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.