Eu2+-activated Ba2ZnS3 has been reported as a red phosphor with a broad emission band peaking at 650 nm under blue excitation for white-LED. In this study, Ba2ZnS3:Eu2+, X– (X = F, Cl, Br, I) phosphors doped with halide ions were prepared by traditional high-temperature solid-state reaction. Phase identification of powders was performed by X-ray powder diffraction analysis, confirming the existence of single-phase Ba2ZnS3 crystals without dopant. The corresponding excitation spectra showed an additional broad band in the green region peaking at 550 nm when the phosphor was halogenated except by the smallest F–. It was proved that the green-excitation efficiency successively strengthened from Cl–, to Br–, to I–, which suggested larger halide ions made a greater contribution to the further splitting of the t2g energy level of the doped Eu2+ ions in the host Ba2ZnS3, and the optimized formula Ba1.995ZnS2.82:Eu2+0.005, I–0.18 showed a potential application in solar spectral conversion for agricultural greenhouse and solar cell. Defect chemistry theory and crystal field theory provided insights into the key role of halide ions in enhancing green-excitation efficiency.

A series of color-tunable Ce3+ single-doped and Ce3+, Mn2+ codoped Ca2.5Sr0.5Al2O6 phosphors were synthesized by a high-temperature solid-state reaction. The crystal structure, luminescent properties, and energy transfer were studied. For Ca2.5Sr0.5Al2O6:Ce3+ phosphors obtained with Al(OH)3 as the raw material, three emission profiles were observed. The peak of photoluminescence (PL) spectra excited at ∼360 nm shifts from 470 to 420 nm, while that of the PL spectra excited at 305 nm stays unchanged at 470 nm with the increase of Ce3+ content. Furthermore, the peak of PL spectra is situated at 500 nm under excitation at ∼400 nm. The relationship between the luminescent properties and crystal structure was studied in detail. Ce3+, Mn2+ codoped Ca2.5Sr0.5Al2O6 phosphors also showed interesting luminescent properties when focused on the PL spectra excited at 365 nm. Obvious different decreasing trends of blue and cyan emission components were observed in Ca2.5Sr0.5Al2O6:0.11Ce3+,xMn2+ phosphors with the increase in Mn2+ content, suggesting different energy transfer efficiencies from blue- and cyan-emitting Ce3+ to Mn2+. Phosphors with high color-rendering index (CRI) values are realized by adjusting the doping content of both Ce3+ and Mn2+. Studies suggest that the Ca2.5Sr0.5Al2O6:Ce3+,Mn2+ phosphor is a promising candidate for near UV-excited w-LEDs.

•The phosphor BaZnSiO4: Eu2+/Tm2+/Yb2+ can be prepared in air.•The self-reduction of RE3+ to RE2+ result from the defect chemistry at Ba2+ sites.•BaZnSiO4 can stabilize low-valence ions is due to its three-dimensional network.•The white-emitting phosphor BaZnSiO4:0.03Eu show a superior synthesis route.Nominal composition phosphors BaZnSiO4:RE (RE = Eu, Tm, Sm, Yb) were synthesized by the conventional solid-state method in air (oxidizing atmosphere, OA) and under 10%H2/90%N2 flow (reductive atmosphere, RA) conditions, respectively. Self-reduction phenomenon, which is a valence change from trivalent rare-earth ions (RE3+) to corresponding divalent state (RE2+) without reducing agent, were observed in the samples of Eu/Tm/Yb-doped BaZnSiO4 but have not observed in Sm-doped samples. Compared with RA condition, the ca. percent of the reductive concentration of Eu2+/Tm2+/Yb2+ in the phosphors BaZnSiO4 prepared in OA condition are 21%, 15% and 35%, respectively. It was found that both of BaZnSiO4:Eu and BaZnSiO4:Tm prepared in air show greatly broad band emission peak at 500 nm under the excitation of ultraviolet (UV), which indicated they are potential phosphors for UV-based LED. The factors which are related to self-reduction degree have been also discussed. The successful synthesis of BaZnSiO4:RE2+/3+ phosphors in air provides a new route to obtain other novel orthosilicate-based materials.The self-reduction ability of RE (RE = Eu/Tm/Yb/Sm) in BaZnSiO4-based phosphors prepared in air was compared and explained. The white-emitting of BaZnSiO4:0.03Eu prepared in oxidizing atmosphere shows a potential environmentally friendly route to obtain phosphor for UV-based LED.Download high-res image (174KB)Download full-size image

•Emission of Mn2+ in Ca5(BO3)3F:Ce3+, Mn2+ is very weak under excitation at 360 nm.•Introduction of RE3+ ions increases the amount of Mn2+ on Ca(2) site with 6 O atoms.•Introduction of RE3+ ions into Ca5(BO3)3F:Ce3+, Mn2+ enhances the emission of Mn2+.Ce3+, Mn2+ codoped and Ce3+, RE3+, Mn2+ (RE3+ = Tb3+, La3+, Gd3+, Lu3+) codoped Ca5(BO3)3F phosphors have been synthesized by a high-temperature solid-state reaction. Crystal structure and site occupancy of the doped ions were determined by Rietveld refinement. Photoluminescence (PL) spectra as well as fluorescence lifetimes were investigated. Under the excitation of 360 nm, Ca5(BO3)3F:Ce3+, Mn2+ phosphors show a strong emission band at 392 nm and a very weak emission band at ∼630 nm, which belong to Ce3+ and Mn2+, respectively. The introduction of Tb3+ and even non-luminescent ions such as La3+, Gd3+ and Lu3+ to the Ce3+, Mn2+ codoped phosphors leads to the enhancement of Mn2+ emission. The analyses of fluorescence lifetimes suggest that there is energy transfer from Ce3+ and especially Tb3+ to Mn2+. However, Rietveld refinement results indicate that the introduction of RE3+ ions leads to the adjustment of Mn2+ ions from Ca(3) site to Ca(2) site, which is the key factor for the enhancement of Mn2+ emission. The crystal-site engineering is expected to provide a promising route to tune the luminescent properties of phosphors with more than one site for activators.Figure optionsDownload full-size imageDownload as PowerPoint slide

Photoluminescence evolution in single-composition phosphors for solid state lighting can be realized through an energy transfer strategy, which is commonly based on one sensitizer and one activator. In this work, we focus on a dual energy transfer process, that is, energy transfers from the sensitizer Eu2+ at two different cation sites to the activator Mn2+ at another site in whitlockite-type β-Ca2.7Sr0.3(PO4)2. Firstly, we refined the crystal structure of the β-Ca2.7Sr0.3(PO4)2 host by a Rietveld method. Then we studied the phase, morphology and photoluminescence of Eu and Mn co-activated β-Ca2.7Sr0.3(PO4)2 phosphors, which are synthesized by a conventional solid-state reaction. By adjusting the ratio of Eu2+/Mn2+, the emission hue can be controlled from cyan (0.211, 0.281) to white-light (0.332, 0.290) and eventually to red (0.543, 0.276). The dual energy transfers from Eu2+ to Mn2+ are demonstrated to be dipole–dipole and quadrupole–quadrupole mechanisms, respectively. Moreover, white light-emitting diodes (LEDs) were fabricated through the integration of a 375 nm NUV chip and the white-emitting phosphor Ca2.7Sr0.3(PO4)2:0.008Eu2+, 0.03Mn2+ into a single package, which shows a warm white light with color coordinates of (0.40, 0.41) and correlated color temperature of 3731 K.

In this paper, Ce3+ doped and Ce3+, Tb3+ co-doped Ca5(BO3)3F phosphors were synthesized by a high-temperature solid-state reaction. Upon excitation at 360 nm, the emission spectra of Ce3+ doped phosphors exhibit a broad emission band peaking at 392 nm, which originates from the 5d to 4f transition of Ce3+. The Ce3+, Tb3+ co-doped phosphors show strong energy transfer from Ce3+ to Tb3+, and the emission color can be tuned from purplish blue to green by changing the Tb3+ content. The excitation band in the 300–400 nm region broadens when monitored at 541 nm compared to that monitored at 392 nm. Furthermore, the co-doping of Tb3+ facilitates the appearance of green emitting Ce3+, which originates from Ce3+ on the Ca site other than that for purplish-blue Ce3+. The relationship between the luminescence properties of Ce3+ and its coordination environments, namely, different Ca sites, is discussed based on the calculations of centroid shift and crystal field splitting of 5d energy levels of Ce3+. Results suggest that Ce3+, Tb3+ co-doped Ca5(BO3)3F phosphors may be a candidate for near-UV chip based white light-emitting diodes.

Many strategies, including double substitution, addition of charge compensation, cation-size-mismatch and neighboring-cation substitution, have contributed to tuning photoluminescence of phosphors for white light-emitting diodes. These strategies generally involve modification of a certain special site where the activator occupies; tuning strategy based on multiple cation sites is very rare and desirable. Here we report that isovalent (Sr2+) and aliovalent (Gd3+) substitutions for Ca2+ tune the photoluminescence from one band to multiple bands in whitlockite β-Ca3–xSrx(PO4)2:Eu2+ and β-Ca3–3y/7Gd2y/7(PO4)2:Eu2+ phosphors. The saltatory variation of the emission spectra is caused by the removal of Eu2+ from the site M(4) to other sites. Moreover, we found the mechanisms of dopant redistribution tuning the luminescence are different. The incorporation of Gd3+ makes the site M(4) empty according to the scheme 3Ca2+ = 2Gd3+ + □, while Sr2+ substitution causes the cation sites to be enlarged due to cation size mismatch. Additionally, the influence of the cation substitutions on the photoluminescence thermal stability of phosphors is researched. The strategies, emptying and enlarging sites, developed herein are expected to provide a general route for tuning luminescence of phosphors with multiple sites in the future.