Lattice Structure and Pure Green Upconversion Luminescence of a BiTa ₇ O ₁₉ :Er ³⁺ /Yb ³⁺ Single Crystal

Using K2Mo3O10 and B2O3 as the solvent and BiTa7O19:Er3+/Yb3+ powder as the precursor, BiTa7O19:Er3+/Yb3+ single crystals were synthesized by the molten salt method in a platinum crucible. Utilizing scanning transmission electron microscopy (STEM) images recorded with a high-angle annular dark field (HAADF) detector and an annular bright field (ABF) detector, it is demonstrated that Er3+/Yb3+ can replace the sites in the (Ta/Bi)O6 layer and can also exist in the interstices between the (Ta/Bi)O6 layer and the TaO7 layer. BiTa7O19:Er3+/Yb3+ in the (Ta/Bi)O6 layer has a disordered structure and belongs to the P63/mcm (193) space group, rather than the ordered P6̅c2 (188) space group. Since three-layer TaO7 stacking was observed, the growth mode of the single crystal was inferred to be a piece-by-piece stacking process of the precursor blocks. This is because using raw materials rather than precursors is not sufficient to grow large-sized single crystals with high-quality. Under 980 nm laser excitation, the sample emits pure green upconversion luminescence, which is caused by the 2D distribution of Er3+/Yb3+. The shift rate of the peak centroids reached 0.173 nm GPa–1 under high pressure. The absolute and relative temperature sensitivities reached 0.851% and 0.83% K–1, respectively. In phosphor-converted 980 nm LEDs fabricated based on the samples, the CIE color coordinate can reach (0.21, 0.75), which exceeds the NTSC standard. This indicates that this crystal can be used for pressure and temperature sensing, along with UCL display applications.