Reduction of Eu3+ to Eu2+ in Ba1−xSrxGd1−yYyB9O16:Eu phosphor: Synthesis, composition controlling, and tunable luminescence

Journal of Alloys and Compounds 698 (2017) 565e570

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Reduction of Eu3þ to Eu2þ in Ba1xSrxGd1yYyB9O16:Eu phosphor: Synthesis, composition controlling, and tunable luminescence Wanping Chen*, Xiaomei Chen, Fangfang Sun, Xiping Wang Key Laboratory of Rare Earth Optoelectronic Materials and Devices, School of Chemistry and Materials Engineering, Huaihua University, Huaihua, 418008, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 October 2016 Received in revised form 17 December 2016 Accepted 20 December 2016 Available online 21 December 2016

Eu2þ and Eu3þ codoping phosphor have attracted much attention due to their tunable luminescence color and potential application in lighting and display. In this work, Eu3þ ions are controllably reduced to Eu2þ ions in Ba1-xSrxGd1-yYyB9O16 to synthesize Eu2þ and Eu3þ codoping phosphors by using a hightemperature solid-state method. In the obtained phosphors, the Eu2þ shows a blue emission of ~460 nm, and Eu3þ shows a characteristic red emission with a maximum at ~617 nm. The relative emission intensity of Eu2þ and Eu3þ can be tuned by the incorporation of Sr2þ or Y3þ, which results in a tunable luminescence color in the range of purplish blue to purplish red. The results indicate that BaGd0.2Y0.76B9O16:0.04Eu simultaneously show bright red and blue emission, and its chromatic coordinate is the closest to the white-light region. The tunable luminescence color is explained on the basis of the crystal structure of BaGdB9O16. © 2016 Elsevier B.V. All rights reserved.

Keywords: Photoluminescence Borate phosphor Eu2þ and Eu3þ codoping BaGdB9O16

1. Introduction Nowadays, white light-emitting diodes (LEDs), as the nextgeneration solid-state lighting, have attracted much attention owing to their energy saving, reasonable cost, long lifetime, high light efficiency, and environmental friendliness [1]. In general, multicomponent phosphor mixture is required to achieve whitelight emitting. However, single-host phosphors to generate multicolor light or white light have also attracted great attention due to overcoming the drawbacks of self-absorption and complex blending operations. Multicolor-light or white-light emission of single-host phosphors can be achieved by the codoping of multiactivators such as Eu2þ/Ce3þ and Eu2þ/Dy3þ [2]. Different from the phosphors codoped with different kinds of rare earth ions, mixed valences of Eu ions in some specific inorganic compounds have been widely obtained [3e10], and the optimal combination of an individual spectrum for each valence can be explored to generate multicolor light or white light. For examples, in CaYAlO4 [6] Ca2NaSiO4F [7], Ca2Tb8(SiO4)6O2 [8] LaF3 [9], BaZnSiO4 [10] phosphors, white-light emitting was observed due to the codoping of Eu2þ and Eu3þ excited with UV light.

* Corresponding author. E-mail address: [email protected] (W. Chen). http://dx.doi.org/10.1016/j.jallcom.2016.12.258 0925-8388/© 2016 Elsevier B.V. All rights reserved.

Recently, polyborate RLnB9O16 (Ln ¼ rare earths, R ¼ Sr and Ba) have been considered as a host crystal to develop new phosphor, although the crystal structure of RLnB9O16 is controversial [11,12]. Many rare earth ions such as Eu3þ, Tb3þ, Ce3þ, Sm3þ, and Dy3þ were doped or codoping in LnRB9O16 [12e14], and the obtained phosphors have been considered to be suitable candidates as phosphors for white LEDs, tricolor lamps, and plasma display panels [12]. However, as far as we known, Eu2þ and Eu3þ codoping RLnB9O16 phosphor has not been reported. In this work, Eu3þ ions are partially reduced to synthesize Eu2þ and Eu3þ codoping Ba1xSrxGd1-yYyB9O16:Eu phosphors by using a high-temperature solidstate method with CO as a weak reducing atmosphere. The Sr2þ and Y3þ are intentionally doped in the phosphor to controllably reduce Eu3þ and to effectively achieve tunable emission color. 2. Experimental All phosphor were synthesized by using a conventional hightemperature solid-state reaction. Starting materials contain Gd2O3 (99.99%), Y2O3 (99.99%), Eu2O3 (99.99%), H2BO3 (analytical reagent, AR), BaCO3 (AR), and SrCO3 (AR). Appropriate amounts of raw materials were stoichiometrically mixed and thoroughly ground in an agate mortar, and then heated at 900  C for 12 h with CO as reducing atmosphere. Some desired white phosphors were obtained after cooled and reground.

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The purity and crystal structure of phosphors was measured by using X-ray diffraction with CuKa radiation (l ¼ 1.5045 Å) on a Rigaku Ultima IV type powder X-ray diffractometer. Photoluminescence behaviors of all phosphors were analyzed with a Hitachi F-7000 fluorescence spectrophotometer. All measurements were performed at room temperature. 3. Results and discussion 3.1. XRD patterns of products

and the impurity peaks around ~30 were denoted with a red triangle. The impurity peak is attributed to the existence of Ba2B2O5 (PDF no. 24-0087). Fig. 1f is the XRD pattern of BaGd0.2Y0.76B9O16:0.04Eu. When Y3þ is gradually instituted by Eu3þ, BaGd0.2Y0.8-xB9O16:xEu is the predominant phase in the assynthesis phosphors. Besides Ba2B2O5, diffraction peak of BaB2O4 (PDF no. 24-0086) also is observed (denoted by a red triangle in Fig. 1f). However, the diffraction peaks of the two impurities almost have no change in intensity with increasing doping concentration of Eu. When Ba2þ is gradually instituted by Eu2þ in Ba1-xGd0.2Y0.8B9O16:xEu, no obvious impurity diffraction peaks can be observed. The XRD pattern of a representative phosphor Ba0.98Gd0.2Y0.8B9O16:0.02Eu is presented in Fig. 1g.

In RLnB9O16 crystal, due to the co-existence of Ln3þ and R2þ, when a small quantity of Eu were incorporate in this host crystal, it is undoubted that the Eu should enter into the two distinct lattice sites as the form of Eu3þ and Eu2þ, respectively. Therefore, it is not difficult to obtain two kinds of emission of Eu3þ and Eu2þ from a single-phase phosphor. Herein, the compound BaGdB9O16 is selected to synthesize Eu2þ and Eu3þ codoping phosphor. Fig. 1 presents some typical XRD patterns of the as-synthesized phosphors. Fig. 1b is XRD patterns of host crystal BaGdB9O16, which well agree with a PDF file (no. 40-0646) of BaGdB9O16 (Fig. 1a). This indicates that a pure BaGdB9O16 phase was obtained. When Ba2þ is partially substituted by Eu2þ, the obtained phosphors show completely similar XRD pattern to the pure BaGdB9O16 phase. Fig. 1c is the XRD pattern of representative phosphor Ba0.98GdB9O16:0.02Eu. This indicates that the incorporation of Eu does not change the crystal structure of host BaGdB9O16. When Ba2þ is gradually substituted by Sr2þ, the corresponding XRD pattern shows a slight change in shape. At low doping concentration, XRD pattern is in agreement with the pure BaGdB9O16 phase. When doping concentration is 0.4 (mole fraction), an impurity X-ray diffraction peak can be observed around 33.6 denoted by a red triangle in Fig. 1d. The intensity of the diffraction peak gradually enhances with the increasing Sr2þ concentration, and a similar intensity to the diffraction peak (114) of BaGdB9O16 occurs when the Sr2þ concentration increases to 0.99. The XRD pattern of the impurity well matches with PDF file (no. 13-0485) of EuBO3 (no present). Therefore, the impurity diffraction peak is attributed to the formation of EuBO3. When Gd3þ is gradually substituted by Y3þ, except for two very weak diffraction peaks, the X-ray diffraction patterns of all phosphors are consistent with the PDF card file of BaGdB9O16. Fig. 1e is the XRD pattern of BaGd0.57Y0.4B9O16:0.03Eu,

Fig. 2 presents normalized emission and excitation spectra of phosphor Ba0.98GdB9O16:0.02Eu. Upon 260 nm excitation, the corresponding emission spectra (denoted with a) contain a broadband emission and some line emissions. Broadband blue emission centered at ~460 nm is attributed to Eu2þ emission. The line emission is typical red emission of Eu3þ. Among these line emissions, the 5D07F2 emission around 617 nm is predominant, and the relative intensity of 5D07F1 around 592 nm is much less than that of 5D07F2. Upon 274 nm excitation, a similar emission spectrum b can be obtained, and the relative intensity of Eu2þ shows a little decrease. However, when the phosphor were excited upon 340 nm, only broadband blue emission of Eu2þ can be observed in the emission spectrum e (inserted in Fig. 2), and the red emission of Eu3þ is completely negligible. By monitoring the emission at 460 nm of Eu2þ, corresponding excitation spectrum c extends from ~225 nm to 425 nm and contains two strong absorption bands denoted with bands I and II. The band I (centered at ~260 nm) and II (centered at ~340 nm) are attributed to the 4f5d transition absorption of Eu2þ. On the longwavelength side of the band I have an obvious shoulder peak around ~280 nm. By monitoring the emission of 617 nm, the corresponding excitation spectrum d contains a broadband absorption and several line absorptions. The broadband absorption with a maximum at ~238 nm is attributed to the charge-transfer band (CTB) of O2-Eu3þ. Among these line absorptions, the strongest

Fig. 1. PDF file of BaGdB9O16 (no. 40-0646) and XRD pattern of BaGdB9O16 doped with various cations.

Fig. 2. Emission and excitation spectra of phosphor Ba0.98GdB9O16:0.02Eu.

3.2. Luminescence properties of Ba1-xGdB9O16:xEu

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3.3. Impact of Sr2þ on luminescence of Ba1xSrxGdB9O16:Eu

Fig. 3. Emission spectra of phosphors Ba1-xGdB9O16:xEu with x ¼ 0.005, 0.01, 0.02, 0.03, 0.04, and 0.06 upon 260 nm excitation.

absorption at ~274 nm is attributed to 8S7/26I7/2 transition absorption of Gd3þ. On the basis, the shoulder peak around ~280 nm in spectrum c is also attributed to the absorption of Gd3þ. Obviously, at a weak reducing atmosphere, Eu3þ ions in BaGdB9O16 are partially reduced to Eu2þ ions, and there exist two different luminescence centers of Eu2þ and Eu3þ in phosphor BaGdB9O16:Eu. This indicates that Eu inevitably enters two different cation lattices of Ba2þ and Gd3þ. However, a nominal formula Ba0.98GdB9O16:0.02Eu still have been used to representative of the actual product, although Eu is artificially designed as the form of Eu2þ to substitute Ba2þ in the synthesis process. The existence of 8 S7/26I7/2 absorption band (~274 nm) indicates that the excitation energy of Gd3þ can be transferred to Eu2þ and Eu3þ. That is, an energy transfer process occurs between Gd3þ and Eu2þ/Eu3þ. However, an energy transfer from Eu2þ to Eu3þ did not occur because the red emission of Eu3þ in the spectrum e is not observed. Fig. 3 presents emission spectra of phosphors Ba1xGdB9O16:xEu with x ¼ 0.005, 0.01, 0.02, 0.03, 0.04, and 0.06 upon 260 nm excitation. All phosphors show similar emission spectra in shape, which consists of a broadband emission of Eu2þ and some line emissions of Eu3þ. However, the Eu2þ and Eu3þ in BaGdB9O16 show different change trends in intensity with increasing doping concentration of Eu. Among these line emissions, the 5D07F2 emission is much stronger than the D07F1 emission. Theses line emissions show the same change trend in intensity. Enlarged emission peaks of 5D07F2 are inserted on the right side of Fig. 3, in which the arrow and corresponding lower-case letters are used to indicate the change trend in emission intensity. It can be seen that the red emission of Eu3þ gradually enhance with increasing doping concentration of Eu. However, the emission intensity of the phosphor Ba1xGdB9O16:xEu with x ¼ 0.03 is slightly stronger than that of the phosphor with x ¼ 0.04, and their emission line almost completely overlap one another. Upon 260 nm excitation, the emission of Eu2þ gradually increases when the x value is less than 0.02, and then gradually decreases when the x value exceeds 0.02. The change trend in emission intensity of Eu2þ can be obviously observed from enlarged emission spectra inserted on the left side of Fig. 3. This indicates that a concentration quenching phenomenon occurs for the Eu2þ emission. Because the doped Eu ions always simultaneously occupy two different cation lattice sites of Ba2þ and Gd3þ, the optimum doping concentration of Eu2þ in BaGdB9O16 should be less than 0.02.

Fig. 4 presents normalized emission and excitation spectra of phosphor Ba0.9Sr0.1Gd0.97B9O16:0.03Eu. Upon 238 nm excitation, the corresponding emission spectrum (denoted with a) consists of a weak broadband emission and several strong line emissions. Obviously, these line emissions are attributed to the red emission of Eu3þ and the broadband emission is attributed to the blue emission of Eu2þ. Compared with the emission of Ba0.98GdB9O16:0.02Eu, an obvious difference is that the emission of Eu2þ in Ba0.9Sr0.1Gd0.97B9O16:0.03Eu becomes weaker and shows an obvious blue shift. The maximum emission of Eu2þ shifts from ~460 nm in Ba0.98GdB9O16:0.02Eu to ~420 nm in Ba0.9Sr0.1Gd0.97B9O16:0.03Eu. By monitoring the emission of 617 nm, the excitation spectrum of Ba0.9Sr0.1Gd0.97B9O16:0.03Eu (line b) is similar to that of Ba0.98GdB9O16:0.02Eu (line d in Fig. 2). The CTB of O2-Eu3þ is the strongest absorption with a maximum at ~238 nm. The medium absorption centered at ~274 nm is 8S7/26I7/2 transition absorption of Gd3þ. By monitoring the emission of 420 nm, the corresponding excitation spectrum c consists of two broadband absorption denoted by band I and band II. Obviously, excitation spectrum c of Ba0.9Sr0.1Gd0.97B9O16:0.03Eu is similar to that of Ba0.98GdB9O16:0.02Eu (line c in Fig. 2) in shape. However, herein, the band I obviously contain two absorption peaks centered at ~270 and 280 nm respectively, and the maximum absorption of the band II shifts to ~330 nm. Fig. 5 presents the emission spectra of phosphors Ba1xSrxGd0.97B9O16:0.03Eu with x ¼ 0.1, 0.2, 0.4, 0.6, 0.8, and 0.99 upon 238 nm excitation. All emission spectra simultaneously contain broadband emission of Eu2þ and line emission of Eu3þ. The red emissions of Eu3þ show same change trend in the intensity, and the intensity gradually decreases with increasing Sr2þ concentration. The change trend can be obviously observed from the enlarged emission of 5D07F2 inserted in Fig. 5. Among these line emissions, the 5D07F2 emission is predominant. However, the emission intensity ratio of 5D07F2 to 5D07F1 gradually decreases from 2.90 to 1.25. On short-wavelength side of these emission spectra, there exist two broadband emissions of Eu2þ denote by band I (centered at ~370 nm) and band II (centered at ~420 nm), respectively. The emission intensity of band II is very weak and slowly enhances with increasing doping concentration of Sr2þ. At low doping

Fig. 4. Normalized emission Ba0.9Sr0.1Gd0.97B9O16:0.03Eu.

and

excitation

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phosphor

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Fig. 5. Emission spectra of phosphors Ba1-xSrxGd0.97B9O16:0.03Eu with x ¼ 0.1, 0.2, 0.4, 0.6, 0.8, and 0.99 upon 238 nm excitation.

Fig. 6. Emission spectra of BaGd0.97-xYxB9O16:0.03Eu with x ¼ 0.1, 0.2, 0.4, 0.6, 0.8, and 0.97 upon 260 nm excitation.

concentration of Sr2þ such as x ¼ 0.1 and 0.2, the emission band I is very weak and almost negligible. However, when the x value exceeds 0.2, the intensity of band I show a rapid enhancement with increasing Sr2þ concentration. It can be seen from Figs. 4 and 5 that the incorporation of Sr2þ resulted in the obvious change in the emission spectra. This can be explained according to the crystal structure of BaGdB9O16. The cation size sequence is Y3þ < Gd3þ < Eu3þ < Eu2þ < Sr2þ < Ba2þ [15]. Due to the ionic size of Sr2þ is less than that of Ba2þ, it is proposed that the crystal cell size will be severely shrunk with the incorporation of a large number of Sr2þ (named cell shrinking effect). This will prevent Eu from entering the crystal to a certain extent. That is, the Eu3þ is difficult to enter the small Gd3þ lattice site, and the Eu2þ is difficult to enter the Ba2þ lattice site. Sequentially, the red emission of Eu3þ gradually decreases with the increasing Sr2þ concentration. Maybe this is a reason that more and more GdBO3 is obtained as an impurity in the phosphors (discussed in Fig. 1). Two broadband emission of Eu2þ in the emission spectra shows the existence of two distinct Ba2þ lattice sites in the host crystal. Generally, Eu2þ should occupy large Ba2þ lattice site. However, the Eu2þ was compelled to enter small Ba2þ lattice site owing to the incorporation of Sr2þ (named site occupying effect) [16]. Eu2þ suffers a strong crystal field effect and shows an obvious blue-shift emission due to the cell shrinking effect. Therefore, the blue emission of Eu2þ in large Ba2þ site shifts from ~460 nm to ~420 nm. Under the synergism of the site occupying effect and the cell shrinking effect, the 420 nm emission of Eu2þ in large Ba2þ site becomes very weak and the 370 nm emission of Eu2þ in small Ba2þ site show gradual increase in intensity. In a word, due to the incorporation of Sr2þ, the number of Eu atom entered the host crystal lattice is lowered, but the relative quantity of Eu2þ occupied the smaller Ba2þ site is increased.

and the 5D07F1 emission around 592 nm become predominant. The 5D07F1 emission slowly decreases when x value is less than 0.4, and then obviously increase when x value exceeds 0.4. Interestingly, the emission peak around ~603 nm become very weak when x ¼ 0.4, however, a new emission peak around ~600 nm occurs and its intensity gradually increase when the x value exceeds 0.4 (see the insert on the right side of Fig. 6). It is noted that the phosphor with x ¼ 0.8 show a rapid enhancement in emission intensity of 5D07F1. The 5D07F1 emissions of the phosphor with x ¼ 0.8 and 0.97 completely overlap and can not be identified. The blue emission of Eu2þ always enhances until the x value increases to 0.8. When x value is 0.97, the blue emission shows an obvious decrease in intensity. It can be seen that the Eu2þ and Eu3þ in BaGd0.97xYxB9O16:0.03Eu show different change trend in emission intensity, and that the 5D07F1 and 5D07F2 emission of Eu3þ also show different change trend in intensity. Their change trends can be obviously observed from the insert on the left side of Fig. 6. In the insert, the symbols represent actual integrated intensity of three emissions at the different doping concentration of Y3þ. Line (1) shows the integrated intensity change of 5D07F2 emission in the range of 609e625 nm; Line (2) shows the integrated intensity change of 5D07F1 emission in the range of 580e609 nm; Line (3) shows the integrated intensity change of broadband emission of Eu2þ in the range of 400e550 nm. Obviously, the 5D07F2 emission gradually decreases, and then the 5D07F1 emission gradually enhances with increasing Y3þ concentration. Based on the above observation, the influence of Y3þ on the luminescence behavior is different from that of Sr2þ. The incorporation of Y3þ results in the change of Gd3þ site environment. First, the site size of Gd3þ becomes small to some extent. This prevents Eu3þ from entering Gd3þ lattice site and result in the decrease of the red emission of Eu3þ, especially for the 5D07F2 emission. Therefore, more Eu2þ ions enter large Ba2þ lattice sites and the emission intensity of Eu2þ gradually enhance with increasing Y3þ concentration. Second, the incorporation of Y3þ effectively changes the symmetry of Gd3þ lattice site. When Y3þ content is much larger than Gd3þ content, the host crystal should be BaYB9O16 in fact, in which the site symmetry of Y3þ is different from that of Gd3þ in BaGdB9O16 [13]. It is well known that electric-dipole 5D07F2 is forbidden in centrosymmetric sites, and magnetric-dipole 5D07F1 transition is insensitive to site symmetry. It is proposed that the site

3.4. Impact of Y3þ on luminescence of BaGd1xYxB9O16:Eu Fig. 6 presents the emission spectra of Ba0.99Gd0.98xYxB9O16:0.03Eu with x ¼ 0.1, 0.2, 0.4, 0.6, 0.8, and 0.97 upon 260 nm excitation. All emission spectra consist of blue emission of Eu2þ and red emission of Eu3þ. The 5D07F2 emission around 617 nm gradually decreases with increasing doping concentration of Y3þ. When x ¼ 0.1, the 5D07F2 emission is predominant. Whereas x ¼ 0.97, the 5D07F2 emission is almost negligible

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Fig. 7. Emission spectra of phosphors BaGd0.2Y0.8-xB9O16:xEu with x ¼ 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, and 0.15 upon 260 nm excitation.

Fig. 8. Emission spectra of phosphors Ba1-xGd0.2Y0.8B9O16:xEu with x ¼ 0.005, 0.01, 0.02, 0.03, 0.04, and 0.06 upon 260 nm excitation.

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symmetry of Eu3þ converts from noncentrosymmetry to centrosymmetry with increasing Y3þ concentration. This will decrease the emission of 5D07F2 and enhance the emission of 5D07F1. Therefore, the emission of 5D07F2 was gradually decreased, and the emission of 5D07F1 was decreased in first and then gradually enhanced with increasing Y3þ concentration. Obviously, the incorporation of Y3þ hardly has influence on the site environment of Ba2þ. BaGdB9O16 is considered to be a layered compound, in which Gd3þ and B3þ constitute the layer and Ba2þ locates in interlayer position [17]. The different influence of Y3þ and Sr2þ on the host crystal structure may be related to the layered structure. Fig. 7 presents emission spectra of phosphors BaGd0.2Y0.8xB9O16:xEu with x ¼ 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, and 0.15 upon 260 nm excitation. With increasing doping concentration of Eu, the red emission of Eu3þ gradually enhances. The enhancement rate of 5 D07F2 is faster than that of 5D07F1. When x ¼ 0.01, the 5D07F1 emission is dominant, whereas x ¼ 0.15, the 5D07F2 emission exceed the 5D07F1 emission in intensity. The arrow and lower-case letters indicate the enhancement trends. This shows that the incorporation of a large quantity of Eu3þ also changes the symmetry of Gd3þ/Y3þ site. Compared to the red emission of Eu3þ, the blue emission of Eu2þ around 460 nm shows a different change trend in intensity, which can be seen from enlarged emission spectra of Eu2þ inserted in Fig. 7. The blue emission has an obvious enhancement with increasing doping concentration, but show a rapid decrease when the x value exceeds 0.04. When x ¼ 0.15, the blue emission around 460 nm become very weak and a new emission band around 420 nm occurs. This indicates that a little Eu2þ enter small Ba2þ lattice sites in a high doping concentration of Eu2þ. Fig. 8 presents emission spectra of phosphors Ba1xGd0.2Y0.8B9O1:xEu 6 with x ¼ 0.005, 0.01, 0.02, 0.03, 0.04, and 0.06 upon 260 nm excitation. All phosphors show similar emission spectra. The blue emission of Eu2þ shows a rapid enhancement when x are less than 0.01, and then shows same emission intensity in the doping range of 0.01e0.04. The 5D07F1 of Eu3þ emission gradually enhance with increasing doping concentration of Eu. The two kinds of emissions shows an obvious decrease when x exceed 0.04. This indicates that their optimum doping concentration is about 0.04. However, the 5D07F2 emission always enhance with increasing doping concentration. Compared to Fig. 7, it can be concluded that the incorporation of a large quantity of Y3þ or Eu3þ in Gd3þ sites will obviously change the site symmetry of Gd3þ, but the incorporation of Eu2þ, specially for low concentration Eu2þ, have little

Fig. 9. Chromatic coordination of phosphor Ba1-xGdB9O16:xEu (a), BaGd0.97-xYxB9O16:0.03Eu (b), and Ba1-xGd0.2Y0.8B9O16: xEu (c) upon 260 nm excitation.

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influence on the site symmetry of Gd3þ. This results in the difference in the optimum doping concentration of Eu3þ in Figs. 7 and 8. In addition, it is proposed that the impact of Y3þ on Gd3þ sites in different positions is different, which results in the occurrence of two distinct Eu3þ luminescence centers. Therefore, in Ba1xGd0.2Y0.8B9O16:xEu, the change tends in the emission intensity of 5 D07F1 and 5D07F2 is different.

incorporation of Y3þ. Simultaneously, the blue emission of Eu2þ is enhanced to a certain extent. Therefore, the luminescence colors of the phosphors doped with Y3þ more close to the white region. The results show that it will become possible that the phosphor is considered as a potential white phosphor when the synthesis conditions are further optimized. Acknowledgment

3.5. Chromatic coordinate of phosphors Fig. 9 presents the chromatic coordinates of phosphor Ba1xGdB9O16:xEu, Ba0.99Gd0.98-xYxB9O16:0.03Eu, and Ba1xGd0.2Y0.8B9O16: xEu upon 260 nm excitation. Obviously, the luminescence color can be easily tuned from purplish blue to purplish red. In Fig. 9a, the chromatic coordinate (0.653, 0.347) of Eu3þ in BaGdB9O16 is showed in the red region. For Ba1xGdB9O16:xEu phosphor, with increasing concentration of Eu (x value), the luminescence color of the phosphor show an obvious shift from purple to purplish red due to the enhance red emission of Eu3þ (Fig. 3). The chromatic coordinate of the phosphor with the smallest and largest x value is (0.238, 0.161) and (0.387, 0.223), respectively. Fig. 9b shows that the chromatic coordinate shifts from purple region to purplish blue region when the Gd3þ ions were gradually substituted by Y3þ in BaGd0.97xYxB9O16:0.03Eu. This is mainly attributed to the quick enhancement in the blue emission of Eu2þ (Fig. 6). The corresponding chromatic coordinate is (0.304, 0.178) and (0.202, 0.139) for the smallest and largest x value, respectively. In Fig. 9c, an abnormal change in chromatic coordinate occurs for Ba1xGd0.2Y0.8B9O16: xEu phosphor. That is, when x value is the smallest (0.005), the chromatic coordinate is (0.409, 0.279) and locates the purplish red region. Whereas, with increasing x value, the chromatic coordinate shifts from purple region to purplish red region, and the corresponding chromatic coordinate is (0.287, 0.195) and 0.367, 0.238) for the x value of 0.01 and 0.06, respectively. More importantly, compared to another two kinds of phosphors Ba1xGdB9O16:xEu and BaGd0.97xYxB9O16:0.03Eu (Fig. 9a and b), the chromatic coordinate of phosphor Ba1xGd0.2Y0.8B9O16: xEu more close to the white region overall. It is attributed to the predominant emission of Eu3þ shift from 5D07F2 (617 nm) to 5D07F1 (592 nm) due to the existence of a large quantity of Y3þ ions and the existence of bright blue emission of Eu2þ (Fig. 8). 4. Conclusions A series of Eu2þ and Eu3þ codoping Ba1xSrxGd1-yYyB9O16:Eu phosphors were synthesized by using a high-temperature solidstate reaction. The incorporation of Sr2þ or Y3þ can change the site occupancy of Eu ion and the reduction of Eu3þ to Eu2þ in BaGdB9O16:xEu. In Ba1xGdB9O16:xEu, Eu2þ shows a blue emission of ~460 nm, and Eu3þ shows a typical red emission with a predominant 5D07F2 (~617 nm)emission. The incorporation of Sr2þ result in a shrinking crystal cell and a blue shift of Eu2þ emission, which is bad for the application; while the site symmetry of Eu3þ undergoes obvious change and the predominant emission of Eu3þ changes from 5D07F2 (~617 nm) to 5D07F1 (~592 nm) due to the

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