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 45SiO(2)-15Al(2)O(3)-12Na(2)CO(3)-21BaF(2)-(7-x) LaF3-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 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. (C) 2017 Elsevier B.V. All rights reserved.