Lanthanide ions doped tetragonal LiYF₄ has became an investigative focus of upconversion luminescence (UCL) materials for its well properties of multi-photon UCL and as a comparable matrix material with hexagonal NaYF₄. While the cause for its well performance on short bands emission is still unrevealed. After the exploration of crystal structure characteristic of tetragonal LiYF₄, a hexagonal circle sublattice structure of Y³⁺ with 0.3710 nm interval between adjacent Y³⁺ ions and larger than 0.5 nm interval between meta-position and para-position Y³⁺ ions were revealed. The energy transfer of rare earth ions are easy take place around the hexagonal circles or among the cluster of five adjacent trivalent ions. Base on the sublattice structure characteristic of tetragonal LiYF₄, we have an idea to study UCL mechanism systematically of tetragonal LiYF₄:RE by the construction of sublattice energy cluster 1M-xYb (M=Er, Ho, Tm) and the manipulation of crystal field symmetry by introducing different amount Yb³⁺ ions and Sc³⁺ or Hf⁴⁺ ions, respectively. Hydrothermal method was employed to prepare LiY_0.98-xYb_xEr_0.02F₄, LiY_0.98-xYb_xHo_0.02F₄, LiY_0.995-xYb_xTm_0.005F₄, LiY_0.68-xYb_0.3Er_0.02Sc_xF₄ and LiY_0.68-xYb_0.3Er_0.02Hf_xF₄ series samples. A typical preparation process demonstrate as follows, at first, (1-x) mmol Y(NO₃)₃ (0.2 mol/L), x mmol (x=0.2, 0.5, 0.7 and 0.9) Yb(NO₃)₃ (0.20 mol/L) and Er(NO₃)₃ (0.02 mmol) solution was dropwise added into 20 mL deionized (DI) water with 1 mmol EDTA to form a solution under vigorous stirring for 30 min. Secondly, 3.0 mL LiOH (1.0 mol/L) and 4.0 mL NH₄HF₂(1.0 mol/L) aqueous solution were dropwise added to the solution under thorough stirring for 30 min until the solution completely became a white emulsion, the pH value of the emulsion is 3~4. Finally, the white emulsion was slowly transferred into a 50 mL Teflon-lined autoclave, sealed and heated at 190℃ for 18 h. The final products were collected by centrifugation, and then washed with DI water several times. The collected samples were dried at 60℃ over night. X-ray powder diffraction (XRD) and Rietveld refinement method were employed to reveal the variation of crystal structure, field emission scanning electron microscopy (FESEM) and field emission transmission electron microscopy (FETEM) were employed to the analysis of crystal morphology and crystal structure. UCL performance was analyzed by Edinburgh fluorescence spectrophotometer FSP920. After investigation, we found excited energy levels distribution of different RE ions is diverse, and the level matching with Yb³⁺ are different too, it result in different luminescence quenching of energy cross relaxation, so the different sublattice energy clusters 1Er-2Yb, 1Ho-2Yb and 1Tm-4Yb of different active rare earth ions can be constructed for the best UCL performance. The cystal field symmetry of tetragonal LiYF₄:Yb/Er were manipulated successfully by 6 mol% Sc³⁺ or 4 mol% Hf⁴⁺ doping, and UCL intensity were enhanced about 50% with 6 mol% Sc³⁺, while the UCL intensity were weaken after Hf⁴⁺ doping. After Sc³⁺ or Hf⁴⁺ doping, there are only three Yb³⁺ ions in the five trivalence ions cluster that can't realize two-photon cooperation upconversion synchronous electron population of ⁴F_5/2 excited state level of Er³⁺ ions and ²G_7/2 or ⁴F_5/2^o excited state level of Sc³⁺ or Hf⁴⁺ respevticely, and then Sc³⁺ and Hf⁴⁺ ions become a quenching center in the asymmetric crystal field that is conversed with them doped hexagonal NaYF₄:Yb/Er that Sc³⁺ and Hf⁴⁺ ions were taken as energy storage ions and dramatically enhanced UCL performance. In this work, the UCL mechanism of sublattice energy cluster construction and crystal field manipulation were revealed that may be an inspiration for high efficient UC luminescence materials design and preparation.

Key words: LiYF₄, rare earth, sublattice structure, crystal field manipulation, upconversion luminescence