Ba 1.8−x Sr x SiO 4 :0.1Ce 3+ ,0.1Na + (x = 0−1.8) phosphors were prepared by a high-temperature solid-state reaction. The emission peaks of Ba 1.8−x Sr x SiO 4 :0.1Ce 3+ ,0.1Na + shift from 391 to 411 nm with increasing Sr 2+ content under excitation by a UV light at around 360 nm. Ba 0.4 Sr 1.4 SiO 4 :0.1Ce 3+ ,0.1Na + phosphor exhibits the best performance of luminescence, whose absolute quantum efficiency is 97.2%, and the emission intensity at 150 °C remains 90% of that at room temperature. The effect of replacing Ba 2+ by Sr 2+ on the red shift of the emission band and the increase of quantum efficiency (QE) and thermal stability (TS) was investigated in detail based on the Rietveld refinements, Raman spectra, thermoluminescence, and decay curves, etc. The performance of UV chip-based pc-LEDs indicates that Ba 0.4 Sr 1.4 SiO 4 :0.1Ce 3+ ,0.1Na + can be a promising blue phosphor for white-emitting pc-LEDs.
Photoluminescence quantum efficiency (QE) and thermal stability are important for phosphors used in phosphor-converted light-emitting diodes (pc-LEDs). LiSrCa(SiO):0.03Ce (-0.7 ≤ x ≤ 1.0) phosphors were designed from the initial model of LiSrCa(SiO):Ce, and their single-phased crystal structures were found to be located in the composition range of -0.4 ≤ x ≤ 0.7. Depending on the substitution of Sr for Ca ions, the absolute QE value of blue-emitting composition-optimized LiSrCa(SiO):0.03Ce reaches ∼94%, and the emission intensity at 200 °C remains 95% of that at room temperature. Rietveld refinements and Raman spectral analyses suggest the increase of crystal rigidity, increase of force constant in CeO, and decrease of vibrational frequency by increasing Sr content, which are responsible for the enhanced quantum efficiency and thermal stability. The present study points to a new strategy for future development of the pc-LEDs phosphors based on local structures correlation via composition screening.
Intrinsic defect-related luminescence has recently been attracting more research interest for the modification of phosphors. However, the connection between defect formation and crystal structure has never been considered. In this work, we report that in the absence of an impurity activator, under a reducing atmosphere, apatite-type compound M(PO)X (M = Ca, Sr, or Ba; X = F, Cl, or Br) can emit tunable colors ranging from blue to orange depending on the content of M and X. To better understand the cause, BaSr (PO)Br (BSPOB; m = 0-5) solid solutions were analyzed in detail. The dependency of self-activated luminescence on atmospheric conditions and solid solution compositions was investigated by combining experimental characterizations and theoretical calculations using density functional theory. Crystal structures of these solid solutions were verified by X-ray diffraction patterns as well as Rietveld refinements. With the defect formation energy and electron paramagnetic resonance measurement, we propose that an oxygen vacancy (V) should be mainly responsible for the peculiar super wide band emission. Moreover, the enhanced distortion of solid solution crystal structures augments V concentrations and leads to luminescence intensities in solid solutions that are higher than that in end point compounds. Variations of the electronic structure of BSPOB matrices with gradual tuning of the Sr/Ba ratio were also investigated. As a result, the introduction of V defect levels within the band gap leads to the formation of donors and acceptors, allowing for a modulation of the photoluminescence throughout the visible part of the spectrum. As the first report in the literature to demonstrate fine-tunable emissions over a wide wavelength range as a consequence of native defective levels in a series of continuous apatite-type solid solutions, our results illustrate the feasibility of defect-meditated systems by carefully tailoring defect chemistry and nonstoichiometric chemical composition under controlled conditions to engineer phosphor properties.
A single‐phase multicolor emitting phosphor, Ca3Al2O6:Ce3+,Li+, was prepared by a solid‐state reaction. When the Ce3+ concentration is lower than 0.030 (molar ratio in Ca3Al2O6), yellow and greenish blue emissions can be observed under the excitation by a blue and a near UV light, respectively. The yellow‐emitting phosphor possesses an internal quantum efficiency of 89%. Additional purplish blue emission turns up when Ce3+ concentrations are higher than 0.040. Tunable emission bands are originated from Ce3+ ions on different Ca sites in Ca3Al2O6. Although the emission band of purplish blue or greenish blue overlaps the excitation band of yellow emission, and the distances between the unlike Ce3+ ions are in the range of electric dipole–dipole interaction, no energy transfer is observed. Furthermore, emission wavelengths for the yellow, greenish blue, and purplish blue emission show little change upon increasing Ce3+ concentrations.
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