Design and Synthesis of a Garnet Structural NaGd₂InGa₂Ge₂O₁₂ Crystal Matrix for Trivalent Bismuth Doping and the Realization of Broadband Cyan Light Emission for WLEDs

A series of Bi3+-doped NaGd2InGa2Ge2O12 (NGIGG) inorganic phosphors were synthesized via a high-temperature solid-state reaction. X-ray diffraction (XRD), transmission electron microscopy (TEM) and fluorescence spectroscopy were used to explore the phase structure, micromorphology, and luminescence properties of the synthesized phosphors, respectively. Results revealed that the NGIGG:Bi3+ phosphors are structurally the same as garnets and that the luminescence center, Bi3+, in the NGIGG can provide broadband emission ranging from 350 to 550 nm when excited by a wavelength of 330 nm. Such a spectral feature is related to the energy level of the metal-to-metal charge transfer (MMCT) of Bi3+. The CIE coordinates of the NGIGG:Bi3+ phosphor reach the point of (0.1814, 0.1937), corresponding to the cyan color region. By the combined use of ZSO:Mn2+ green-emitting and MGOF:Mn4+ red-emitting phosphors together with the NGIGG:Bi3+ phosphor, a standard white light spectrum with CIE coordinates located at (0.3210, 0.3216) is successfully obtained.