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.