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Y3 + co-doped oxyfluoride glass-ceramics
Journal of Non-Crystalline Solids 404 (2014) 37–42
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Infrared emission from Er3 +/Y3 + co-doped oxyfluoride glass-ceramics Zhiyong Zhao a, Chao Liu a,⁎, Yang Jiang b, Jihong Zhang a, Haizheng Tao a, Jianjun Han a, Xiujian Zhao a, Jong Heo a,c a b c
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 122 Luoshi Road, Hongshan, Wuhan, Hubei 430070, PR China China Building Material Institute of Solar-Energy Application, Zhenjiang, Jiangsu 212009, PR China Center for Information Materials, Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, South Korea
a r t i c l e
i n f o
Article history: Received 3 June 2014 Received in revised form 23 July 2014 Available online xxxx Keywords: Nanocrystals; Glass-ceramics; Infrared emission; Clustering; Optical properties
a b s t r a c t Infrared emission properties of oxyfluoride glasses with nominal compositions of 55SiO2–10Al2O3–(35 − x − y) PbF2–xErF3–yYF3 (in mol%) with x = 0.1, 0.2, 0.3, or 0.5 and y = 0, or 1.0, were investigated. YF3 was doped into the glasses to overcome the concentration quenching and achieve efficient infrared emission from Er3+ doped oxyfluoride glass-ceramics. It was found that introduction of YF3 can efficiently promote the formation of Er3+doped fluoride nanocrystals even when the doping concentration of ErF3 was low. With the doping of YF3, infrared emission was significantly enhanced and strong infrared emissions at 980 nm and 2730 nm bands were observed. Prolongation of lifetimes of Er3+:4I13/2 and 4I11/2 energy levels showed that concentration quenching of the infrared emissions was greatly suppressed. These results indicate that oxyfluoride glass-ceramics co-doped with Y3+ have potentials for efficient infrared applications. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth ions doped fluoride glasses and crystals have been intensively investigated for the infrared applications due to the relative low phonon energy of the host and subsequent high emission efficiency [1,2]. In recent years, rare-earth doped oxyfluoride glass-ceramics has been considered as one of the candidates for the efficient infrared emission due to the precipitation of fluoride nanocrystals, which provide the low phonon energy environment for rare-earth ions incorporated inside [3–7]. In oxyfluoride glass-ceramics containing lead fluoride nanocrystals, mid-infrared emission at ~ 2.7 μm from Er3 + [3], ~ 2.87 μm from Dy3 + [7] and ~ 2.9 μm from Ho3 + [7] has been reported. However, efficiency of these mid-infrared emissions from rare-earth doped oxyfluoride glass-ceramics was found to be low, even though the rare-earth ions have been incorporated into the fluoride nanocrystals during ceramming process. Until present, reports on the efficient infrared emission from rare-earth doped oxyfluoride glass-ceramics were still lacking. These weak mid-infrared emissions from rare-earth ions were most probably induced by the high effective concentration of rare-earth ions in the fluoride nanocrystals. Energy dispersive spectroscopy (EDS) analysis [8] and X-ray diffraction (XRD) analysis [9] have proved that rare-earth ions were highly enriched in the fluoride nanocrystals, and it was further directly evidenced by the electron energy loss spectroscopy (EELS) analysis [10,11]. Such high
⁎ Corresponding author. E-mail address: [email protected] (C. Liu).
http://dx.doi.org/10.1016/j.jnoncrysol.2014.07.043 0022-3093/© 2014 Elsevier B.V. All rights reserved.
concentration of rare-earth ions in the fluoride nanocrystals is favorable for the efficient visible up-conversion emission, while, detrimental for the infrared emission from rare-earth ions due to the possible energy transfer processes such as cross relaxation, etc. [12,13]. For example, the lifetime of the Er3+:4I11/2 energy level was strongly dependent on the concentration of Er3 + ions. When the concentration of ErF3 in polycrystalline PbF2 increased from 1 mol% to 16.6 mol%, the lifetime of the Er3 +:4I11/2 energy level decreased from 8.8 ms to 3.9 ms [14]. Therefore, to achieve efficient infrared emission from rare-earth ions doped oxyfluoride glass-ceramics, it is necessary to control or reduce the effective concentration of rare-earth ions in the fluoride nanocrystals. For rare-earth doped oxyfluoride glass-ceramics, it is generally accepted that rare-earth ions act as the nucleation agent for the formation of fluoride nanocrystals [15]. However, concentration of rare-earth dopants needs to be high enough to initiate the nucleation process, and as a result, concentration of rare-earth ions in the fluoride nanocrystals will be expected to be high enough to promote the energy transfer processes [10]. Therefore, to dilute the concentration of rareearth ions in the fluoride nanocrystals and achieve efficient infrared emission from rare-earth doped oxyfluoride glass-ceramics, one possible route is to introduce some non-active ions such as Y3+, La3+, and Gd3 + ions as the nucleation agents for the formation of fluoride nanocrystals. In this work, YF3 was used to support the formation of Er3+-doped lead fluoride nanocrystals and decrease the effective concentration of Er3+ ions in the fluoride nanocrystals. As a result, efficient near-infrared emissions at ~ 980 nm, ~ 1540 nm and ~ 2730 nm were observed, indicating the great potentials of this doping strategy for efficient infrared emission materials.
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2. Experimental Oxyfluoride glasses with nominal compositions of 55SiO2–10Al2O3– (35 − x − y)PbF2–xErF3–yYF3 (in mol%) with x = 0.1, 0.2, 0.3, or 0.5 and y = 0, or 1.0, were prepared using the conventional meltquenching method. Raw chemical powders (purity ≥ 99.9%) were thoroughly mixed and melted in alumina crucibles at 1250 °C for 30 min under ambient atmosphere. The melts were quenched by pouring the melt onto the brass mold and pressing with another brass plate. The as-quenched glasses were annealed at 350 °C for 2 h to reduce the thermal stress. Glasses thus obtained were cut into small pieces for heat treatments at 430 °C, 450 °C, 470 °C, or 490 °C for 10 h. Structures of the as-prepared glasses and glass-ceramics were characterized using the X-ray diffractometer (D8 Advance, Bruker) with radiation wavelength of 1.5406 Å and a scanning rate of 2°/min. High resolution transmission electron microscope (HR-TEM, JEOL, JEM-2200FS with an image Cs-corrector and an Ω-filter) at the National Institute for Nanomaterials Technology (NINT) at POSTECH was used for the characterization of nanostructures and electron energy loss spectroscopy (EELS) analysis. The specimens for TEM analyses were prepared through the standard disk grinding, dimple grinding and ion milling. To reduce the effect of electron beam irradiation, both sides of these TEM specimens were coated with carbon. EELS analyses were used to characterize the distribution of several elements in the glass matrices. The energy losses associated with the Er (N4,5), Pb (O2,3), F (K), Si (L2,3), Al (L2,3), O (K), and Y (M4,5) were measured. The elemental mapping was conducted using the three-window technique using one post-edge image and two pre-edge images for the background correction [16]. Absorption spectra were recorded using an UV/Vis/NIR spectrophotometer (UV3600, Shimadzu). An 800 nm laser from Ti:Sapphire laser (3900S, Spectra Physics) was used as the excitation source for the emission property measurement, and a mechanical chopper was used to modulate the excitation laser beam. The excitation laser beam was focused onto the polished specimens. Emission was collected using two-concave lenses and dispersed into a 0.25 meter monochromator. Intensities of the emitted light were detected using photomultiplier tube detector (for visible emission) and InSb detector (for infrared emission). Decay curves of the infrared emission were recorded with a digital oscilloscope (HP54503a). All the optical characterizations were carried out at room temperature. 3. Results and discussion Formation of fluoride nanocrystals has been confirmed using the X-ray diffraction patterns, which were very similar to previous works [10,11]. Upon thermal treatment, diffraction peaks similar to that of cubic fluorite structured lead fluoride crystal (JCPDS#: 72-2304) were observed from all the specimens (Fig. 1). Due to the similarities of diffraction patterns of Pb1 − x − yErxYyF2+ x + y crystal [11,17], it was difficult to determine the exact composition of fluoride nanocrystals formed in the glass-ceramics. Formation of rare-earth doped fluoride nanocrystals has been confirmed using the EELS analysis in our previous work [10]. Effect of YF3 doping on the formation of fluoride nanocrystals was clearly evidenced by the diffraction patterns recorded from the as-prepared specimens (Fig. 1a). For the as-prepared specimens, only broad halos were observed when the glass was singly doped with 0.5 mol% ErF3. While, when 1.0 mol% YF3 was doped in addition to the ErF3, diffraction patterns similar to the glass-ceramics were observed, indicating the formation of fluoride nanocrystals in the as-prepared specimens. For oxyfluoride glasses doped with high concentration of ErF3, similar phenomena were also observed in the as-quenched glasses [18]. Since YF3 can also act as efficient nucleation agent [19], addition of 1.0 mol% YF3 was high enough to promote the formation of fluoride nanocrystals in the as-quenched glasses. To further confirm the formation of and incorporation of Er3+ ions and Y3+ ions into the fluoride nanocrystals, EELS analysis was carried
Fig. 1. X-ray diffraction patterns of (a) as-prepared specimens and (b) glass-ceramic specimens heat-treated at 490 °C for 10 h. Patterns were vertically shifted for clarity.
out using the 0.5 mol% ErF3 and 1.0 mol% YF3 co-doped glass heattreated at 490 °C for 10 h as an example (Fig. 2). Fig. 2a shows the nanocrystals formed in the heat-treated glass, and mapping results of each element are shown in Fig. 2b to h. These results clearly demonstrated that these nanocrystals were deficient in oxygen, silicon and aluminum, rich in lead, fluorine, erbium and yttrium, confirming that these nanocrystals were mainly made of lead fluoride and erbium and yttrium were incorporated into these fluoride nanocrystals. Besides the formation of fluoride nanocrystals, doping of YF3 also promoted the incorporation of Er3+ ions into the fluoride nanocrystals. Fig. 3a showed the absorption spectra of Er3+ ions in the visible and near-infrared spectral region. Bands at ~ 486 nm, ~ 520 nm, ~ 540 nm, and ~ 1525 nm were induced by the transition from ground state (4 I15/2) of Er 3 + ions to the excited states 4 F7/2 , 2 H11/2 , 4S 3/2 , and 4 I13/2, respectively. It is known that Er3 +:4I15/2 ➔ 2H11/2 transition is very sensitive to the local symmetry of ligand field and the covalency of chemical bonds [20]. For 0.5 mol% ErF3 doped specimens, it has been observed that absorption coefficient of Er3+:4I15/2 ➔ 2H11/2 transition decreased ~40% when Er3+ ions were incorporated into the lead fluoride nanocrystals [11]. For the specimens doped with 0.1, 0.2, 0.3 and 0.5 mol% ErF3 along with 1.0 mol% YF3, the peak absorption coefficient decreased 44.5%, 38.4%, 36.0%, and 40.8%, respectively, upon thermal treatments at 490 °C for 10 h. Variation in the decrease of peak absorption coefficients was induced by the formation of some fluoride nanocrystals upon quenching (Fig. 1), where a portion of Er3 + ions was already incorporated. In addition, the ~1525 nm absorption bands showed clear Stark splitting upon thermal treatment, indicating that the Er3 + ions resided in the crystalline-like environment. On the other hand, for specimens singly doped with 0.1 mol% or 0.2 mol%
Z. Zhao et al. / Journal of Non-Crystalline Solids 404 (2014) 37–42
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Fig. 2. (a) Transmission electron microscope image of 0.5 mol% ErF3 and 1.0 mol% YF3 co-doped glass heat-treated at 490 °C for 10 h. (b) to (h) show the element mapping of Si, Al, Pb, O, F, Er and Y in the region shown in (a), respectively.
Fig. 3. Absorption spectra of (a) Er3+/Y3+ co-doped as-prepared specimens (solid lines) and glass-ceramic specimens heat-treated at 490 °C for 10 h (symbol lines), and (b) 0.2 mol% ErF3 doped specimens heat-treated at various conditions. Spectra were vertically shifted for clarity.
ErF3, fluoride nanocrystals cannot form due to the low concentration of nucleation agents, and as a consequence, absorption spectra in the visible and near-infrared spectral regions did not show any changes (Fig. 3b). Both XRD patterns and absorption spectra showed that YF3 acted as the nucleation agents for the formation of fluoride nanocrystals, and promoted the incorporation of Er3 + ions into the fluoride nanocrystals formed in the glass-ceramics. Because the co-doped YF3 can promote formation of fluoride nanocrystals and preferential incorporation of Er3+ into the fluoride nanocrystals, it can be expected that YF3 co-doping can disperse Er3+ ions and control the effective concentration of Er3+ ions in the fluoride nanocrystals, and tune the infrared emission properties of Er3+ ions. For Er3+ ions, there exist several concentration dependent energy transfer (cross relaxation) path ways such as (4I11/2, 4I13/2) ➔ (4I15/2, 4F9/2), which lead to the quenching of 980 nm and 2730 nm band emissions [13,14]. Decrease in the effective concentration of Er3 + ions in the fluoride nanocrystals will significantly enhance the emission intensity of these two bands. Fig. 4a shows the infrared emission spectra of as-prepared and heat-treated (490 °C/10 h) specimens singly doped with Er3+ or co-doped with Er3+/Y3+. For all the specimens, 980 nm and 2730 nm bands were enhanced upon thermal treatment due to the incorporation of Er3+ ions into the fluoride nanocrystals with low phonon energy. For 0.5 mol% ErF3 doped glass and glass-ceramics, 980 nm and 2730 nm band emissions were very weak. It has been shown that most of Er3+ ions have been incorporated into the fluoride nanocrystals and only trace amount of Er3+ ions was left in the glass matrix upon thermal treatment at 490 °C for 10 h [10]. As a result, concentration of Er3+ ions in the fluoride nanocrystals was very high and efficient cross relaxation among Er3 + ions occurred in the fluoride nanocrystals, leading to the quenching of 980 nm and 2730 nm band emissions. When 0.5 mol% ErF3 and 1.0 mol% YF3 were co-doped, emission intensities of the 980 nm and 2730 nm bands were enhanced, confirming that co-doping with YF3 can decrease to the effective concentration of Er3+ ions in the fluoride nanocrystals, and thus, reduce cross-relaxation processes and enhance the infrared emission. With YF3 acting as the nucleation agent, further enhancement in the intensities of 980 nm and 2730 nm band emissions can be expected when the doping
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Fig. 4. (a) Infrared emission spectra of as-prepared specimens (solid lines) and glassceramic specimens heat-treated at 490 °C for 10 h (symbol lines), (b) integrated intensity ratio between 980 nm and 1540 nm band emissions, and (c) integrated intensity ratio between 2730 nm and 1540 nm band emissions.
concentration of ErF3 decreased. As shown in Fig. 4a, when the doping concentration of ErF3 decreased to 0.3 mol% or lower, strong 980 nm and 2730 nm band emissions were observed. When normalized to the 1540 nm band emission, the integral emission intensity ratio of 980 nm band gradually increased from 0.2% (0.5 mol% ErF3 single doping) to 10.05% (0.1 mol% ErF 3 and 1.0 mol% YF 3 co-doping) for the as-prepared specimens, and from 0.08% (0.5 mol% ErF3 single doping) to 53.19% (0.1 mol% ErF3 and 1.0 mol% YF3
co-doping) for the specimens heat-treated at 490 °C for 10 h (Fig. 4b). For the 2730 nm band emission, the integral emission intensity ratios were found to be 0.1–2.2% for all the as-prepared specimens, and it gradually increased from 1.65% (0.5 mol% ErF3 single doping) to 10.95% (0.1 mol% ErF3 and 1.0 mol% YF3 co-doping) for the specimens heat-treated at 490 °C for 10 h (Fig. 4c). Observation of the strong 980 nm/2730 nm band emissions and changes in the integrated emission intensity ratios indicated that the effective concentration of Er3 + ions in the fluoride nanocrystals was decreased and the cross relaxation processes were suppressed, due to the doping of YF3. All these changes can be further confirmed using the decay curves of 1540 nm and 2730 nm band emissions. Fig. 5a shows the decay curves of 1540 nm band recorded from the glass-ceramic specimens heattreated at 490 °C for 10 h. For 0.5 mol% ErF3 singly doped specimen (curve 1), the 1540 nm band emission showed a double-exponential decay behavior. Using a double-exponential fitting, it was found that the fast decay process had a time constant of 0.7 ms, and slow decay process of 6.51 ms. Compared to the Er3 + doped single lead fluoride crystals, the time constant of 0.7 ms indicated that effective concentration of Er3+ ions was higher than 20% (cationic concentration) and the time constant of 6.51 ms showed that effective concentration of Er3+ ions was lower than 10% (cationic concentration) [12]. This observation is consistent with our previous finding on the inhomogeneous distribution of Er3+ ions in the fluoride nanocrystals [11], there existed regions highly enriched in Er3 + ions even in single nanocrystal. It was also found that the fast decay process contributed 58.7% to the total emission and the slow decay process 41.3%. For 0.5 mol% ErF3 and 1.0 mol% YF3 co-doped glass-ceramic specimen (curve 2), a double exponential decay was also observed for the 1540 nm band emission. The fast and slow decay time constants were found to be 2.22 ms with a contribution to the total emission intensity of 85.9% and 5.80 ms with a contribution to the total emission intensity of 14.1%, respectively. For the ErF3 and YF3 co-doped glass-ceramic specimens, when the concentration of ErF3 was 0.3 mol%, 0.2 mol% and 0.1 mol%, the 1540 nm band emission nearly showed single exponential decay behaviors, and the time constants of this decay were found to be 5.31 ms, 6.20 ms and 7.94 ms, respectively (curves 3, 4, and 5). Compared to the lifetimes of Er3+ ions in lead fluoride crystals [12], effective cationic concentration of Er3 + ions in the fluoride nanocrystals decreased to below 10% for 0.1 mol% ErF3/1.0 mol% YF3 co-doped glass-ceramics. Effective lifetimes of the 1540 nm band emission recorded from all these specimens heattreated at various temperatures were summarized in Fig. 4b. For 0.5 mol% ErF3 singly doped and 0.5 mol% ErF3/1.0 mol% YF3 co-doped specimens, effective lifetimes of the 1540 nm band emission decreased with the increase in the heat-treatment temperatures, since the Er3+ ions were preferentially incorporated into the fluoride nanocrystals and the effective concentration significantly increased, leading to the concentration quenching and shortening of lifetimes. For the Er3+/Y3+ co-doped glass-ceramic specimens, when concentration of ErF3 decreased to 0.3 mol% or lower, effective lifetimes of the 1540 nm band emission were much longer than those doped with 0.5 mol% ErF3. For 0.1 mol% ErF3 doped specimens, the lifetimes of the 1540 nm band emission increased from 5.72 ms (as-prepared) to 7.94 ms (490 °C/10 h). For 0.2 mol% ErF3 doped specimens, the lifetimes of the 1540 nm band emission were found to be ~6.2 ms except the specimen heat-treated at 430 °C for 10 h (6.8 ms). However, when the doping concentration of ErF3 was 0.3 mol%, the lifetimes of the 1540 nm band decreased from 6.38 ms (as-prepared) to 5.31 ms (490 °C/10 h), indicating that the concentration quenching of the 1540 nm band emission started to occur. Thus, to achieve efficient near-infrared emission, decreasing the effective concentration of dopants is necessary and co-doping with YF3 is efficient to control the effective concentration rare-earth dopants in the fluoride nanocrystals. Decay curves of the 2730 nm band emission of glass-ceramic specimens heat-treated at 490 °C for 10 h were recorded (Fig. 5c). All the decay curves showed nearly exponential decay behaviors.
Z. Zhao et al. / Journal of Non-Crystalline Solids 404 (2014) 37–42
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Fig. 5. Decay curves of (a) 1540 nm and (c) 2730 nm band emissions recorded from glass-ceramic specimens heat-treated at 490 °C for 10 h, and effective lifetimes of (b) 1540 nm and (d) 2730 nm band emissions recorded from all the specimens. In (b) and (d), solid square, solid circle, open square, open circle and solid diamond represent glasses doped with 0.1 ErF3/1.0YF3, 0.2 ErF3/1.0YF3, 0.3 ErF3/1.0YF3, 0.5 ErF3/1.0YF3, and 0.5 ErF3, respectively.
Specimens heat-treated at other temperatures showed similar exponential decay behaviors. The lifetimes of the 2730 nm band emission recorded from all the specimens were summarized in Fig. 5d. It was found that the lifetimes of the 2730 nm band emission increased with decrease in the concentration of ErF 3 . Changes in the lifetimes were consistent with the emission spectra, indicating that the effective concentration of Er 3 + ions in the fluoride nanocrystals was decreased and cross relaxation processes were suppressed. Unlike the changes observed in Fig. 4c, changes in the lifetime of the 2730 nm band emission were not so obvious. Intensity of the 2730 nm band increased monotonically since more and more Er 3 + ions were incorporated into the fluoride nanocrystals with an increase in the heat-treatment temperature. However, the lifetime of the 2730 nm band (Fig. 5d) was mainly determined by the Er3+ ions incorporated in the fluoride nanocrystals, since Er3 + ions in the glass matrices only emitted very weakly at 2730 nm.
4. Summary Efficient 980 nm and 2730 band emissions from Er3 + doped oxyfluoride glass-ceramics were achieved through the co-doping of YF3. It was found that YF3 co-doping can efficiently promote the formation of fluoride nanocrystals and incorporation of Er3 + ions into these fluoride nanocrystals. Intensities of the 980 nm and 2730 band emissions were greatly enhanced compared to the 1540 nm band emission with the doping of 1 mol% YF3. Prolonged lifetimes of 1540 nm and 2730 nm band emissions evidenced that the effective concentration of Er3+ ions in the fluoride nanocrystals decreased and the cross relaxation processes leading to the quenching of Er3+ infrared emission were suppressed. These findings have great potential for
the development of efficient infrared emitting materials based on rare-earth doped oxyfluoride glass-ceramics. Acknowledgments This work was financially supported by the Fundamental Research Funds for the Central Universities of China (WUT: 2013-II-010), Natural Science Foundation of Hubei Province (Grant Nos.: 2012FFA024, 2013CFA008), National Natural Science Foundation of China (Grant No.: 51202170), Program for New Century Excellent Talents in University (Grant No.: NCET-13-0943), Research Fund for the Doctoral Program of Higher Education of China (FRDP: 20130143110013), and Chutian Scholar Program of Hubei Province. References [1] Y. Tian, J. Zhang, X. Jing, Y. Zhu, S. Xu, J. Lumin. 138 (2013) 94–97. [2] L. Su, J. Xu, H. Li, D. Zhang, X. Xu, G. Zhao, J. Lumin. 122–123 (2007) 17–20. [3] G. Wu, S. Fan, Y. Zhang, G. Chai, Z. Ma, M. Peng, J. Qiu, G. Dong, Opt. Lett. 38 (2013) 3071–3074. [4] W. Zhang, Q. Chen, Q. Qian, Q. Zhang, J. Am. Ceram. Soc. 95 (2012) 663–669. [5] V.K. Tikhomirov, C. Görller-Walrand, K. Driesen, J. Alloys Compd. 451 (2008) 542–544. [6] F. Lahoz, J.M. Almenara, U.R. Rodríguez-Mendoza, I.R. Martín, V. Lavín, J. Appl. Phys. 99 (2006) 053103. [7] W.J. Chung, K.H. Kim, B.J. Park, H.S. Seo, J.T. Ahn, Y.G. Choi, J. Am. Ceram. Soc. 93 (2010) 2952–2955. [8] H. Lin, D. Chen, Y. Yu, Z. Shan, P. Huang, A. Yang, Y. Wang, J. Alloys Compd. 509 (2011) 3363–3366. [9] N. Hu, H. Yu, M. Zhang, P. Zhang, Y. Wang, L. Zhao, Phys. Chem. Chem. Phys. 13 (2011) 1499–1505. [10] C. Liu, J. Heo, J. Am. Ceram. Soc. 95 (2012) 2100–2102. [11] C. Liu, X. Zhao, J. Heo, J. Non-Cryst. Solids 365 (2013) 1–5. [12] G. Dantelle, M. Mortier, G. Patriarche, D. Vivien, J. Solid State Chem, J. Solid State Chem. 179 (2006) 1995–2003. [13] L. Zhang, H. Hu, J. Non-Cryst. Solids 326–327 (2003) 353–358.
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[14] M. Mortier, P. Goldner, C. Chateau, M. Genotolle, J. Alloys Compd. 323–324 (2001) 245–249. [15] M. Mortier, J. Non-Cryst. Solids 318 (2003) 56–62. [16] R.F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, Plenum Press, New York, 1996. [17] A.K. Tyagi, S.J. Patwe, S.N. Achary, M.B. Mallia, J. Solid State Chem. 177 (2004) 1746–1757.
[18] F. Bao, Y. Wang, Z. Hu, Mater. Res. Bull. 40 (2005) 1645–1653. [19] P.A. Tick, N.F. Borrelli, L.K. Cornelius, M.A. Newhouse, J. Appl. Phys. 78 (1995) 6367–6374. [20] S. Tanabe, H. Hayashi, T. Hanada, N. Onodera, Opt. Mater. 19 (2002) 343–349.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Infrared emission from Er3 +/Y3 + co-doped oxyfluoride glass-ceramics Zhiyong Zhao a, Chao Liu a,⁎, Yang Jiang b, Jihong Zhang a, Haizheng Tao a, Jianjun Han a, Xiujian Zhao a, Jong Heo a,c a b c
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 122 Luoshi Road, Hongshan, Wuhan, Hubei 430070, PR China China Building Material Institute of Solar-Energy Application, Zhenjiang, Jiangsu 212009, PR China Center for Information Materials, Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, South Korea
a r t i c l e
i n f o
Article history: Received 3 June 2014 Received in revised form 23 July 2014 Available online xxxx Keywords: Nanocrystals; Glass-ceramics; Infrared emission; Clustering; Optical properties
a b s t r a c t Infrared emission properties of oxyfluoride glasses with nominal compositions of 55SiO2–10Al2O3–(35 − x − y) PbF2–xErF3–yYF3 (in mol%) with x = 0.1, 0.2, 0.3, or 0.5 and y = 0, or 1.0, were investigated. YF3 was doped into the glasses to overcome the concentration quenching and achieve efficient infrared emission from Er3+ doped oxyfluoride glass-ceramics. It was found that introduction of YF3 can efficiently promote the formation of Er3+doped fluoride nanocrystals even when the doping concentration of ErF3 was low. With the doping of YF3, infrared emission was significantly enhanced and strong infrared emissions at 980 nm and 2730 nm bands were observed. Prolongation of lifetimes of Er3+:4I13/2 and 4I11/2 energy levels showed that concentration quenching of the infrared emissions was greatly suppressed. These results indicate that oxyfluoride glass-ceramics co-doped with Y3+ have potentials for efficient infrared applications. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth ions doped fluoride glasses and crystals have been intensively investigated for the infrared applications due to the relative low phonon energy of the host and subsequent high emission efficiency [1,2]. In recent years, rare-earth doped oxyfluoride glass-ceramics has been considered as one of the candidates for the efficient infrared emission due to the precipitation of fluoride nanocrystals, which provide the low phonon energy environment for rare-earth ions incorporated inside [3–7]. In oxyfluoride glass-ceramics containing lead fluoride nanocrystals, mid-infrared emission at ~ 2.7 μm from Er3 + [3], ~ 2.87 μm from Dy3 + [7] and ~ 2.9 μm from Ho3 + [7] has been reported. However, efficiency of these mid-infrared emissions from rare-earth doped oxyfluoride glass-ceramics was found to be low, even though the rare-earth ions have been incorporated into the fluoride nanocrystals during ceramming process. Until present, reports on the efficient infrared emission from rare-earth doped oxyfluoride glass-ceramics were still lacking. These weak mid-infrared emissions from rare-earth ions were most probably induced by the high effective concentration of rare-earth ions in the fluoride nanocrystals. Energy dispersive spectroscopy (EDS) analysis [8] and X-ray diffraction (XRD) analysis [9] have proved that rare-earth ions were highly enriched in the fluoride nanocrystals, and it was further directly evidenced by the electron energy loss spectroscopy (EELS) analysis [10,11]. Such high
⁎ Corresponding author. E-mail address: [email protected] (C. Liu).
http://dx.doi.org/10.1016/j.jnoncrysol.2014.07.043 0022-3093/© 2014 Elsevier B.V. All rights reserved.
concentration of rare-earth ions in the fluoride nanocrystals is favorable for the efficient visible up-conversion emission, while, detrimental for the infrared emission from rare-earth ions due to the possible energy transfer processes such as cross relaxation, etc. [12,13]. For example, the lifetime of the Er3+:4I11/2 energy level was strongly dependent on the concentration of Er3 + ions. When the concentration of ErF3 in polycrystalline PbF2 increased from 1 mol% to 16.6 mol%, the lifetime of the Er3 +:4I11/2 energy level decreased from 8.8 ms to 3.9 ms [14]. Therefore, to achieve efficient infrared emission from rare-earth ions doped oxyfluoride glass-ceramics, it is necessary to control or reduce the effective concentration of rare-earth ions in the fluoride nanocrystals. For rare-earth doped oxyfluoride glass-ceramics, it is generally accepted that rare-earth ions act as the nucleation agent for the formation of fluoride nanocrystals [15]. However, concentration of rare-earth dopants needs to be high enough to initiate the nucleation process, and as a result, concentration of rare-earth ions in the fluoride nanocrystals will be expected to be high enough to promote the energy transfer processes [10]. Therefore, to dilute the concentration of rareearth ions in the fluoride nanocrystals and achieve efficient infrared emission from rare-earth doped oxyfluoride glass-ceramics, one possible route is to introduce some non-active ions such as Y3+, La3+, and Gd3 + ions as the nucleation agents for the formation of fluoride nanocrystals. In this work, YF3 was used to support the formation of Er3+-doped lead fluoride nanocrystals and decrease the effective concentration of Er3+ ions in the fluoride nanocrystals. As a result, efficient near-infrared emissions at ~ 980 nm, ~ 1540 nm and ~ 2730 nm were observed, indicating the great potentials of this doping strategy for efficient infrared emission materials.
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2. Experimental Oxyfluoride glasses with nominal compositions of 55SiO2–10Al2O3– (35 − x − y)PbF2–xErF3–yYF3 (in mol%) with x = 0.1, 0.2, 0.3, or 0.5 and y = 0, or 1.0, were prepared using the conventional meltquenching method. Raw chemical powders (purity ≥ 99.9%) were thoroughly mixed and melted in alumina crucibles at 1250 °C for 30 min under ambient atmosphere. The melts were quenched by pouring the melt onto the brass mold and pressing with another brass plate. The as-quenched glasses were annealed at 350 °C for 2 h to reduce the thermal stress. Glasses thus obtained were cut into small pieces for heat treatments at 430 °C, 450 °C, 470 °C, or 490 °C for 10 h. Structures of the as-prepared glasses and glass-ceramics were characterized using the X-ray diffractometer (D8 Advance, Bruker) with radiation wavelength of 1.5406 Å and a scanning rate of 2°/min. High resolution transmission electron microscope (HR-TEM, JEOL, JEM-2200FS with an image Cs-corrector and an Ω-filter) at the National Institute for Nanomaterials Technology (NINT) at POSTECH was used for the characterization of nanostructures and electron energy loss spectroscopy (EELS) analysis. The specimens for TEM analyses were prepared through the standard disk grinding, dimple grinding and ion milling. To reduce the effect of electron beam irradiation, both sides of these TEM specimens were coated with carbon. EELS analyses were used to characterize the distribution of several elements in the glass matrices. The energy losses associated with the Er (N4,5), Pb (O2,3), F (K), Si (L2,3), Al (L2,3), O (K), and Y (M4,5) were measured. The elemental mapping was conducted using the three-window technique using one post-edge image and two pre-edge images for the background correction [16]. Absorption spectra were recorded using an UV/Vis/NIR spectrophotometer (UV3600, Shimadzu). An 800 nm laser from Ti:Sapphire laser (3900S, Spectra Physics) was used as the excitation source for the emission property measurement, and a mechanical chopper was used to modulate the excitation laser beam. The excitation laser beam was focused onto the polished specimens. Emission was collected using two-concave lenses and dispersed into a 0.25 meter monochromator. Intensities of the emitted light were detected using photomultiplier tube detector (for visible emission) and InSb detector (for infrared emission). Decay curves of the infrared emission were recorded with a digital oscilloscope (HP54503a). All the optical characterizations were carried out at room temperature. 3. Results and discussion Formation of fluoride nanocrystals has been confirmed using the X-ray diffraction patterns, which were very similar to previous works [10,11]. Upon thermal treatment, diffraction peaks similar to that of cubic fluorite structured lead fluoride crystal (JCPDS#: 72-2304) were observed from all the specimens (Fig. 1). Due to the similarities of diffraction patterns of Pb1 − x − yErxYyF2+ x + y crystal [11,17], it was difficult to determine the exact composition of fluoride nanocrystals formed in the glass-ceramics. Formation of rare-earth doped fluoride nanocrystals has been confirmed using the EELS analysis in our previous work [10]. Effect of YF3 doping on the formation of fluoride nanocrystals was clearly evidenced by the diffraction patterns recorded from the as-prepared specimens (Fig. 1a). For the as-prepared specimens, only broad halos were observed when the glass was singly doped with 0.5 mol% ErF3. While, when 1.0 mol% YF3 was doped in addition to the ErF3, diffraction patterns similar to the glass-ceramics were observed, indicating the formation of fluoride nanocrystals in the as-prepared specimens. For oxyfluoride glasses doped with high concentration of ErF3, similar phenomena were also observed in the as-quenched glasses [18]. Since YF3 can also act as efficient nucleation agent [19], addition of 1.0 mol% YF3 was high enough to promote the formation of fluoride nanocrystals in the as-quenched glasses. To further confirm the formation of and incorporation of Er3+ ions and Y3+ ions into the fluoride nanocrystals, EELS analysis was carried
Fig. 1. X-ray diffraction patterns of (a) as-prepared specimens and (b) glass-ceramic specimens heat-treated at 490 °C for 10 h. Patterns were vertically shifted for clarity.
out using the 0.5 mol% ErF3 and 1.0 mol% YF3 co-doped glass heattreated at 490 °C for 10 h as an example (Fig. 2). Fig. 2a shows the nanocrystals formed in the heat-treated glass, and mapping results of each element are shown in Fig. 2b to h. These results clearly demonstrated that these nanocrystals were deficient in oxygen, silicon and aluminum, rich in lead, fluorine, erbium and yttrium, confirming that these nanocrystals were mainly made of lead fluoride and erbium and yttrium were incorporated into these fluoride nanocrystals. Besides the formation of fluoride nanocrystals, doping of YF3 also promoted the incorporation of Er3+ ions into the fluoride nanocrystals. Fig. 3a showed the absorption spectra of Er3+ ions in the visible and near-infrared spectral region. Bands at ~ 486 nm, ~ 520 nm, ~ 540 nm, and ~ 1525 nm were induced by the transition from ground state (4 I15/2) of Er 3 + ions to the excited states 4 F7/2 , 2 H11/2 , 4S 3/2 , and 4 I13/2, respectively. It is known that Er3 +:4I15/2 ➔ 2H11/2 transition is very sensitive to the local symmetry of ligand field and the covalency of chemical bonds [20]. For 0.5 mol% ErF3 doped specimens, it has been observed that absorption coefficient of Er3+:4I15/2 ➔ 2H11/2 transition decreased ~40% when Er3+ ions were incorporated into the lead fluoride nanocrystals [11]. For the specimens doped with 0.1, 0.2, 0.3 and 0.5 mol% ErF3 along with 1.0 mol% YF3, the peak absorption coefficient decreased 44.5%, 38.4%, 36.0%, and 40.8%, respectively, upon thermal treatments at 490 °C for 10 h. Variation in the decrease of peak absorption coefficients was induced by the formation of some fluoride nanocrystals upon quenching (Fig. 1), where a portion of Er3 + ions was already incorporated. In addition, the ~1525 nm absorption bands showed clear Stark splitting upon thermal treatment, indicating that the Er3 + ions resided in the crystalline-like environment. On the other hand, for specimens singly doped with 0.1 mol% or 0.2 mol%
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Fig. 2. (a) Transmission electron microscope image of 0.5 mol% ErF3 and 1.0 mol% YF3 co-doped glass heat-treated at 490 °C for 10 h. (b) to (h) show the element mapping of Si, Al, Pb, O, F, Er and Y in the region shown in (a), respectively.
Fig. 3. Absorption spectra of (a) Er3+/Y3+ co-doped as-prepared specimens (solid lines) and glass-ceramic specimens heat-treated at 490 °C for 10 h (symbol lines), and (b) 0.2 mol% ErF3 doped specimens heat-treated at various conditions. Spectra were vertically shifted for clarity.
ErF3, fluoride nanocrystals cannot form due to the low concentration of nucleation agents, and as a consequence, absorption spectra in the visible and near-infrared spectral regions did not show any changes (Fig. 3b). Both XRD patterns and absorption spectra showed that YF3 acted as the nucleation agents for the formation of fluoride nanocrystals, and promoted the incorporation of Er3 + ions into the fluoride nanocrystals formed in the glass-ceramics. Because the co-doped YF3 can promote formation of fluoride nanocrystals and preferential incorporation of Er3+ into the fluoride nanocrystals, it can be expected that YF3 co-doping can disperse Er3+ ions and control the effective concentration of Er3+ ions in the fluoride nanocrystals, and tune the infrared emission properties of Er3+ ions. For Er3+ ions, there exist several concentration dependent energy transfer (cross relaxation) path ways such as (4I11/2, 4I13/2) ➔ (4I15/2, 4F9/2), which lead to the quenching of 980 nm and 2730 nm band emissions [13,14]. Decrease in the effective concentration of Er3 + ions in the fluoride nanocrystals will significantly enhance the emission intensity of these two bands. Fig. 4a shows the infrared emission spectra of as-prepared and heat-treated (490 °C/10 h) specimens singly doped with Er3+ or co-doped with Er3+/Y3+. For all the specimens, 980 nm and 2730 nm bands were enhanced upon thermal treatment due to the incorporation of Er3+ ions into the fluoride nanocrystals with low phonon energy. For 0.5 mol% ErF3 doped glass and glass-ceramics, 980 nm and 2730 nm band emissions were very weak. It has been shown that most of Er3+ ions have been incorporated into the fluoride nanocrystals and only trace amount of Er3+ ions was left in the glass matrix upon thermal treatment at 490 °C for 10 h [10]. As a result, concentration of Er3+ ions in the fluoride nanocrystals was very high and efficient cross relaxation among Er3 + ions occurred in the fluoride nanocrystals, leading to the quenching of 980 nm and 2730 nm band emissions. When 0.5 mol% ErF3 and 1.0 mol% YF3 were co-doped, emission intensities of the 980 nm and 2730 nm bands were enhanced, confirming that co-doping with YF3 can decrease to the effective concentration of Er3+ ions in the fluoride nanocrystals, and thus, reduce cross-relaxation processes and enhance the infrared emission. With YF3 acting as the nucleation agent, further enhancement in the intensities of 980 nm and 2730 nm band emissions can be expected when the doping
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Fig. 4. (a) Infrared emission spectra of as-prepared specimens (solid lines) and glassceramic specimens heat-treated at 490 °C for 10 h (symbol lines), (b) integrated intensity ratio between 980 nm and 1540 nm band emissions, and (c) integrated intensity ratio between 2730 nm and 1540 nm band emissions.
concentration of ErF3 decreased. As shown in Fig. 4a, when the doping concentration of ErF3 decreased to 0.3 mol% or lower, strong 980 nm and 2730 nm band emissions were observed. When normalized to the 1540 nm band emission, the integral emission intensity ratio of 980 nm band gradually increased from 0.2% (0.5 mol% ErF3 single doping) to 10.05% (0.1 mol% ErF 3 and 1.0 mol% YF 3 co-doping) for the as-prepared specimens, and from 0.08% (0.5 mol% ErF3 single doping) to 53.19% (0.1 mol% ErF3 and 1.0 mol% YF3
co-doping) for the specimens heat-treated at 490 °C for 10 h (Fig. 4b). For the 2730 nm band emission, the integral emission intensity ratios were found to be 0.1–2.2% for all the as-prepared specimens, and it gradually increased from 1.65% (0.5 mol% ErF3 single doping) to 10.95% (0.1 mol% ErF3 and 1.0 mol% YF3 co-doping) for the specimens heat-treated at 490 °C for 10 h (Fig. 4c). Observation of the strong 980 nm/2730 nm band emissions and changes in the integrated emission intensity ratios indicated that the effective concentration of Er3 + ions in the fluoride nanocrystals was decreased and the cross relaxation processes were suppressed, due to the doping of YF3. All these changes can be further confirmed using the decay curves of 1540 nm and 2730 nm band emissions. Fig. 5a shows the decay curves of 1540 nm band recorded from the glass-ceramic specimens heattreated at 490 °C for 10 h. For 0.5 mol% ErF3 singly doped specimen (curve 1), the 1540 nm band emission showed a double-exponential decay behavior. Using a double-exponential fitting, it was found that the fast decay process had a time constant of 0.7 ms, and slow decay process of 6.51 ms. Compared to the Er3 + doped single lead fluoride crystals, the time constant of 0.7 ms indicated that effective concentration of Er3+ ions was higher than 20% (cationic concentration) and the time constant of 6.51 ms showed that effective concentration of Er3+ ions was lower than 10% (cationic concentration) [12]. This observation is consistent with our previous finding on the inhomogeneous distribution of Er3+ ions in the fluoride nanocrystals [11], there existed regions highly enriched in Er3 + ions even in single nanocrystal. It was also found that the fast decay process contributed 58.7% to the total emission and the slow decay process 41.3%. For 0.5 mol% ErF3 and 1.0 mol% YF3 co-doped glass-ceramic specimen (curve 2), a double exponential decay was also observed for the 1540 nm band emission. The fast and slow decay time constants were found to be 2.22 ms with a contribution to the total emission intensity of 85.9% and 5.80 ms with a contribution to the total emission intensity of 14.1%, respectively. For the ErF3 and YF3 co-doped glass-ceramic specimens, when the concentration of ErF3 was 0.3 mol%, 0.2 mol% and 0.1 mol%, the 1540 nm band emission nearly showed single exponential decay behaviors, and the time constants of this decay were found to be 5.31 ms, 6.20 ms and 7.94 ms, respectively (curves 3, 4, and 5). Compared to the lifetimes of Er3+ ions in lead fluoride crystals [12], effective cationic concentration of Er3 + ions in the fluoride nanocrystals decreased to below 10% for 0.1 mol% ErF3/1.0 mol% YF3 co-doped glass-ceramics. Effective lifetimes of the 1540 nm band emission recorded from all these specimens heattreated at various temperatures were summarized in Fig. 4b. For 0.5 mol% ErF3 singly doped and 0.5 mol% ErF3/1.0 mol% YF3 co-doped specimens, effective lifetimes of the 1540 nm band emission decreased with the increase in the heat-treatment temperatures, since the Er3+ ions were preferentially incorporated into the fluoride nanocrystals and the effective concentration significantly increased, leading to the concentration quenching and shortening of lifetimes. For the Er3+/Y3+ co-doped glass-ceramic specimens, when concentration of ErF3 decreased to 0.3 mol% or lower, effective lifetimes of the 1540 nm band emission were much longer than those doped with 0.5 mol% ErF3. For 0.1 mol% ErF3 doped specimens, the lifetimes of the 1540 nm band emission increased from 5.72 ms (as-prepared) to 7.94 ms (490 °C/10 h). For 0.2 mol% ErF3 doped specimens, the lifetimes of the 1540 nm band emission were found to be ~6.2 ms except the specimen heat-treated at 430 °C for 10 h (6.8 ms). However, when the doping concentration of ErF3 was 0.3 mol%, the lifetimes of the 1540 nm band decreased from 6.38 ms (as-prepared) to 5.31 ms (490 °C/10 h), indicating that the concentration quenching of the 1540 nm band emission started to occur. Thus, to achieve efficient near-infrared emission, decreasing the effective concentration of dopants is necessary and co-doping with YF3 is efficient to control the effective concentration rare-earth dopants in the fluoride nanocrystals. Decay curves of the 2730 nm band emission of glass-ceramic specimens heat-treated at 490 °C for 10 h were recorded (Fig. 5c). All the decay curves showed nearly exponential decay behaviors.
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Fig. 5. Decay curves of (a) 1540 nm and (c) 2730 nm band emissions recorded from glass-ceramic specimens heat-treated at 490 °C for 10 h, and effective lifetimes of (b) 1540 nm and (d) 2730 nm band emissions recorded from all the specimens. In (b) and (d), solid square, solid circle, open square, open circle and solid diamond represent glasses doped with 0.1 ErF3/1.0YF3, 0.2 ErF3/1.0YF3, 0.3 ErF3/1.0YF3, 0.5 ErF3/1.0YF3, and 0.5 ErF3, respectively.
Specimens heat-treated at other temperatures showed similar exponential decay behaviors. The lifetimes of the 2730 nm band emission recorded from all the specimens were summarized in Fig. 5d. It was found that the lifetimes of the 2730 nm band emission increased with decrease in the concentration of ErF 3 . Changes in the lifetimes were consistent with the emission spectra, indicating that the effective concentration of Er 3 + ions in the fluoride nanocrystals was decreased and cross relaxation processes were suppressed. Unlike the changes observed in Fig. 4c, changes in the lifetime of the 2730 nm band emission were not so obvious. Intensity of the 2730 nm band increased monotonically since more and more Er 3 + ions were incorporated into the fluoride nanocrystals with an increase in the heat-treatment temperature. However, the lifetime of the 2730 nm band (Fig. 5d) was mainly determined by the Er3+ ions incorporated in the fluoride nanocrystals, since Er3 + ions in the glass matrices only emitted very weakly at 2730 nm.
4. Summary Efficient 980 nm and 2730 band emissions from Er3 + doped oxyfluoride glass-ceramics were achieved through the co-doping of YF3. It was found that YF3 co-doping can efficiently promote the formation of fluoride nanocrystals and incorporation of Er3 + ions into these fluoride nanocrystals. Intensities of the 980 nm and 2730 band emissions were greatly enhanced compared to the 1540 nm band emission with the doping of 1 mol% YF3. Prolonged lifetimes of 1540 nm and 2730 nm band emissions evidenced that the effective concentration of Er3+ ions in the fluoride nanocrystals decreased and the cross relaxation processes leading to the quenching of Er3+ infrared emission were suppressed. These findings have great potential for
the development of efficient infrared emitting materials based on rare-earth doped oxyfluoride glass-ceramics. Acknowledgments This work was financially supported by the Fundamental Research Funds for the Central Universities of China (WUT: 2013-II-010), Natural Science Foundation of Hubei Province (Grant Nos.: 2012FFA024, 2013CFA008), National Natural Science Foundation of China (Grant No.: 51202170), Program for New Century Excellent Talents in University (Grant No.: NCET-13-0943), Research Fund for the Doctoral Program of Higher Education of China (FRDP: 20130143110013), and Chutian Scholar Program of Hubei Province. References [1] Y. Tian, J. Zhang, X. Jing, Y. Zhu, S. Xu, J. Lumin. 138 (2013) 94–97. [2] L. Su, J. Xu, H. Li, D. Zhang, X. Xu, G. Zhao, J. Lumin. 122–123 (2007) 17–20. [3] G. Wu, S. Fan, Y. Zhang, G. Chai, Z. Ma, M. Peng, J. Qiu, G. Dong, Opt. Lett. 38 (2013) 3071–3074. [4] W. Zhang, Q. Chen, Q. Qian, Q. Zhang, J. Am. Ceram. Soc. 95 (2012) 663–669. [5] V.K. Tikhomirov, C. Görller-Walrand, K. Driesen, J. Alloys Compd. 451 (2008) 542–544. [6] F. Lahoz, J.M. Almenara, U.R. Rodríguez-Mendoza, I.R. Martín, V. Lavín, J. Appl. Phys. 99 (2006) 053103. [7] W.J. Chung, K.H. Kim, B.J. Park, H.S. Seo, J.T. Ahn, Y.G. Choi, J. Am. Ceram. Soc. 93 (2010) 2952–2955. [8] H. Lin, D. Chen, Y. Yu, Z. Shan, P. Huang, A. Yang, Y. Wang, J. Alloys Compd. 509 (2011) 3363–3366. [9] N. Hu, H. Yu, M. Zhang, P. Zhang, Y. Wang, L. Zhao, Phys. Chem. Chem. Phys. 13 (2011) 1499–1505. [10] C. Liu, J. Heo, J. Am. Ceram. Soc. 95 (2012) 2100–2102. [11] C. Liu, X. Zhao, J. Heo, J. Non-Cryst. Solids 365 (2013) 1–5. [12] G. Dantelle, M. Mortier, G. Patriarche, D. Vivien, J. Solid State Chem, J. Solid State Chem. 179 (2006) 1995–2003. [13] L. Zhang, H. Hu, J. Non-Cryst. Solids 326–327 (2003) 353–358.
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