Effect of Bromine on NaF Crystallization in Photo‐Thermo‐Refractive Glass

Abstract

Photo‐thermo‐refractive (PTR) glass is a Na2O–K2O–ZnO–Al2O3–SiO2 silicate optical glass which also contains fluorine, a small amount of bromine, and dopants that yield photo‐sensitivity in the UV range. PTR glass undergoes crystallization of NaF nanocrystals after UV‐exposure followed by thermal treatment, resulting in permanent refractive index change. In this study, where we explore only the thermally activated transformations in the UV‐unexposed glass, we show that bromine decreases the solubility of NaF, i.e., increases the super‐saturation of NaF thus increasing the thermodynamic driving force for crystallization. This feature causes a decrease in the maximum volume fraction of crystallized NaF with decreasing bromine content in the parent glass. The evolution of the glass transition temperature, Tg, with increasing isothermal treatment time revealed a minimum resulted from interplay between two concurring processes, liquid–liquid phase separation that led to decrease in Tg, and Br‐controlled NaF crystallization that acted in the opposite direction. In glasses with lower bromine content, fewer and larger crystals appeared and a surface‐initiated crystallization was dominant. A surface layer of F‐depleted glass imprinted residual macro‐stresses, which were not alleviated by annealing.