Investigation On Preparation And Upconversion Luminescence Of Rare-Earth Doped CaWO₄ Polycrystals

CaWO₄:Ho³⁺/Yb³⁺, CaWO₄:Tm³⁺/Yb³⁺, CaWO₄:Er³⁺/Yb³⁺ and CaWO₄: Ho³⁺/Yb³⁺/ Tm³⁺ polycrystals are synthesized via high temperature solid state method. XRD analysis shows that the prepared samples maintain CaWO₄ lattice structure. Using the ultraviolet-visible absorption spectrum to do research on the movements of the ultraviolet absorption boundary, and determine the regulation of the doped rare earth. Along with the increase of dosage concentration of rare-earth ions, the energy needed for the rare-earth ions to transition changes accordingly.Under the excitation of 980 nm, the upconversion luminescence performance of the rare-earth doped different CaWO₄ polycrystals is investigated. The upconversion spectra of the CaWO₄:Ho³⁺/Yb³⁺ polycrystals have two obvious emission peaks at 545 nm and 659 nm, the upconversion luminescence is from the 5F4/5S2â†'5I8 and 5F5â†'5I8 transitions of Ho³⁺, the power curve shows the upconversion luminescence processes are all two-photon processes; the upconversion spectra of CaWO₄:Tm³⁺/Yb³⁺ polycrytals have an obvious blue emission peak at 473 nm, its upconversi0n luminescence is from the 1G4â†'3H6 transition of Tm³⁺, the power curve shows the upconversion process includes the two-photon process and three-photon process simultaneously. The upconversion spectra of CaWO₄:Er³⁺/Yb³⁺ polycrystals have two obvious green emission peaks respectively at 530 nm and 552 nm, and they corresponds to the 2H11/2â†'4I15/2 and 4S3/2â†'4I15/2 transitions of Er³⁺, respectively. Meanwhile the experiment tests the luminescence decay curve of these samples, and analyzes the fluorescence lifetime of upconversion luminescence energy level, and further study the mechanism of upconversion.Under the excitation of 980 nm, there are three obvious emission peaks in the upconversion spectra of CaWO₄:Ho³⁺/Yb³⁺/Tm³⁺ polycrystals, emission centers at 474 nm, 545 nm, and 659 nm. Upconversion blue emission is from 1G4â†'3H6 of transition Tm³⁺, upconversion green emission and red emission correspond to 5F4/5S2â†'5I8 and 5F5â†'5I8 transitions of Ho³⁺. Therefore, it can control the relative intensity of three primary colors fluorescence through regulating the pump power and concentration of doped rare earth (Yb³⁺ and Ho³⁺). Along with the increase of the pump power, CIE coordinate gradually moves to the bottom left; along with the increase of the concentrations of doped Yb³⁺ and Ho³⁺, CIE coordinates separately move to bottom left and top right.