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Intense red emission from Sr4Nb2O9:Eu3+ phosphor by introducing with SrF2 as flux and charge compensator
Journal Pre-proof Intense red emission from Sr4Nb2O9:Eu and charge compensator
3+
phosphor by introducing with SrF2 as flux
Jingshan Hou, Yanrong Cao, Guangxiang Jiang, Yu Liu, Ganghua Zhang, Yufeng Liu, Jun Zou, Fuqiang Huang, Yang Li, Yongzheng Fang PII:
S0022-2313(19)31249-9
DOI:
https://doi.org/10.1016/j.jlumin.2019.116771
Reference:
LUMIN 116771
To appear in:
Journal of Luminescence
Received Date: 21 June 2019 Revised Date:
8 September 2019
Accepted Date: 21 September 2019
Please cite this article as: J. Hou, Y. Cao, G. Jiang, Y. Liu, G. Zhang, Y. Liu, J. Zou, F. Huang, Y. Li, Y. 3+ Fang, Intense red emission from Sr4Nb2O9:Eu phosphor by introducing with SrF2 as flux and charge compensator, Journal of Luminescence (2019), doi: https://doi.org/10.1016/j.jlumin.2019.116771. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Intense red emission from Sr4Nb2O9:Eu3+ phosphor by introducing with SrF2 as flux and charge compensator Jingshan Hou a, Yanrong Cao a, Guangxiang Jiang a, Yu Liu a, Ganghua Zhang b ,Yufeng Liu a, Jun Zou c, Fuqiang Huang d, *, Yang Li a, * and Yongzheng Fang a, * a
School of Materials Science and Engineering, Shanghai Institute of Technology,
Shanghai, 201418, China. b
Shanghai Key Laboratory of Engineering Materials Application and Evaluation,
Shanghai Research Institute of Materials, Shanghai 200437, P. R. China. c
Institute of New Materials and Industrial Technology, Wenzhou University,
Wenzhou, 325035, China. d
State Key Laboratory of High Performance Ceramics and Superfine Microstructure,
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. Abstract: Sr4Nb2O9:Eu3+ phosphors were prepared by flux method. SrF2 was found to be the appropriate flux and its effect on the crystal structure and optical properties were investigated. The structure of Sr4Nb2O9:Eu3+ has been determined by X-ray powder diffraction with Rietveld refinement. The optimal Eu3+ doping concentrations and the SrF2 usage in Sr4Nb2O9 were determined to be 0.13 mol and 5wt% respectively. The spectra analysis indicated that the phosphors could be excited by near UV light of 394 nm and blue light of 465 nm, and yield an intense red light with a maximum at 614 nm. The as prepared phosphor shows its advantages in quantum yield and thermal stability when compared with that of CaMoO4:Eu3+, Y3Al5O12:Ce3+, CaAlSiN3:Eu2+ phosphors. Finally, a white LED with improved Ra (from 75.6 to 82.5) was fabricated by using a 450 nm blue chip with Y3Al5O12:Ce3+ and the as-prepared Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphor. Keywords: Phosphors, Solid-state reaction, Eu3+-doped, Flux, Luminescence ∗ Corresponding authors: [email protected] (F. Huang); [email protected]; [email protected]. (Y.Z.
1. Introduction Phosphor-converted white light-emitting diodes (WLEDs) have been widely used in the fields of display and general lighting applications due to their high luminescence efficiency, energy conservation, long lifetime and environmental friendliness characteristics [1-20]. Currently, the mass production of high-efficiency commercial WLEDs are manufactured by packaging a blue InGaN chip with the yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+ ) phosphor. However, this combination exhibits a high correlated color temperature and poor color-rendering index (Ra) due their weak emission intensity in the red and green region. As it is well known that, phosphor with red component emission is essential in a high Ra light source. Therefore, great efforts have been made for the development of red phosphor to complement the missed red emitting components [5-20]. Eu3+ ion is well known to be a narrow line red-emitting activator around 610 nm responding to the near-UV or blue light excitation attributed to its characteristic 5
D0-7FJ (J = 1, 2, 3, 4) transitions [6-19]. Such properties made Eu3+ doped
compounds became popular red phosphors for high Ra WLEDs. It was reported that bright red emissions can be achieved in some particular Eu3+-activated host materials, such as CaMoO4:Eu3+ [7-10], BaNb2O6:Eu3+ [11], LiGd(WO4)2:Eu3+ [12], Na3GaF6:Eu3+ [13], CaLa1-xMgM’O6:xEu3+ (M’ = Nb, Ta) [14]. However, considering the subsequent practical applications in WLEDs, the low-cost Eu3+ ions doped red phosphors with high quantum efficiency and good thermal stability still urgently required. Niobate is excellent host material for optical materials, especially those inorganic materials niobate containing Nb5+ ions have attracted increasing attention owing to their non-linear optical, piezoelectric, thermal stability, and luminescence properties [21-23]. Moreover, Eu3+ ion doped niobate containing compounds are also known as interesting luminescent materials. In general, they exhibit good self-laser luminescence due to the transfer of O2- to Nb5+ ions in the NbO6 group, which have a Fang).
strong broad band in the UV region. It has been reported that Eu3+ doped Nb5+ -containing niobate red phosphors exhibit excellent luminescent properties. Meanwhile, perovskite structural materials are excellent hosts for optical materials own to their high chemical-physical stability and diversity of their structure and composition. The luminescent properties of perovskite-type materials have also been widely investigated in recent years [24-26]. D. Pasero and R.J.D. Tilley have reported the structure of Sr4Nb2O9 in 1999, this compound with double perovskite compound in which Sr is accommodated in the B site with 1:1 ordering. Two thermodynamically stable polymorphs of Sr-Nb-O were confirmed to occur above and below 1250 above 1250
. The high-temperature polymorph (HT
). HT exhibits an average cryolite-like structure with NaCl like (1:1)
ordering of Sr and Nb on the B sites. On the other hand, the low temperature polymorph (LT below 1250
) has apparent P21/n monoclinic symmetry, and features
a different but undetermined type of B-site ordering [27-30]. However, to the best of our knowledge, the synthesis and luminescence properties of Eu3+-doped double perovskite niobate compound Sr4Nb2O9 has not been reported yet. Thus, in this work, we report the synthesis, crystal structure and luminescence properties of =Eu3+-doped the HT Sr4Nb2O9 phosphor. 2. Experimental 2.1. Synthesis of Sr4Nb2O9:xEu3+ phosphors A series of Sr4Nb2O9:xEu3+ (x = 0.01-0.20) phosphors were prepared by high temperature solid state reaction method from powder based precursors. The precursors SrCO3 (99.00%), Nb2O5 (99.99%), Eu2O3 (99.99%) were used as the starting materials and SrF2 (99.50%), Li2CO3 (99.99%), LiF (99.00%), NH4Cl (99.50%), H3BO3 (99.00%) as the flux, weighed according to the stoichiometric ratio. After mixing and grinding the starting materials, the mixture was calcined at 1400 °C for 12h in air atmosphere; finally, the samples were cooled to room temperature in the furnace and reground again to obtain final samples for the following characterization. 2.2. Characterizations Phase confirmation of the synthesized phosphors were characterized by powder
X-ray diffraction (XRD) analysis on a D/max 2200PC diffractometer with Cu Kα radiation operated at 40 kV and 20 mA. All patterns were recorded in the range of 2θ from 15 to 75°. The UV-vis diffuse reflectance spectra (DRS) were performed by UH-4150 spectrophotometer (HITACHI). The photoluminescence excitation (PLE), photoluminescence (PL), and the temperature-dependent (25-250
) spectra were
measured with Hitachi F-7000 spectrophotometer equipped with a 150W xenon lamp and a successional temperature controller. Fluorescence quantum yields units (S-68) attached to the Model F-7000 was used to carry out the measurements of the Commission International de L’Eclairage (CIE) chromaticity coordinates and photoluminescence quantum yield. 3. Results and discussion 3.1. Phase analysis Sr4Nb2O9 with pure phase can be easily synthesized by traditional solid state reaction. However, impurity phases appeared after Eu3+ ions introduced. We speculated that this is due to differences in both of ionic radius and charge numbers between Eu3+ ions (r = 0.947 Å, CN = 6) and Sr2+ ions (r = 1.18 Å, CN = 6). SrF2, H3BO3, NH4Cl, LiF and Li2CO3 as flux to improve the purity of samples. The XRD patters of the selected samples with SrF2 and other flux are shown in Fig. 1 and Fig. S1 (H3BO3, NH4Cl, LiF and Li2CO3). Fig. 1 shows that the impurity diffraction peaks decrease with the dosage of SrF2 from 0% to 5%wt. After that, the impurity diffraction peaks appeared again which illustrates that the suitable dose flux can make the crystallinity of phosphor better. The phase purity of the as-prepared compounds was examined by XRD, and the results indicated that only SrF2 is the right flux when its usage locates in the range of 5% - 7%wt. The detailed crystal structure information on the samples were further determined by Rietveld refinement of Sr4Nb2O9 and Sr4Nb2O9:0.13Eu3+, 0.05SrF2 (Rietveld refinement were performed with EXPGUI and GSAS program packages) . The Rietveld analysis patterns of Sr4Nb2O9 (Rp=6.37%, Rwp=10.05%) and Sr4Nb2O9: 0.13Eu3+, 0.05SrF2 (Rp=5.93%, Rwp=9.79%) were presented in Figure 2a and b, respectively.
The
XRD
patterns
indicate
that
the
compound
Sr4Nb2O9:
0.13Eu3+,0.05SrF2 is isostructural to Sr4Nb2O9. The phase purity of Sr4Nb2O9:xEu3+,0.05SrF2 were analyzed by XRD. As shown in Fig. 3a, all the diffraction peaks of samples are in excellent agreement with the standard card of Sr4Nb2O9 (PDF#48-0558) without impurity phase, which demonstrates that Eu3+ ions occupy the Sr2+-sites in Sr4Nb2O9 and the doped Eu3+ ions have no obvious changes to the crystal structure. The simulated perovskite structure of Sr4Nb2O9 was shown in Fig. 3b. It is reported that the Sr4Nb2O9 has a cryolite-like structure with space group Fm3m (No. 225, a ≈ 2ac, where “c” refers to the cubic ~ 4Å perovskite unit cell). The structure of Sr4Nb2O9 also can be written as Sr2(Sr2/3Nb1/3)NbO6 (A2B’B’’O6; B’ and B’’ denote different B sites with ordering structure) in which mixed Sr/Nb sites and Nb sites are ordered along the [111] direction. [28, 29]. In Sr4Nb2O9, the ionic radius of Sr2+, Nb5+ are 1.18 Å, and 0.64 Å, respectively. The ionic radius of Eu3+ (r = 0.947 Å) is close to Sr2+ (r = 1.18 Å). Based on the comparison of the effective ion radius and valence, we assumed that the Eu3+ (r = 0.947 Å) ions are expected to randomly occupy the Sr2+ sites in Sr4Nb2O9. 3.2. The UV-vis absorption and luminescence properties of Eu3+-doped Sr4Nb2O9 phosphors The absorption spectra of Sr4Nb2O9 and selected Sr4Nb2O9:Eu3+ samples are shown in Fig. 4. All the samples were investigated from 200 to 800 nm. For the Sr4Nb2O9:0.13Eu3+ samples, the absorption peak at 383, 394, 465 and 533 nm are associated with 7F0-5L7, 7F0-5L6, 7F0-5D2, 7F0-5D1 transition of Eu3+ ions. In particular, it can be seen that the absorption peak intensity of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 is similar to that of Sr4Nb2O9:0.13Eu3+. This also indicates that there will be some relation with their luminescent behaviors. The excitation spectra and emission spectra of Sr4Nb2O9:0.13Eu3+ phosphors are shown in Fig. 5. The wide excitation band appearing below 350 nm is assigned to the charge-transfer band (CTB) between Eu3+ and the surrounding oxygen anions. The sharp lines at wavelengths exists at about 283, 394, 465 and 533nm, are assigned to the intra-configurational 4f-4f forbidden transitions of Eu3+ 7F0-5L7, 7F0-5L6, 7F0-5D2 and 7F0-5D1 respectively. It can be seen clearly that the excitation peaks mainly exist
at 394 nm and 465 nm, which indicate that the as-prepared Sr4Nb2O9:0.13Eu3+ phosphors match well with the emission spectra of near-UV and blue LEDs, respectively, thus making the synthesized phosphors suitable for solid-state light sources application. For the emission spectra of Sr4Nb2O9:0.13Eu3+ samples, peaks locate at 581 nm, 595nm, 614nm, 656nm and 710 nm in emission spectra can be ascribed to 5D0-7FJ (J = 0-4) transitions of Eu3+ ions [31]. Among these peaks, the red emission peak at 614 nm is more dominant than the other peaks, which is from the Eu3+ electric dipole transitions of 5D0-7F2, indicating Eu3+ occupies a non-symmetry site in the host lattice. This indicates that the Sr4Nb2O9:x0.13Eu3+ phosphor is suitable to be used as blue and near-UV LED excited red phosphor. The PL spectra of Sr4Nb2O9:xEu3+, 0.05SrF2, Sr4Nb2O9:0.13Eu3+, mSrF2 phosphors with different concentrations of Eu3+ and SrF2 are shown in Fig. 6a and b (The PL spectra of Sr4Nb2O9:xEu3+ showing in Fig. S2). When the doping concentration reaches 0.13, the emission intensity decreased due to the concentration quenching effect. According to Blasse’s theory: Rc ≈ 2[3V/ (4πxcN)] 1/3, where V is the unit cell volume of the host lattice, xc is the concentration of the dopant and N is the number of sites available for the dopant [32]. In Sr4Nb2O9:0.13Eu3+, V = 423.23 Å3, N = 2, given xc = 0.13, the critical transfer distance Rc of Eu3+ in Sr4Nb2O9 is calculated to be about 14.59 Å. We know that the exchange interaction is possible only when the Rc is smaller than 5 Å. The calculated results Rc of these samples are much bigger than 5 Å, indicating that no exchange interaction take place. Based on above experiment and analysis, we noticed that the purity and emission improved by introducing SrF2 as flux. Fig. 6b shows the effect of flux mass ratio on the luminescent intensity of phosphor Sr4Nb2O9:0.13Eu3+ excited by 394 nm. The PL intensity increases with the flux dosage until it reached 5 wt%. This indicates that all Eu3+ has entered the host crystal lattice and 5 wt% is the optimal flux mass ratio. In accordance with the XRD patterns of Sr4Nb2O9:0.13Eu3+, mSrF2. Flux in the process of synthesizing phosphor contributes to the reaction of powders . The PL intensity of Sr4Nb2O9:0.13Eu3+, Sr4Nb2O9:0.13Eu3+, 0.05SrF2, and CaMoO4:Eu3+ under different excitation wavelength is shown in Fig. 7 (The
excitation spectra and emission spectra of CaMoO4:Eu3+ showing in Fig. S3 and Fig. S4). It is reported that flux can enhance the emission intensity of Eu3+ ions [33-37]. In this work, we think the improvement in luminous intensity is because the flux acts not only as a lattice repair agent, but also as a charge compensator. On the other hand, (according to the Judd–Ofelt theory [38]), the luminescence intensity of Eu3+ is also relate to its symmetry. The (5D0-7F2)/ (5D0-7F1) emission ratio (the asymmetry ratio) can be used as an index to evaluate the site symmetry of Eu3+ ions [39-41]. Then we calculate the (5D0-7F2)/ (5D0-7F1) ratio of Eu3+ ions of all the as prepared samples. The obtained values found to be located between 3.9 and 4.6. This implies that most of the excitation energy of 5D0 level has contributed to the 614nm red emission. Studies have shown that Eu3+ will have strong luminescence properties when it is in a suitable lattice environment and low symmetry structure. In perovskite structure compound, structural symmetry can be characterized by tolerance factors (which can be used to describe the relationship between the stability of the perovskite structure and the ion size (the relationship with the ion size can be expressed as t = (RA+RO)/√2 (RB+RO), RA, RB, RO represent the ionic radius. Tolerance factors (the size represent different spatial groups of perovskite compounds). When the tolerance factor is close to 1, the crystal structure is cubic, away from 1, the crystal structure of the perovskite compound is distorted, symmetry decreased. In Sr4Nb2O9, RA = Sr2+ (r = 1.18 Å), RB = Nb5+ (r = 0.64 Å). When Eu3+ ions (r = 0.947 Å) replace the Sr2+ ions into the lattice acts as the A-site ion, lead to the tolerance factor of the perovskite compound becomes small, the structure is distorted [42], and the symmetry is lowered, then bringing about an increase in luminous intensity. In the Sr4Nb2O9:Eu3+ phosphor, Eu3+ ion is incorporated into a host lattice and substitutes for Sr2+ ion, in which the substitution is imbalanced. The defects existed in system will affect the luminescent intensity of Sr4Nb2O9:Eu3+ [43]. Another reason why Eu3+ ions exhibit strong luminescence properties in materials is that SrF2 not only was the flux but also used as a charge compensator to achieve the charge compensation and further improve the PL performance of the phosphors. The above factors make the Sr4Nb2O9:Eu3+ phosphor show very strong red light
emission. From Fig. 7 it is obviously seen that the relative intensity of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 (λexc=394nm) is close to that of CaMoO4: Eu3+ (λexc=289nm) (under 394/395nm excitation, the PL intensity of Sr4Nb2O9: 0.13Eu3+, 0.05SrF2 is about 177% than that of Sr4Nb2O9:0.13Eu3+ and 231% of CaMoO4:Eu3+). The obtained result indicated that the Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphor could have high quantum yield. 3.3. Thermal stability of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 and the references at different temperature. The thermal stability is another indispensable factor for phosphors in practical application. The PL intensity dependence of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 and the references at different temperature is depicted in Fig. 8. All the samples were investigated from 25
to 250
.
The emission intensity of Sr4Nb2O9:0.13Eu3+,
0.05SrF2 decreased to about 65% (Y3Al5O12:Ce3+ = 82%, CaAlSiN3:Eu2+ = 84%, CaMoO4:Eu3+ = 20%) of the initial value, respectively, when the temperature increased to 200 °C. The good thermal stability of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 can be attributed to its special double perovskite structure. Moreover, as an oxides-based compound, the Sr4Nb2O9:0.13Eu3+, 0.05SrF2 could have better physicochemical stability than unstable alkaline-earth sulfides to emit red light and meet the demand of high-power LEDs. 3.4. Quantum yield and CIE chromaticity coordinates of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphors Table 1 Quantum yield of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 and the references at different excitation wavelength. Samples
Excitation wavelength（nm）
η (%)
Sr4Nb2O9:0.13Eu3+,0.05SrF2
394
42.73
CaMoO4:0.24Eu3+
289
30.59
CaMoO4:0.24Eu3+
395
14.96
Y3Al5O12:Ce3+
447
91.58
CaAlSiN3:Eu2+
450
96.79
Quantum yield of selected samples were calculated according to the method described by L.A. Moreno. Briefly, the method allows determining the sample quantum yield Φf by measuring the internal quantum yield using direct excitation (Φd)/indirect excitation (Φi) and the number of those absorbed (Ad) by the sample using the relation: Φ = Φd－(1－Ad) Φi Where Φd is the internal quantum yield using direct excitation. (Internal quantum yield = Amount of Fluorescence/Amount of absorbed excitation light.), Ad is the absorptance for direct excitation. (This is the ratio of the amount of excitation beam absorbed by the sample), Φi is the internal quantum yield using indirect excitation [44]. The quantum yield of Sr4Nb2O9:0.13Eu3+, 0.05SrF2, together with CaMoO4:Eu3+ are listed in Table. 1 (the data are obtained under similar conditions). The quantum yield
of
commercial
red
phosphor
CaAlSiN3:Eu2+
and
yellow
phosphor
Y3Al5O12:Ce3+ under their optimal excitation band were provided as references. Sr4Nb2O9:0.13Eu3+, 0.05SrF2 performed quantum yield when excited by 394nm near-UV light. Compared with other phosphors, the composition-optimized Sr4Nb2O9:0.13Eu3+, 0.05SrF2 exhibits quantum yield of 140% that of CaMoO4:Eu3+, and 47% of the commercial yellow phosphor Y3Al5O12:Ce3+, 44% red phosphor CaAlSiN3:Eu2+ under their optimal excitation wavelength. The Commission Internationale de L’Eclairage (CIE) chromaticity coordinates of the selected Sr4Nb2O9:0.13Eu3+, 0.05SrF2 sample was calculated based on the corresponding PL spectra and shown in Fig. 9a. The CIE chromaticity coordinates of was (0.6583, 0.3414) which are close to the standard of National Television Standards Committee (NTSC, x = 0.670, y = 0.330). To further demonstrate the potential application of the Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphors, We fabricated a LED lamp device by using the blue chip and YAG:Ce3+ commercial yellow phosphor together with Sr4Nb2O9:0.13Eu3+, 0.05SrF2 red phosphor. Fig. 9b shows the EL spectra of the as-fabricated LED. The peak located at 450nm is the emission of InGaN LED chip
while the peak at 614 nm is attributed to the emission of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 red phosphors. The as fabricated white LED with the as-prepared Sr4Nb2O9:0.13Eu3+, 0.05SrF2 red phosphor exhibits high Ra (82.5) than that of white LED without red phosphor (75.6). Obviously, due to the introduction of the red phosphor, this parameter become better and more suitable for practical application. These results demonstrate that the addition of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphor could be favorable to obtain high Ra emission light LED. 4. Conclusion In summary, a series of double perovskite structured Sr4Nb2O9:Eu3+ phosphors were successfully synthesized by flux method (several different kinds of fluxes are used in the synthesis of of Sr4Nb2O9:Eu3+ phosphors, and SrF2 proved to be the best). The structure of the phosphors was identified by Rietveld refinements to crystallize with spacegroup Fm3m, which is isostructural to Sr4Nb2O9.The optimal concentration of Eu and usage of SrF2 were determined to be 0.13 mol and 5 wt%. The spectra analysis indicated that the phosphors could be excited by near UV light of 394 nm and blue light of 465 nm. Furthermore, the red-emitting efficiency can be effectively increased own to the improved of the crystal structure and charge compensation by introducing the SrF2 both as the flux and the charge compensator. Upon excitation of near UV light, the relative emission intensity of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 is 231% than that of CaMoO4:Eu3+. The composition-optimized Sr4Nb2O9:0.13Eu3+, 0.05SrF2 exhibits high quantum yield and 140% that of CaMoO4:Eu3+, 47% of the Y3Al5O12:Ce3+, 44% of CaAlSiN3:Eu2+ under their optimal excitation wavelength. Finally, a white light emitting LED with high Ra was fabricated by using a 450 nm blue chip with the as-obtained Sr4Nb2O9:0.13Eu3+, 0.05SrF2 and yellow phosphor Y3Al5O12:Ce3+. The results demonstrated the potential of employing the Sr4Nb2O9 as the host for Eu3+ ions doping and displayed the possible applications in WLEDs and other optical applications. Acknowledgment Jingshan Hou and Yanrong Cao have equally contributed to this work. This work is financially supported by the National Natural Science Foundation of China (NSFC)
(grant numbers: 51902203, 51672177, 51772184). Shanghai Institute of Technology Development Fund of Science, Technology Talents for Young and Middle Teachers (Grant Numbers ZQ2018-1) and the Program of Shanghai Academic/Technology Research Leader (19XD1434700).
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Figure captions Fig. 1 Fig. 1 XRD patterns of Sr4Nb2O9:0.13Eu3+ with different concentrations of SrF2. Fig. 2 Measured (x) and calculated (red line) powder XRD patterns of (a) Sr4Nb2O9 and (b) Sr4Nb2O9 :0.13Eu3+,0.05SrF2 sample. The Bragg reflection positions are labled as the pink sticks, and the bottom blue solid lines reprent the difference between Measured and calculated data. Fig. 3(a) XRD patterns of Sr4Nb2O9: xEu3+,0.05SrF2, and (b) the crystal structure of Sr4Nb2O9 Fig. 4 Absorption spectra of Sr4Nb2O9 and selected Sr4Nb2O9:0.13Eu3+ samples. Fig. 5 Excitation and emission spectra of Sr4Nb2O9:0.13Eu3+ Fig. 6(a) PL spectra of Sr4Nb2O9:xEu3+, 0.05SrF2, and (b) Sr4Nb2O9:0.13Eu3+, mSrF2 phosphors with different Eu3+ and SrF2 concentrations Fig. 7 Fig. 7 PL intensity of Sr4Nb2O9:0.13Eu3+, Sr4Nb2O9:0.13Eu3+, 0.05SrF2, and CaMoO4:Eu3+ Fig. 8 PL intensity of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 and the references at different temperature Fig. 9(a) CIE chromaticity coordinates (the inset picture is the phosphor under the 365 nm UV light excitation in dark background), and (b) EL spectrum of the fabricated LED device of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphors.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Highlights
Sr4Nb2O9:Eu3+ phosphor was synthesized by flux method. Sr4Nb2O9:Eu3+ presented red light emission under UV light excitation. Sr4Nb2O9:Eu3+ have high PL intensity and internal quantum efficiency.
3+
phosphor by introducing with SrF2 as flux
Jingshan Hou, Yanrong Cao, Guangxiang Jiang, Yu Liu, Ganghua Zhang, Yufeng Liu, Jun Zou, Fuqiang Huang, Yang Li, Yongzheng Fang PII:
S0022-2313(19)31249-9
DOI:
https://doi.org/10.1016/j.jlumin.2019.116771
Reference:
LUMIN 116771
To appear in:
Journal of Luminescence
Received Date: 21 June 2019 Revised Date:
8 September 2019
Accepted Date: 21 September 2019
Please cite this article as: J. Hou, Y. Cao, G. Jiang, Y. Liu, G. Zhang, Y. Liu, J. Zou, F. Huang, Y. Li, Y. 3+ Fang, Intense red emission from Sr4Nb2O9:Eu phosphor by introducing with SrF2 as flux and charge compensator, Journal of Luminescence (2019), doi: https://doi.org/10.1016/j.jlumin.2019.116771. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Intense red emission from Sr4Nb2O9:Eu3+ phosphor by introducing with SrF2 as flux and charge compensator Jingshan Hou a, Yanrong Cao a, Guangxiang Jiang a, Yu Liu a, Ganghua Zhang b ,Yufeng Liu a, Jun Zou c, Fuqiang Huang d, *, Yang Li a, * and Yongzheng Fang a, * a
School of Materials Science and Engineering, Shanghai Institute of Technology,
Shanghai, 201418, China. b
Shanghai Key Laboratory of Engineering Materials Application and Evaluation,
Shanghai Research Institute of Materials, Shanghai 200437, P. R. China. c
Institute of New Materials and Industrial Technology, Wenzhou University,
Wenzhou, 325035, China. d
State Key Laboratory of High Performance Ceramics and Superfine Microstructure,
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. Abstract: Sr4Nb2O9:Eu3+ phosphors were prepared by flux method. SrF2 was found to be the appropriate flux and its effect on the crystal structure and optical properties were investigated. The structure of Sr4Nb2O9:Eu3+ has been determined by X-ray powder diffraction with Rietveld refinement. The optimal Eu3+ doping concentrations and the SrF2 usage in Sr4Nb2O9 were determined to be 0.13 mol and 5wt% respectively. The spectra analysis indicated that the phosphors could be excited by near UV light of 394 nm and blue light of 465 nm, and yield an intense red light with a maximum at 614 nm. The as prepared phosphor shows its advantages in quantum yield and thermal stability when compared with that of CaMoO4:Eu3+, Y3Al5O12:Ce3+, CaAlSiN3:Eu2+ phosphors. Finally, a white LED with improved Ra (from 75.6 to 82.5) was fabricated by using a 450 nm blue chip with Y3Al5O12:Ce3+ and the as-prepared Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphor. Keywords: Phosphors, Solid-state reaction, Eu3+-doped, Flux, Luminescence ∗ Corresponding authors: [email protected] (F. Huang); [email protected]; [email protected]. (Y.Z.
1. Introduction Phosphor-converted white light-emitting diodes (WLEDs) have been widely used in the fields of display and general lighting applications due to their high luminescence efficiency, energy conservation, long lifetime and environmental friendliness characteristics [1-20]. Currently, the mass production of high-efficiency commercial WLEDs are manufactured by packaging a blue InGaN chip with the yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+ ) phosphor. However, this combination exhibits a high correlated color temperature and poor color-rendering index (Ra) due their weak emission intensity in the red and green region. As it is well known that, phosphor with red component emission is essential in a high Ra light source. Therefore, great efforts have been made for the development of red phosphor to complement the missed red emitting components [5-20]. Eu3+ ion is well known to be a narrow line red-emitting activator around 610 nm responding to the near-UV or blue light excitation attributed to its characteristic 5
D0-7FJ (J = 1, 2, 3, 4) transitions [6-19]. Such properties made Eu3+ doped
compounds became popular red phosphors for high Ra WLEDs. It was reported that bright red emissions can be achieved in some particular Eu3+-activated host materials, such as CaMoO4:Eu3+ [7-10], BaNb2O6:Eu3+ [11], LiGd(WO4)2:Eu3+ [12], Na3GaF6:Eu3+ [13], CaLa1-xMgM’O6:xEu3+ (M’ = Nb, Ta) [14]. However, considering the subsequent practical applications in WLEDs, the low-cost Eu3+ ions doped red phosphors with high quantum efficiency and good thermal stability still urgently required. Niobate is excellent host material for optical materials, especially those inorganic materials niobate containing Nb5+ ions have attracted increasing attention owing to their non-linear optical, piezoelectric, thermal stability, and luminescence properties [21-23]. Moreover, Eu3+ ion doped niobate containing compounds are also known as interesting luminescent materials. In general, they exhibit good self-laser luminescence due to the transfer of O2- to Nb5+ ions in the NbO6 group, which have a Fang).
strong broad band in the UV region. It has been reported that Eu3+ doped Nb5+ -containing niobate red phosphors exhibit excellent luminescent properties. Meanwhile, perovskite structural materials are excellent hosts for optical materials own to their high chemical-physical stability and diversity of their structure and composition. The luminescent properties of perovskite-type materials have also been widely investigated in recent years [24-26]. D. Pasero and R.J.D. Tilley have reported the structure of Sr4Nb2O9 in 1999, this compound with double perovskite compound in which Sr is accommodated in the B site with 1:1 ordering. Two thermodynamically stable polymorphs of Sr-Nb-O were confirmed to occur above and below 1250 above 1250
. The high-temperature polymorph (HT
). HT exhibits an average cryolite-like structure with NaCl like (1:1)
ordering of Sr and Nb on the B sites. On the other hand, the low temperature polymorph (LT below 1250
) has apparent P21/n monoclinic symmetry, and features
a different but undetermined type of B-site ordering [27-30]. However, to the best of our knowledge, the synthesis and luminescence properties of Eu3+-doped double perovskite niobate compound Sr4Nb2O9 has not been reported yet. Thus, in this work, we report the synthesis, crystal structure and luminescence properties of =Eu3+-doped the HT Sr4Nb2O9 phosphor. 2. Experimental 2.1. Synthesis of Sr4Nb2O9:xEu3+ phosphors A series of Sr4Nb2O9:xEu3+ (x = 0.01-0.20) phosphors were prepared by high temperature solid state reaction method from powder based precursors. The precursors SrCO3 (99.00%), Nb2O5 (99.99%), Eu2O3 (99.99%) were used as the starting materials and SrF2 (99.50%), Li2CO3 (99.99%), LiF (99.00%), NH4Cl (99.50%), H3BO3 (99.00%) as the flux, weighed according to the stoichiometric ratio. After mixing and grinding the starting materials, the mixture was calcined at 1400 °C for 12h in air atmosphere; finally, the samples were cooled to room temperature in the furnace and reground again to obtain final samples for the following characterization. 2.2. Characterizations Phase confirmation of the synthesized phosphors were characterized by powder
X-ray diffraction (XRD) analysis on a D/max 2200PC diffractometer with Cu Kα radiation operated at 40 kV and 20 mA. All patterns were recorded in the range of 2θ from 15 to 75°. The UV-vis diffuse reflectance spectra (DRS) were performed by UH-4150 spectrophotometer (HITACHI). The photoluminescence excitation (PLE), photoluminescence (PL), and the temperature-dependent (25-250
) spectra were
measured with Hitachi F-7000 spectrophotometer equipped with a 150W xenon lamp and a successional temperature controller. Fluorescence quantum yields units (S-68) attached to the Model F-7000 was used to carry out the measurements of the Commission International de L’Eclairage (CIE) chromaticity coordinates and photoluminescence quantum yield. 3. Results and discussion 3.1. Phase analysis Sr4Nb2O9 with pure phase can be easily synthesized by traditional solid state reaction. However, impurity phases appeared after Eu3+ ions introduced. We speculated that this is due to differences in both of ionic radius and charge numbers between Eu3+ ions (r = 0.947 Å, CN = 6) and Sr2+ ions (r = 1.18 Å, CN = 6). SrF2, H3BO3, NH4Cl, LiF and Li2CO3 as flux to improve the purity of samples. The XRD patters of the selected samples with SrF2 and other flux are shown in Fig. 1 and Fig. S1 (H3BO3, NH4Cl, LiF and Li2CO3). Fig. 1 shows that the impurity diffraction peaks decrease with the dosage of SrF2 from 0% to 5%wt. After that, the impurity diffraction peaks appeared again which illustrates that the suitable dose flux can make the crystallinity of phosphor better. The phase purity of the as-prepared compounds was examined by XRD, and the results indicated that only SrF2 is the right flux when its usage locates in the range of 5% - 7%wt. The detailed crystal structure information on the samples were further determined by Rietveld refinement of Sr4Nb2O9 and Sr4Nb2O9:0.13Eu3+, 0.05SrF2 (Rietveld refinement were performed with EXPGUI and GSAS program packages) . The Rietveld analysis patterns of Sr4Nb2O9 (Rp=6.37%, Rwp=10.05%) and Sr4Nb2O9: 0.13Eu3+, 0.05SrF2 (Rp=5.93%, Rwp=9.79%) were presented in Figure 2a and b, respectively.
The
XRD
patterns
indicate
that
the
compound
Sr4Nb2O9:
0.13Eu3+,0.05SrF2 is isostructural to Sr4Nb2O9. The phase purity of Sr4Nb2O9:xEu3+,0.05SrF2 were analyzed by XRD. As shown in Fig. 3a, all the diffraction peaks of samples are in excellent agreement with the standard card of Sr4Nb2O9 (PDF#48-0558) without impurity phase, which demonstrates that Eu3+ ions occupy the Sr2+-sites in Sr4Nb2O9 and the doped Eu3+ ions have no obvious changes to the crystal structure. The simulated perovskite structure of Sr4Nb2O9 was shown in Fig. 3b. It is reported that the Sr4Nb2O9 has a cryolite-like structure with space group Fm3m (No. 225, a ≈ 2ac, where “c” refers to the cubic ~ 4Å perovskite unit cell). The structure of Sr4Nb2O9 also can be written as Sr2(Sr2/3Nb1/3)NbO6 (A2B’B’’O6; B’ and B’’ denote different B sites with ordering structure) in which mixed Sr/Nb sites and Nb sites are ordered along the [111] direction. [28, 29]. In Sr4Nb2O9, the ionic radius of Sr2+, Nb5+ are 1.18 Å, and 0.64 Å, respectively. The ionic radius of Eu3+ (r = 0.947 Å) is close to Sr2+ (r = 1.18 Å). Based on the comparison of the effective ion radius and valence, we assumed that the Eu3+ (r = 0.947 Å) ions are expected to randomly occupy the Sr2+ sites in Sr4Nb2O9. 3.2. The UV-vis absorption and luminescence properties of Eu3+-doped Sr4Nb2O9 phosphors The absorption spectra of Sr4Nb2O9 and selected Sr4Nb2O9:Eu3+ samples are shown in Fig. 4. All the samples were investigated from 200 to 800 nm. For the Sr4Nb2O9:0.13Eu3+ samples, the absorption peak at 383, 394, 465 and 533 nm are associated with 7F0-5L7, 7F0-5L6, 7F0-5D2, 7F0-5D1 transition of Eu3+ ions. In particular, it can be seen that the absorption peak intensity of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 is similar to that of Sr4Nb2O9:0.13Eu3+. This also indicates that there will be some relation with their luminescent behaviors. The excitation spectra and emission spectra of Sr4Nb2O9:0.13Eu3+ phosphors are shown in Fig. 5. The wide excitation band appearing below 350 nm is assigned to the charge-transfer band (CTB) between Eu3+ and the surrounding oxygen anions. The sharp lines at wavelengths exists at about 283, 394, 465 and 533nm, are assigned to the intra-configurational 4f-4f forbidden transitions of Eu3+ 7F0-5L7, 7F0-5L6, 7F0-5D2 and 7F0-5D1 respectively. It can be seen clearly that the excitation peaks mainly exist
at 394 nm and 465 nm, which indicate that the as-prepared Sr4Nb2O9:0.13Eu3+ phosphors match well with the emission spectra of near-UV and blue LEDs, respectively, thus making the synthesized phosphors suitable for solid-state light sources application. For the emission spectra of Sr4Nb2O9:0.13Eu3+ samples, peaks locate at 581 nm, 595nm, 614nm, 656nm and 710 nm in emission spectra can be ascribed to 5D0-7FJ (J = 0-4) transitions of Eu3+ ions [31]. Among these peaks, the red emission peak at 614 nm is more dominant than the other peaks, which is from the Eu3+ electric dipole transitions of 5D0-7F2, indicating Eu3+ occupies a non-symmetry site in the host lattice. This indicates that the Sr4Nb2O9:x0.13Eu3+ phosphor is suitable to be used as blue and near-UV LED excited red phosphor. The PL spectra of Sr4Nb2O9:xEu3+, 0.05SrF2, Sr4Nb2O9:0.13Eu3+, mSrF2 phosphors with different concentrations of Eu3+ and SrF2 are shown in Fig. 6a and b (The PL spectra of Sr4Nb2O9:xEu3+ showing in Fig. S2). When the doping concentration reaches 0.13, the emission intensity decreased due to the concentration quenching effect. According to Blasse’s theory: Rc ≈ 2[3V/ (4πxcN)] 1/3, where V is the unit cell volume of the host lattice, xc is the concentration of the dopant and N is the number of sites available for the dopant [32]. In Sr4Nb2O9:0.13Eu3+, V = 423.23 Å3, N = 2, given xc = 0.13, the critical transfer distance Rc of Eu3+ in Sr4Nb2O9 is calculated to be about 14.59 Å. We know that the exchange interaction is possible only when the Rc is smaller than 5 Å. The calculated results Rc of these samples are much bigger than 5 Å, indicating that no exchange interaction take place. Based on above experiment and analysis, we noticed that the purity and emission improved by introducing SrF2 as flux. Fig. 6b shows the effect of flux mass ratio on the luminescent intensity of phosphor Sr4Nb2O9:0.13Eu3+ excited by 394 nm. The PL intensity increases with the flux dosage until it reached 5 wt%. This indicates that all Eu3+ has entered the host crystal lattice and 5 wt% is the optimal flux mass ratio. In accordance with the XRD patterns of Sr4Nb2O9:0.13Eu3+, mSrF2. Flux in the process of synthesizing phosphor contributes to the reaction of powders . The PL intensity of Sr4Nb2O9:0.13Eu3+, Sr4Nb2O9:0.13Eu3+, 0.05SrF2, and CaMoO4:Eu3+ under different excitation wavelength is shown in Fig. 7 (The
excitation spectra and emission spectra of CaMoO4:Eu3+ showing in Fig. S3 and Fig. S4). It is reported that flux can enhance the emission intensity of Eu3+ ions [33-37]. In this work, we think the improvement in luminous intensity is because the flux acts not only as a lattice repair agent, but also as a charge compensator. On the other hand, (according to the Judd–Ofelt theory [38]), the luminescence intensity of Eu3+ is also relate to its symmetry. The (5D0-7F2)/ (5D0-7F1) emission ratio (the asymmetry ratio) can be used as an index to evaluate the site symmetry of Eu3+ ions [39-41]. Then we calculate the (5D0-7F2)/ (5D0-7F1) ratio of Eu3+ ions of all the as prepared samples. The obtained values found to be located between 3.9 and 4.6. This implies that most of the excitation energy of 5D0 level has contributed to the 614nm red emission. Studies have shown that Eu3+ will have strong luminescence properties when it is in a suitable lattice environment and low symmetry structure. In perovskite structure compound, structural symmetry can be characterized by tolerance factors (which can be used to describe the relationship between the stability of the perovskite structure and the ion size (the relationship with the ion size can be expressed as t = (RA+RO)/√2 (RB+RO), RA, RB, RO represent the ionic radius. Tolerance factors (the size represent different spatial groups of perovskite compounds). When the tolerance factor is close to 1, the crystal structure is cubic, away from 1, the crystal structure of the perovskite compound is distorted, symmetry decreased. In Sr4Nb2O9, RA = Sr2+ (r = 1.18 Å), RB = Nb5+ (r = 0.64 Å). When Eu3+ ions (r = 0.947 Å) replace the Sr2+ ions into the lattice acts as the A-site ion, lead to the tolerance factor of the perovskite compound becomes small, the structure is distorted [42], and the symmetry is lowered, then bringing about an increase in luminous intensity. In the Sr4Nb2O9:Eu3+ phosphor, Eu3+ ion is incorporated into a host lattice and substitutes for Sr2+ ion, in which the substitution is imbalanced. The defects existed in system will affect the luminescent intensity of Sr4Nb2O9:Eu3+ [43]. Another reason why Eu3+ ions exhibit strong luminescence properties in materials is that SrF2 not only was the flux but also used as a charge compensator to achieve the charge compensation and further improve the PL performance of the phosphors. The above factors make the Sr4Nb2O9:Eu3+ phosphor show very strong red light
emission. From Fig. 7 it is obviously seen that the relative intensity of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 (λexc=394nm) is close to that of CaMoO4: Eu3+ (λexc=289nm) (under 394/395nm excitation, the PL intensity of Sr4Nb2O9: 0.13Eu3+, 0.05SrF2 is about 177% than that of Sr4Nb2O9:0.13Eu3+ and 231% of CaMoO4:Eu3+). The obtained result indicated that the Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphor could have high quantum yield. 3.3. Thermal stability of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 and the references at different temperature. The thermal stability is another indispensable factor for phosphors in practical application. The PL intensity dependence of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 and the references at different temperature is depicted in Fig. 8. All the samples were investigated from 25
to 250
.
The emission intensity of Sr4Nb2O9:0.13Eu3+,
0.05SrF2 decreased to about 65% (Y3Al5O12:Ce3+ = 82%, CaAlSiN3:Eu2+ = 84%, CaMoO4:Eu3+ = 20%) of the initial value, respectively, when the temperature increased to 200 °C. The good thermal stability of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 can be attributed to its special double perovskite structure. Moreover, as an oxides-based compound, the Sr4Nb2O9:0.13Eu3+, 0.05SrF2 could have better physicochemical stability than unstable alkaline-earth sulfides to emit red light and meet the demand of high-power LEDs. 3.4. Quantum yield and CIE chromaticity coordinates of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphors Table 1 Quantum yield of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 and the references at different excitation wavelength. Samples
Excitation wavelength（nm）
η (%)
Sr4Nb2O9:0.13Eu3+,0.05SrF2
394
42.73
CaMoO4:0.24Eu3+
289
30.59
CaMoO4:0.24Eu3+
395
14.96
Y3Al5O12:Ce3+
447
91.58
CaAlSiN3:Eu2+
450
96.79
Quantum yield of selected samples were calculated according to the method described by L.A. Moreno. Briefly, the method allows determining the sample quantum yield Φf by measuring the internal quantum yield using direct excitation (Φd)/indirect excitation (Φi) and the number of those absorbed (Ad) by the sample using the relation: Φ = Φd－(1－Ad) Φi Where Φd is the internal quantum yield using direct excitation. (Internal quantum yield = Amount of Fluorescence/Amount of absorbed excitation light.), Ad is the absorptance for direct excitation. (This is the ratio of the amount of excitation beam absorbed by the sample), Φi is the internal quantum yield using indirect excitation [44]. The quantum yield of Sr4Nb2O9:0.13Eu3+, 0.05SrF2, together with CaMoO4:Eu3+ are listed in Table. 1 (the data are obtained under similar conditions). The quantum yield
of
commercial
red
phosphor
CaAlSiN3:Eu2+
and
yellow
phosphor
Y3Al5O12:Ce3+ under their optimal excitation band were provided as references. Sr4Nb2O9:0.13Eu3+, 0.05SrF2 performed quantum yield when excited by 394nm near-UV light. Compared with other phosphors, the composition-optimized Sr4Nb2O9:0.13Eu3+, 0.05SrF2 exhibits quantum yield of 140% that of CaMoO4:Eu3+, and 47% of the commercial yellow phosphor Y3Al5O12:Ce3+, 44% red phosphor CaAlSiN3:Eu2+ under their optimal excitation wavelength. The Commission Internationale de L’Eclairage (CIE) chromaticity coordinates of the selected Sr4Nb2O9:0.13Eu3+, 0.05SrF2 sample was calculated based on the corresponding PL spectra and shown in Fig. 9a. The CIE chromaticity coordinates of was (0.6583, 0.3414) which are close to the standard of National Television Standards Committee (NTSC, x = 0.670, y = 0.330). To further demonstrate the potential application of the Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphors, We fabricated a LED lamp device by using the blue chip and YAG:Ce3+ commercial yellow phosphor together with Sr4Nb2O9:0.13Eu3+, 0.05SrF2 red phosphor. Fig. 9b shows the EL spectra of the as-fabricated LED. The peak located at 450nm is the emission of InGaN LED chip
while the peak at 614 nm is attributed to the emission of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 red phosphors. The as fabricated white LED with the as-prepared Sr4Nb2O9:0.13Eu3+, 0.05SrF2 red phosphor exhibits high Ra (82.5) than that of white LED without red phosphor (75.6). Obviously, due to the introduction of the red phosphor, this parameter become better and more suitable for practical application. These results demonstrate that the addition of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphor could be favorable to obtain high Ra emission light LED. 4. Conclusion In summary, a series of double perovskite structured Sr4Nb2O9:Eu3+ phosphors were successfully synthesized by flux method (several different kinds of fluxes are used in the synthesis of of Sr4Nb2O9:Eu3+ phosphors, and SrF2 proved to be the best). The structure of the phosphors was identified by Rietveld refinements to crystallize with spacegroup Fm3m, which is isostructural to Sr4Nb2O9.The optimal concentration of Eu and usage of SrF2 were determined to be 0.13 mol and 5 wt%. The spectra analysis indicated that the phosphors could be excited by near UV light of 394 nm and blue light of 465 nm. Furthermore, the red-emitting efficiency can be effectively increased own to the improved of the crystal structure and charge compensation by introducing the SrF2 both as the flux and the charge compensator. Upon excitation of near UV light, the relative emission intensity of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 is 231% than that of CaMoO4:Eu3+. The composition-optimized Sr4Nb2O9:0.13Eu3+, 0.05SrF2 exhibits high quantum yield and 140% that of CaMoO4:Eu3+, 47% of the Y3Al5O12:Ce3+, 44% of CaAlSiN3:Eu2+ under their optimal excitation wavelength. Finally, a white light emitting LED with high Ra was fabricated by using a 450 nm blue chip with the as-obtained Sr4Nb2O9:0.13Eu3+, 0.05SrF2 and yellow phosphor Y3Al5O12:Ce3+. The results demonstrated the potential of employing the Sr4Nb2O9 as the host for Eu3+ ions doping and displayed the possible applications in WLEDs and other optical applications. Acknowledgment Jingshan Hou and Yanrong Cao have equally contributed to this work. This work is financially supported by the National Natural Science Foundation of China (NSFC)
(grant numbers: 51902203, 51672177, 51772184). Shanghai Institute of Technology Development Fund of Science, Technology Talents for Young and Middle Teachers (Grant Numbers ZQ2018-1) and the Program of Shanghai Academic/Technology Research Leader (19XD1434700).
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Figure captions Fig. 1 Fig. 1 XRD patterns of Sr4Nb2O9:0.13Eu3+ with different concentrations of SrF2. Fig. 2 Measured (x) and calculated (red line) powder XRD patterns of (a) Sr4Nb2O9 and (b) Sr4Nb2O9 :0.13Eu3+,0.05SrF2 sample. The Bragg reflection positions are labled as the pink sticks, and the bottom blue solid lines reprent the difference between Measured and calculated data. Fig. 3(a) XRD patterns of Sr4Nb2O9: xEu3+,0.05SrF2, and (b) the crystal structure of Sr4Nb2O9 Fig. 4 Absorption spectra of Sr4Nb2O9 and selected Sr4Nb2O9:0.13Eu3+ samples. Fig. 5 Excitation and emission spectra of Sr4Nb2O9:0.13Eu3+ Fig. 6(a) PL spectra of Sr4Nb2O9:xEu3+, 0.05SrF2, and (b) Sr4Nb2O9:0.13Eu3+, mSrF2 phosphors with different Eu3+ and SrF2 concentrations Fig. 7 Fig. 7 PL intensity of Sr4Nb2O9:0.13Eu3+, Sr4Nb2O9:0.13Eu3+, 0.05SrF2, and CaMoO4:Eu3+ Fig. 8 PL intensity of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 and the references at different temperature Fig. 9(a) CIE chromaticity coordinates (the inset picture is the phosphor under the 365 nm UV light excitation in dark background), and (b) EL spectrum of the fabricated LED device of Sr4Nb2O9:0.13Eu3+, 0.05SrF2 phosphors.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Highlights
Sr4Nb2O9:Eu3+ phosphor was synthesized by flux method. Sr4Nb2O9:Eu3+ presented red light emission under UV light excitation. Sr4Nb2O9:Eu3+ have high PL intensity and internal quantum efficiency.