Modification of the coordination environment of Eu2+ in Sr2SiO4:Eu2+ phosphors to achieve full color emission

Sr2SiO4:Eu2+ phosphors were synthesized by a conventional solid state reaction method. After a low amount of nitrogen (similar to 1 mol% of oxygen) was incorporated to modify the local coordination environment of Eu2+, the phosphor showed a single intense broad band emission centered at 625 nm under blue light (453 nm) excitation, and three emission bands (480, 555 and 625 nm) under ultraviolet irradiation. The incorporation of nitrogen was confirmed by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR) and absorption spectroscopy. 480 and 555 nm emissions originated from Eu2+ ions occupying the Sr(I) sites and Sr(II) sites in the Sr2SiO4 crystal, respectively, while 625 nm emission originated from the nitrogen coordinated Eu2+ ions. The local coordination structure around Eu2+ ions in the red phosphors was analyzed with the aid of density functional theory based first principles calculations. The analysis showed that nitrogen should preferentially substitute the O5' sites around Eu2+ in Sr(II) sites, which agreed fairly well with the experimental results from the X-ray absorption fine structure (XAFS) and the electron paramagnetic resonance (EPR) spectra. The electronic structure analysis confirmed the lowered center of gravity of Eu 5d energy states and the broadened Eu 4f energy states, which are due to the tightened coordination environment and the hybridization of the 4f states of Eu and 2p states of nitrogen-oxygen, leading to a red emission. The novel nitrogen modified Sr2SiO4:Eu2+ could serve as a full color phosphor for near-UV LEDs or a red-emitting phosphor for blue LEDs.