Hexagonal NaY_0.95Yb_0.03Er_0.02F₄ microcrystals with different morphologies were prepared by hydro-thermal synthesis with three controlling methods, including addition of different amount of EDTA, addition of different surfactant including CTAB, P123 and SDBS and using different thermal solvents including water, ethanediol and polyethyleneglycol. The synthesis method can be described as following: trivalent nitrate stock solutions of 1 mol/L with ratio 0.95/0.03/0.02 of Y³⁺/Yb³⁺/Er³⁺were prepared by dissolving the corresponding metal oxide in concentrated nitric acid at elevated temperatures and label as A. Certain concentration solution of EDTA, P123, CTAB, SDBS, NaF, and NH₄HF₂ were prepared as candidate. In a typical procedure, a certain amount of EDTA, CTAB, P123, and SDBS were added into 20 mL A sulution. After vigorous stirring for 30 min, certain mol NaF was added drop by drop, the pH value of solution was adjusted to 3.0 by the addition of 1 mol/L HF or NaOH, and then stirring continued for 30 min. Then the emulsion mixture was moved to Teflon-lined autoclave and incubated in oven at 195 ℃ for 4 h. The products were washed by deionized water and ethanol three times respectively, and then dried in oven at 60 ℃. Phase structure and crystal morphologies were characterized by XRD (X-ray powder diffraction), SEM (scanning electron microscope) and TEM (transmission electron microscope) methods. Analysis results indicate that the crystal growing direction changed from[001] to[100] and bond distances between F^- and Y^3-/Yb^3-/Er^3-were enlarged while bond distance between F^- and Na^- was reduced with the increasing EDTA amount. The crystal morphology was not changed by adding CTAB surfactant compared with no surfactant adding sample, while microtubes were obtained when P123 was added as surfactant, and micrograins morphology mixed with some smaller hexagonal NaYF₄ when SDBS was added as surfactant; crystal sizes and exposure lattice plane were also tuned with different thermal solvents. Crystal growing mechanism of different morphologies was discussed in the manuscripture. Upconversion (UC) luminescence emission spectra and decay time curves of different-morphological samples were obtained using Edinburgh instruments FSLP920 fluorescence spectrophotometer with 980 nm OPTEK OPO LASER excitation. The lifetimes of 521 and 541 nm emission were fitted by single exponential decay function. Luminescence property analysis results show the intensity and lifetime of spectra emission is interrelated with crystal morphology, the emission intensity is enhanced with reducing of the thickness of micrograins, and the intensity of the smallest microstick is the strongest among all the samples. The lifetime fitted results show the decay time constants and rising time constants are different of different-morphological NaY_0.95Yb_0.03Er_0.02F₄, indicating that their energy transfer models are different during upconversion UC emission. This manuscripture may be helpful for morphology-controllable synthesis of upconversion UC materials with super luminescence performance.

Key words: rare earth, upconversion, crystal morphology, luminescence, NaYF₄