X-ray diffraction study of NaF nano-crystals in photo-thermo-refractive glass

• NaF growth in PTR glass is studied using various characterization techniques.
• Average NaF crystals' size in UV-exposed PTR glass is lower than ~ 20 nm.
• NaF crystals' growth is limited by the exhaustion of Na+F− within the glass.
• Increase of viscosity of PTR glass around NaF crystals acts as a kinetics barrier.
• Rayleigh scattering model is used to accurately predict scattering.

PTR glass is a photosensitive silicate glass which imparts refractive index change after UV-exposure and thermal development. The origin of photosensitivity is explained by the precipitation of NaF crystals inside the glass matrix. While nucleation of NaF in PTR glass has been thoroughly studied, the mechanisms of growth of NaF crystals have drawn a limited interest. To study the NaF growth, several techniques were combined: optical spectroscopy, interferometry, DSC and XRD. It was found that the size of NaF crystals in UV-exposed PTR glass, as determined by the broadening of the diffraction lines does not exceed ~ 20 nm. The NaF crystals' growth is limited by the exhaustion of Na+F− within the glass matrix as well as the increase of the viscosity of the glass surrounding the crystals that act as a kinetics barrier for further growth. X-ray diffraction analysis of NaF powder allowed calculating the crystals' volume fraction and their average size. A combination of these parameters with Rayleigh scattering model allowed the prediction of scattering at any wavelength and any thermal treatment, proving that scattering is determined by single crystals.