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

Abstract In this study, Tb 3+ -Yb 3+ co-doped transparent oxyfluoride glass-ceramics were prepared respectively from melt-quenched glass samples with the molar compositions of 45SiO 2 -15Al 2 O 3 -12Na 2 CO 3 -21BaF 2 -(7-x)LaF 3 -xYF 3 (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 YF 3 /LaF 3 . X-ray diffraction (XRD) and transmission electron microscopy (TEM) results revealed that the crystal structure transformed from cubic Ba 2 LaF 7 phase to cubic intermediate phase and finally to cubic BaYF 5 phase, the BaYF 5 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 Tb 3+ ions was infected by different types of irregularly arranged interstitial F- ions, which further affected the effective phonon energy around Tb 3+ ions. The high effective phonon energy around Tb 3+ ions in the intermediate phase could strongly enhance the up-conversion (UC) luminescence intensity, and thus improved the UC luminescence efficiency.