Oxyfluoride glasses with the mol% compositions (63-x) SiO2, xAl2O3, 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 550-620 deg C. This led to the crystallization of cubic CaF2 with extremely broadened XRD-lines caused by mean crystallite sizes ranging from 14-38 nm. An interface controlled crystallization mechanism is proposed. Phase separation was observed in the as coated glasses by electron microscopy. Droplet like particles occur with a size of ca. 360nm 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 E43+ triggers CaF2 nanocrystallization by decreasing the crystallization temperature.
