Synthesis of Tetragonal Phase LiYF4: Yb and Tm Microcrystals with Strong UV Upconversion Fluorescence

In this work, we synthesized pure tetragonal phase LiYF4 microcrystals by a hydrothermal method. In the reaction, sodium ions acted as an intermedium species. The long axis of the quasi-octahedron crystals was about 5 to 20 micrometers. Strong UV (I6H6)-I-1-H-3 (291nm), (I6F4)-I-1-F-3 (346nm), and (D2H6)-D-1-H-3 (362nm) and blue emissions (D2F4)-D-1-F-3 (450nm) and (1)G(4)(3)H(6) (475nm) could be observed in our samples. We could see blue luminescence from the microcrystals by our naked eyes in the daylight even if the pump power of the 980nm laser diode was less than 10mW. Obvious energy level splitting was also observed, which indicated that Tm3+ ions were embedded into the crystal lattice very well. The (1)G(4) energy level split into four subenergy levels owing to the effect of crystal field. The upconversion mechanisms were discussed in detail.