Ordering of fluorite-type phases in erbium-doped oxyfluoride glass ceramics

In this study, novel transparent Er³⁺ doped glass ceramics were prepared from melt-quenched oxyfluoride glasses with general composition of Na₂O-NaF-BaF₂-YbF₃-Al₂O₃-SiO₂. The crystallization of fluorite (BaF₂, BaF₂-YbF₃, NaF-BaF₂-YbF₃ and Na_0.5-xYb_0.5+xF_2+2x) and distorted fluorite (rhombohedral Ba₄Yb₃F₁₇ and tetragonal NaF-BaF₂-YbF₃) phases was analysed in glass ceramics with different BaF₂ and YbF₃ ratio. The phase composition and microstructure were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Intense red upconversion luminescence (UCL) was detected under near-infrared excitation resulting from three photon upconversion followed by cross-relaxation between Er³⁺ and Yb³⁺ ions. The local environment of Er³⁺ ions in fluorite and distorted phases was analysed using site-selective spectroscopy. The Er³⁺ ions were found to act as nucleation centres in the glass ceramics containing BaF₂. The phase transition from metastable fluorite to rhombohedrally and tetragonally distorted fluorite phases was detected using Er³⁺ ions as a probe.