Luminescence Properties of Calcium Tungstate Activated by Lanthanide(III) Ions

Calcium tungstate phosphors activated by the Ln³⁺ ions (Ln=Pr, Nd, Tb, Yb) were synthesized by a traditional high-temperature solid-state method. The crystal structures and morphologies of the products were characterized by scanning electron microscopy (SEM), X-ray powders diffraction (XRD) and infrared spectra (FT-IR). The samples were found to show luminescence properties (down-conversion, DC, at excitation wavelength 254 nm and up-conversion, UC, at excitation wavelength 980 nm). CaWO4 doped with Tb³⁺/Yb³⁺showed green DC and UC luminescence characteristic of Tb(III) ion in the range of 470-660 nm, cor-responding to the ⁵D₄→⁷F_6,5,4,3,2 electronic transition. CaWO₄ doped with Pr³⁺/Yb³⁺showed week blue, green and red (DC and UC) luminescence of Pr(III) ion, in the wavelength region of 450-700 nm. Emission peaks were ascribed to the ³P₁→³H_4,5,6, ³P₀→³H_4,5,6, ³P₁→³F₂ and ³P₀→³F₂ transitions, respectively. CaWO₄ doped with Nd³⁺/Yb³⁺phosphor emitted orange UC luminescence at 450-690 nm (²P_3/2→⁴I_15/2, ⁴G_7/2→⁴I_{9/2,11/2,13/2}) and strong near-infrared UC luminescence at 720-900 nm (⁴F_7/2+⁴S_3/2→⁴I_9/2, ⁴F_5/2+²H_3/2→⁴I_9/2, ⁴F_3/2→⁴I_9/2) which is the characteristic of Nd(III) ion.

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