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Eu3+ doped Li3Ba2La3(WO4)8 red phosphors were synthesized by the solid state reaction method. A pure phase was confirmed by the X-ray diffraction pattern. Diffuse reflection spectra, photoluminescence spectra, decay curves, quantum yields, and temperature-dependence luminescence spectra were measured. The Eu3+ ions can substitute the La3+ ions completely without decreasing the emission intensity obviously and the quantum yields can keep at about 95%. The activation energy from the bottom of 5D0 level to the 5D0-charge transfer state crossover is 0.24eV. All the results indicate that the Eu3+ doped Li3Ba2La3(WO4)8 can serve as a near ultraviolet excited red phosphor for white light emitting diodes.
© 2015 Optical Society of America
1. Introduction
White light emitting diodes (LEDs) have attracted much attention due to their energy saving, long lifetime, and environmental friendliness [1–3]. Although a blue diode chip combined with Y3Al5O12:Ce3+ (YAG:Ce3+) yellow phosphor is still the most mature method for fabricating commercial white LEDs, it has some drawbacks such as low color rendering index and high correlated color temperature owe to lack of red light component [4, 5]. Therefore, searching for red phosphors gained a lot of attention and resulted in the invention of (Ca,Sr)AlN3:Eu2+ and (Ca,Sr,Ba)2Si5N8:Eu2+, which have been widely used in warm-white LEDs. However, both kinds of phosphor show the relatively broad absorption and emission bands, which result in serious re-absorption of visible light and lower total luminous efficiency of white LED [4, 6, 7]. Furthermore, the rigorous synthesis conditions of these nitride compounds also lead to a high production cost [4]. According to Ref [8], the red phosphors with a narrow emission band (FWHM < 30 nm) located between 615 and 655 nm is optimal for a warm-white LED with high lumen output and color rendering. Eu3+ and Mn4+ doped red phosphors which meet this demand have aroused much interest [4, 9–12].
Eu3+ doped materials have been studied as red phosphors excited by NUV-LEDs in recent years [9]. However, these materials suffer from low absorption in NUV because of the f-f transitions [9]. There are several ways to solve this problem, the first one is to add a sensitizer such as Sm3+ and Bi3+ [13, 14], the second one is to move a charge transfer band to NUV area [15, 16], and the third one is to seek a compound in which Eu3+ ions can be highly doped without fluorescence quenching [12, 17]. In this work, the third option has been adopted.
Rare earth based ternary molybdate and tungstate Li3Ba2Ln3(AO4)8 (Ln = La–Lu, Y, A = Mo, W) have been studied as luminescent materials because of their low sintering temperature and low phonon energy [18–24]. The structures of Li3Ba2Ln3(MoO4)8 (Ln = Gd, Tm) have been solved by R. F. Klevtsova in 1992 with monoclinic space group C2/c [25]. The Li3Ba2Gd3(MoO4)8:Eu3+ and Li3Ba2La3(MoO4)8:Eu3+ have been reported with bright red emission and high quantum efficiency under blue light excitation [19, 22]. These materials have a broad and strong excitation band due to the ligand-to-metal charge transfer (LMCT) in the UV and NUV region resulting from Mo(W)O4 groups. The LMCT of Li3Ba2Ln3(MoO4)8:Eu3+ locates in the spectral region of 250-370 nm and is close to the 5DJ level position of Eu3+, which may lead to the thermal quenching of 5DJ and thus decrease the quantum efficiency of these red phosphors [19]. The quantum efficiency of Li3Ba2Y3-x(MoO4)8:xEu3+ red phosphor is only 63% for x = 2.4 under excitation at 395 nm [20]. The LMCT of the Eu3+ doped phosphors containing Mo(W)O4 groups would shifts toward shorter wavelengths with WO4 content increasing [16]. Therefore, the Eu3+ doped Li3Ba2La3(WO4)8 (LBLW) under excitation at NUV to achieve more efficient red emission is worth investigating.
In this work, Eu3+ doped LBLW have been synthesized by the solid state reaction method. The crystalline structure is checked by the X-ray powder diffraction (XRD). The photoluminescence properties of the Eu3+ doped LBLW are measured and its emission intensity is compared with that of the commercial phosphor Y2O2S:Eu3+ and CaAlSiN3:Eu2+. The quantum yield and thermal stability are reported as well.
2. Experimental
A series of Li3Ba2La3-xEux(WO4)8 (LBLW:xEu3+) (x = 0.1, 0.3, 0.6, …, 3.0) were synthesized by the solid state reaction. The starting materials of Li2CO3 (A.R.), BaCO3 (A.R.), La2O3 (99.99%), WO3 (A.R.), and Eu2O3 (99.99%) were weighed according to the stoichiometric ratio. A 13% excess of Li2CO3 was added as a self-flux. The raw materials were mixed in an agate mortar employing ethanol as the grinding medium, transfered to crucibles and then dried at 90 °C for 1h. After this, they had been sintered in air at 800 °C for 10 h in muffle furnace and were reground for further characterization.
XRD measurements were carried out on a MiniFlex 600 powder diffractometer with Cu Kα radiation (1.5405 Å), operating at 40 kV and 15mA. The scanning speed for phase determination was 0.2°/min. Diffuse refection spectra (DRS) of the powder samples were measured by an UV−vis-NIR spectrophotometer (Eclipse, Cary 500) and using BaSO4 as a standard reference. Photoluminescence spectra, decay curves, and temperature-dependent luminescence spectra were recorded by a spectrometer (Edinburgh Instruments, FLS980) equipped with both continuous (450 W) and pulsed xenon lamps as the light source. Quantum yield (QY) was measured by a spectrometer (Edinburgh Instruments, FLS920) using a barium sulfate coated integrating sphere attached to the spectrometer.
3. Results and discussion
XRD patterns of the LBLW:xEu3+ (x = 0.1, 0.3, 0.6, …, 3.0) are displayed in Fig. 1. All the peak positions agree well with those of LBLW (ICSD #187059) [23] despite the different radii of La3+ and Eu3+. From the magnified XRD curves in the range of 24−30° in Fig. 2, it can be found that the peaks have a gradual shift toward larger 2θ angel with the increasing of Eu3+ concentration. This may be due to the smaller radius of Eu3+ ion (CN = 8, r = 1.066Å) than that of La3+ (CN = 8, r = 1.160 Å) [26].
Figure 3 shows the diffuse reflection spectra of the LBLW:3.0Eu3+, i.e. Li3Ba2Eu3(WO4)8, and LBLW. The un-doped LBLW shows no absorption in the visible region, while Li3Ba2Eu3(WO4)8 contains several absorption bands related to the f-f transitions of Eu3+. The strongest absorption band is around 395nm which is attributed to the 7F0→5L6 transition of Eu3+ ions [27].
Figure 4 shows the excitation spectra of the Li3Ba2Eu3(WO4)8, the commercial red phosphor Y2O2S:6.3%Eu3+, and CaAlSiN3:Eu2+. The strong broad band around 340nm of Y2O2S:6.3%Eu3+ is attributed to the charge transfer transition of the Eu3+-O2− and Eu3+-S2− [28]. The excitation spectra of Li3Ba2Eu3(WO4)8 contains several narrow excitation bands ascribed to the 7F0 →5D4 (361nm), 7F0→5L7 (382nm), 7F0→5L0 (395nm), 7F0→5D3 (416nm), 7F0→5D2 (465nm), and 7F0→5D1 (535nm) transitions of Eu3+ ions [27, 29], and the no obviously LMCT band appears. Particularly, Li3Ba2Eu3(WO4)8 shows a stronger emission than that of Y2O2S:6.3%Eu3+ under excitation around 395nm, therefore, the LBLW:xEu3+ can be excited by the NUV LEDs more effectively than the Y2O2S:6.3%Eu3+. The commercial CaAlSiN3:Eu2+ red phosphor has a stronger and broader excitation band from 200 to 550nm via the 4f-5d transition. Obviously, the CaAlSiN3:Eu2+ has serious absorption in the visible region, which will lower the total luminous efficiency of white LED.
Fig. 4 Excitation spectra of Li3Ba2Eu3(WO4)8 (λem = 615nm), Y2O2S:6.3%Eu3+ (λem = 626nm), and CaAlSiN3:Eu2+ (λem = 630nm).
The emission spectra of the Li3Ba2Eu3(WO4)8, Y2O2S:6.3%Eu3+, and CaAlSiN3:Eu2+ under the same experimental condition are compared in Fig. 5. The emission spectrum of the Li3Ba2Eu3(WO4)8 exhibit a strong red emission band around 615nm assigned to the 5D0→7F2 transition, and several weak bands around 580, 591, 655, and 701nm corresponding to the 5D0→7F0, 5D0→7F1, 5D0→7F3, and 5D0→7F4 transitions, respectively. The emission band around 626nm of the Y2O2S:6.3%Eu3+ is also attributed to the 5D0→7F2 transition. The integrated emission intensity of the 5D0→7F2 transition for the Li3Ba2Eu3(WO4)8 is 3.8 times of that of the Y2O2S:6.3%Eu3+. The electrical dipole transition 5D0→7F2 is hypersensitive and its intensity is sensitive to the local environment, while the magnetic dipole transition 5D0→7F1 is not sensitive to the surround. Hence the integrated intensity ratio of R = I(5D0→7F2)/I(5D0→7F1) can be used as a measure of the site symmetry of Eu3+ [17]. A lower symmetry of the crystal field around Eu3+ will result in a larger ratio value [30]. The intensity ratio R of Li3Ba2Eu3(WO4)8 is 8.82 and in good agreement with the crystal structure of LBLW [23], in which the local symmetry of La3+ site is C1. However, it is worth noting that the emission of Li3Ba2Eu3(WO4)8 and Y2O2S:6.3%Eu3+ via the internal 4f-4f transition is weaker than that of the 5d-4f for CaAlSiN3:Eu2+ shown in Fig. 5. The Commission International del’Eclairage (CIE) chromaticity coordinate for the emission of Li3Ba2Eu3(WO4)8 is (0.67, 0.33) as shown in Fig. 6, which is the same as the standard red chromaticity (0.67, 0.33) for the National Television Standard Committee (NTSC) system.
Fig. 5 Emission spectra of Li3Ba2Eu3(WO4)8 (λex = 395nm), Y2O2S:6.3%Eu3+ (λex = 396nm) and CaAlSiN3:Eu2+ (λex = 395nm).
The emission spectra of LBLW:xEu3+ under excitation at 395nm are shown in Fig. 7, the inset shows the concentration-dependent integrated emission intensity of the 5D0→7F2 transition. The emission intensity increases with the increment of Eu3+ concentration and reaches the highest point at x = 2.7. It can be found from the unit cell of LBLW along a-axis shown in Fig. 8 that, in the same layer, the shortest distance between La3+ is 3.987 Å; but in different layers, the shortest distance between La3+ is 8.43 Å and they are separated by the WO4 groups. G. Blasse et al. have reported that if the Eu-Eu distance is shorter than 5 Å, the exchange interaction between Eu3+ ions becomes effective [31]. It can be imaged that the interaction between Eu3+ in the Li3Ba2Eu3(WO4)8 is two-dimensional. This is resemble to the NaEuTiO4, in which the shortest Eu-Eu distance is 3.630 Å, and the interaction between Eu3+ is also two-dimensional [32]. However, the shortest distance between Eu3+ in the LBLW is longer than that in the NaEuTiO4. Therefore, the exchange interaction between Eu3+ ions is weaker in the LBLW and Eu3+ ions can substitute La3+ ions completely without decreasing the emission intensity obviously whereas the NaEuTiO4 shows weak red emission.
Fig. 7 Emission spectra of LBLW:xEu3+ (x = 0.1, 0.3, 0.6, …, 3.0) (λex = 395nm). The inset shows the dependence of integrated emission intensity of 5D0→7F2 transition on the Eu3+concentration.
The effect of Eu3+ concentration on luminescence decay curves at 615nm under excitation at 395nm is shown in Fig. 9. It can be found that the decays can be fitted by a single-exponential function [33] as
where I represents the luminescence intensity at time t after the cutoff of the excitation light, I0 is the initial emission intensity at t = 0 and τ is the fluorescence lifetime of the 5D0 level. The inset of Fig. 8 shows that the values of τ decrease slightly with the increment of Eu3+ concentration. It also reveals that the concentration dependent fluorescence quenching is weak for the LBLW:xEu3+.The quantum yield and absorption efficiency of LBLW:xEu3+ under excitation at 395nm are listed in Table 1. The values for x = 2.1, 2.4, 2.7 are around 95% while for LBLW:3.0Eu3+ is 83%, which may be due to the increasing cross relaxation among Eu3+ ions.
Table 1. Quantum yield and absorption efficiency of LBLW:xEu3+
Figure 10 demonstrates the thermal stability of emission from the LBLW:2.7Eu3+, which was measured under excitation at 395nm and at temperature from 300K to 550K. The integrated emission intensity of the 5D0→7F2 transition decreases with the increasing of temperature. However, there are no alteration for the CIE coordinate of LBLW:2.7Eu3+ at different temperature. As the temperature reaches to 450K, the emission intensity decreases to 65% of that at 300K. The activation energy ∆E is the energy from the bottom of 5D0 level to the 5D0-charge transfer state crossover [34] and can be calculated by [35]
From Fig. 11, the values of ln(I0/I-1) decrease linearly with the increasing of 1/kT and the ∆E can be calculated to be 0.24 eV, which is smaller than that of Li3Ba2Gd3(MoO4)8:2.4Eu3+ (∆E = 0.283 eV) [19]. This may attribute to different Eu3+ concentrations and different ionic radii of La3+ (CN = 8, r = 1.160Å) and Gd3+ (CN = 8, r = 1.053 Å) [26, 34, 36].Fig. 10 Temperature dependence of emission intensity of LBLW:2.7Eu3+ (λex = 395nm). The inset shows the temperature dependence of emission intensity of 5D0→7F2 transition.
4. Conclusion
LBLW:Eu3+ with different Eu3+ concentrations were synthesized by the solid state reaction method. XRD diffraction demonstrated a pure phase was achieved and the lattice shrinks with the increasing of Eu3+ concentration. The Li3Ba2Eu3(WO4)8 red phosphor shows strong absorption of NUV light and its emission intensity is 3.8 times of that of Y2O2S:6.3%Eu3+ under the same experimental condition though it is weaker than that of CaAlSiN3:Eu2+. As the temperature reaches to 450K, the red emission intensity decreases to 65% of that at 300K. All the results indicate that the LBLW:Eu3+ has potential applications for NUV excited white LEDs.
Acknowledgements
This work was supported by the Science and Technology Major Project of Fujian Province (Grant No. 2011HZ0001-2).
References and links
1. Y. Narukawa, “White-light LEDS,” Opt. Photonics News 15, 24–29 (2004).
2. E. F. Schubert and J. K. Kim, “Solid-state light sources getting smart,” Science 308(5726), 1274–1278 (2005). [CrossRef] [PubMed]
3. C. C. Lin and R.-S. Liu, “Advances in phosphors for light-emitting diodes,” J. Phys. Chem. Lett. 2(11), 1268–1277 (2011). [CrossRef] [PubMed]
4. H. Zhu, C. C. Lin, W. Luo, S. Shu, Z. Liu, Y. Liu, J. Kong, E. Ma, Y. Cao, R. S. Liu, and X. Chen, “Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes,” Nat. Commun. 5, 4312–4322 (2014). [PubMed]
5. P. Pust, V. Weiler, C. Hecht, A. Tücks, A. S. Wochnik, A. K. Henss, D. Wiechert, C. Scheu, P. J. Schmidt, and W. Schnick, “Narrow-band red-emitting Sr[LiAl₃N₄]:Eu²⁺ as a next-generation LED-phosphor material,” Nat. Mater. 13(9), 891–896 (2014). [CrossRef] [PubMed]
6. X. Piao, K.-i. Machida, T. Horikawa, H. Hanzawa, Y. Shimomura, and N. Kijima, “Preparation of CaAlSiN3: Eu2+ phosphors by the self-propagating high-temperature synthesis and their luminescent properties,” Chem. Mater. 19(18), 4592–4599 (2007). [CrossRef]
7. X. Piao, T. Horikawa, H. Hanzawa, and K.-i. Machida, “Preparation of (Sr1−x Cax)2Si5N8/Eu2+ solid solutions and their luminescence properties,” J. Electrochem. Soc. 153, H232–H235 (2006). [CrossRef]
8. A. Žukauskas, R. Vaicekauskas, F. Ivanauskas, H. Vaitkevičius, and M. S. Shur, “Spectral optimization of phosphor-conversion light-emitting diodes for ultimate color rendering,” Appl. Phys. Lett. 93(5), 051115 (2008). [CrossRef]
9. S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma, and Q. Y. Zhang, “Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties,” Mater. Sci. Eng. Rep. 71(1), 1–34 (2010). [CrossRef]
10. X. Jiang, Y. Pan, S. Huang, X. Chen, J. Wang, and G. Liu, “Hydrothermal synthesis and photoluminescence properties of red phosphor BaSiF6:Mn4+ for LED applications,” J. Mater. Chem. C Mater. Opt. Electron. Devices 2(13), 2301 (2014). [CrossRef]
11. S. H. Park, K. H. Lee, S. Unithrattil, H. S. Yoon, H. G. Jang, and W. B. Im, “Melilite-structure CaYAl3O7:Eu3+ phosphor: structural and optical characteristics for near-UV LED-based white light,” J. Phys. Chem. C 116(51), 26850–26856 (2012). [CrossRef]
12. J. Zhong, D. Chen, W. Zhao, Y. Zhou, H. Yu, L. Chen, and Z. Ji, “Garnet-based Li6CaLa2Sb2O12:Eu3+ red phosphors: a potential color-converting material for warm white light-emitting diodes,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(17), 4500–4510 (2015). [CrossRef]
13. X. Wang, Y. Xian, G. Wang, J. Shi, Q. Su, and M. Gong, “Luminescence investigation of Eu3+–Sm3+ co-doped Gd2−x−yEuxSmy(MoO4)3 phosphors as red phosphors for UV InGaN-based light-emitting diode,” Opt. Mater. 30(4), 521–526 (2007). [CrossRef]
14. A. Xie, X. Yuan, Y. Shi, F. Wang, and J. Wang, “Photoluminescence characteristics of energy transfer between Eu3+ and Bi3+ in LiEu1−xBix(WO4)0.5(MoO4)1.5,” J. Am. Ceram. Soc. 92(10), 2254–2258 (2009). [CrossRef]
15. H. Li, R. Zhao, Y. Jia, W. Sun, J. Fu, L. Jiang, S. Zhang, R. Pang, and C. Li, “Sr(1.7)Zn(0.3)CeO4: Eu3+ novel red-emitting phosphors: synthesis and photoluminescence properties,” ACS Appl. Mater. Interfaces 6(5), 3163–3169 (2014). [CrossRef] [PubMed]
16. J. Huang, B. Hou, H. Ling, J. Liu, and X. Yu, “Crystal structure, electronic structure, and photoluminescence properties of La₃BW(1-x)Mo(x)O₉:Eu³⁺ red phosphor,” Inorg. Chem. 53(18), 9541–9547 (2014). [CrossRef] [PubMed]
17. R. Zhu, Y. Huang, and H. J. Seo, “A Red-Emitting Phosphor of Eu-Based Borotungstate Eu3BWO9 for White Light-Emitting Diodes,” J. Electrochem. Soc. 157(12), H1116–H1120 (2010). [CrossRef]
18. H. Li, L. Zhang, and G. Wang, “Growth, structure and spectroscopic characterization of a new laser crystals Nd3+:Li3Ba2Gd3(WO4)8,” J. Alloys Compd. 478(1-2), 484–488 (2009). [CrossRef]
19. Y.-C. Chang, C.-H. Liang, S.-A. Yan, and Y.-S. Chang, “Synthesis and photoluminescence characteristics of high color purity and brightness Li3Ba2Gd3(MoO4)8:Eu3+ red phosphors,” J. Phys. Chem. C 114(8), 3645–3652 (2010). [CrossRef]
20. M. Shang, G. Li, X. Kang, D. Yang, and J. Lin, “Synthesis and luminescent properties of Li3Ba2Y3(MoO4)8:Ln3+ (Ln = Eu, Tb, Dy) phosphors for UV-LEDs,” J. Electrochem. Soc. 158(5), H565–H571 (2011). [CrossRef]
21. X. Han, R. Calderón-Villajos, F. Esteban-Betegón, C. Cascales, C. Zaldo, A. Jezowski, and P. Stachowiak, “Crystal growth and physical characterization of monoclinic Li3Lu3Ba2(MoO4)8. a spectrally broadened disordered crystal for ultrafast mode-locked lasers,” Cryst. Growth Des. 12(8), 3878–3887 (2012). [CrossRef]
22. A. Katelnikovas, J. Plewa, S. Sakirzanovas, D. Dutczak, D. Enseling, F. Baur, H. Winkler, A. Kareiva, and T. Jüstel, “Synthesis and optical properties of Li3Ba2La3(MoO4)8:Eu3+ powders and ceramics for pcLEDs,” J. Mater. Chem. 22(41), 22126–22134 (2012). [CrossRef]
23. Y. Pan, Y. Chen, Y. Lin, X. Gong, J. Huang, Z. Luo, and Y. Huang, “Structure, spectral properties and laser performance of Tm3+-doped Li3Ba2La3(WO4)8 crystal,” CrystEngComm 14(11), 3930–3935 (2012). [CrossRef]
24. M. Song, W. Zhou, M. Wu, and G. Wang, “Structure, thermal and spectroscopic properties of Tm3+-doped Li3Ba2Y3(MoO4)8 crystal as a promising candidate for 2 μm lasers,” CrystEngComm 15(1), 168–174 (2013). [CrossRef]
25. R. F. Klevtsova, A. D. Vasil’ev, L. A. Glinskaya, A. I. Kruglik, N. M. Kozhevnikova, and V. P. Korsun, “Crystal structure investigation of triple molybdates Li3Ba2Ln3(Mo04)8 (Ln = Gd, Tm),” J. Struct. Chem. 33(3), 131–136 (1992). [CrossRef]
26. R. D. Shannon, “Revised effective ionic radii and systematic studies of interatomie distances in halides and chaleogenides,” Acta Crystallogr. A 32(5), 751–767 (1976). [CrossRef]
27. W. T. Carnall, “Electronic energy levels of the trivalent lanthanide aquo ions. IV. Eu3+,” J. Chem. Phys. 49(10), 4450 (1968). [CrossRef]
28. K. R. Reddy, K. Annapurna, and S. Buddhudu, “Fluorescence spectra of Eu3+ Ln2O2S (Ln = Y, La, Gd) powder phosphors,” Mater. Res. Bull. 31(11), 1355–1359 (1996). [CrossRef]
29. Y. Huang and H. J. Seo, “A novel red-emitting nano-phosphor of Eu3+-doped LaBWO6,” Mater. Lett. 84, 107–109 (2012). [CrossRef]
30. Y. Huang, Y. Nakai, T. Tsuboi, and H. J. Seo, “The new red-emitting phosphor of oxyfluoride Ca2RF4PO4:Eu3+ (R=Gd, Y) for solid state lighting applications,” Opt. Express 19(7), 6303–6311 (2011). [CrossRef] [PubMed]
31. G. Blasse and B. C. Grabmaier, “Luminescent materials,” in Luminescent Materials (Springer-Verlag, Berlin and New York, 1995), pp. 97–99.
32. P. A. M. Berdowski and G. Blasse, “Luminescence and energy migration in a two-dimensional system: NaEuTiO4,” J. Lumin. 29(5-6), 243–260 (1984). [CrossRef]
33. X. Qiao, Y. Cheng, L. Qin, C. Qin, P. Cai, S. I. Kim, and H. J. Seo, “Coprecipitation synthesis, structure and photoluminescence properties of Eu3+-doped sodium barium borate,” J. Alloys Compd. 617, 946–951 (2014). [CrossRef]
34. W. H. Fonger, “Eu+3 5D resonance quenching to the charge-transfer states in Y2O2S, La2O2S, and LaOCl,” J. Chem. Phys. 52(12), 6364–6372 (1970). [CrossRef]
35. D. Wen, J. Feng, J. Li, J. Shi, M. Wu, and Q. Su, “K2Ln(PO4)(WO4):Tb3+,Eu3+(Ln = Y, Gd and Lu) phosphors: highly efficient pure red and tuneable emission for white light-emitting diodes,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(9), 2107–2114 (2015). [CrossRef]
36. L. Li, S. Zhou, and S. Zhang, “Investigation on relationship between charge transfer position and dielectric definition of average energy gap in Eu3+-doped compounds,” J. Phys. Chem. C 111(7), 3205–3210 (2007). [CrossRef]
