Eu Sites in Eu-Doped AlON Phosphors: Anomalous Eu Occupancy

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Eu Site in Eu Doped AlON Phosphor: Anomalous Eu Occupancy Layers Meng Wang, Sheng-Hui Zhang, Qiang-Qiang Zhu, Zhong-Wei Zhang, Li Zhang, Xin Wang, Lin-Bo Zhang, Yu-Jie Zhao, Xin Xu, and Liangjun Yin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11263 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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The Journal of Physical Chemistry

Eu Site in Eu Doped AlON Phosphor: Anomalous Eu Occupancy Layers Meng Wang, † Sheng-Hui Zhang, † Qiang-Qiang Zhu,‡ Zhong-Wei Zhang,§ Li Zhang,＃ Xin Wang,＃ Lin-Bo Zhang,＃ Yu-Jie Zhao,※ Xin Xu,Ⅱ Liang-Jun Yin,*,† †

School of Energy Science and Engineering, University of Electronic Science and Technology of China, 2006 Xiyuan Road, Chengdu, P.R.China. ‡ Key Laboratory of Materials for High Power Lasers, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai, 201800, China. §Laboratory of Energy Storage and New Energy Materials Technology, Central Research Institute, Dongfang Electric Corporation. ＃National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, 2006 Xiyuan Road, Chengdu, P.R. China. ※College of Materials, Xiamen University, Xiamen, 361005, China. ⅡLaboratory of materials for energy conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, P.R.China

ABSTRACT: Eu2+ doped AlON phosphor shows interesting luminescent properties. However, up to now, the exact sites of Eu2+ ions in AlON lattice are still unknown on account of large mismatch of ionic radius between Eu2+ and Al3+. In present paper, Eu occupancy sites in AlON lattice are investigated, which proves that Eu2+ ions are located at the layered structure like EuMgAl10O17, to release the lattice strain. Clarifying the Eu structure in AlON favors comprehensive understanding of the luminescent properties of AlON: Eu phosphors and supports an insight to explore other AlON based phosphors.

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INTRODUCTION Currently, there is increasing interest in developing light-emitting diodes (LEDs) because of their prolonged lifetime, remarkable efficiency, reliable stability, and environmental friendly features 1-7. In many cases, inorganic phosphors play important roles in luminescence which is accompanied by energy conversion from high excitation energy, by electromagnetic wave, electronic beam or electric field to low emission energies 8. Because of the excellent optical properties, low thermal conductivity and expansion, nitride and oxynitride based phosphors have received great attention in recent years 9-11. Among them, aluminum oxynitride (AlON) is widely researched because of its excellent luminous performance. Aluminum oxynitride (AlON) is a stable single-phase solid solution, with the structure of spinel type 12. It has good thermal stability and optical transparency properties which make it a potential candidate for applications as high-performance transparent ceramics

13-16.

These

properties also confer AlON as a main material for developing advanced phosphors for fluorescent lamps, display and white light LED applications 17. Many phosphors of AlON have been synthesized recently by different methods

18-20

and their luminescence properties have

been in-depth studied. In the current, AlON phosphors doped with different rare earth ions have been proved to exhibit quite efficient luminescence, covering blue to green emissions 21-23. However, research on the occupancy site of large rare earth ions (Eu2+, Ce3+, etc.) in AlON based phosphors is quite limited. Because of the large radius mismatch between Eu2+ and Al3+, the knowledge on the occupancy site of Eu2+ is extremely limited, which hinders the better understanding of luminescent mechanism. In this study, we focus on the study of Eu2+ doped aluminum oxide phosphor. The microstructure of Eu2+ in AlON lattice is analyzed experimentally, and the results indicate that Eu2+ ions are located at the layered structure like EuMgAl10O17.

EXPERIMENTAL SECTION 2


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Synthesis. 0.1mol% Eu and 10mol% Mg doped AlON (Al23O27N5) phosphor (hereafter AlON: Eu2+) was synthesized by conventional solid-state reaction, according to the procedure in a previous research 18. The purpose of Mg doping is to promote the formation of AlON and improve its phase purity. The powder mixtures of Al2O3 (Sinopharm Chemical Reagent Co. Ltd, Shanghai, China), AlN (Sinopharm Chemical Reagent Co. Ltd, Shanghai, China), MgO (Sinopharm Chemical Reagent Co. Ltd, Shanghai, China), and Eu2O3 (Sinopharm Chemical Reagent Co. Ltd, Shanghai, China) were well mixed in a Si3N4 mortar by grinding. The resulting mixtures were fired in BN crucibles under N2 atmosphere for 8h at 1800°C. The samples were heated at a constant rate of 300°C/h and cooled naturally. Characterization.

Photoluminescence

spectra

were

measured

by

a

fluorescent

spectrophotometer (Model F-4600, Hitachi, Tokyo, Japan) with a 200 W Xe lamp as an excitation source. The phase formation was analyzed by an X-ray diffractometer (Model PW 1700, Philips Research Laboratories, Eindhoven, the Netherlands) using Cu Kα radiation at a scanning rate of 0.5 degree/min. Rietveld refinements of the X-ray diffractograms were conducted using the GSAS (General Structure Analysis System) program

24.

Z-contrast

Scanning transmission electron microscopy (STEM) was performed using high angle annulardark field scanning electron microscopy (HAADF-STEM) equipped with an energy dispersion X-ray spectroscopy (JEM-ARM 200F). The X-ray absorption spectra at the Eu L3-edge were measured at the beamline of BL14W1 Shanghai Synchrotron Radiation Facility. Finally, we used IFEFFIT-1.2.11 software package to analyze the results of the EXAFS data (Extended Xray absorption fine structure) by standard methods.

RESULTS AND DISCUSSION In the Photoluminescence (PL) spectra of Fig. 1-a, AlON: Eu2+ only exhibits a broad emission band centered at 479 nm when excited by 325 nm, which is an obvious characteristic of the 4f65d-4f7 transitions of Eu2+ ions in AlON crystal lattice. There is no detection of sharp lines 3



assigned to Eu3+ intra-4f6 transitions, indicating that there are no Eu3+ ions in the crystal. Figure 1-b shows the normalized Eu L3-edge X-ray absorption near edge structure (XANES) of AlON: Eu2+ phosphor, Eu2O3 and Sr2Si5N8: Eu2+. For AlON: Eu2+ phosphor, two peaks can be seen at about 6977 and 6984 eV, which can be attributed to the divalent and trivalent oxidation states of Eu, respectively 25. Obviously, Eu is dominantly present in the form of Eu2+ while the peak for Eu3+ ions is much smaller. This is possibly because of the unreacted starting material Eu2O3 and sample surface oxidation. The XANES result indicates that Eu2+ is quite stable in the AlON: Eu2+ system. It is a common sense that Eu3+ should be detected easily in case of Eu substitution for Al3+ ions, considering the charge balance. Thus, it seems that a kind of unique structure containing Eu2+ is formed inside AlON lattice.

Figure 1. (a) PL spectra; (b) Eu L3-edge XANES of AlON: Eu2+ phosphors. The data of Eu2O3 and Sr2Si5N8: Eu2+ powders are given as standards. Figure 2 shows the X-ray diffraction (XRD) pattern of AlON: Eu2+ powders, which well matches with the standard data (Reference code: 00-047-0099). To further confirm its reliability, the Rietveld Refinement is performed without considering the Eu2+ doping. The final reliability factors for the whole pattern are Rp = 7.77％ and Rwp = 10.27％. The refined details can be found in table S1. It is observed that Al has two positions in AlON: tetrahedral site (Al1) and octahedral site (Al2). The doped Mg only substitutes the Al atoms in the tetrahedral sites. This indicates that the Eu2+ doping does not induce change to the spinel AlON structure. Note 4


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that Eu2+ owns a larger ionic radius than Al3+. The lattice parameter of AlON will expand if Eu2+ occupies Al3+ site, which is controversial to the Rietveld Refinement results. It is expected that some kind of new structure containing Eu2+ is stacked inside AlON lattice.

Figure 2. Rietveld refinement of crystal structure of AlON: Eu2+ powders. Experimental, calculated, background and difference signal, and hkl of the XRD pattern are plotted in the same range. As a spinel structure, there are 64 tetrahedron and 32 octahedron sites totally in AlON lattice (Al(64+z)/3V(8−z)/3O32−zNz, z = 5). Figure 3 shows the crystal structure of AlON according to the refinement results, in which the tetrahedral and octahedral are stacked closely. One octahedron is connected to the next octahedron by edge sharing and connected to the tetrahedron by point sharing

26.

Previous investigations have proved the incorporation of Mn2+ and Mg2+ ions on

tetrahedral Al sites via experimental and theoretical results on account of the similar ion radius between Al3+, Mn2+ and Mg2+ 26, 27. As for the Eu2+, the radius of Eu2+ (1.17Å, CN=6) is quite 5



larger than the radius of Al3+ (0.54Å, CN=6) 28, so it cannot be accommodated in the tetrahedral or octahedral sites directly.

Figure 3. Crystal structure of AlON. (a) Structure details of AlON showing each Al, O, N atoms. (b) Polyhedron structure showing the tetrahedron and octahedron. The tetrahedral and octahedral are shown in green and blue, respectively. Figure 4 shows the High Resolution Transmission Electron Microscope (HRTEM) of AlON: Eu2+. In some parts, many layered structures are present inside AlON lattice due to Eu2+ doping. The interplanar spacing of 2.03Å, 2.20Å, 2.64Å and 2.90Å in the flat area corresponds to the (400), (321), (221) and (212) of AlON host structure. And the interplanar spacing of 2.24Å, 3.22Å, 3.38Å and 4.51Å in the flat area corresponds to the (109), (105), (105) and (102) of EuMgAl10O17 host structure, which mismatch with the interplanar parameters of AlON. Therefore, Eu doping introduces unique layered structure in the host lattice, similar to the EuMgAl10O17 host structure.

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Figure 4. The HRTEM images of AlON: Eu2+ in different spot (a, b). White and orange color represents the lattice of EuMgAl10O17 and AlON respectively. For a very low doping crystal, the exact doping structure could only be resolved via direct viewing in the atomic scale. By taking advantage of atom-resolved Cs-corrected STEM, we are able to differentiate light and heavy atoms in the crystal lattice, which can be highlighted in the Z-contrast High-Angle Annular Dark Field (HAADF) images. Derived from the result of Fig. 5-a, Eu atom columns match the layer projection as observed in the HRTEM. In order to study the elemental distribution of the layered structure in AlON, energy-dispersive X-Ray spectrometry (EDS) mapping analyses is performed, as shown in Fig. 5-b. It is seen that Eu is almost concentrated in the layered column. However, N element is poor in the Eu concentrated area. This proves that the layered substance is composed of Eu, Mg, Al and O elements. Considering the Eu valence and composition, the layer structure possibly corresponds to EuMgAl10O17, which works as a popular layered material. Previous research reported that the solubility of Si-N in BaMgAl10O17 was less than 3%, which can explain the lack of N concentration in the layered strcuture 29, 30. As shown in the HAADF images, the Eu-Eu distance is 18.17Å. Theoretically, in EuMgAl10O17, Eu is located in the spinel layer and the Eu-Eu distance is 3.21Å along the X-Y plane

31.

This means the calculated Eu-Eu distance is

approximately 5 times larger than it in EuMgAl10O17. Low Eu doping concentration can result in the poor Eu occupancy and thereof longer periodic structure.

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Figure 5. (a) HAADF-STEM image of AlON: Eu2+. (b) Energy-dispersive X-Ray spectrometry (EDS) mappings showing the elemental distribution of Al, Mg, O, N, and Eu, respectively. To confirm the similarity between the structure of Eu-contained layer and EuMgAl10O17, the PL spectra are compared with each other, as shown in Fig. S1. Similar spectra shape and excitation and emission bands are observed in both phosphors. Note that the full width at half maximum of the emission band is in AlON: Eu2+ (97nm) is larger than that that in EuMgAl10O17 (57nm), indicating that there is minor difference in their compositions. In addition, local environmental variations of Eu in AlON: Eu2+ and EuMgAl10O17 are analyzed using the Eu Ledge EXAFS spectra, as shown in the Fig. S2. Although it is difficult to give the real coordination envelops, the Fourier transformed Eu-L3-edge EXAFS spectra resemble each other, indicating the similar coordination environment. The new discovery of this layered structure in AlON: Eu2+ is interesting, which is composed of Eu, Mg, Al and O elements. In an area of such high Eu concentration, at the Eu layer position, the luminescence is expected to be quenched. However, this phosphor emits intense cyan luminescence. This could be due to the long Eu–Eu interatomic distance larger than 18.17Å as shown in the HAADF images. As shown in Fig. 5, the distance between the Eu layers, 8


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corresponding to the 6 times distance of Eu-Eu distance in EuMgAl10O17 along X-Y plane, is so large that there is no energy migration between the Eu layers. Thus, the energy migration between the luminescent centers is controlled and concentration quenching is suppressed by the long Eu–Eu interatomic distance. Although Eu-contained layer is present in AlON: Eu2+, there is no indication of secondary phase associated with EuMgAl10O17 in the XRD pattern, due to its low Eu concentration.

CONCLUSION In summary, through the investigations and analyses of the structure of AlON: Eu2+ phosphor, we can prove that Eu2+ ions are confirmed to be located at layered structure, to release the lattice strain. It is found that Eu mostly exists in the form of Eu2+ ions and the Eu2+ doping did not introduce a change to the spinel AlON structure. The layered structure like EuMgAl10O17 (or BaMgAl10O17) is embedded in the spinel layer. Present results favor an insight to understand the rare-earth sites and luminescence properties of other AlON or SiON based phosphors.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The refinement parameters of AlON:Eu confirms the its high phase purity ; Emission spectra and excitation spectra of AlON: Eu2+ (λex=325 nm, λem=479 nm) and EuMgAl10O17 (λex=335 nm, λem=475 nm) powders; The Fourier transform of the Eu-L3-edge EXAFS spectra of AlON: Eu2+ and EuMgAl10O17 phosphors indicate the structure similarity between Eu-contained layer and EuMgAl10O17. AUTHOR INFORMATION

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Corresponding Author *E-mail: [email protected] (Liangjun Yin). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Meng Wang and Sheng-Hui Zhang contributed equally to this paper. Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (Grant No. 51302029, 51802037) and Sichuan Science and Technology Program (Grant No. 2018FZ0100).

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Eu Sites in Eu-Doped AlON Phosphors: Anomalous Eu Occupancy

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