Cation Substitution-Dependent Phase Transition and Color-Tunable Emission in (Ca1-xBax)2SiO4∶Eu Phosphor Series
WANG Yu1, LUO Lan1, 2*, GUO Rui1, SUN Chuan-yao1, GAO Ming-yuan1
1. School of Materials Science and Engineering，Nanchang University，Nanchang 330001，China
2. Key Laboratory of Lightweight and High Strength Structural Materials of Jiangxi Province，Nanchang University，Nanchang 330001，China
Abstract：(Ca1-xBax)1.95SiO4∶0.05Eu (x=0, 0.1, 0.3, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) new phosphor series were prepared by high temperature solid-state reaction at 1 170 ℃ under a reduction atmosphere for 3.5 hours in this paper. The matrix crystal structure, Eu ion valence, photoluminescence spectra and fluorescence lifetime and quantum efficiency had been investigated. The matrix phase constituents change as γ-Ca2SiO4 (x=0)→T phase and γ-Ca2SiO4 mixture (0＜x＜0.7) →T phase (0.7≤x＜0.9)→Ba2SiO4(0.9≤x≤1) as Ba ion content increasing. By XRD analysis, it is known that (Ca1-xBax)2SiO4 powders form solid solution phases at the Ba-rich end, which are T-phase and Ba2SiO4-phase. Precision measurement of lattice parameters had been also done to T-phase (0.7≤x<0.9) and Ba2SiO4 phase (x≥0.9). For the former phase powder, the lattice parameters would increase because coordinated numbers for M1, M2, M5 sites are increasing as Ba ion content increasing, While, change from the latter one’s lattice parameters would be neglecting. Moreover, Eu ions enter into the crystal lattice by substituting for alkaline earth ions, with a minor impact. The surveys of X-ray photoelectron spectroscopy (XPS) spectra are similar, which all show characteristic electron binding energy peaks of Ba（3p3/2）, Ba（3d3/2）, Ba（3d5/2）, O（1s）, Eu（4d） and Si（2p3/2）. The high-resolution spectrum of O（1s） has two peaks, corresponding to lattice oxygen and interstitial oxygen defects (caused by Eu3+ substitution to alkaline earth ion+2), respectively. Moreover, the high-resolution XPS spectrums of the Eu（4d5/2） shows that the Eu2+/Eu3+ ratio would increase as the increasing of Ba ions in the T-phase powders, while the Eu2+/Eu3+ ratio of Ba2SiO4-phase powders is not obviously changed. Ultraviolet photoluminescence photographs show that Ca1.95SiO4∶0.05Eu (γ-Ca2SiO4 phase phosphors) could be used as red phosphors, while (Ca1-xBax)1.95SiO4∶0.05Eu (x≥0.7, T-phase (green emission centered at 455 nm) or Ba2SiO4-phase phosphors (green emission peak centered at 510 nm) could green phosphors. T-phase phosphors emission is blue shift comparing with Ba2SiO4 phase phosphor. Both T-phase and Ba2SiO4-phase phosphors are blue-shifted with the increase of x value. The brightest phosphor is (Ca0.1Ba0.9)1.95SiO4∶0.05Eu (fluorescence lifetime 571.8 ns, quantum efficiency 55%, which is the shortest lifetime and highest efficiency among the green phosphors). The high-resolution photoluminescence emission spectra of green phosphor (x≥0.7) show that that Eu2+ prefers more the site with 10 coordination than the site with 9 coordination as x value decreasing in Ba2SiO4-phase crystal (the Eu2+ activators with 10-coordination contribute more in green emission), but the site preference phenomenon is vague in the case of T-phase crystal. Overall, cation substitution (i. e. controlling the x value) should be a valid way to adjust the phase constituent, lattice, ion valence, photoluminescence CIE value and intensity for the phosphor.
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