Cation Substitution (Sr2+â Ba2+, Al3+â Si4+, N3

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Influence of Anion/Cation Substitution (Sr2+ # Ba2+, Al3+ # Si4+, N3- # O2-) on Phase Transformation and Luminescence Properties of Ba3Si6O15:Eu2+ Phosphors Mengmeng Shang, Sisi Liang, Nianrui Qu, Hongzhou Lian, and Jun Lin Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b05493 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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Chemistry of Materials

Influence of Anion/Cation Substitution (Sr2+ → Ba2+, Al3+ → Si4+, N3− → O2−) on Phase Transformation and Luminescence Properties of Ba3Si6O15:Eu2+ Phosphors Mengmeng Shang,a Sisi Liang,a,b Nianrui Qu,c Hongzhou Lian,a and Jun Lin*a a

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China b

University of Science and Technology of China, Hefei 230026, P. R. China

c

Key Laboratory of Applied Chemistry, Yanshan Universty, Qinhuangdao, 066004, P. R. China

ABSTRACT: A series of promising cyan, green, and yellow emission (Ba, Sr)3(Si, Al)6(O, N)15:Eu2+ phosphors were synthesized by a Pechini-type sol−gel ammonolysis method. Variations in luminescence properties and crystal structure caused by the modification of phosphor composition were studied in detail. The prefired temperatures of the precursors play a key role in the process of forming the final products. Under UV light excitation, the as-prepared Ba3Si6O15:Eu2+ phosphor presents a strong cyan-emitting located at 498 nm. Moreover, the as-prepared oxynitride phosphors, Eu2+-activated (Ba1ySry)3Si6-xAlxO15-µNδ (x = 0-1.2, y = 0-0.6), display a broader excitation band covering the entire visible region. Under blue light excitation, Ba3Si6-xAlxO15-µNδ:Eu2+ phosphors show a intense and narrow green emission at 520 nm and the luminescent intensity can be enhanced by increasing Al content within a certain range. However, (Ba1-ySry)3Si6O152+ 2+ µNδ:Eu phosphors exhibit green (520 nm) to yellow (554 nm) emission with increasing the Sr content. Unexpectedly, Eu doped Ba3Si6O9N4-type Ba3Si6O15-µNδ–1300 °C phosphor exhibits a bluish green emission and a strong thermal quenching behavior. The (Ba1-ySry)3Si6-xAlxO15-µNδ:Eu2+ phosphors exhibit a small thermal quenching and the quantum yields measured under 460 nm excitation could reach up to 89% for green Ba3Si6-xAlxO15-µNδ:Eu2+ phosphor and 71% for yellow (Ba12+ ySry)3Si6xO15-µNδ:Eu phosphor. White LEDs with tunable color temperature and higher color rendering index were fabricated by combining the prepared cyan Ba3Si6O15:Eu2+/green Ba2.91Eu0.09Si6-xAlxO15-µNδ (x = 0.06)/yellow (Ba0.97ySry)3Eu0.09Si6O15-µNδ( y = 0.4) phosphor and a red phosphor with a UV or blue LED chip, indicating that they are promising phosphors for white LEDs.

1 INTRODUCTION Currently, phosphor-conversion white LEDs are considered to be the new generation lighting sources owing to the energy efficiency, high efficiency, environmentally friendliness and so on.1-4 The common white LEDs are manufactured by combination of a blue GaInN LED chip with a yellow Y3Al5O12:Ce3+ (YAG:Ce) phosphor. As we all know, the YAG:Ce phosphor shows an excellent chemical stability and high luminescence efficiency excited with the blue light. However, there is an insurmountable problem that the white LEDs fabricated with YAG:Ce have a lower color rendering index (Ra < 80) because of lacking red components in the emission spectrum of YAG:Ce phosphor. In recent years, in order to obtain high-quality white LEDs (Ra ＞80), a new white LEDs that are fabricated with n-UV or blue chips and color-tunable phos-

phors has been rapidly developed. Consequently, the highly efficient green (e.g. (Ba, Sr)2SiO4:Eu2+ 5-6 and β7-8 SiAlON:Eu2+ ), yellow (e.g. CaAlSiN3:Ce3+,9 3+ 10 2+ SrAlSi4N7:Ce ) and red (e.g. Eu -doped red (Ca, Sr, Ba)2Si5N8 11-12 and CaAlSiN3 13-15) phosphors have been developed with the purpose of improving the color rendering index of general lighting and color reproducibility of liquid crystal display (LCD) backlight. Especially, Eu2+ doped M2Si5N8 and CaAlSiN3 red phosphors, Ca-aSiAlON:Eu2+ yellow phosphor,16 and β-SiAlON:Eu2+ green phosphor have been applied to the market. It is noteworthy that the use of traditional high-temperature solidstate method to synthesize of (oxygen) nitride phosphors still has many difficulties. Solid state reaction conditions are harsh, and usually require higher temperature and pressure, and longer calcination time. For the sake of obtaining the fine powder, it is usually necessary to further

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grind after calcinations because of the hard agglomerates caused by the high calcination temperature. Therefore, seeking alternative synthesis methods and studying the corresponding luminescence properties become particularly important. Recently substitution strategy has been used as a general way for the photoluminescence tuning.17-18 The silicate phosphors have been researched for many years due to their good stability, easy to synthesize, and low production cost.19-20 Moreover, one can easily achieve color tunable silicate-phosphors through modifying their chemical composition and crystal structure. For instance, the Ba2SiO4:Eu2+ phosphor exhibits green emission; When the Ba2+ ion is partially substituted with the Sr2+ ion, SrxBa2−xSiO4:Eu2+ phosphor gradually emits green into orange-yellow with increasing the Sr content.21 Recently, it is shown that the substitution of O with N not only affects the luminescence behavior but also causes the phase transformation in the SrxBa2−xSi(O,N)4:Eu2+ 22 and Ca2Si(O, N)4:Ce3+ 23 phosphors. So in this paper, we chose the silicate (Ba3Si6O15) as the luminescent material host and studied the influence of substitution of anion / cation (Sr2+ → Ba2+, Al3+ → Si4+, N3− → O2−) on the phase transformation and luminescence properties of (Ba, Sr)3(Si, Al)6(O, N)15:Eu2+ phosphors for the first time. Different from conventional solid state reaction using expensive Si3N4 as the N source, the (Ba, Sr)3(Si, Al)6(O, N)15:Eu2+ phosphors in our work are synthesized through the Pechini–type sol–gel reaction, combining with the calcination of the precursors in ammonia atmosphere. Compared with the traditional solid state reaction method, the product obtained by Pechini–type sol–gel method has uniform morphology and no further ground. In particular, pure Ba3Si6OxNy:Eu2+ with (x, y) = (12, 2), (9, 4) phosphors were obtained by this method, which are usually synthesized by SSR.24-28 Our study shows that the method sol-gel combined with ammonia nitriding is convenient to prepare oxynitrides, in which the ammonia can simultaneously play the role of nitriding and reduction. The photoluminescence properties of the as-prepared phosphors were studied according to the photoluminescence spectra, thermal quenching, quantum efficiency, and so on. Finally, we demonstrated the application of the cyan Ba3Si6O15:Eu2+, green Ba3Si6-xAlxO15-µNδ:Eu2+ and yellow (Ba1-ySry)3Si6O15-µNδ:Eu2+ phosphors in white LEDs. All the results indicate that these phosphors can be suitable candidates for making n-UV or blue-based white LEDs. 2 MATERIALS AND EXPERIMENTAL PROCEDURES 2.1 Preparation. The Eu2+-activated (Ba1-ySry)3Si6-xAlxO15µNδ (0 ≤ x ≤ 1.2, 0 ≤ y ≤ 0.6, and the value of µ, δ will be fixed in the next part) samples were prepared by the Pechini−type sol−gel method combining with the calcination of the precursors in ammonia atmosphere. Stoichiometric amounts of Eu(NO3)3, Ba(NO3)2, Sr(NO3)2 and Al(NO3)3·9H2O were dissolved in deionized water with stirring for 15 min. Later, citric acid was added as a complexing agent in 1:2 ratio (metal to citric acid) and was

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capped and stirred for 30 min more. Then, the pH of the solution was adjusted to ∼1 with HNO3. In next, a stoichiometric amount of tetraethyl orthosilicate was added. Finally, a certain amount of poly(ethylene glycol) (PEG; molecular weight = 20000) with stirring for several minutes. The resultant mixtures were stirred for 2 h and then held in a water bath maintained at 75 °C until homogeneous gels formed. The final gel was collected and dried in the oven at 100 °C for 10 h and then was prefired at 500-800 °C for 4 h in air in the muffle furnace. The obtained precursors were put into a corundum crucible and subsequently kept at 1200−1300 °C under a NH3 flow rate of 0.5−1.5 L·min-1 for 12−36 h. Finally, the as-synthesized samples were slowly cooled to room temperature inside the tube furnace under NH3 atmosphere. 2.2 Characterization. The X-ray diffraction (XRD) measurements were carried out on a D8 Focus diffractometer using Cu Kα radiation (λ = 0.15405 nm), operating at 40 kV and 40mA in the 2θ range of 10°−100°. Structure refinement was done using the GSAS program.29 Scanning electron microscopy (SEM) micrographs, energydispersive X-ray (EDX) spectra and the element distribution mapping images were obtained using a fieldemission scanning electron microscope (Philips XL30). Fourier transform infrared (FTIR) spectra were measured on a Vertex Perkin-Elmer 580 BIR spectrophotometer (Bruker) with the KBr pellet technique. Thermogravimetry analysis (TGA) was carried out on a Netzsch STA 449F3 thermoanalyzer with a heating rate of 10 °C/min in air atmospheres. Diffuse reflectance spectra were recorded on a Hitachi U-4100 UV-Vis-NIR spectrophotometer using BaSO4 as a reference. Photoluminescence (PL) measurements were performed on a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The temperature-dependent (0−300 °C) PL spectra were obtained on a fluorescence spectrophotometer equipped with a 450 W xenon lamp as the excitation source (Edinburgh Instruments FLSP-920) with a temperature controller. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 digital osilloscope (1 GHz) using a tunable laser (pulse width = 4 ns; gate = 50 ns) as the excitation (Contimuum Sunlite OPO). Photoluminescence absolute quantum yields (QY) were measured using an absolute PL quantum yield measurement system (C9920-02, Hamamatsu Photonics K. K., Japan) with a 150 W Xe lamp as an excitation source, a monochromator, a barium sulfate coated integrating sphere and a CCD spectrometer for detecting the whole spectral range simultaneously. The electronic structure (band structure) is calculated by Vienna Ab Initio Simulation Package (VASP),30-33 based on the projector augmented-wave (PAW) method.34-35 The exchangecorrelation energy was treated by the generalized gradient approximation (GGA-PBE).36 The plane wave cut-off energy was chosen to be 500 eV. For the self-consistent field iterations, the meshes of 9×5×3, 6×6×6, 4×4×5 k-points are used for BaSi2O5, Ba3Si6N9O4 and Ba3Si6N12O2, respec-


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tively. The convergence tolerances are selected as the difference in total energy and the maximum force being within 1.0×10-5 eV and 1.0×10-2 eV/Å, respectively. The performance of the WLEDs was measured by Starspec SSP6612.

precursor at 1200 °C for 12 h under NH3 atmosphere; when using 500 °C–prefired sample as precursor, a mesophase Ba3Si6O15-µNδ–1200 °C and a pure Ba3Si6O9N4 phase were

3 RESULTS AND DISCUSSION 3.1 Ba3Si6(O, N)15:Eu2+ 3.1.1 Crystallization Conditions and Phase Identification. Figure S1 gives the XRD profiles of the precursors prefired at 500−800 °C. It can be observed that the precursor prefired at 500 °C is composed of BaO and SiO2 besides un-decomposed organic matters from the raw materials. However, after prefired at 800 °C, the precursor is the mixture of BaSiO3, Ba2SiO4 and SiO2. The next experiment indicates that the prefired temperatures of the precursors play a key role in the process of forming the final products. We put the 800°C-precursor into a corundum crucible and then calclined them at 1200 °C under a NH3 flow rate of 0.5 L·min-1 for 12 h. Finally, the white powder samples are obtained. Figure 1 presents the XRD patterns of the representative products. As displayed in Figure 1, all the diffraction peaks of the samples doped with and without Eu2+ ion can be ascribed to a pure rhombohedral Ba3Si6O15 phase (BaSi2O5, JCPDS No. 720171). No detectable XRD peaks corresponding to impurity phases are observed and Eu2+ ions have been successfully incorporated into the Ba3Si6O15 crystal lattice. However, when we used the 500 °C-prefired samples as the precursors and calclined them at 1200−1300 °C under a NH3 flow rate of 0.5 L·min-1 for 12 h, the completely different products were obtained. Figure S2 compares with the XRD patterns of the products calcined at 1200 °C using the different precursors. It can be seen that the 1200 °C–product using 500°C–prefired precursor forms a solid solution with compostion of Ba3Si6O15-µNδ, which may be a mesophase of Ba3Si6O15 (JCPDS No. 72-0171) and Ba3Si6O12N2 [ref. 27]. In order to obtain pure Ba3Si6O12N2 phase, we prepared a series of samples using 500°Cprefired precursor through changing the calcined time, temperature and the NH3 flow rate. The XRD patterns of the corresponding products are shown in Figures S3 and S4. As seen from Figure S3, niether prolonging the annealing time nor increasing the NH3 flow rate contribute the formation of the Ba3Si6O12N2 phase. Interestingly, as shown in Figure S4, we get a new phase when raising the annealing temperature to 1250 °C and 1300 °C. Figure 2 gives the XRD patterns of the products using 500°C– prefired precursor calcined at 1200 °C and 1300°C, respectively. The XRD patterns of Ba3Si6O15, Ba3Si6O12N2[ref. 27], and Ba3Si6O9N4 [ref. 37] are shown for comparison. As presented in Figure 2, the XRD pattern of the product calcined at 1300°C (Ba3Si6O15-µNδ–1300 °C) is different from that of the product calcined at 1200°C (Ba3Si6O15-µNδ–1200 °C) but consistent with the diffraction peaks of the trigonal Ba3Si6O9N4. Based on the results of the XRD characterization, we make a conclusion that the Ba3Si6O15 phase was obtained by calcining the 800 °C–prefired

Figure 1. XRD patterns of the representative product prepared by using 800 °C-prefired precursor. As a reference, the standard XRD pattern for Ba3Si6O15 (JCPDS No. 72-0171) is shown.

Figure 2. XRD patterns of the products using 500°Cprefired precursor calcined at 1200 °C and 1300°C, respectively. The XRD patterns of Ba3Si6O15, Ba3Si6O12N2, and Ba3Si6O9N4 are shown for comparison. obtained by calclined at 1200 °C and 1300 °C for 12 h under a NH3 flow rate of 0.5 L·min-1, respectively. This is to say, NH3 only plays a role of reduction in the formation process of Ba3Si6O15:Eu2+ phosphor, in which Eu3+ is reduced to Eu2+. Whereas, for the formation of Eu2+ activitatedoxynitrides (Ba3Si6O15-µNδ–1200 °C and Ba3Si6O9N4), NH3 not only plays a role of reduction but also provides a source of nitrogen. Moreover, it can be concluded that the prefired temperature affects the formation of the



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Figure 3. (a) The SEM images and the corresponding element distribution mapping; the EDX spectra of (b) Ba3Si6O15, (c) Ba3Si6O15-µNδ–1200 °C and (d) Ba3Si6O15-µNδ–1300 °C. crystalline phase, and the lower prefired temperature is favorable for the formation of oxynitrides. As discussed above (Figure S1), the precursor prefired at 500 °C is composed of BaO and SiO2. Whereas, after prefired at 800 °C, the silicate phases (BaSiO3 and Ba2SiO4) have been crystallized. When the precursor were treated at 1200 °C/1300 °C temperature under NH3 atmosphere, the oxide-precursor is easier to react with ammonia. The reaction can be shown as follows: 3BaO + 6SiO2 + NH3 → Ba3Si6O15-µNδ + H2O However, the crystallized silicates may be not easy to ammonolysis, and they will react with each other, as expressed bellow38: Ba2SiO4 + SiO2 = 2BaSiO3 2BaSiO3 + SiO2 = Ba2Si3O8 3Ba2Si3O8 + 3SiO2 = 2Ba3Si6O15 Similar phenomenon was observed by Yoon et al. in 2012.39 In their study, Ba3Si6O12N2:Eu2+ phosphor was synthesized by liquid phase precursor method. They observed that the product prefired at 500 °C was a pure Ba3Si6O12N2:Eu2+ phase, but the product prefired at 600 °C showed an unknown phase. So, they conclude that the pre-calcination temperature plays an important part in the process of forming the crystalline phase. However, the related explanation and discussion were not given in their study. To further validate whether the desired samples have formed or not, FTIR spectra characterization was performanced. Figure S5 shows the FTIR spectra for the 500°Cprecursor, Ba3Si6O15, Ba3Si6O15-µNδ–1200 °C Ba3Si6O15-µNδ– 1300 °C samples. The absorption band near 3440 cm–1 for

all samples could be attributed to the stretching vibration of O–H bonds.40 However, there is a large difference in the spectra below 2000 cm–1 for these samples. For the 500°C–precursor, the absorption band at 1498 cm–1 can be assined to the organic impurities from the starting materials (citric acid and PEG).41 The absorption bands of Si– O–Si (υas, 1062 cm–1; υs, 812 cm–1), and Si–O (δ, 462 cm–1) bonds (υas = asymmetric stretching, υs = symmetric stretching, δ = bending) are also observed.42 The FTIR spectra of the Ba3Si6O15-µNδ–1200 °C/1300 °C samples exhibits many extra absorption peaks at 1099, 1091, and 890 cm–1, etc, compared with the Ba3Si6O15 sample. These peaks may be from the stretching of Si–O(N) and Ba– O(N) bonds in the Ba3Si6O15-µNδ–1200 °C/1300 °C samples. The result provides the evidence that the chemical compositions of Ba3Si6O15-µNδ–1200 °C/1300 °C samples are quite different from the Ba3Si6O15 sample. Fiure 3a presents the energy dispersive X-ray mapping of the Ba3Si6O15-µNδ–1200 °C sample with the purpose of further substantiate the composition and uniform distribution of elements. Seen from Figure 3a, the sample is comprised of Ba, Si, O and N, and the distribution of the elements is homogeneous. The EDX results presented in Figure 3b-d demonstrate that nearly stoichiometric Ba3Si6O15 (Figure 3b) and Ba3Si6O9N4 (Figure 3d) is obtained. However, the Ba3Si6O15-µNδ–1200 °C sample (Figure 3b) has the atomic ratio of Ba: Si: O: N = 3: 5.47: 13.63: 1.10. EDX analysis is consistent with the XRD result and provides strong evidences for the formation of the desired products. For the sake of examining the stability and further determining the as-prepared samples composition, the TGA


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Figure 4. Rietveld refinement XRD patterns of un-doped Ba3Si6O15 (a), Ba3Si6O15-µNδ–1200 °C (b) and Ba3Si6O15-µNδ–1300 °C (c). Observed, calculated, background and difference signal are plotted. The short vertical lines represent the positions of the Bragg reflections for the main and secondary phases; the crystal structure and coordination enviroments of Ba2+ ions for Ba3Si6O15 (d), Ba3Si6O12N2 (e) and Ba3Si6O9N4 (f). characterization was carried out in air and nitrogen, respectively. Figure S6 presents the TGA analysis carried out in air of the Ba3Si6O15-µNδ–1200 °C and Ba3Si6O15-µNδ– 1300 °C samples. The nitrogen contents can be also determined by thermogravimetric analysis. The TGA curve measured in air reveals that below 800 °C, the weight of the products have no obvious change. So, we can conclude that the as-prepared oxynitride samples have impressive chemical stability and antioxidation property. From 800 °C to 1200 °C, the sample weight increases gradually by about 1.3% and 4.5% for Ba3Si6O15-µNδ–1200 °C and Ba3Si6O15-µNδ–1300 °C, respectively. Since the chemical valence state for Ba2+ and Si4+ ions in the Ba3Si6O15-µNδ host are unchanged, Ba3Si6O15-µNδ can finally be oxidized to

Ba3Si6O15 (in form) in air when the temperature is higher than 800 °C. So, the nitrogen contents (δ value) are determined by the weight increment to be 1.1 and 3.6 for Ba3Si6O15-µNδ–1200 °C and Ba3Si6O15-µNδ–1300 °C, respectively. Thus, we can further confirm that the composition of the oxynitride samples is Ba3Si6O13.35N1.1 and Ba3Si6O9.6N3.6 for Ba3Si6O15-µNδ–1200 °C and Ba3Si6O15-µNδ– 1300 °C, respectively. It is worthy to note that the oxygen stoichiometries are calculated based on charge balancing. This experimental value matches well with the EDX results. The weight of the sample is almost uniformity under nitrogen during the process of heating up, which is not shown here. Therefore, it is confirmed the formation of the oxynitride product by ammonolysis method.



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To further identify the phase and crystal structure of the as-prepared samples, the crystal structure refinements of the representative Ba3Si6O15, Ba3Si6O15-µNδ–1200 °C and Ba3Si6O15-µNδ–1300 °C host were performed using the general structure analysis system (GSAS) method. The refinement patterns of the samples and the crystal structure are presented in Figure 4. In the refinement process, the BaSi2O5 (ICSD–10162) structure, and Ba3Si6O9N4 (ICSD– 415918) structure were utilized as the original model for Ba3Si6O15 and Ba3Si6O15-µNδ–1300 °C samples, respectively. Figures 4a and 4c demonstrate that all atom coordinates, fraction factors, and thermal vibration parameters were well adapted under the reflection conditions. The impurity phases were not observed at the current detection level. The parameters for the refined crystallography data are given in Table S1. However, the crystal structure of Ba3Si6O15-µNδ–1200 °C was analyzed using the BaSi2O5 (ICSD–10162) and Ba3Si6O12N2 [ref. 27] structures as starting models for refinement. The results, shown in Figure 4b, indicate that a predominant Ba3Si6O12N2 (61%) phase is formed accompanying with a small portion (39%) of the Ba3Si6O15 phase. So the formula of the Ba3Si6O15-µNδ–1200 °C sample can be calculated from the refinement result to be Ba3Si6O13.17N1.22, which is well consistent with the results of EDX and TGA. Figure 4d-f presents the crystal structures of Ba3Si6O15, Ba3Si6O12N2 and Ba3Si6O9N4, and the coordination environment of Ba2+ ions in the corresponding crystal structures, respectively. The Ba3Si6O15 is crystallized in orthorhombic with the space group of Pcmn. The SiO4 tetrahedra shared the corner forms corrugated layers, and the Ba2+ ions are located between the layers. The Ba2+ ions in Ba3Si6O15 have only one crystallographic site coordinated with nine oxygen atoms. For the Ba3Si6O12N2 and Ba3Si6O9N4 phases, they belong to the trigonal crystal system. The Ba3Si6O12N2 compound is composed by corrugated layers formed by SiO3N tetrahedra. Because of the addition of N elements, as shown in Figure 4e, there are two Ba2+ ions crystallographic sites: one is coordinated with six oxygen atoms, and the other is coordinated with six oxygen atoms and a nitrogen atom.27 The crystal structure and chemical formula of Ba3Si6O9N4 compound appears close to Ba3Si6O12N2. The compound is also constructed by corner sharing SiO2N2 tetrahedral (Figure 4f). Different from Ba3Si6O12N2, the Ba2+ ions fill three crystallographic sites: two of them are coordinated with six oxygen atoms with different bond lengths; the other is connected with six oxygen atoms and one nitrogen atom.37 These Ba2+ crystallographic sites in Ba3Si6O9N4 is similar to those in Ba3Si6O12N2. But there is an important difference between them: the Ba-N distance is about 3.156 Å in Ba3Si6O9N4 whereas about 2.968 Å in Ba3Si6O12N2. All the structure features will affect their corresponding luminescent properties. 3.1.2 Photoluminescence properties at room temperature. The Eu2+–doped Ba3Si6(O, N)15 phosphor exhibits a broad excitation band covering the entire visible light region. So, it can match well with the LED chips. For the

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Ba3Si6O15:Eu2+ phosphor, the sample exhibits white powder under day light whereas bright cyan emission under 365 nm UV lamp. Figure 5a presents the PL excitation and emission spectra of the typical Ba3Si6O15:Eu2+ phosphor. Since the host absorption is below 300 nm, as shown in Figure S7, the broad absorption band from 200 to 400 nm in the excitation spectrum is deriving from the allowed 4f7→5d transition of Eu2+. Upon 340 nm UV excitation, the emission spectrum presents a symmetric broad band extending from 400 to 600 nm with peak at about 498 nm, which is ascribed to the Eu2+ 4f65d1→4f7 allowed transition.43 The full width at half maximum (FWHM) is about 104 nm. The dependence of emission intensity for Ba3-mSi6O15:mEu2+ phosphors on Eu2+ concentration (m) are presented in Figure 5b. With increasing Eu2+ concentration, the spectral shape and the emission peak position no change (not shown here). However, the emission intensity at 498 nm enhances until m = 0.09. After that, the Eu2+ emission intensity decreases gradually because of the concentration quenching effect induced by the energy transfer between Eu2+ ions. The greater the doping Eu2+ concentration, the shorter the distance between Eu2+ ions. As a result, the probability of energy transfer between Eu2+ increases. Because the overlap between the excitation and emission spectra is small (as shown in Figure 5a), the concentration quenching caused by reabsorption effect can be neglectted. The following eqn is usually used to calculate the critical distance (Rc) for occurring energy transfer44

 3V   Rc = 2  4πxc N 

1

3

(1)

where xc is the optimal Eu2+ doping concentration, V represents the volume of the unit cell, and N is the number of cations that can be replaced by Eu2+ ions. For Ba32+ (namely Ba(3-m)/3Si2O5:m/3Eu2+), N = 4. TakmSi6O15:mEu ing xc = 0.09/3, V = 315.58 Å3 in eqn (1), the Rc is then computed to be about 17.13 Å. Another important parameter for measuring phosphor performance is the quantum yield. The absolute quantum yield (ηQY) is measured and calculated by the following eqn(2)45-46:

η QY =

∫ LS ∫ ER − ∫ ES (2)

where LS means the emission spectrum; ER is the spectrum of the excitation light from the empty integrated sphere (without the sample); ES means the excitation spectrum for exciting the sample. The absolute quantum yield of Ba3-mSi6O15:mEu2+ samples under 340 nm and 365 nm wavelength excitation is listed in Table S2. The change trend of the absolute quantum yield is basically consistent with the emission intensity. For m = 0.09 and 0.10 samples, the ηQY (λex = 340 nm) can reach up to 91%


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Figure 5. (a) PL excitation and emission spectra of asprepared Ba3Si6O15:Eu2+ sample. (b) The emission intensity as a fuction of Eu2+ concentration.

1200 °C in theory. However, an unexpected result appears. Figure 6a and 6b presents PL excitation and emission spectra of as-prepared Ba2.91Eu0.09Si6O15-µNδ–1200 °C and Ba2.91Eu0.09Si6O15-µNδ–1300 °C samples, respectively. It is seen from Figure 6a that the Ba2.91Eu0.09Si6O15-µNδ–1200 °C phosphor exhibits a broad excitation band which is extending to 500 nm. Similar to Ba3Si6O15:Eu2+ phosphor, the broad absorption band is also from the 4f→5d transition of Eu2+ as a result of the host absorption below 300 nm (Figure S7). Excited with near-UV or blue light, a green emission band with peak at 520 nm is observed Its FWHM (= 72 nm) is smaller than the promising phosphors such as Sr2.975-xBaxCe0.025AlO4F (110 nm)47 and SrSi2O2N2:Eu2+ (82 nm)48, indicating the high color saturation of Ba2.91Eu0.09Si6O15-µNδ–1200 °C phosphor. In order to appraise the thermal stability, we calculated the Stokes shift (ΔS). Compared with the a-sialon:Eu2+ of 7000–8000 cm-1 49, the ΔS of Ba2.91Eu0.09Si6O15-µNδ–1200 °C demonstrates a relatively small value of 5000 cm-1. These suggest that Ba2.91Eu0.09Si6O15-µNδ–1200 °C is a potential green phosphor for white LEDs. As discussed above, the Ba2.91Eu0.09Si6O15-µNδ–1200 °C phosphor with chemical formula of Ba2.91Eu0.09Si6O13.35N1.1, is a solid solution of the Ba3Si6O15 and Ba3Si6O12N2 compounds. So there would be two Ba2+ ions crystallographic sites in Ba3Si6O13.35N1.1 like Ba3Si6O12N2: one coordinated with six oxygen atoms, the other coordinated with six oxygen atoms and one nitrogen atom (seen in Figure 4e). On the basis of the crystal field splitting (CFS) theory, the following equation 50-51

1 r4 Dq = Ze2 5 6 R

Figure 6. PL excitation and emission spectra of asprepared (a) Ba2.91Eu0.09Si6O15-µNδ–1200 °C and (b) Ba2.91Eu0.09Si6O15-µNδ–1300 °C sample. and 96%, respectively. The ηQY still reach 63% and 75% even if they are excited with 365 nm wavelength. Combined with the optimal emission intensity and quantum efficiency, the doping concentration of Eu2+ is fixed to be m = 0.09 in the Ba3-mSi6(O, N)15:mEu2+ samples in the subsequent experiment. As we all know, the position of the 5d energy level of Eu2+ ions is strongly affected by the nephelauxetic effect and crystal-field splitting. Since the N atom has less electronegative and more polarizable than O atom, the nitrogen-metal bonds can lower the energy of Eu2+ 5d level. So, the Ba2.91Eu0.09Si6O15-µNδ–1200 °C and Ba2.91Eu0.09Si6O15-µNδ– 1300 °C phosphors are expected to exhibit a redshift emission than Ba2.91Eu0.09Si6O15 under UV light excitation. Furthermore, due to having more N atoms in the Ba2.91Eu0.09Si6O15-µNδ–1300 °C phosphor, it should exhibit a longer emission wavelength than Ba2.91Eu0.09Si6O15-µNδ–

can demonstrate the crystal field acting on activators. Dq is a measure of the energy level separation, Z is the anion charge, e is the electron charge, r is the radius of the d wavefunction, and R is the bond length. Therefore, two different Eu2+ sites generate two different emission bands. The two different emission bands overlapped with each other, leading to a broad asymmetric emission band.52-53 However, as seen in Figure 6a, the Ba2.91Eu0.09Si6O15-µNδ– 1200 °C phosphor exhibits a symmetric emission band. Compared with the emission spectrum of the Ba2.91Eu0.09Si6O15 phosphor, its emission peak shifts to longer wavelength (from 498 nm to 520 nm). As discussed earlier, Eu-N ligand would lead to red shift due to the more covalent, which indicates that Eu2+ mainly occupied the Ba2 site (shown in Figure 4e). This result coincides with with the Mikami et al.’s report about the photoluminescence properties of Ba3Si6O12N2:Eu2+, 27 whereas, Braun suggested Eu2+ primarily occupying the Ba1 site.54 It is worth mentioning that the emission peak of Ba2.91Eu0.09Si6O15-µNδ–1200 °C shifts toward shorter wavelength (λem = 520 nm) as well as broadened (FWHM = 72 nm) compared with the Ba3Si6O12N2:Eu2+ (λem ≈ 525-530 nm, FWHM ≈ 68 nm).24, 27, 54 This is because the Ba2.91Eu0.09Si6O15-µNδ–1200 °C phosphor is composed of major Ba3Si6O12N2 phase and minor Ba3Si6O15 phase (as



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shown in Figures 2 and 4b). As a consequence, the emission peak of the phosphor exhibits blue shift and a broadened FWHM because of the overlap of emission bands of Ba3Si6O12N2: Eu2+ (λem ≈ 525-530 nm, FWHM ≈ 68 nm) and Ba3Si6O15: Eu2+ (λem ≈ 498 nm, FWHM ≈ 104 nm) phosphors. For the Ba2.91Eu0.09Si6O15-µNδ–1300 °C phosphor, an unexpected cyan emission is obtained, which is similar with Ba3Si6O15: Eu2+. The excitation spectrum presented in Figure 6b exhibits a broad absorption band ranging from 250 to 500 nm, which is composed of an intense excitation band at 296 nm and a weak absorption band from 350 to 500 nm. The emission spectrum presents a weak emission peak at 405 nm and a strong emission band centered at 494 nm under the excitation of 296 nm, which is attributed to the 4f65d1 → 4f7 transition of Eu2+. The Ba2.91Eu0.09Si6O15-µNδ–1300 °C phosphor with chemical formula of Ba2.91Eu0.09Si6O9.6N3.6 has been proved to have the same crystal structure with Ba3Si6O9N4, as discussed earlier. Eu2+-doped Ba3Si6O9N4 phosphor is known to exhibit a bluish green emission only at lower temperatures55, but the PL intensity of Ba2.91Eu0.09Si6O15-µNδ–1300 °C phosphor prepared by soft-chemical ammonolysis method was intense enough to be measured even at room temperature. Moreover, a broad nonuniform emission band should be observed because there are three different Ba2+ ion sites can be occupied by Eu2+ ions, as presented in Figure 4f. According to Uitert’s report56, the Eu2+ ions emission position is strongly influenced by the coordination environment. In various compounds, the relationship between the position of Eu2+ energy level and the local environment follows an empirical formula suggested by Van Uitert56

(

E cm

−1

)

1  − ( nEar )  V V    = Q 1 −   × 10 80   4    *

Here E represents the energy of the Eu2+ ion emission peak, Q* is the position in energy for the lower d-band edge for the free Eu2+ ion (Q* = 34000 cm−1)57, V is the valence of the Eu2+ ion (V = 2), n is the number of anions in the immediate shell about the Eu2+ ion, Ea is the electron affinity of the atoms that form anions (eV), and r is the radius of the host cation replaced by the Eu2+ ion (Å). So the value of E is in direct proportion to the value of n and r. As shown in Figure 4f, Ba1 and Ba3 sites coordinate with six oxygen atoms, and Ba2 site coordinates with six oxygen atoms and one nitrogen atom. r is calculated to 135 pm (n = 6) for Ba1 and Ba3 and 138 pm (n = 7) for Ba2, respectively.58 So, it is concluded that the weak band with peak at 405 nm is assigned to the 5d−4f emission of Eu2+ ions locating at the Ba1 and Ba3 sites, and the strong band at 494 nm is ascribed to the 5d−4f emission of Eu2+ ions in the seven coordinated Ba2 site. The emission spectrum also implys that the Eu2+ ions mainly occupy the Ba2 site due to the stronger emission intensity. However, Yin et al. proposed that the emission mainly come from the Eu2+

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locating at the Ba3 site,59 which is conflicting with the Mikami et al.’s report.27 Since the Ba-N bond length of Ba3Si6O9N4 (about 3.2 Å) is longer than that of Ba3Si6O12N2 (about 3.0 Å), the crystal field splitting of Eu2+ 5d levels in Ba3Si6O9N4 is narrower than that in Ba3Si6O12N2. So the Ba2.91Eu0.09Si6O15-µNδ−1300 °C phosphor shows an emission wavelength of 494 nm, shorter than that of Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphor. Figure 7 compares the luminescence colors, CIE chromaticity coordinates (x, y) and absolute quantum yields of the Ba2.91Eu0.09Si6O15, Ba2.91Eu0.09Si6O15-µNδ−1200 °C, Ba2.91Eu0.09Si6O15-µNδ−1300 °C phosphors. It can be clearly seen that Ba2.91Eu0.09Si6O15 phosphor is the white powder under daylight and exhibits bluish green emission with (x, y = 0.17, 0.32) when excited with 365 nm UV lamp. Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphor presents a green body color as a result of 4f7 − 4f65d1 absorption of Eu2+ in the range of blue to green. The Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphor shows high color purity with (x, y = 0.21, 0.59), which is similar to (Ba, Sr, Eu)2SiO4. Therefore, it can be applied to LED backlight field. However, different from Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphor, the Ba2.91Eu0.09Si6O15-µNδ−1300 °C phosphor shows cream body color, and emits cyan light with (x, y = 0.14, 0.31) under 365 nm UV lamp excitation, which is similar with those of Ba2.91Eu0.09Si6O15 phosphor. It is worth mentioning that the traditional devices usually combine with UV-LED, red, green and blue phosphor. However, there will be a cyan cavity in the region of 480 – 520 nm. Cyan phosphors hence play an important role to compensate the cavity. So the Ba2.91Eu0.09Si6O15 and Ba2.91Eu0.09Si6O15-µNδ−1300 °C phosphors could be a potential candidate for full spectrum lighting. Under 365 nm excitation, the Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphor has the highest absolute quantum yield (75%) than the other two phosphors. Moreover, the inset in Figure 7 shows that the Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphor presents a wider absorption in near-UV and blue region comparing with the Ba2.91Eu0.09Si6O15 phosphor. So the Ba2.91Eu0.09Si6O15µNδ−1200 °C phosphor can well match with the blue chip and have a potential application in the field of phosphorconverted white LEDs. Under the 440 nm blue light excitation, its absolute quantum yield can reach up to 68%. The decay curves are shown in Figure S8, indicating the decay behavior of the Ba2.91Eu0.09Si6O15, Ba2.91Eu0.09Si6O15-µNδ−1200 °C, and Ba2.91Eu0.09Si6O15°C samples. The decay curves of the µNδ−1300 Ba2.91Eu0.09Si6O15 and Ba2.91Eu0.09Si6O15-µNδ−1200 °C samples can be fitted well with a single exponential function60: I(t) = I0exp(-t/τ), where I(t) and t are the luminescence intensity and time, respectively; I0 is a constant; and t is the lifetime for the exponential component. Based on the above formula, the decay times were determined to be 0.95 and 1.38 µs for Ba2.91Eu0.09Si6O15 and Ba2.91Eu0.09Si6O15µNδ−1200 °C samples, respectively. This result supports the spectral analysis that Eu2+ ions only occupy one kind of Ba2+ crystallographic site in these two phosphors.


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Figure 7. (left) The luminescence photographs and the corresponding CIE chromaticity coordinates of Ba2.91Eu0.09Si6O15 (a), Ba2.91Eu0.09Si6O15-µNδ−1200 °C (b), Ba2.91Eu0.09Si6O15-µNδ−1300 °C (c) under daylight and UV lamp (365 nm) irradiation. (right) Absolute quantum yield under 365 nm and 440 nm excitation. The insert shows the excitation spectra of samples a and b. Conversely, the decay curve of the Ba2.91Eu0.09Si6O15µNδ−1300 °C sample can be well matched with the double order exponential decay: I(t) = A1exp(-t/τ1) + A2exp(-t/τ2), where A1 and A2 are constants, and τ1 and τ2 are rapid and slow time for the exponential components, respectively. The average decay times can be approximately calculated to be 0.33 µs using the formula: τ* = (A1τ12 + A2τ22)/(A1τ1 + A2τ2). Obviously, this result coincides with the fact of two emission peaks in the emission spectrum of the Ba2.91Eu0.09Si6O15-µNδ−1300 °C sample. 3.1.3 Temperature-dependent photoluminescence properties. In general, the strong thermal quenching behavior of the luminescence is not desirable for applications in white LEDs, which will change the chromaticity and efficiency of the LEDs. Figure 8a-c presents the temperature-dependent emission spectra of Ba2.91Eu0.09Si6O15, Ba2.91Eu0.09Si6O15-µNδ−1200 °C, and Ba2.91Eu0.09Si6O15-µNδ−1300 °C, respectively. It can be observed that all these phosphors demonstrate a constant decrease in PL intensity. The Ba2.91Eu0.09Si6O15 phosphor is a typical oxide phosphor. The thermal stability of the oxides is usually poor than that of oxynitrides. The higher the temperature, the lower the emission intensity of phosphors. So, when the temperature is higher than 100°C, the emission intensity of the Ba2.91Eu0.09Si6O15 phosphor presents an abrupt decrease compared with the Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphor. As presented in Figure 8d, the Ba2.91Eu0.09Si6O15-µNδ−1200 °C has the best thermal stability than the other two phosphors, and the thermal stability of Ba2.91Eu0.09Si6O15-µNδ−1300 °C is the worst. When the phosphor is heated up to 100°C , 150 °C

and 200 °C, the luminescence of the Ba2.91Eu0.09Si6O15µNδ−1200 °C phosphor still remains 96%, 93%, 84% of the initial intensity measured at room temperature, respectively. However, the luminescence of the Ba2.91Eu0.09Si6O15µNδ−1300 °C phosphor only remains 20% of the initial intensity when higher than 200 °C. For the Ba2.91Eu0.09Si6O15 phosphor, it exhibit good thermal stability below 150 °C (curve 1 in Figure 8d). When the temperature is higher than 200 °C, there is a sudden sharp in the emission intensity which only remains 49% of its initial intensity. In Figure 8a-c, the FWHM of Eu2+ emission band broadens with the temperature increasing: FWHMs of Ba2.91Eu0.09Si6O15, Ba2.91Eu0.09Si6O15-µNδ−1200 °C and Ba2.91Eu0.09Si6O15-µNδ−1300 °C phosphors at 25°C and 300 °C are 104 and 124 nm, 72 and 85 nm, 77 and 96 nm, respectively. The broadened FWHM and the decreased emission intensity are discussed with the configurational coordinate diagram, as shown in Figure 9a.6, 61 In the diagram, parabolas G and E represent the ground state and excited state energy levels, respectively. The point O is the crossing point of G and E, and the point A′ represents the excited state energy level. The point O locates at higher energy level than point A′. Generally, the luminescent centers relax to the lowest level through radiative transition, donated by point B′. When the temperature increases, the luminescent center locating at excited state energy level can be thermally activated through phonon interaction, and then get back to the ground state through the crossing point O. This is to say the excited Eu2+ ions can easily return the ground state through the phonon vibration rather than emitting photons. Consequently, the



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Figure 8. Emission spectra of (a) Ba2.91Eu0.09Si6O15, (b) Ba2.91Eu0.09Si6O15-µNδ−1200 °C, and (c) Ba2.91Eu0.09Si6O15-µNδ−1300 °C samples excited at 365 nm at varying temperatures. (d) Temperature dependence of the relative emission intensity: curve 1, 2, 3 represents the a, b and c samples, respectively. emission from the transition of B′ to B is quenched. This nonraditive transition probability by thermal activation is greatly influenced by temperature, which can reduce the emission intensity. The nonraditive transition probability is closely related to the energy barrier (ΔE) between B′ and O. The energy barrier ΔE the greater, the thermal stability the better. The energy barrier ΔE can be determined by the following eqn62-63:

IT =

I0  − ∆E  1 + c exp   kT 

Where c is a constant, T represents the temperature (K) and k is the Boltzmann constant. I0 is the initial emission intensity and and IT is the intensity at different operating temperatures. The relationship of In(I0/IT - 1) versus 1/kT for Ba2.91Eu0.09Si6O15, Ba2.91Eu0.09Si6O15-µNδ−1200 °C and Ba2.91Eu0.09Si6O15-µNδ−1300 °C phosphors is shown in Figure S9. The ΔE was calculated to be 0.18, 0.22, 0.09 eV for the Ba2.91Eu0.09Si6O15, Ba2.91Eu0.09Si6O15-µNδ−1200 °C and Ba2.91Eu0.09Si6O15-µNδ−1300 °C, respectively.

As discussed above, the luminescent center with thermal activation is strongly interaction with thermally active phonon, causing the change of FWHM. The higher the temperature, the greater the population density of phonon. The interaction between electron and phonon becomes dominant, resulting in the FWHM broadened.6 Moreover, it can be found that the emission spectra of the Ba2.91Eu0.09Si6O15 and Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphors (Figure 8a and b) exhibit blue-shift with increasing temperature, which is due to the interaction between 5d electron of Eu2+ and thermally active phonon, as illustrated in Figure 9b. The decrease in the average free path of phonon will increase the collision probability between 5d electron of Eu2+ and phonon. When increasing the temperature, more 5d electrons of Eu2+ at low energy state (E1) will move to high energy state (E2) by overcoming the energy barrier (ΔE) with assistant of the thermally active phonon, then go back to the ground state (G), resulting in a shorter wavelength emission.61 So the temperature-dependent PL spectra of Ba2.91Eu0.09Si6O15 and Ba3Si6O15-μNδ-1200oC with increasing the temperatures show the blue-shift. Especially, the emission peak of Ba2.91Eu0.09Si6O15-µNδ−1300 °C phosphor shows red-shifted


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with increasing the temperature. When the temperature is higher than 150 °C, the blue-shift in peak positions of emission spectra occurs with further increasing the temperature. The Varshini equation explain the red-shift phenomenon:64

E (T ) = E0 −

aT 2 T +b

where E(T) is the energy difference between excited states and ground states at a special temperature, E0 is the energy difference at 0 K, and a and b are constants. Therefore,

Figure 9. Configurational coordinate diagram of (a) the thermal quenching process and (b) the tunneling of Eu2+ from low-energy level to high-energy level with the help of thermally active phonon. we can concluded that E(T) will decrease due to the raise of the temperature, resulting in a red-shift emission. Moreover, the bond length between the activators and its ligand ions will be lengthened, which will distort the symmetry of coordination environment. So the John– Teller effect gradually plays a leading role. These two effects result in the splitting of degenerate excited state or ground state,6, 61 and decrease the transition energy. So, the emission peak is red-shifted with the temperature increasing. Continuing to raise the temperature, the transition probability that 5d electrons of Eu2+ ion tune back from the lower excited states to the higher excited states

with the help of thermally active phonon increases and gradually becomes dominant. As a result, the blue-shift in emission spectra was observed at higher temperature. On the basis of the above discussion, it is concluded that Eu2+-activated the Ba3Si6O15-µNδ−1200 °C (with chemical formula Ba3Si6O13.35N1.1) phosphor exhibits strong green emission at room temperature with good thermal stability. On the other hand, Eu2+-activated the Ba3Si6O15µNδ−1300 °C (with chemical formula Ba3Si6O9.6N3.6) phosphor gives weak blue-green emission and has strong thermal quenching. Moreover, its excitation band is narrower than that of Eu2+-activated Ba3Si6O15-µNδ−1200 °C phosphor. The Ba3Si6O13.35N1.1 and Ba3Si6O9.6N3.6 have the similar crystal structure with Ba3Si6O12N2 and the Ba3Si6O9N4, respectively. What is more, the crystal structure of Ba3Si6O9N4 appears close to Ba3Si6O12N2, and Eu2+ ions occupy the similar Ba2+ site in both compounds. Our spectral analysis has suggested that the Ba-N bond length in Ba2.91Eu0.09Si6O15-µNδ−1200 °C is shorter than that in Ba2.91Eu0.09Si6O15-µNδ−1300 °C, and so the crystal field strength of Eu2+ ions in the former is stronger than that in the latter. A longer wavelength emission is observed in °C compared with Ba2.91Eu0.09Si6O15-µNδ−1200 Ba2.91Eu0.09Si6O15-µNδ−1300 °C. However, why is the thermal quenching behavior of Ba2.91Eu0.09Si6O15-µNδ−1300 °C quite different from that of Ba2.91Eu0.09Si6O15-µNδ−1200 °C. To explain this question, DFT calculations of the two compound hosts are used to compute the band gap.65 The calculated electronic band structure and density of states (DOS) are given in Figure S10. The result (Figure S10a and S10c) presents an electronic band gap of about 4.7 eV and 4.3 eV for Ba3Si6O12N2 and Ba3Si6O9N4, respectively, which is smaller than the experimental result (Figure S7). This common phenomenon is a result of the insufficient description of the exchange correlation in DFT calculations, while this value coincides with the previous DFT result of 4.63 eV and 4.37eV for Ba3Si6O12N2 and Ba3Si6O9N4, respectively.27 The highest occupied states and the lowest unoccupied states are comprised of O 2p and N 2p for both compounds, as shown in Figure S10bd. Due to the smaller band gap of Ba3Si6O9N4, it is boldly supposed that the lowest 5d states of Eu2+ are close to the conduction bands in Ba3Si6O9.6N3.6. And the photo-excited 5d electrons of Eu2+ can be thermally ionized to the conduction bands. So the stronger thermal quenching in Ba2.91Eu0.09Si6O15-µNδ−1300 °C may be due to smaller band gap and longer Ba-N bond length. 3.2 (Ba, Sr)3(Al, Si)6(O, N)15:Eu2+ As the above discussion, the Ba3Si6O15-µNδ -1200 °C production, which is prepared by ammonolysis method using 500 °C-precursor, is a intermediate phase of Ba3Si6O12N2 and Ba3Si6O15. Pure Ba3Si6O12N2 phase is hard to obtain even if changing the synthesis conditions such as calcined temperature, time and the flow rate of ammonia. We notice that when the N3− substitutes O2− forming Ba3Si6O12N2 phase in the process of ammonolysis, the charge is imbalance. This would be the reason that blocks the forming of



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the pure Ba3Si6O12N2 phase. In recent studies, Al-N as ion pair, makes up for the mismatching resulting from N3− element instead of O2− and is usually doped into silicate phosphors to tune their luminescence properties.66-67 Moreover, the as-prepared Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphor has the highest quantum yield under blue light excitation and the best thermal stability than the asprepared Ba2.91Eu0.09Si6O15 and Ba2.91Eu0.09Si6O15-µNδ−1300 °C phosphors. However, an ideal white LEDs (with higher color rendering index and luminous efficiency) require a longer wavelength emitting phosphor (λem = 550-570 nm). Obviously, the 520 nm emission of the Ba2.91Eu0.09Si6O15µNδ−1200 °C does not meet that requirement. It is reported that partially replacing Ba2+ (rBa2+ = 149 pm) ions in the host lattice with smaller Sr2+ (rSr2+ = 132 pm) ions can lead to the unit cell volume contraction. As a consequence, the local coordination environment around Eu2+ ions is modified, and the corresponding luminescence properties are improved.68 Based on the above consideration, we prepared a series of (Ba, Sr)3(Al, Si)6(O, N)15:Eu2+ samples, and studied the structure and luminescence properties in detail.

Figure 10. Observed (×) and calculated ( red line) X-ray powder diffraction patterns of Ba3Si6-xAlxO15-µNδ (x = 0.03−0.18) samples together with their difference curve after Rietveld refinement. 3.2.1 Phase identification. Figures S11 and S12 give the XRD patterns of the as-prepared Ba3Si6-xAlxO15-µNδ (x = 0.03−1.2) and (Ba1-ySry)3Si6O15-µNδ (y = 0.1−0.6) samples synthesized by 500 °C-precursor calcined in NH3 atmosphere at 1200 °C, respectively. It can be seen that all diffraction peaks can fit well with the pure Ba3Si6O12N2 phase, indicating the formation of solid solutions. The impurities (extra diffraction peaks at 22.5 °and 26°) due to the Ba3Si6O15 phase began to appear when x, y value exceeds 0.18 and 0.5, respectively. So we carry out the Rietveld analysis of the powder XRD profiles of the Ba3Si6xAlxO15-µNδ (x = 0.03−0.18) and (Ba1-ySry)3Si6O15-µNδ (y = 0.1−0.5) samples to confirm their crystal structure, using

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the Ba3Si6O12N2 (ref. 27) and Ba3Si6O15 (ICSD-10162) structures as starting models. Figures 10 and 11a show the observed (×) and calculated (red line) and difference (blue line) XRD patterns for the XRD Rietveld refinement of Ba3Si6-xAlxO15-µNδ (x = 0.03−0.18) and (Ba1-ySry)3Si6O15µNδ (y = 0.1−0.5) samples, respectively. Table 1 gives the corresponding XRD refinement result. These results indicate that the leading Ba3Si6O9N2 phase forms associated with a small portion of the Ba3Si6O15 phase. What’s more, there is fewer impurity phase (Ba3Si6O15) for the Ba3Si6xAlxO15-µNδ (x = 0.03−0.18) and (Ba1-ySry)3Si6O15-µNδ (y = 0.1−0.5) samples than the as-prepared Ba3Si6O15-µNδ−1200 °C samples. This indicates that the substitution of Sr/Al for Ba/Si is conducive to the formation of Ba3Si6O9N2 phase. Al−N ion pair makes up for the mismatching resulting from N3− element instead of O2−, which would facilitate the formation of Ba3Si6O9N2 phase. On the other hand, it was reported that it is easier for the nonstoichiometric raw materials at Si/Ba = 3 than the stoichiometric ones at Si/Ba = 2 to form Ba3Si6O12N2 phase. The excess Si source formed SiO2 glass, which can promote the formation of Ba3Si6O12N2.69 So the (Ba1-ySry)3Si6-xAlxO15-µNδ is easier to form Ba3Si6O12N2−type phase than Ba3Si6O15µNδ−1200 °C sample. As shown in Table 1, the Ba3Si6xAlxO15-µNδ (x = 0.06 and 0.12) and (Ba1-ySry)3Si6O15-µNδ (y = 0.3) samples are closer to Ba3Si6O12N2−type phase. Moreover, substituting Sr2+ (ionic radius 132 pm) for smaller Ba2+ (ionic radius 149 pm) in (Ba1-ySry)3Si6O15-µNδ samples cause an obvious lattice contraction, as presented in Figure 11b. Similarly, the bigger Al3+ (ionic radius 53 pm) substitution for Si4+ (ionic radius 40 pm) would expand the unit cell. As the substitution concentration of Al3+ is low, the expansion of the unit cell parameters is not obvious, as listed in Table 1. These substitutions have a great influence on the coordination environment of activators and the luminescence properties which are discussed in next part. In order to further confirm the content of the N element, EDX characterization is carried out and the results are listed in Table S3. It can be found that the N content is close to the theoretical value (δ = 2) in (Ba,Sr)3(Si, Al)6O15-µNδ samples. Moreover, the value of the N content from EDX is greater than the calculated value from Rietveld refinement, which is in the error tolerance. 3.2.2 Photoluminescence properties at room temperature. Under UV light excitation, all the Ba2.91Eu0.09Si6xAlxO15-µNδ (x = 0.03−0.6) phosphors with different x values exhibit bright green emission. Figure 12 presents the PL excitation and emission spectra of as-prepared Ba2.91Eu0.09Si6-xAlxO15-µNδ (x = 0.03−0.6) phosphors, and compares with the excitation and emission spectra of Ba2.91Eu0.09Si6-xAlxO15-µNδ (x = 0.3) and Ba2.91Eu0.09Si6O15µNδ−1200 °C phosphors. As displayed in Figure 12a, the emission peak position does not vary with the Al content and only the emission intensity decreases with the Al


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Table 1. Crystallographic details for Ba3Si6-xAlxO15-µNδ (x = 0.03−0.18) and (Ba1-ySry)3Si6O15-µNδ (y = 0.1−0.5) derived from Rietveld refinement. (A = Ba3Si6O9N2; B = Ba3Si6O15) Sample

unit cell parameters (Å)

phase

a=b

A

c

RP

RWP

B

0.03

7.5035

6.4727

88%

12%

8.37%

13.61%

Ba3Si6-xAlxO15-µNδ

0.06

7.5033

6.4717

93%

7%

6.38%

9.19%

x=

0.12

7.5031

6.4711

93%

7%

5.83%

9.23%

0.18

7.5025

6.4710

89%

11%

3.58%

5.17%

0.10

7.4864

6.4577

86%

14%

4.55%

6.58%

(Ba1-ySry)3Si6O15-µNδ

0.20

7.4738

6.4412

90%

10%

7.09%

11.41%

y=

0.30

7.4665

6.4293

93%

7%

6.99%

10.84%

0.40

7.4524

6.4117

92%

8%

6.20%

10.93%

0.50

7.4500

6.3991

86%

14%

4.69%

7.28%

Figure 11. (a) Observed (×) and calculated ( red line) X-ray powder diffraction patterns of (Ba1-ySry)3Si6O15-µNδ (y = 0.1−0.5) samples together with their difference curve after Rietveld refinement. (b) Lattice parameters of a, b, c, and cell volume of (Ba1-ySry)3Si6O15-µNδ (y = 0.1−0.5) obtained from Rietveld refinement. content increasing. The excitation spectra of these phosphors show a broad absorption band from 250 nm to 500 nm, which covers the whole UV and blue light region. Furthermore, compared with the non Al-substituted Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphor (Figure 12b), the excitation spectrum of the representative Ba2.91Eu0.09Si6xAlxO15-µNδ (x = 0.3) sample shows a stronger absorption in the near UV and blue light region from 370 to 500 nm, which indicates the Al-substituted samples are more suitable for white LEDs than the Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphor. The absolute quantum yield also supports this fact, as illustrated in Figure S13. Under the excitation of 440 nm, the Ba2.91Eu0.09Si6-xAlxO15-µNδ with x = 0.03 and0.06 phosphors have the higher quantum yield (89% for x = 0.03 and 72% for x = 0.06) than the non Alsubstituted Ba2.91Eu0.09Si6O15-µNδ−1200 °C (68%) phosphor. Similar with the variation trend of the emission intensity, the quantum yield also decreases with the increase of Al

content. So the optimal doping concentration of Al is fixed to be x = 0.03 and the Ba2.91Eu0.09Si6-xAlxO15-µNδ (x = 0.03) sample has the best luminescence performance among the series of Ba2.91Eu0.09Si6-xAlxO15-µNδ (x = 0.03−0.6) samples. The

excitation

and emission spectra of (Ba0.97= 0.1−0.5) samples are displayed in Figure 13a and b. Similar with the excitation spectra of the Ba2.91Eu0.09Si6-xAlxO15-µNδ samples, (Ba0.97Sr ) Eu Si O N also show a broad band absorption. y y 3 0.09 6 15-µ δ The emission intensity enhances with the increase of Sr content until y = 0.3. Moreover, the emission peak shifts toward the longer wavelength. The curves in Figure 13c more intuitively show the change trend of emission intensity and emission peak position with various Sr concentration (y). The emission peak shifts from 520 nm for y = 0 ySry)3Eu0.09Si6O15-µNδ (y



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ly, the contraction of (Ba/Eu)O6 octahedral favors the presence of Eu3+. According to the above mentioned emission red-shift properties for the (Ba0.97-ySry)3Eu0.09Si6O152+ into the crystal µNδ phosphors, the introduction of Sr structure directly cause the shrinkage of EuO6 octahedral and therefore, the Eu3+ was observed. However, the existence of Eu3+ does not affect the intrinsic luminescence of Eu2+ in both two phosphors due to its very little content.

Figure 12. (a) PL excitation and emission spectra of asprepared Ba2.91Eu0.09Si6-xAlxO15-µNδ phosphors with different x values (x = 0.03−0.6). (b) The comparison of the PL excitation and emission spectra of Ba2.91Eu0.09Si6-xAlxO15µNδ (x = 0.3) and Ba2.91Eu0.09Si6O15-µNδ−1200 °C phosphors. to 554 nm for y = 0.5. So, the phosphors with green to yellow emitting are observed with increasing y in (Ba0.97ySry)3Eu0.09Si6O15-µNδ phosphors, as shown in Figure 13d. Moreover, the body colors of the as-prepared sample powders also change from green to yellow. The result of red-shift emission coincides with the unit cell volume contraction with increasing Sr concentrations, as discussed earlier. Therefore, the introduction of Sr2+ into the host lattice modifies the octahedral crystal field around Eu2+ ions, enlarging the splitting of 5d octahedral crystal field, and resulting in the emission red-shift. Similar shifts are observed in (Sr1-xBax)Si2O2N2:Eu2+ phorsphors70 and (Ba, Sr)2Si5N8:Eu2+9. Here, it is worth noting that (Ba0.97ySry)3Eu0.09Si6O15-µNδ samples have the more efficient excitation in the blue light region from 400 to 470 nm in the spectra ( Figure 13a), perfectly matching with the InGaN LEDs (∼465 nm), which is also indicated by the quantum yield shown in Figure S14. Under 465 nm excitation, the quantum yield of the (Ba0.97-ySry)3Eu0.09Si6O15-µNδ phosphors with y = 0.2 can reach up to 71%. Moreover, an unexpected phenomenon was observed in the excitation spectra for both Ba2.91Eu0.09Si6-xAlxO15-µNδ and (Ba0.97-ySry)3Eu0.09Si6O15-µNδ phosphors, as shown in Figure 12a and 13a. There are some sharp excitation peaks ranging from 370 nm to 500 nm, which is attributed to the characteristic 4f–4f electron transitions of Eu3+. So we deduce that the change of the crystal field around Eu ions due to the introduction of Sr/Al makes the reduction of Eu3+ to Eu2+ become difficult.66-67 Based on the ions radius: r (Si4+) = 40 pm, r(Al3+) = 53 pm58, the replacement of Si4+ with Al3+ results in the expansion of (Si/Al)(O/N)4 tetrahedral, and thereby shrinks the (Ba/Eu)O6 octahedral. For Ca12Al14-zSizO32F2-z:Eu, the occupation sites of Eu3+ in CaO6F get to expand after corresponding substitutions, which could favor the reduction of Eu3+ to Eu2+.71 Similar-

Figure S15 shows the decay properties of the Ba2.91Eu0.09Si6-xAlxO15-µNδ and (Ba0.97-ySry)3Eu0.09Si6O15-µNδ samples. Similar with the Ba2.91Eu0.09Si6O15-µNδ–1200 °C sample, the decay curves is fitted well with a single exponential function: I(t) = I0exp(-t/τ). So, the decay times were determined as shown in Figure S15. The introduction of Al/Sr has little influence on the decay time of Eu2+ in Ba3Si6O2N2–type phosphor. 3.2.3 Temperature-dependent photoluminescence properties. The thermal stability of the series of the Ba2.91Eu0.09Si6-xAlxO15-µNδ and (Ba0.97-ySry)3Eu0.09Si6O15-µNδ samples was also investigated. Figures S16 and S17 present the temperature-dependent PL emission spectra of Ba2.91Eu0.09Si6-xAlxO15-µNδ and (Ba0.97-ySry)3Eu0.09Si6O15-µNδ samples, respectively. All the samples demonstrate a constant decrease in PL intensity with increasing temperature. Moreover, the emission peaks exhibit blue-shift with the temperature increasing. As the detailed discussion earlier, this is because the transition probability that the back tunneling of 5d electron of Eu2+ ion from the lower excited states to the higher excited states by assistance of thermally active phonon increases. The emission intensity demonstrates a decline trend with increasing the x and y in Ba2.91Eu0.09Si6-xAlxO15-µNδ and (Ba0.97-ySry)3Eu0.09Si6O15-µNδ samples, especially at the higher temperature, as shown in Figure 14a-b. But all the samples still present a very favorable thermal quenching behavior due to the fact that the emission intensities remain more than 50% of its initial value even if the temperature is higher than 200 °C. So the thermal quenching of the as-prepared samples is very small. More importantly, as green emission phosphors, the as-prepared Ba2.91Eu0.09Si6-xAlxO15-µNδ (x = 0.06) and Ba2.91Eu0.09Si6O15-µNδ–1200 °C phosphors have better thermal stability than a commercial Ba2SiO4:Eu2+ phosphor, as shown in Figure 14c. Similarly, Figure 14d confirms that the thermal quenching of (Ba0.97-ySry)3Eu0.09Si6O15-µNδ (y = 0.3) phosphor with yellow emission is smaller than the commercial YAG:Ce3+ yellow phosphor. Thus, the Ba2.91Eu0.09Si6-xAlxO15-µNδ and (Ba0.97-ySry)3Eu0.09Si6O15-µNδ phosphors appear promising green and yellow phosphors for LEDs. 3.3 LED applications For appraising the device performance for the synthesized (Ba0.97-ySry)3Eu0.09Si6-xAlxO15-µNδ phosphors, we fabricated LEDs with various CCTs by combining UV (365 nm)/blue (440 nm) chips and the representative cyan Ba2.91Eu0.09Si6O15 phosphor, green Ba2.91Eu0.09Si6-xAlxO15-µNδ


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Figure 13. PL excitation (a) and relative emission spectra (b) of as-prepared (Ba0.97-ySry)3Eu0.09Si6O15-µNδ phosphors as a function of y value (y = 0.1−0.5). (c) The emission peak shift and emission intensity of the (Ba0.97-ySry)3Eu0.09Si6O15-µNδ phosphors with y = 0−0.5. (d) CIE chromaticity coordinate diagram and luminescent photographs of the as-prepared (Ba0.97-ySry)3Eu0.09Si6O15-µNδ phosphors with y = 0−0.5.

Figure 14. Thermal quenching behaviour of photoluminescence for (a) Ba2.91Eu0.09Si6-xAlxO15-µNδ phosphors (x = 0.03–0.18) and (b) (Ba0.97-ySry)3Eu0.09Si6O15-µNδ phosphors (y = 01–0.5). The comaprison of the thermal stability for (c) Ba2.91Eu0.09Si62+ 2+ xAlxO15-µNδ (x = 0.06, abbreviated as BSAON:Eu ), Ba2.91Eu0.09Si6O15-µNδ–1200 °C (abbreviated as BSON:Eu ) and 2+ 2+ 3+ Ba2SiO4:Eu phorsphors; and (d) (Ba0.97-ySry)3Eu0.09Si6O15-µNδ (y = 0.3, abbreviated as BSSON:Eu ) and YAG:Ce phosphors.



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Page 16 of 20

Figure 15. The emission spectra of fabricated white LEDs based on bluish green Ba2.91Eu0.09Si6O15, green Ba2.91Eu0.09Si6xAlxO15-µNδ (x = 0.06) phosphor, and yellow (Ba0.97-ySry)3Eu0.09Si6O15-µNδ (y = 0.4) phosphor. (a), (b), (c) Electroluminescent (EL) spectra and photographs of the as-fabricated lighted LEDs under a 20 mA drive current. (d) The chromaticity coordinates of as-fabricated LEDs in Commission Internationale de l’Ećlairage (CIE) 1931 color spaces. Table 2. Important photoelectric parameters for four typical LEDs with different correlated color temperatures (CCTs) Devices LED 1

Composition

LED 4

Efficacy (lm/W)

chip/nm

Phosphor blend

CCT (K)

Ra

R9

x

y

365

B+1113

3846

92.9

60.7

0.3937

0.4016

17.57

BA2+1113

4768

84.3

68.3

0.3443

0.2979

43.31

BS4

5805

53.3

-16.8

0.3249

0.3590

60.68

BS4+1113

2948

70

56.5

0.4188

0.3614

49.89

LED 2 LED 3

Chromaticity coordinate

440 nm

note: B = Ba2.91Eu0.09Si6O15; BA2 = Ba2.91Eu0.09Si6-xAlxO15-µNδ (x = 0.06); BS4 = (Ba0.97-ySry)3Eu0.09Si6O15-µNδ (y = 0.4) (x = 0.06) phosphor, yellow (Ba0.97-ySry)3Eu0.09Si6O15-µNδ (y = 0.4) phosphor and commercial red CaAlSiN3:Eu2+ phosphor. The electroluminescence (EL) spectra, photographs, and the chromaticity coordinates of typical LEDs are illustrated in Figure 15. Table 2 gives the detail information and important photoelectric parameters such as CCT, CRI and luminous efficacy for the typical fabricated LEDs. As seen, White LEDs with various CCTs are obtained. High color rendition is obtained in LED1 (Figure 15a) which is combined by 365 nm-UV LED chip, cyan Ba2.91Eu0.09Si6O15 phosphor and red CaAlSiN3:Eu2+ phosphor. Particularly noteworthy is the value of R9, which measures the satura-

tion of red degree. The R9 value of LED1 is also quite high. However, this type of white LED has lower luminous efficiency. But it is worth noting that the fabricated LED1 no longer need to add additional green or blue phosphor, which will greatly reduce the reabsorption between the phosphors. For green Ba2.91Eu0.09Si6-xAlxO15-µNδ (x = 0.06) phosphor and yellow (Ba0.97-ySry)3Eu0.09Si6O15-µNδ (y = 0.4) phosphor, they can well match with blue LED chips, as shown in Figure 16b-c. We can observe that warm white with lower CCT are readily obtained in LED2 and LED4. However, the LED3 has a higher CCT and lower CRI. Its negative R9 value coincides the deficiency of the red


Page 17 of 20

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emission in yellow (Ba0.97-ySry)3Eu0.09Si6O15-µNδ (y = 0.4) phosphor similar with YAG:Ce3+ phosphor. Nevertheless, the R9 value of LED4 added red CaAlSiN3:Eu2+ phosphor becomes positive, suggesting that the longer-wavelength red phosphor significantly enhanced the red spectral saturation. It is worth pointing out that lower luminous efficiency for LED2 and LED4 can be improved by by optimizing the processing conditions and controlling the particle size, morphology, and crystallinity of the phosphor. Therefore, the as-prepared (Ba0.97-ySry)3Eu0.09Si6-xAlxO15-µNδ phosphors are excellent candidates for UV or blue-excited white LEDs. 4 CONCLUSIONS In this work, Eu2+ doped (Ba, Sr)3(Si, Al)6(O, N)15 phosphors [(Ba1-ySry)3Si6-xAlxO15-µNδ, x = 0.03–1.2, y = 0–0.6] with multicolor emissions were synthesized by Pechinitype sol−gel method combining with calcination of the precursors in ammonia atmosphere. The as-prepared Ba3Si6O15:Eu2+ phosphor displayed a symmetrical emission band at 498 nm with FWHM = 104 nm, and a broad excitation band from 200 to 400 nm. Green emitting Ba2.91Eu0.09Si6O15-µNδ–1200 °C phosphor with chemical formula Ba2.91Eu0.09Si6O13.35N1.1 was synthesized successfully by ammonolysis of the 500 °C-prefired precursor at 1200 °C for 12 hours with a NH3 flow rate of 0.5 L·min-1; Refinement result indicated the Ba3Si6O13.35N1.1 phase was composed of a predominant Ba3Si6O12N2 (61%) phase accompanied by a small portion (39%) of the Ba3Si6O15 phase. Its excitation spectrum showed a more broad absorption band than Ba3Si6O15:Eu2+ phosphor, covering the UV to blue light region. The as-prepared Ba3Si6O15-µNδ–1300 °C with chemical formula Ba3Si6O9.6N3.6 is attributed to a pure Ba3Si6O9N4 phase. However, Eu2+ doped Ba3Si6O15µNδ–1300 °C exhibited an unexpected bluish green emission due to the longer Ba/Eu-N bond length. In particular, Eu2+ doped cyan Ba3Si6O15 and green Ba3Si6O15-µNδ–1200 °C phosphors exhibited excellent thermal stability. Stronger thermal quenching in Ba2.91Eu0.09Si6O15-µNδ–1300 °C may be ascribed to smaller band gap and longer Ba/Eu–N distance. By substituting Al/Sr with Si/Ba, a pure Ba3Si6O12N2–type phase was obtained in Ba3Si6-xAlxO15-µNδ and (Ba1-ySry)3Si6O15-µNδ samples by calcining the 500 °Cprefired precursor at 1200 °C under NH3 atmosphere. Compared to the unsubstituted Ba2.91Eu0.09Si6O15-µNδ–1200 °C sample, the Al-substituted Ba2.91Eu0.09Si6-xAlxO15-µNδ samples showed green emission with higher quantum yield under blue (440 nm) light excitation, but Srsubstituted (Ba0.97-ySry)3Eu0.09Si6O15-µNδ samples exhibited green (520 nm) to yellow (554 nm) emission with increasing the Sr content due to the shrinkage of EuO6 octahedral. The excitation bands of (Ba0.97-ySry)3Eu0.09Si6-xAlxO15µNδ at a range of 370-460 nm perfectly match with InGaNbased LEDs, indicating they are attractive phosphors for white-LED applications. Moreover, the (Ba0.97Sr ) Eu Si Al O N phosphors exhibited smaller y y 3 0.09 6-x x 15-µ δ thermal quenching than green Ba2SiO4:Eu2+ phosphor and yellow Y3Al5O12:Ce3+ phosphor. High color rendering white

LEDs can be achieved by using Ba3Si6O15:Eu2+/Ba2.91Eu0.09Si6-xAlxO15-µNδ (x = 0.06)/ (Ba0.97ySry)3Eu0.09Si6O15-µNδ( y = 0.4) and a red phosphor, indicating that they are promising phosphors for white LEDs.

ASSOCIATED CONTENT Supporting Information. XRD, TGA, FTIR spectra, Crystallographic details, Diffuse reflection spectra, Absolute quantum yield, Luminescence decay curves, The relationship of ln[(I0/IT)-1] versus 1/kT, Band structures and the density of states (DOS), N atom content, and Temperature-dependent PL emission spectra This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author

*J. Lin, E-mail: [email protected] Author Contributions

All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (NSFC 51672265, 91433110, 51472234, 51672259), the Joint Funds of the National Natural Science Foundation of China and Guangdong Province (U1301242), the National Basic Research Program of China (2014CB643803), Young Elite Scientist Sponsorship Program by CAST (YESS) as well as the Fund of Jilin Province (20150520029JH).

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Cation Substitution (Sr2+â Ba2+, Al3+â Si4+, N3

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