Luminescent spetra regulation mechnism of (Sr1-xMex)2SiO4:Eu

Abstract:

The (Sr1-xMex)1.95SiO4：0.05Eu phosphor powders were synthesized by high temperature solid-state reaction. The effects of solid solution of different large (Ba2+) and small (Mg2+) ionic on the phase, the coordination crystal structure and the valence state of Eu ions were investigated firsly, and then regulation mechanism in the luminscent spectra was disccused. With increasing temperature, α-Sr2SiO4 increases and β-Sr2SiO4 phase decreases in Sr2SiO4 powder. Mg2+ ion dopant would increase the stability of α-Sr2SiO4 phase. Ba2+ ion dopant would lead the phase tranformation as β-Sr2SiO4+α-Sr2SiO4→α-Sr2SiO4→α-Sr2SiO4+Ba2SiO4→Ba2SiO4, in the sequence as β-Sr2SiO4, α-Sr2SiO4, Ba2SiO4, the Si–O–Me(I)–O–Me(II) chain changes from zigzag to straight chain, and the Me-O length increases. Under the excitation of 254nm (365nm), Eu-activated β-Sr2SiO4, α-Sr2SiO4, and Ba2SiO4 powders have bright green fluorescence emission (in the same sequence, the intensity increases and the overall spectrum is slightly blue shifted) and weak red light emission. For β-Sr2SiO4 →α-Sr2SiO4, Eu (I) emission peak is blue-shifted while the one of Eu (II) is red-shifted (Si–O–Me(I)–O–Me(II) chain changes from zigzag to straight, Me-O length increases). For α -Sr2SiO4 →Ba2SiO4, both emission peaks of Eu (I) and Eu (II) are blue-shifted (Me-O bond is enlonged). In the thermoluminescence spectra, there exit Eu2+ and Eu3+ defect energy levels in all these phosphors, and Eu2+ concentration looks larger. The high resolution XPS spectrum of Eu3d indicates that there are higher possibilities for Eu2+ in the sequence as β-Sr2SiO4 → α-Sr2SiO4 → Ba2SiO4, which is confirmed by Eu ion ESR spetra. By Ba2+ ions solid solution into Sr2SiO4 crystal, phase transforamtion as β-Sr2SiO4 → α-Sr2SiO4 → Ba2SiO4 would occurs, which would means the adjustmens in Si–O–Me(I)–O–Me(II) chain type and Me-O bond length. These would lead to the regulation in the coordination environment and valence state (Eu2+/ Eu3+ ratio) of Eu ion, and result to regular in emission wavelength and intensity.

Key words:Crystal structure, photoluminescence spectra, X-ray photoelectron spectroscopy，electron spin resonance