Molten salt synthesis of tetragonal LiYF₄:Yb³⁺/ Ln³⁺ (Ln = Er, Tm, Ho) microcrystals with multicolor upconversion luminescence

Well-defined tetragonal LiYF₄:Yb³⁺/Ln³⁺ (Ln = Er, Tm, Ho) microcrystals were successfully prepared by a surfactant-free molten salt method for the first time. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra were used to characterize the samples. It is found that with prolonged time, the phase of product transforms from orthorhombic YF₃ through hexagonal NaYF₄ to tetragonal LiYF₄, while the morphology changes from nanoparticles via microtubes to micro-sized bi-pyramids. By changing the dopant species (Ln³⁺), multicolor (yellow, green, and blue) upconversion (UC) emissions can be obtained in LiYF ₄:Yb³⁺/Ln³⁺ under 980 nm laser diode (LD) excitation. The UC mechanisms in co-doped LiYF₄ samples were analyzed in detail based on the emission spectra. Importantly, this synthetic methodology may offer a new alternative in the fabrication of high quality rare earth fluorides and open the possibility to meet the increasing commercial demand.