Crystallization of BaF2 from Droplets of a Phase Separated Glass-Evidence of Core-Shell Structure by ASAXS
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. 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. Anomalous small-angle X-ray scattering (ASAXS) gave evidence of a core shell structure. In agreement with earlier studies, it is assumed that the shell acts as 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 large as in fluoride phosphate glasses.