Highly efficient rare-earth free vanadate phosphors for WLEDs

Yellow-green emitting phosphors of vanadate Ca₅Mg₄(VO₄)₆ (CMV) doped with different concentrations of Ta⁵⁺ ions were synthesized by a solid-state reaction method. The formation of single-phase compounds with a garnet structure was verified by X-ray diffraction (XRD), Rietveld refinement calculations and energy-dispersive X-ray spectroscopy. Different luminescence properties of CMV phosphors such as spectral shift, luminescence lifetime, quantum efficiency, color coordinates and Stokes shift were measured and have been discussed in detail. PLE and PL spectra showed that CMV : xTa⁵⁺ (0 ≤ x ≤ 5%) phosphors could match well to 365 nm LED chips, and showed bright yellow-green emission in the visible range of 400-750 nm, with a peak at 544 nm, which is attributed to the charge transfer (CT) of an electron from the 2p orbital of the oxygen atom to the vacant 3d orbital of V⁵⁺ ions in the tetrahedral [VO₄]^3- group. Compared with the CMV host, the integrated luminescence intensity of CMV : 0.5%Ta⁵⁺ increased by 26.31%, and the quantum efficiency increased by 15.98%. The phenomenon can be ascribed to the substitution of V⁵⁺ ions by the large Ta⁵⁺ ions, which resulted in the squeezed and distorted VO₄ tetrahedron. Finally, the white light emitting diode (WLED) devices prepared with UV WLED chips and the CMV : 0.5%Ta⁵⁺ phosphor exhibited excellent color temperature (4083 K) and CIE coordinates (0.3677, 0.3409). The CMV : 0.5%Ta⁵⁺ phosphor can be considered as a potential yellow-green emitting phosphor in the solid-state lighting field.