Crystallization of BaF2 from droplets of phase separated glass – evidence of a core–shell structure by ASAXS

Abstract

Glasses with the mol% compositions 1.88 Na₂O·15.04 K₂O·7.52 Al₂O₃·69.56 SiO₂·6.00 BaF₂ and 1.88 Na₂O·15.03 K₂O·7.52 Al₂O₃·69.52 SiO₂·6.00 BaF₂·0.05 SmF₃ were studied using X-ray diffraction, transmission electron microscopy, and anomalous small-angle X-ray scattering (ASAXS). While the glass doped with samarium showed liquid/liquid phase separation of droplets with sizes of around 100 nm, the glass without samarium did not. The samples were annealed at 580 °C or at 600 °C which led to the crystallization of cubic BaF₂. The X-ray diffraction patterns showed strongly broadened lines. Hence, the BaF₂ crystals possess sizes in the nm range. ASAXS gave evidence of a core shell structure. In agreement with earlier studies, it is assumed that the shell acts as a diffusion barrier that hinders crystal growth. Surprisingly, the cores and shells from the crystallization of the homogeneous glass and from the second glass, which is Sm-doped and shows liquid/liquid phase separation, both possess similar dimensions, even though the origin of the barrier is very different. The doped samples show long luminescence lifetimes of nearly 5 ms at a wavelength of 600 nm, which is nearly as long as those in fluoride phosphate glasses.