different fluorescence emission spectra and longer fluorescence lifetimes than cubic NaGdF 4 .