Self-organized nano-crystallisation of BaF2 from Na2O/K2O/BaF2/Al2O3/SiO2 glasses

In glasses with the compositions (100 − x)(2Na2O·16K2O·8Al2O3·74SiO2)xBaF2 (with x = 0 to 6), the glass transition temperature decreases with increasing BaF2-concentration. Samples with x = 6 were thermally treated at temperatures in the range from 500 to 600 °C for 5–160 h. This leads to the crystallisation of BaF2. The quantity of crystalline BaF2 increases with increasing time of thermal treatment, while the mean crystallite size remains constant within the limits of error. The glass transformation temperature of partially crystallised samples increases with increasing crystallisation time and approaches a value equal to the temperature, at which the samples were treated. This is explained by the formation of a highly viscous layer enriched in SiO2 which is formed during crystallisation. This layer acts as a diffusion barrier and hinders further crystal growth.