Self-organized nano-crystallisation of BaF 2 from Na 2O/K 2O/BaF 2/Al 2O 3/SiO 2 glasses

Abstract

In glasses with the compositions (100 − x)(2Na 2O·16K 2O·8Al 2O 3·74SiO 2) xBaF 2 (with x = 0 to 6), the glass transition temperature decreases with increasing BaF 2-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 BaF 2. The quantity of crystalline BaF 2 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 SiO 2 which is formed during crystallisation. This layer acts as a diffusion barrier and hinders further crystal growth.