Crystal nucleation and growth kinetics of NaF in photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is an optically transparent photosensitive oxide glass. Upon heating, the UV-exposed regions of this glass undergo copious crystallization of NaF nano-crystals giving rise to a permanent, localized refractive index change. But the unexposed parts of the glass also undergo some crystallization which causes unwanted light scattering. Holographic optical elements produced from PTR glass have been used in special laser systems. In this article we report, for the first time since the invention of PTR glass about half a century ago, the steady-state nucleation rates, nucleation time-lags and crystal growth rates for UV-unexposed PTR glass estimated in a wide temperature range, from Tg ~ 470 °C up to 750 °C. A self-consistent description of these data is presented in the framework of classical nucleation theory using the interfacial free energy of the critical nuclei and the effective diffusion coefficient as adjustable parameters. The diffusivity calculated from crystallization kinetics and that estimated from viscous flow via the Stokes–Einstein–Eyring equation show a decoupling phenomenon.