Synthesis and photoluminescence properties of a novel BaGe4O9:Eu3+ red emitting phosphor for warm white LEDs

Synthesis and photoluminescence properties of a novel BaGe4O9:Eu3+ red emitting phosphor for warm white LEDs

Accepted Manuscript 3+ Synthesis and photoluminescence properties of a novel BaGe4O9:Eu red emitting phosphor for warm white LEDs Changyan Ji, Zhi Hua...

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Accepted Manuscript 3+ Synthesis and photoluminescence properties of a novel BaGe4O9:Eu red emitting phosphor for warm white LEDs Changyan Ji, Zhi Huang, Xiuying Tian, Wei Xie, Jin Wen, Hengping He, Cai Zhou, Ting Zeng PII:

S0143-7208(18)31488-8

DOI:

10.1016/j.dyepig.2018.09.012

Reference:

DYPI 6994

To appear in:

Dyes and Pigments

Received Date: 6 July 2018 Revised Date:

31 August 2018

Accepted Date: 5 September 2018

Please cite this article as: Ji C, Huang Z, Tian X, Xie W, Wen J, He H, Zhou C, Zeng T, Synthesis and 3+ photoluminescence properties of a novel BaGe4O9:Eu red emitting phosphor for warm white LEDs, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.09.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Graphical and textual abstract for the contents page

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Novel BaGe4O9:Eu3+ red-emitting phosphors for warm light-emitting diodes were synthesized by a high-temperature solid-state reaction.

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Synthesis and photoluminescence properties of a novel BaGe4O9:Eu3+ red emitting phosphor for warm white LEDs Changyan Jia,*, Zhi Huangb, Xiuying Tiana, Wei Xiec, Jin Wena, Hengping Heb, Cai Zhoub and Ting Zengb a

Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials, School of Materials and Environmental

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Engineering, Hunan University of Humanities, Science and Technology, Loudi 417000, China b

National electronic ceramics product quality supervision and inspection center (Hunan), Loudi 417000, China;

c

School of Physical Science and Technology, Lingnan Normal University, Zhanjiang 524048, P. R. China;

Abstract: Novel BaGe4O9:Eu3+ red-emitting phosphors were synthesized by a high-temperature solid-state reaction. The crystal structures, morphological properties, elements analysis, optical properties and fluorescence decay time were investigated systematically. In contrast, the optimal Eu3+-doping concentration is

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30 mol% when monitored at 591 nm with a 394 nm excitation. The energy transfer of Eu3+ ions in BaGe4O9 host lattice is dominated by the exchange interaction with the critical distance (Rc) of 5.28 Å. As expected, the red-emitting phosphor BaGe3.7O9:0.3Eu3+ exhibited a better quantum yield (QY) of 57.9% and an excellent

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lifetime of 2.25 ms. Consequently, the red-emitting phosphors BaGe3.7O9:0.3Eu3+ revealed a promising performance in warm WLED device with the CIE coordinates of (0.4365, 0.4130), the color temperature (Tc) of 3079 K and the color rendering index (Ra) of 85.8 under a driven current of 20 mA. These results indicate that the BaGe4O9:Eu3+ phosphors are prospective candidates as red-light component for warm white light-emitting diodes (WLEDs) devices.

Key words: White light-emitting diodes; Red-emitting phosphors; Solid-state reaction; BaGe4O9

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1 Introduction

In the past few decades, the WLEDs has been a hot topic in the field of lighting for their distinctive characteristics of energy-efficient, long-lifetime, good stability, and quick response [1]. To date, two important strategies on device fabrication have been developed to achieve WLEDs: (1) blue LED chip (450-470 nm) with yellow phosphors [2]; (2) near-ultraviolet (370-410 nm) LED chip with red, green and blue phosphors [3]. Though the

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most common method towards WLEDs is combining a blue LED chip with commercial Y3Al5O12:Ce3+ yellow phosphors, the high correlated color temperature (CCT) and poor color rendering index (CRI) of WLEDs for lacking of red-light component restrict its practical application[4]. Therefore, several kinds of red red emitting

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phosphors were developed to settle this problem [5]. Besides, more and more researchers have focused their efforts on pumping a near-ultraviolet LED chip with the red, green and blue phosphors. This approach is beneficial to improve the optical performance of WLEDs on account of the enhancement of red region spectrum. It is thus important to develop novel phosphors especially the red-light components with better luminescent properties for efficient WLEDs. To meet this need, an effective method of developing novel efficiency red-emitting phosphors is to dope rare earth ions into a host lattice [3a, 6]. Among many studies, great interests in Eu3+acting as activator ions have been received due to its unique 5D0-7F2 transition, corresponding to the strong red emission at about 615 nm wavelength[7]. So far, the Eu3+-activated red-emitting phosphors have widely appeared in tungstates, borates, phosphates, molybdates, aluminates and vanadates host lattices [7b, 8]. Besides, metal germanates acting as well functional materials have also been extensively used in many fields for their excellent physical and chemical characteristics. For instance, Fu et al. reported several germanium materials Zn2GeO4, Bi2GeO5 and Cd2Ge2O6, which can be used as photocatalysts for the decomposition of organic pollutants in water [9]. Ding et al. prepared a

ACCEPTED MANUSCRIPT series of Mn4+-activated red phosphors with potassium germanium as the host lattice and present strong emission intensity [10]. In addition, barium germanates materials BaGe4O9 has been extensively used in lithium-ion battery fields owing to its high capacity [11]. Noticeably, good luminescent properties were observed for phosphors containing BaGe4O9 as host lattice [12]. Consequently, we envisioned that the Eu3+-activated metal germanate BaGe4O9 could be a red-emitting phosphors candidate for WLEDs. Yet, it is worth to mention that the Eu3+-activated BaGe4O9 red emitting phosphor was seldom reported. To examine the effect of Eu3+ ions as the activator on properties of BaGe4O9 host lattice, a series of red-emitting

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phosphors BaGe4-xO9:xEu3+ (0.05≤x≤0.6) were synthesized successfully via high-temperature solid-state reaction method, and their crystal structure, morphological features, elements analysis and optical properties as well as the application in WLEDs were fully investigated. This work clearly indicates that BaGe4O9:Eu can be a promising red phosphor candidate in warm WLEDs application.

2 Experimental

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2.1 Synthesis

A series of Eu3+-activated red emitting BaGe4-xO9:xEu3+ (0.05≤x≤0.6) phosphors were prepared by

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high-temperature solid-state reaction method. The high purity powders of BaCO3 (99.99%), GeO2 (99.999%), Eu2O3 (99.999%) were weighted according to a certain stoichiometric ratio. Then the starting materials were mixed thoroughly with ethanol using an alumina pestle and mortar. Finally, the mixtures were placed in a crucible and annealed at 1100 °C for 4 h in air. After that, these mixtures were cooled down naturally to room temperature. 2.2 Fabrication of warm WLED device

The synthesized BaGe3.7O9:0.3Eu3+ red phosphors, commercially purchased BAM:Eu2+ blue phosphors, (Ba, Sr)2SiO4:Eu2+ green phosphors and a 395 nm near-ultraviolet chip (Shenzhen looking long technology co., LTD) were used to fabricate the warm WLED device. Firstly, these phosphors were mixed with organic silica gel

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thoroughly. Then the mixtures were coated on a 395 nm chip and solidified. The obtained device was dried at 120 °C for 1.0 h, followed by electroluminescence tests under different drive currents from 20 mA to 120 mA. 2.3. Characterizations

The crystal structures of these phosphors were identified through a Shimadzu 6100 X-ray diffractometer with Cu

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Kα radiation. The Rietveld structure refinements were performed using the general structure analysis systerm (GSAS) program. The photoluminescence (PL), PL excitation (PLE) spectra and quanturm yeild (QY) were characterized using an F-7000 fluorescence spectrophotometer (Hitachi) equipped with a Xe lamp. The

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morphological characteristics of these phosphors were analyzed by FEI Quanta 200 Thermal FE Environment scanning electron microscope (SEM) with an attached energy-dispersive spectrometer (EDS). The luminescence decay curve was obtained from an FLS920 fluorescence spectrophotometer. The performance of the warm WLED device was investigated using a high accuracy array spectrometer (HSP6000). 3 Results and discussion 3.1 Crystal Structure The XRD measurements were performed to investigate the effect of Eu3+contents on crystalline phases of BaGe4-xO9:xEu3+ (0.05≤x≤0.6) phosphors and the corresponding XRD patterns are shown in Fig. 1. The results showed that the relatively intense and sharp peaks observed in the XRD patterns revealed the crystalline nature of BaGe4-x O9:xEu3+. All the diffraction peaks were well matched with the standard BaGe4O9 phase (JCPDS NO. 43-0644), and no obvious peaks shifting and other impurity phase were observed, indicating that the Eu3+ ions

ACCEPTED MANUSCRIPT were well diffused in the BaGe4O9 host lattices and a small amount of Eu3+ ions doping did not influence the crystal structure significantly. To get more crystal structure information, Rietveld refinement of the XRD patterns has been done[13]. The refinement results of the representative BaGe3.7O9:0.3Eu3+ samples are shown in Fig. 2(a) with a 2 theta range of 5-120º. The important refinement data and crystal structural parameters are summarized in Table 1 and Table 2. For the refinement of BaGe3.7O9:0.3Eu3+, the structre parameters of BaGe4O9 (ICSD No. 28203) were used as startingstructral model. As shown in Table 1, the reliability factors RP = 8.98%, Rwp = 9.23% and χ2 = 2.12,

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indicated that all atom positions, fraction factors of the refinement phosphor BaGe3.7O9:0.3Eu3+ well satisty the reflection condition. Fig. 2(b) shows the unit cell structure of BaGe3.7O9:0.3Eu3+. The BaGe4O9 host is crystallized in a P321 (150) space group with hexagonal unit cell parameters of a = b = 11.609 Å, c = 4.743 Å, V = 553.588 Å3 and Z = 1. In comparision with standard crystal, it is interesting to note that there are slightly variation for the values of unit cell parameters a, b and V, indicating that the Eu3+ ions can partially replace the Ge4+ ions without

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obviously changes of crystal structure. It can be seen that from Table 2, there are four kinds of Ge4+ sites in the BaGe4O9 host lattice. The Ge1 and Ge2 are surrounded by six oxygen atoms to form GeO6 octahedra, and the Ge3 and Ge4 are surrounded by four oxygen atoms to form GeO4 tetrahedra.

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3.2 Size and Morphology

The morphological features of Eu3+-activated red BaGe4O9 powder were characterized by scanning electron microscope (SEM), and the representative SEM image of BaGe3.7O9:0.3Eu3+ is shown in Fig. 3(a). The SEM image revealed that the samples possess irregular crystal shapes. Bearing in mind that the better morphology indicated strong crystallization. It was interesting to note that the samples exhibit clear geometric edges and smooth surfaces. Moreover, the powder exhibited obvious agglomerations, which maybe result from the high temperature solid-state reaction processed [14].

The energy dispersive spectrometer (EDS) taken from SEM image was investigated, and the corresponding EDS

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spectrum is shown in Fig. 3(b). It is obviously to be found that the elements Ba, Ge, O and Eu are all present in Fig. 3(b), which is consistent with the XRD analysis. The results further determining the construction of BaGe3.7O9:0.3Eu3+ phosphor. 3.3 Luminescence properties

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The PLE spectra of BaGe3.7O9:0.3Eu3+ phosphors monitored at 591 nm and 613 nm is shown in Fig. 4 (a). Obviously, the relative intensity of PLE spectra monitored at 613 nm is relatively lower than that of monitored at 591 nm. In addition, both the PLE spectra are similar and exhibit broad band covering from

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350 nm to 550 nm with a series of sharp peaks. The excitation peaks wavelength located at approximately 364, 382, 394, 416, 466 and 534 nm, respectively, attribute to the 7F0→5D4, 7F0→3G5, 7F0→5L6, 7F0→5D3, 7

F0→5D2, 7F0→5D1 transitions of Eu3+ ions, respectively [15]. Among them, the excitation peak located at

394 nm shows much stronger intensity, indicating that BaGe3.7O9:0.3Eu3+ phosphors can be excited efficiently by near ultraviolet lights. Fig. 4(b) gives the PL profile of BaGe3.7O9:0.3Eu3+ excited at 394 nm. The PL spectra exhibited wide emission band between 570 nm and 640 nm with two visibly maximum peaks at 591 nm and 613 nm, respectively. These emissions can be ascribed to the transitions of 5D0→7F1 and 5D0→7F2 of Eu3+ ions, respectively. Acorrding to the magnetic dipole (MD) transition rule (∆J = 0, ±1), the 5D0→7F1 belongs to the MD transition and its intensity is insensitive to the crystal environment surrounding the Eu3+ ions, whereas the 5D0→7F2 transition is related to the electric dipole (ED) transition of Eu3+ ions and its PL intensity is largely dependent on the surrounded crystal field of Eu3+ ions [7b, 10, 16]. Generally, the 5D0→7F1 transition plays a leading role in

ACCEPTED MANUSCRIPT the PL spectra when the Eu3+ ions occupied the high symmetry sites, whereas the 5D0→7F2 transition becomes the strongest one. It is worth noting that the ratio of 5D0→7F2/D0→7F1 peak was estimated to be about 0.93 for BaGe3.7O9:0.3Eu3+ phosphors indicated that Eu3+ ions occupied the positions with high symmetry. In addition, a weak PL emission peak located at about 654 nm also be observed, owing to the 5D0→7F3 transition. In contrast, the emission intensity of 613 nm is slightly lower than that of 591 nm, indicating that the Eu3+ ions occupied two

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nonequivalent sites in the BaGe4O9 host lattice [17]. Moreover, the BaGe3.7O9:0.3Eu3+ phosphors exhibited bright red light when excited by 395 nm near ultraviolet light, as shown in inset of Fig.4(b).

To better understand the correlation between the PLE and PL emission intensity and the Eu3+-doping concentration, the PLE and PL emission spectra of BaGe4-xO9: xEu3+ (0.05≤x≤0.6) with increasing concentration of Eu3+ ions were discussed, the corresponding PLE and PL profiles are shown in Fig. 5. It can be seen that the optimal Eu3+-doping concentration is x = 0.3. In addition, the PLE and PL emission intensity gradually increased

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with the increasing concentration of Eu3+ ions within the range 0 to 30 mol%, and decreased with the further increasing of Eu3+ ions concentration. These results can be attributed to the concentration quenching effect.

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3.4 Mechanism of energy transfer

To investigate the concentration quenching mechanism, the critical distance (Rc) of Eu3+ ions in BaGe4O9 was calculated according to Blasse equation:[18]

Rc ≈ 2[

3V ⅓ ] 4πxcZ

where Rc is the critical distance, V is the volume of the unit cell, xc is the critical concentration of Eu3+ ions and Z is the number of cation sites in the unit cell. In this work, V = 553.77 Å3, xc = 0.3, Z = 3. As a result, the Rc was estimated to be about 5.28 Å between 5 Å and 8 Å, indicating that the nonradiative energy transfer of

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Eu3+ ions in BaGe4O9 is dominated by the exchange interaction[19]. In order to further identify the concentration quenching mechanism between Eu3+ ions, the relationship between PL emission intensity (I) per doping concentration of Eu3+ ions (x) and x was described by the Dexter equation, and the relationship was followed:[20]

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log (I/x) = c- (θ/3) log (x)

where I is the PL emission intensity, x is the doping concentration of Eu3+ ions, c and θ are the constants. The θ values of are 3, 6, 8, 10, which stand for the exchange coupling interaction, dipole-dipole interaction,

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dipole-quadrupole interaction and quadrupole-quadrupole interaction, respectively. From the inset of Fig.4(bottom), it can be seen that log(I/x) is linear with logx, and the slope is fitted to be about -0.69 Therefore, the value of θ is 2.07 close to 3, which further indicating that the concentration quenching mechanism of Eu3+ ions in BaGe4O9 is dominated by the exchang interaction. 3.5 Quantum yield and Photoluminescence Decay The quantum yield (QY) was measurement to evaluate the luminescence performance of the BaGe3.7O9:0.3Eu3+ phosphors, the related QY curve of BaGe3.7O9:0.3Eu3+ is shown in Fig. 6(a). It can be seen that the red-emitting phosphor BaGe3.7O9:0.3Eu3+ exhibited an excellent QY value of 57.9% at around 396 nm, attributed to its relatively better optical properties. This result can be beneficial to improve its performance in WLEDs application. Fig. 6(b) shows the PL decay curve of BaGe3.7O9:0.3Eu3+ phosphors at room tempreature. The corresponding monitoring wavelength is 591 nm with a 394 nm excitation. The decay curve is well in conformity with the single exponential equation: [21]

ACCEPTED MANUSCRIPT I = I0exp(-t/τ)+A in which I and I0 are the luminescence intensities at t and τ= 0, respectively, A is the constent, t is the time, τ is the lifetime. As a result, the lifetime of BaGe3.7O9:0.3Eu3+ was estimated to be 2.25 ms. 3.6 Application in WLEDs To investigate the potential application of BaGe3.7O9:0.3Eu3+ phosphors, the warm WLED device was fabricated with 395 nm near-ultraviolet chip and BAM:Eu2+ blue phosphors, (Ba,Sr)2SiO4:Eu2+ green phosphors, and BaGe3.7O9:0.3Eu3+ red phosphors driven by 20 mA current and 3 V voltage. The mass ratio of BAM:Eu2+ :

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(Ba, Sr)2SiO4:Eu2+ : BaGe3.7O9:0.3Eu3+ = 2:1:5. The EL spectrum and the CIE chromaticity diagram are shown in Fig. 7(a), and the relative performance is summarized in Table 3. As a result, the EL spectrum of the warm WLED device based on BaGe3.7O9:0.3Eu3+ exhibited several red emission peaks located at about 587 nm, 591 nm, 596 nm, 613 nm, 616 nm, 622 nm and 654 nm, respectively, in agreement well with its photoluminescence study results. In addition, the CIE coordinates, Tc and Ra of this warm WLED device were

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(0.4365, 0.4130), 3079 K and 85.8, respectively. The relatively lower Tc and higher Ra can be rationally ascribed to the much favoured structure composition of BaGe3.7O9:0.3Eu3+. Notably, the CIE coordinate with the color point close to the black body locus, as shown in Fig. 7(b). The dependences of this WLED device performance on the

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different driven current between 20 mA and 120 mA were measured to further investigate the WLED application. The related EL spectra are displayed in Fig. 7(c) and the corresponding results are summarized in Table 3. In contrast, the CIE coordinates, Tc and Ra of the WLED changed slightly when the driven current increased. This phenomenon suggests that the potential of BaGe3.7O9:0.3Eu3+ for the application of warm light-emitting diodes. 4. Conclusions

In summary, a series of novel Eu3+ ions activated BaGe4O9 red-emitting phosphors for warm WLEDs

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were synthesized and characterized. The XRD paterns demonstrated the synthesized red phosphors possess a pure hexahedral phase. The PL and PLE tests showed that the optimum Eu3+-doping concentration is 30 mol% when monitored at 591 nm in the excitation of 394 nm. The energy transfer mechanism reveals that the exchange interaction contributed to the concentration quenching. As expected, BaGe3.7O9:0.3Eu3+ revealed a promising performance in the warm WLED device with the CIE of (0.4365,

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0.4130), along with a Ra of 85.8 and a Tc of 3079 K under a driven current of 20 mA, which attributed to the good luminisence properties and QY as well as the excellent PL lifetime. These results indicated that the Eu3+ ions activated BaGe4O9 can be used as a kind of promising red-emitting phosphor in warm

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WLEDs application.

Acknowledgements

The work is supported by the Open Foundation of Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials (No. TC201704 and TC201701), the Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials (No. 2016TP1028), the Scientific Research Fund of Hunan Provincial Education Department (No. 17B138), the Natural Science Foundation of Hunan Province (No. 2018JJ3251), the Natural Science Foundation of China (No. 11747074), the Guangdong Province Science and Technology Plan Project Public Welfare Fund and Ability Construction Project (2017A010103025) and the Zhanjiang Science and Technology Plan Project (2017A03021).

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(c) Z.-w. Zhang, L.-j. Wang, S.-s. Yang, et al. Luminescence properties of a novel promising red phosphor LiY9(SiO4 )6 O2:Eu3+. Mater. Lett., 2017, 204, 101-103.

[15] (a) H. Guo, X. Huang and Y. Zeng. Synthesis and photoluminescence properties of novel highly thermal-stable red-emitting Na3Sc2(PO4 )3:Eu3+ phosphors for UV-excited white-light-emitting diodes. J.

Alloys Compd., 2018, 741, 300-306; (b) P. Du and J. S. Yu. Synthesis and luminescent properties of Eu3+ -activated Na0.5Gd 0.5MoO4: A strong red-emitting phosphor for LED and FED applications. J. Lumin., 2016, 179, 451-456; (c) P. Du, X. Huang and J. S. Yu. Facile synthesis of bifunctional Eu3+-activated NaBiF4 red-emitting nanoparticles for simultaneous white light-emitting diodes and field emission displays. Chem. Eng. J., 2018, 337, 91-100.

[16] (a) Y. Wei, H. Jia, H. Xiao, et al. Emitting-tunable Eu(2+/3+)-doped Ca(8−x)La(2+x) (PO4)6−x(SiO4)xO2 apatite phosphor for n-UV WLEDs with high-color-rendering. RSC Advances, 2017, 7, 1899-1904; (b) P. Du and J. S. Yu. Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors with ultrabroad excitation band for white light-emitting diodes. Sci.

Rep., 2017, 7: 11953 DOI:10.1038/s41598-41017-12161-41595.

ACCEPTED MANUSCRIPT [17] (a) Y. S. Vidya, K. S. Anantharaju, H. Nagabhushana, et al. Euphorbia tirucalli mediated green synthesis of rose like morphology of Gd2O3:Eu3+ red phosphor: Structural, photoluminescence and photocatalytic studies. J. Alloys Compd., 2015, 619, 760-770; (b) L. Zhou, J. Shi and M. Gong. Preparation of SrR2O4:Eu3+ (R=Y, Lu) phosphor and its photoluminescence properties. Mater. Lett., 2005, 59, 2079-2084. [18] G. Blasse. Energy transfer in oxidic phosphors. Phys. Lett., 1968, 28(444-445. [19] J. Zhang, Y. Wang, Z. Zhang, et al. The relationship between photoluminescence quenching concentrations and excitation wavelengths in (Gd,Y)BO3:Tb. Mater. Lett., 2008, 62, 202-205.

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[20] D. L. Dexter and J. H. Schulman. Theory of concentration quenching in inorganic phosphors. J. Chem. Phys., 1954, 22, 1063-1070.

[21] K.-H. Chen, M.-H. Weng, R.-Y. Yang, et al. New NaSrPO4:Sm3+ phosphor as orange-red emitting material.

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Bull. Mater. Sci., 2016, 39, 1171-1176.

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Figure Captions

Fig. 1 The XRD patterns of BaGe4-xO9:xEu3+ (0.05≤x≤0.6) phosphors and the JCPDS card #43-0644 for BaGe4O9. Fig. 2 Rietveld refinement of powder XRD patterns 0of BaGe3.7O9:0.3Eu3+, (b) the crystal structure of BaGe3.7O9:0.3Eu3+ from refinement results.

Fig. 3 (a) SEM image, (b) EDS spectrum of the BaGe3.7O9:0.3Eu3+ phosphor

Fig. 4 (a) PLE spectrum of the BaGe3.7O9:0.3Eu3+ phosphors monitoring at 591 and 613 nm, (b) PL emission spectra of the BaGe3.7O9:0.3Eu3+ phosphors excited at 394 nm.

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Fig. 5 The PLE spectrum of BaGe4-xO9: xEu3+ (0.05≤x≤0.6) phosphors monitoring at 591 nm (top); the PL emission spectra of BaGe4-xO9: xEu3+ (0.05≤x≤0.6) phosphors excited at 394 nm (bottom). The top inset shows the variation of PLE intensity monitored at 591 nm, the bottom inset shows the variation of PL emission intensity under 394 nm excitations.

Fig. 6 (a) the QY curve, (b) the decay curve of BaGe3.7O9:0.3Eu3+ phosphors

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Fig. 7 (a) the EL spectrum and photographs, (b) the CIE chromaticity diagram,

of the fabricated white LED

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device with 395 nm near-ultraviolet chip and BAM:Eu blue phosphors, (Ba,Sr)2SiO4:Eu2+ green phosphors, and BaGe3.7O9:0.3Eu3+ red phosphors driven by 20 mA, (c) the EL spectra of the white LED device under different

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driven currents.

Table Captions

Table 1 Rietveld refinement results and crystal data for the BaGe3.7O9:0.3Eu3+ phosphor Table 2 The crystal structural parameters of BaGe3.7O9:0.3Eu3+ as determined by the rietveld refinement Table 3 Chromaticity parameters for fabricated warm WLED device under different driven currents.

ACCEPTED MANUSCRIPT Table 1 Rietveld refinement results and crystal data for the BaGe3.7O9:0.3Eu3+ phosphor BaGe3.7O9:0.3Eu3+

Formula Radiation type λ(Å)

Cu Kα radiation with λ = 1.5405 Å

2θ range

5-120º

Crystal symmetry

trigonal

Space group;Z

P-321(150;1 a = b =11.609 Å V = 553.59 Å3

RP

8.98%

RWP

9.23%

2

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Cell parameters

2.12

χ

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Table 2 The crystal structural parameters of BaGe3.7O9:0.3Eu3+ as determined by the rietveld refinement BaGe3.7O9:0.3Eu3+ atom (site)

x/a

y/b

z/c

0.66781

0

0

0

0

0

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Ba1 Ge1 Ge2

1/3

1/2

-0.10626

Ge3

0

0.18145

1/2

Ge4

0.15181

0.48584

O1

0.4190

0.4203

0

0.49569

0.08497

0.14936

O4

0.11608

0.32301

O5

0.26930

0.50927

3449 1/2

0.78443 0.27854

0.61365

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O2 O3

0.40643

Table 3 Chromaticity parameters for fabricated warm WLED device under different driven currents. Current (mA)

CIE coordinates

20 40

Ra

y

0.4365

0.4130

3079

85.8

0.4278

0.4154

3248

85.7

0.4276

0.4156

3255

85.9

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60

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x

Tc (K)

80

0.4248

0.4174

3319

85.4

100

0.4219

0.4190

3385

84.9

120

0.4190

0.4203

3449

84.4

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Novel BaGe4O9:Eu3+ red-emitting phosphors were synthesized by a high-temperature solid-state reaction. The optimal Eu3+-doping concentration is 30 mol% when monitored at 591 nm with a 394 nm excitation

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The quantum yield and photoluminescence lifetime of BaGe3.7O9:0.3Eu3+ are 57.9% and 2.25 ms, respectively. The BaGe4O9:Eu3+ phosphors are prospective candidates as red-light component for warm white light-emitting

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diodes (WLEDs) devices