Design of oxy-fluoride glass-ceramics containing NaLaF4 nano-crystals

Fluoride glass-ceramics doped with lanthanide ions are very promising materials for optical devices. In this study, transparent fluoride glass-ceramics have been obtained from oxy-fluoride glasses in the Na2O–Al2O3–SiO2–LaF3 system by heat treatment slightly above Tg, upon which NaLaF4 nano-crystals with a size of up to 20 nm precipitate. The crystallisation mechanism is suggested to involve the formation of a SiO2-enriched shell around the nano-crystals which hampers their further growth, limits Ostwald ripening, and promotes uniformity in size. This desired enrichment of lanthanide ions inside nano-crystals with an adjustable and uniform size distribution is essential for future photonic applications. To validate the crystallisation mechanism of this glass system, the structure and composition of the crystalline phase, glass matrix and the developing interface have been studied by viscosity, dilatometry, X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques.