Synthesis and photoluminescence of Eu3+ doped CaGd2(WO4)4 novel red phosphors for white LEDs applications

- CaGd2(WO4)4:Eu3+ phosphors were synthesized by the solid-state reaction.
- The Eu3+ ions mainly occupy the lattice site without inversion symmetry, which leads to higher 5D0-7F2/5D0 → 7F1 value.
- The dipole-dipole interaction plays an important role in the energy transfer between the adjacent Eu3+ ions.
- The refractive index, Judd-Ofelt parameters, transition branch ratios and radiative transition rate were calculated.

To provide a systematic study of scheelite structure tungstates, potentially better red phosphors, series of CaGd2−x(WO4)4:Eux were successfully synthesized by the conventional solid state method. X-ray powder diffraction, scanning electron microscopy, fluorescence spectra, decay lifetime measurement and Judd-Ofelt theory were used to investigate the properties of CaGd2(WO4)4:Eu3+ phosphors. The results reveal that CaGd2(WO4)4 compound has monoclinic system with space group I2/b. Under the excitation at 393 nm, Eu3+ doped CaGd2(WO4)4 phosphor exhibits the dominant red emission peak located at 616 nm, which is ascribed to 5D0 → 7F2 transition of Eu3+ ion. Effect of the calcination temperature on the photoluminescence properties has also been studied and it is found that the emission intensity reaches the maximum at 1273 K. In addition, the optimal doping concentration of Eu3+ is determined to be x = 0.8. It can be confirmed that the dipole-dipole interaction type plays an important role in the energy transfer in CaGd2(WO4)4:Eu3+ phosphors through the concentration quenching curve. The Judd-Ofelt parameters Ω2 and Ω4 have been calculated, indicating the lower site symmetry and higher covalency around Eu3+ ions. The CIE coordinates of CaGd1.2(WO4)4:0.8Eu3+ phosphors are x = 0.649 and y = 0.349, which are close to the standard of NTSC. These results show that CaGd2(WO4)4:Eu3+ phosphor can be a potentially better red emitting phosphor for white light emitting diodes (LEDs).