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.