Luminescence properties of CaWO₄ :Tb³⁺ phosphors prepared by a soft chemistry method for white leds

Full text: Rare earth-doped nanophosphors have emerged as a new class of luminescence materials, showing advantages of high stability, brightness, and flexible industrial processing ability that are suitable for fluorescent lamps, high definition displays, optical amplifiers, oscilloscopes and scintillators for detecting X and γ-rays in medical diagnostics and display devices. The alkaline earth tungstates absorb efficiently high energy radiation (UV, X-rays). The doping of trivalent rare earth ions (R³⁺) into these matrices, specially with Tb³⁺, allows applications such as phosphors for fluorescent lamps, high definition displays and optical amplifiers. This work reports the synthesis, characterization and photoluminescence study of CaWO₄ doped with Tb³⁺ in concentrations of 0.1 to 10 mole-%. Highly luminescent calcium tungstate (CaWO₄ ) materials doped with rare earth (R³⁺) ions with attractive photoluminescence properties were prepared using coprecipitation method, at room temperature. The materials show white/green (Tb³⁺) luminescence under UV radiation. The phosphors were prepared with aqueous solutions, in stoichiometric amounts of Na₂WO₄ , TbCl₃ and CaCl₂ . The XPD measurements revealed the phase purity as well as crystalline tetragonal scheelite phase with I41/a (#88) space group. The X-ray spectra absorption at XANES of CaWO₄ :Tb³⁺ materials calcined at 1000 deg C indicated only existence of Tb trivalent ions. The excitation spectra show broad bands assigned to overlapping O→W and 4f⁸→4f⁷5d¹ band and narrow peaks assigned to the 4f-4f transitions of Tb³⁺. The emission spectra are dominated by high intensity ⁵D₄→ ⁷F₅ for Tb. The CIE chromaticity coordinates at room temperature for CaWO₄ :Tb³⁺ 0.1 mol-% calcined at 300 deg and 1000 deg C emit in the green region - close white. There is a tuning of emission color as consequence of calcination. The phosphors CaWO₄ :Tb³⁺ showed color emission at green region – almost white. This provides the application these phosphors in white LED materials, and more studies in materials with others rare earths ions. (author)