Effects of crystal structure transformation on cooperative up-conversion luminescence in the Tb³⁺-Yb³⁺ co-doped oxyfluoride glass-ceramics

In this study, Tb³⁺-Yb³⁺ co-doped transparent oxyfluoride glass-ceramics were prepared respectively from melt-quenched glass samples with the molar compositions of 45SiO₂-15Al₂O₃-12Na₂CO₃-21BaF₂-(7-x)LaF₃-xYF₃ (x = 0, 1, 3, 6 and 7). It was found that the crystal structure of the annealed glass-ceramics strongly depended on the ratio of YF₃/LaF₃. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results revealed that the crystal structure transformed from cubic Ba₂LaF₇ phase to cubic intermediate phase and finally to cubic BaYF₅ phase, the BaYF₅ nanocrystal was determined as cubic structure instead of the reported tetragonal structure for the first time via a Rietveld refinement method with TOPAS. In the process of crystal structure transformation, the local symmetry around Tb³⁺ ions was infected by different types of irregularly arranged interstitial F- ions, which further affected the effective phonon energy around Tb³⁺ ions. The high effective phonon energy around Tb³⁺ ions in the intermediate phase could strongly enhance the up-conversion (UC) luminescence intensity, and thus improved the UC luminescence efficiency.