Microstructure and luminescence of erbiumdoped Na2O/K2O/CaO/CaF2/Al2O3/SiO2 nano glass-ceramics

Oxyfluoride glasses with the mol% compositions (63-x) SiO2, x Al2O3, 12.3 CaO, 5.3 K2O, 9.0 Na2O and 10.4 CaF2 with x = (10, 8, 6) were doped with an Er3+-concentration of 1·1020 cm−3. The glasses were thermally treated at temperatures in the range from 550 to 620 °C. This led to the crystallization of cubic CaF2 with extremely broadened XRD-lines caused by mean crystallite sizes ranging from 14 to 38 nm. An interface controlled crystallization mechanism is proposed. Phase separation was observed in the as casted glasses by electron microscopy. Droplet like particles occur with a size of ca. 360 nm after heat treatment. These particles are composed by small crystallites with sizes of around 50 nm. Increasing Al2O3 concentration increases the tendency to liquid/liquid phase separation and decreases the crystallization temperature. Doping with Er3+ triggers CaF2 nanocrystallization by decreasing the crystallization temperature.