Influences of Er3+ content on structure and upconversion emission of oxyfluoride glass ceramics containing CaF2 nanocrystals

Transparent 45SiO2–25Al2O3–5CaO–10NaF–15CaF2 glass ceramics doped with different content of erbium ion (Er3+) were prepared. X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses evidenced the spherical CaF2 nanocrystals homogeneously embedded among the glassy matrix. With increasing of Er3+ content, the size of CaF2 nanocrystals decreased while the number density increased. The crystallization kinetics studies revealed that CaF2 crystallization was a diffusion-controlled growth process from small dimensions with decreasing nucleation rate. Er3+ could act as nucleating agent to lower down crystallization temperature, while some of them may stay at the crystal surfaces to retard the growth of crystal. Intense red and weak green upconversion emissions were recorded for glass ceramics and their intensities increased with the increasing of Er3+ content under 980 nm excitation. However, the concentration quenching effect appeared when Er3+ doping reached 2 mol%. These results could be attributed to the change of ligand field of Er3+ ions due to the incorporation of Er3+ ions into precipitated fluoride nanocrystals.