Structural evolution of CaF2 nanoparticles during the photoinduced crystallization of a Na2O-K2O-CaO-CaF2-Al2O3-ZnO-SiO2 glass

A glass in the system Na2O-SiO2-Al2O3-K2O-CaO-CaF2-ZnO was doped with Ce, Ag, Sn, Sb and Br. Homogeneous and transparent glass ceramics are obtained from this glass by the precipitation of CaF2 nanoparticles. An interface-controlled crystallization mechanism hinders crystal growth after some time. By adding photosensitive agents, a tailored photoinduced CaF2 crystallization was achieved. Structural evolution during irradiation and heat treatment was investigated by different electron microscopic techniques, e.g., scanning transmission electron microscopy (STEM) including energy-dispersive X-ray analysis (EDXS) and scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD). Agglomerated structures with a size of ca. 35 nm were observed after the first thermal treatment at 530 °C. The Ag nucleation is accompanied by clustering of several components within the glass. CaF2 particles with diameters of ca. 300 nm with a spherulitic structure are observed in the glass ceramic after the second heat treatment step at 560 °C. Calcium and fluoride are depleted from the glass matrix during crystallization, while SiO2 is enriched. From (S)TEM micrographs, supplemented by selected area diffraction (SAD), outward growth of CaF2 from a central point is observed. STEM-EDXS analyses show residual glass within the spherulitic structure.