Crystallization and upconversion luminescence of distorted fluorite nanocrystals in Ba²⁺ containing oxyfluoride glass ceramics

In this study novel Er³⁺-doped transparent glass ceramics were prepared from melt-quenched glasses with general composition of Na₂O-NaF-BaF₂-YF₃-Al₂O₃-SiO₂. The phase composition of the annealed glass ceramics strongly depends on the BaF₂ content in glass matrix. Crystallization of β-NaYF₄, cubic fluorite type, tetragonal nanocrystals and rhombohedral Ba₄Y₃F₁₇ was observed. An introduction of BaF₂ in the glass matrix suppresses the crystallization of β-NaYF₄, promotes the formation of Ba²⁺ containing fluoride phases and reduces the cross-relaxation between Er³⁺ ions in fluoride crystals. Small rate of the nonradiative processes and low local symmetry of rhombohedrally disordered Ba₄Y₃F₁₇ enables an efficient upconversion luminescence surpassing that of the glass ceramics containing β-NaYF₄ nanocrystals.