SYMMETRY OF Eu
3+SITE AND ENERGY TRANSFER OF Ce
3+/Eu
3+IN Sr
2Al
2SiO
7MATERIALS
Ho Van Tuyen1*, Đo Thanh Tien2,3, Nguyen Manh Son2, Le Xuan Hung1, Nguyen Thi Thai An1
1Institute of Research and Development, Duy Tan University, 03 Quang Trung, Danang, Vietnam
2Faculty of Physics, University of Sciences, 77 Nguyen Hue, Hue, Vietnam
3Faculty of Basic Science, University of Agriculture and Forestry, 102 Phung Hung, Hue, Vietnam
*Email: [email protected] Abstract:
In this study, Sr2Al2SiO7 (SAS) materials doped Eu3+, Ce3+ ions were synthesized by solid-state reaction. The asymmetry of the Eu3+ site in SAS lattice was estimated by Ωλ (λ = 2, 4, 6) intensity parameters of Judd-Ofelt (JO) theory. Results showed that the introducing of Al2O3 leads to an enhancement of the asymmetry of the Eu3+- surroundings in SAS:Eu3+ materials. Photoluminescence spectra investigation of SAS:Ce3+(x mol%),Eu3+(1 mol%) (x=0.5; 1.0; 1.5) samples indicated that there is an energy transfer from sensitizer Ce3+ to activator Eu3+ in SAS materials.
Keywords: Aluminosilicate, Phonon sideband, Energy Transfer.
INTRODUCTION
Luminescence phosphor based on the alkaline earth aluminum silicate have been extensively studied and they are expected to provide a variety of fluorescence for light-emitting diodes [1-3]. Particularly, rare earth (RE) ions doped strontium aluminum silicate phosphors, Sr2Al2SiO7 (SAS), have been studied on luminescent properties as well as structural characteristic in recent years. For example, Eu2+
ion was doped SAS material to obtain a cyan- green emitting phosphors [4]. Several studies have combined Eu2+ ion with other RE3+ ions, e.g. Dy3+, Ce3+, Nd3+ co-doped into this host lattice to investigate long persistent luminescence, synthesize a promising phosphor for warm-white light emitting diode, and study the thermoluminescence properties [5-8].
Thermoluminescence properties as activation energy, order kinetics were calculated for SAS:Eu/Dy and SAS:Eu3+ [7, 9]. The controllable photoluminescence or thermal stability for SAS compounds have been also published [10, 11]. However, there has been no literature until now presenting information of symmetry of Eu3+ site via Judd-Ofelt (JO) analysis and the vibration energy from the phonon sideband (PSB) in SAS:Eu3+ material. It is known that intensity parameters Ωλ calculated from JO theory contains information about the symmetry of the environment around RE3+ sites [12]. In case of Eu3+ ion, Ωλ intensity parameters
can be calculated from the photoluminescence spectrum [12, 13] and this is a advantage of Eu3+
compare to other RE3+ ions.
Besides that, an energy transfer phenomenon in SAS phosphors was observed for Ce3+/Eu2+ [6]
Ce3+/Tb3+ [14], and Ce3+/Dy3+ [15] co-doped. So we hope that the energy transfer between Ce3+
and Eu3+ also take place in SAS:Ce3+,Eu3+
materials. Therefore, two targets of this work are: (1) estimate the symmetry of Eu3+ site and the vibration modes in SAS:Eu3+ phosphor via the JO intensity parameters and the PSB spectra.
(2) Observe the energy transfer process between Ce3+ and Eu3+ ions in SAS:Ce3+,Eu3+ materials.
EXPERIMENTAL
Sample series including SAS:Eu3+ (0.5 mol%), SAS:Ce3+ (0.5 mol%) and SAS:Ce3+(x mol%), Eu3+ (1.0 mol%) with x=0.5, 1.0, 1.5 were fabricated by solid-state reaction method at high temperature in the same condition and they were denoted by SAS:Eu05, SAS:Ce05, SAS:Ce05Eu10, SAS:Ce10Eu10 and SAS:Ce15Eu10, respectively. Firstly, the raw materials, SrCO3 (AR), Al2O3 (AR), SiO2
(Korea), Ce(NO3)3.6H2O (Sigma) and Eu2O3
(Merck) were weighted according to the nominal composition, mixed homogeneously and milled thoroughly using an agate pestle and mortar to achieve a uniform mixture. A small amount of B2O3 was added during mixing process to serve as a flux to promote the formation of crystal
structure. After that, this mixture was ground and calcined at 1250°C for 2 h in air. The obtained product was finally ground into powder to analyze phase compositions, luminescent property, and Raman spectra.
Experimental measurements such as X-ray diffraction (XRD), Raman scattering, photoluminescence (PL), lifetime were taken out to study structural characteristic and luminescent properties of the prepared sample. XRD pattern was recorded by using X-ray diffractometer D8- Advance (Bruker, Germany). Raman scattering was measured by Xplora plus equipment (Horiba Jobin-Yvon). Photoluminescence (PL) and Photoluminescence excitation (PLE) spectra were taken out by a spectrophotometer (FL3-22, Horiba Jobin-Yvon) at room temperature.
Phonon sideband (PSB) spectrum of the
7F0→5D2 transition of Eu3+ ion was also analyzed from PLE spectra of SAS:Eu05 sample. Lifetime was calculated from the fluorescence decay curves recorded by DeltaTime equipment (Horiba Jobin-Yvon). SEM image of sample was figured out by a Scanning Electron Microscope Jeol 6490 JED 2300 (Japan).
RESULTSANDDISCUSSION
1. XRD pattern, SEM image and Raman spectra of SAS materials
The structure phase of SAS:Ce05 and SAS:Eu05 samples were figured out by X-ray diffraction (XRD) in 20-70o region using Cu K-alpha (0.154 nm) radiation and their XRD patterns are depicted in Fig. 1.
20 30 40 50 60 70
SAS:Ce05
SAS:Eu05
Intensity (a.u.)
2q (Deg.)
Sr2Al2SiO7 (PDF:38-1333)
Figure 1: XRD patterns of SAS:Ce05 and SAS:Eu05 samples.
All the peaks are a match well with the Sr2Al2SiO7 standard PDF card of 38-1333, indicating that the doping ions Ce and Eu of small amount did not affect to the structure of the
matrix lattice. The crystal structure of SAS samples is identified as a tetragonal structure with space group P421m [1, 6] and lattice parameters are a=b=7.820 Å, c=5.264 Å and α=β=γ=90o. A SEM image of SAS:Eu05 sample is presented in Fig. 2. As can be seen in SEM image, the particles tend to agglomerate forming clusters with irregular shapes.
Figure 2: SEM images of SAS:Eu05 sample.
200 400 600 800 1000 1200
Intentsity (a.u.)
Raman shift (cm- 1) a
b c
a: SAS:Ce05 b: SAS:Eu05 c: SAS:Ce05Eu10
Figure 3: Raman spectra of three samples (a) SAS:Ce05, (b) SAS:Eu05, (c) SAS:Ce05Eu10.
Raman scattering spectra of three prepared samples SAS:Ce05, SAS:Eu05, SAS:Ce05Eu10 at room temperature using laser of 532 nm are shown in Fig. 3. Raman spectra of these samples in 200-1200 cm-1 region are the same shape and peak positions. In which, four intense bands locating at 450, 606, 845 and 896 cm-1 are attributed to the bending modes of AlO4 [16], the Si-O-Si bending vibration [16-18], the stretching mode of AlO4 tetrahedra [16, 18] and pyrosilicate [17], respectively. Besides that, three high frequency bands centered at 970, 1067 and 1160 cm-1 are ascribed to non-bridging Si-O- stretching vibration [16, 19], Si-O-Si asymmetric stretching [17, 19, 20] and the feature of silicate networks [17]. A weak band at 782 nm is attributed to Si-O-Si symmetric stretching and the low frequency band around
350 cm-1 corresponds to the lattice modes [16, 17]. Besides the Raman spectra, several vibration modes can be found from phonon sideband spectra of the SAS:Eu3+ sample which is discussed in the next paragraph.
2. Phonon sideband and Judd-Ofelt analysis of Eu3+ doped SAS materials
The PLE spectrum of SAS:Eu05 sample monitoring at 617 nm (5D0→7F2 transition) is presented in Fig. 4 which includes an intense broadband in 240-300 nm and several sharp peaks in 350-575 nm region.
Figure 4: PLE spectra of SAS:Eu05 sample.
The broadband emission is ascribed to the electric charge transfer due to the ligand-to- metal charge transfer states, O2-- Eu3+ [13, 21], meanwhile the sharp peaks are attributed by the excited transitions from the ground sate 7F0 to the excited states of Eu3+ ion in SAS host lattice.
Two intense excited transitions locating at 392 and 462.5 nm belong to the 7F0→5L6 and
7F0→5D2 excitation transitions, respectively.
Some weak peaks are also observed in PLE spectra such as 7F0→5D1 (530 nm), 7F0→5D3
(412 nm) and 7F0→5D4 (361 nm) transitions. It is known that there is no any energy level between the excited 5D2 and 5D3 levels of Eu3+ ion.
Therefore, if any excitation peak locates between those two levels in PLE spectra, then it is due to the phonon energy and is called phonon sideband spectra.
In order to further investigate the phonon energy of the SAS:Eu05 sample, the understanding of vibration behaviors of lattice in immediate
vicinity of rare earth ions is necessary with a help of a powerful tool like PSB spectrum [13, 21]. In case of Eu3+, the 7F0→5D2 is a pure- electric transition (PET) and the barycenter wave number of this transition is as a reference for the zero-phonon line (ZPL), which can be used to calculate the phonon energy from PLE spectrum in Fig. 4. The PSB vibrations associated with the pure electronic transition
7F0→5D2 at 462.5 nm (23446.7 cm-1) of SAS:Eu05 sample are shown in Fig. 5. The PSB spectrum consists of four bands locating at 423.5 nm, 433.5 nm, 441.5 nm and 446 nm which are denoted by P1, P2, P3 and P4, respectively. The phonon energy of P1 at 1967 cm-1 is due to the stretching and bending vibration of O–H groups [22]. The P2 peak at 1423 cm-1 is close with 1400 cm-1 which is Si-O- H modes [23]. Meanwhile, the energy P3 of 1005 cm-1 may be assigned to the stretching vibrations of Si-O-Al [16], and the P4 at 777 cm-1 is close with the vibration energy of Si-O- Si symmetric stretching (782 cm-1) which was analyzed from Raman spectra.
Figure 5: PSB spectra of SAS:Eu05 sample.
The PL spectrum of SAS:Eu05 sample excited by radiation of 392 nm (7F0→5L6 transition) at room temperature is shown in Fig. 6. The emission of Eu3+ ion in 550-875 nm region includes seven sharp peaks which belong to the 5D0→7FJ
transitions (J = 0, 1, …, 6) of 4fn configure [24].
Among these transitions, the 5D0→7F2 is an electric dipole transition, and it is strongly affected by the change of Eu3+- surroundings [24]. Contrary to 5D0→7F2, 5D0→7F1 hardly varies with the evolution of Eu3+-surrounding
because it is a magnetic dipole transition [24].
Hence, the intensity ratio (R) of 5D0→7F2
transition to 5D0→7F1 can thus be used as a measure of site symmetry of rare earth ions and it is defined by the following equation [25-27]:
5 7
0 2
5 7
0 1
( )
( )
I D F
R I D F (1)
The intensity ratio R for SAS:Eu3+ was found to be around 2.25 by using eq.1, a clear indicator implies that the electric- dipole transition
5D0→7F2 red emission is observed to be dominated and Eu3+ ions are located at a low symmetry site in the SAS host lattice [12].
Figure 6: PL spectra of SAS:Eu05 sample under excited by radiation of 392 nm.
Figure 7: The time evolution of the fluorescence of
5D0 level of Eu3+ in SAS:Eu05 under excitation of 295 nm.
Judd-Ofelt theory is often used to describe the intensity of f-f transitions in rare earth ions and it allows the prediction the local structural symmetry of rare earth ions site and excited-
state radiative lifetimes just by using only three empirical parameters, Ωλ (λ=2, 4, 6) [12]. These parameters Ωλ for Eu3+ ion related to the ratios of emission intensity 5D0→7F2,4,6 to emission intensity 5D0→7F1 as below [28]:
4 2 3 2 2
5 ( ) 7 2
0 2,4,6
( ) 64 ( 2)
( ) 3 (2 1) 9
,
J J .
md md md
I d A e n n
I d A h J A
D U F
(2)
Where, AJ and Amd are spontaneous emission probability of 5D0 → 7F2,4,6 transitions and
5D0→7F1 transitions. e is the electron charge, n=1.67 is the refractive index [29], h is the Planck constant, J is the total angular momentum of the excited state, ν is the wavelength number of emission.
5 ( ) 7 2
0 2,4,6
D U F is the square of the matrix elements of the tensor operator and the observed values are found in ref. [12]. Using eq. 2 and emission intensities of 5D0 → 7F1,2,4,6 transitions from PL spectra in Fig. 6, the values of Ωλ
parameters and calculated lifetime (τcal) of 5D0
level were determined and listed in Table 1. It can be seen, Ω2 was greater than Ω4 indicated that higher hypersensitive behavior of 5D0→7F2
transition of Eu3+ ion in SAS:Eu05. The intensity parameters of SAS:Eu05 material were compared with that of Sr2SiO4:Eu3+ [30] as in Table 1. Clearly, Ω2 in SAS:Eu05is larger than the value of Sr2SiO4:Eu3+, this difference can be explain as follow:
The formula of Ωλ in JO theory contains the following factor [31]
( )
4 s 4
E nl f r nl nl r f
. (3)
Where,
4 f r nl
s is the radial integrals andE nl( ) is the energy difference between the 4fN and the excited 4fN-1nl1configurations. It is indicated in eq.3 that Ωλ depend strongly on λ value (s
1) and the Ω2 reflects the change of the energy difference between the 4fN and the excited 4fN-1nl1 configurations [31, 32]. It is known that electronegative of Al ion is smaller than the electronegative of Si ion (1.61 compare to 1.90) and it leads to the bonds of Eu3+ in Sr2Al2SiO7 (including Al ion) compound get larger covalence than that in Sr2SiO4. Hence,value of E nl( )E(4fN)E(4fN15 )d for SAS:Eu05is smaller than that in Sr2SiO4:Eu3+. It is a reason the value of Ω2 in SAS:Eu05is larger than that in Sr2SiO4:Eu3+. It may suggested that the introducing of Al2O3 can enhance the asymmetry of SAS:Eu05 materials. The fluorescence decay curve of 5D0 level of Eu3+ in SAS material was presented in Fig. 7 (obtained with λex=295 nm and λem= 617 nm).
The measured lifetimes (τmes) determined from fluorescence decay curve and calculated lifetimes (τcal) obtained from JO theory of 5D0
excited level of Eu3+ are 2.1 and 3.5 ms, respectively. The measured lifetime (τmes=2.1
ms) is similar with one in Sr2SiO4 material (τ=2.3 ms) (see Table 1), however it is smaller than the calculated value (τcal=3.5 ms). The discrepancy in two lifetime values is due to the contribution of non-radiative processes (multiphonon relaxation, energy transfer process between the rare earth ions and the intrinsic defects) into the measured lifetime [33]. These non-radiative processes reduce the quantum efficiency of the luminescence progress which is defined by as the ratio of the experimental lifetime to calculated lifetime η=τmes/τcal. For SAS:Eu05 in this work, the luminescence quantum efficiency is η=60%.
Table 1: Judd-Ofelt intensity parameters (Ωλ), R ratio, measured lifetimes (τmes) and calculated lifetimes (τcal).
Samples Ω2
(10-20 cm2)
Ω4
(10-20 cm2)
Ω6
(10-20 cm2)
R τmes (ms) τcal (ms) Ref.
Sr2Al2SiO7:Eu3+ 3.60 1.68 1.78 2.25 2.1 3.5 This work
Sr2SiO4:Eu3+ 1.40 0.49 - - 2.3 [30]
3. Energy transfer phenomenon in SAS:Ce3+, Eu3+ materials
The energy transfer phenomenon between Ce3+
and Eu3+ has been observed in some oxide materials such as Sr2CeO4:Eu3+, Tb3Al5O12 [34, 35] and zinc phosphate glasses [36]. It is known that energy transfer process between a sensitizer and activator can only occur when emission band of sensitizer has an overlap with excitation band of activator [37]. Fig. 8 shows PL and PLE spectra of SAS:Eu05 and SAS:Ce05 samples. It can be seen that the emission band of Ce3+
overlapped with the excitation peaks of Eu3+ ion in 360-420 nm region. Remarkably, the strongest excitation of Eu3+ at 394 nm (7F0→5L6 transition) located absolutely in emission band of Ce3+
therefore the energy transfer process from Ce3+
to Eu3+ ions is possible.
Besides, the strongest excitation of Ce3+ centered at 335 nm, this excitation wavelength located out of the excitation area of Eu3+, so it was used to excite for SAS co-doped Ce3+ and Eu3+ ions to investigate the energy transfer. Fig. 9 presents the PL of two sample SAS:Ce05 and SAS:Ce05Eu10 under excitation radiation of 335 nm in the same measurement condition. Two samples prepared with the same Ce concentration (0.5 mol%) but emission intensity of Ce3+ (peak at 400 nm) ion in SAS:Ce05Eu10 is lower than that in SAS:Ce05 sample. Besides,
PL spectra of SAS:Ce05Eu10 sample also included the luminescence of Eu3+ ion in 580- 650 nm region (inset in Figure 9).
250 300 350 400 450 500 550 600 650 700
c d b
7F0-5L6 5D0-7F45D0-7F2
5D0-7F1
Intensity (a.u.)
Wavelengt h (nm) a
a: Ce3+ excit at ion b: Ce3+ emission c: Eu3+ excit at ion d: Eu3+ emission
Figure 8: PL and PLE spectra of SAS:Eu05 and SAS:Ce05 samples.
These results suggested that there is an energy transfer from Ce3+ to Eu3+ in SAS:Ce05Eu10 sample. To confirm this energy transfer process, the PL spectra in 560-650 nm region of SAS:Ce05Eu10, SAS:Ce10Eu10 and SAS:Ce15Eu10 samples under 335 nm radiation are presented in Fig. 10. As can be seen in Fig.
10, fluorescence intensity of Eu3+ increase with increasing of Ce3+ concentration while concentration of Eu3+ is fixed constant (1 mol%).
This result confirms that there is energy transfer
from Ce3+ to Eu3+ center lead to increasing of fluorescence intensity of Eu3+.
350 400 450 500 550 600 650 700
550575600625650 5D0 -- 7F2
(a) (b) 5D0 -- 7F1
(b)
Intensity (a.u.)
Wavelengt h (nm) (a) (a) SAS:Ce05
(b) SAS:Ce05Eu10
Figure 9: PL spectra of (a) SAS:Ce05 and (b) SAS:Ce05Eu10 samples excited by 335 nm
radiation.
Figure 10. PL spectra in 560-650 nm region of SAS:Ce05Eu10, SAS:Ce10Eu10 and SAS:Ce15Eu10 samples excited by 335 nm
radiation.
CONCLUSION
Sr2Al2SiO7 materials doped Eu3+, Ce3+ and Ce3+/Eu3+ have been synthesized successfully by using solid state reaction method at 1250oC. The intense vibration modes assigned to Si-O-Si network, AlO4 units and Si-O-Al bonds in Sr2Al2SiO7:Eu3+ have been determined via Raman spectra and phonon sideband spectra.
Further studies on luminescent property and Judd-Ofelt analysis indicated that higher hypersensitive behavior of 5D0→7F2 transition of Eu3+ ion and the introducing of Al2O3 can enhance the asymmetry of Sr2Al2SiO7:Eu3+
materials. Energy transfer from Ce3+ to Eu3+ ions was observed in Sr2Al2SiO7:Ce3+, Eu3+ material.
ACKNOWLEDGMENTS
This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.03-2018.323.
REFERENCES
[1] Q. Zhang, J. Wang, M. Zhang, Q. Su, Applied Physics B, 92 (2008) 195-198.
[2] H.Y. Jiao, C.R. LiMao, Q. Chen, P.Y. Wang, R.C. Cai, IOP Conference Series: Materials Science and Engineering, 292 (2018) 012058.
[3] K. Van den Eeckhout, D. Poelman, P.F. Smet, PeMaterials, 6 (2013) 2789-2818.
[4] F.C. Lu, L.J. Bai, W. Dang, Z.P. Yang, P. Lin, ECS Journal of Solid State Science and Technology, 4 (2014) R27-R30.
[5] Y. Ding, Y. Zhang, Z. Wang, W. Li, D. Mao, H.
Han, C. Chang, Journal of Luminescence, 129 (2009) 294-299.
[6] G. Li, M. Li, L. Li, H. Yu, H. Zou, L. Zou, S.
Gan, X. Xu, Materials Letters, 65 (2011) 3418-3420.
[7] A. Jadhaw, V.D. Sonwane, A.S. Gour, P. Jha, Luminescence, 32 (2017) 1349-1353.
[8] W. Zhou, X. Ma, M. Zhang, Y. Luo, Z. Xia, Journal of Rare Earths, 33 (2015) 700-705.
[9] I.P. Sahu, D.P. Bisen, N. Brahme, R.K. Tamrakar, Journal of Materials Science: Materials in Electronics, 26 (2015) 10075-10086.
[10] P. Zhang, Z. Lu, Q. Yuan, Q. Hou, T.D. Golden, X. Ren, L. Weng, H. Wang, Materials Chemistry and Physics, 134 (2012) 1190-1196.
[11] H. Zou, D. Peng, Z. Chu, X. Wang, Y. Li, X.
Yao, Journal of Materials Science, 48 (2013) 7981- 7985.
[12] V. Dubey, R. Tiwari, R.K. Tamrakar, J. Kaur, S.
Dutta, S. Das, H.G. Visser, S. Som, Journal of Luminescence, 180 (2016) 169-176.
[13] S. Arunkumar, K. Venkata Krishnaiah, K.
Marimuthu, Physica B: Condensed Matter, 416 (2013) 88-100.
[14] W. Pan, G. Ning, Y. Lin, X. Yang, Journal of Rare Earths, 26 (2008) 207-210.
[15] Y. Gong, Y. Wang, Y. Li, X. Xu, Journal of The Electrochemical Society, 157 (2010) J208.
[16] K. Park, H. Kim, D.A. Hakeem, Dyes and Pigments, 136 (2017) 70-77.
[17] A.K. Yadav, P. Singh, RSC Advances, 5 (2015) 67583-67609.
[18] M.A. Bouhifd, G. Gruener, B.O. Mysen, P.
Richet, Physics and Chemistry of Minerals, 29 (2002) 655-662.
[19] A.M.B. Silva, C.M. Queiroz, S. Agathopoulos, R.N. Correia, M.H.V. Fernandes, J.M. Oliveira, Journal of Molecular Structure, 986 (2011) 16-21.
[20] M. Wang, J. Cheng, M. Li, F. He, Physica B:
Condensed Matter, 406 (2011) 3865-3869.
[21] H. Lin, H. Liang, G. Zhang, Q. Su, The luminescence of Eu3+ activated Ba2Mg(BO3)2
phosphors, Applied Physics A, 105 (2011) 143-147.
[22] I.P. Sahu, D.P. Bisen, N. Brahme, R.K.
Tamrakar, R. Shrivastava, Journal of Materials Science: Materials in Electronics, 26 (2015) 8824- 8839.
[23] A.M. Efimov, V.G. Pogareva, A.V. Shashkin, Journal of Non-Crystalline Solids, 332 (2003) 93- 114.
[24] K. Binnemans, Interpretation of europium(III) spectra, Coordination Chemistry Reviews, 295 (2015) 1-45.
[25] S. Tanabe, S. Todoroki, K. Hirao, N. Soga, Journal of Non-Crystalline Solids, 122 (1990) 59-65.
[26] M. Kolte, V.B. Pawade, S.J. Dhoble, Applied Physics A, 122 (2016).
[27] J.A. Capobianco, P.P. Proulx, M. Bettinelli, F.
Negrisolo, Physical Review B, 42 (1990) 5936-5944.
[28] H.H. Xiong, C. Zhu, X. Zhao, Z.Q. Wang, H.
Lin, Advances in Materials Science and Engineering, 2014 (2014) 1-7.
[29] P.A. Arifov, M.M. Bulatova, Glass Physics and Chemistry, 30 (2004) 198-201.
[30] S.K. Gupta, M. Mohapatra, S. Kaity, V.
Natarajan, S.V. Godbole, Journal of Luminescence, 132 (2012) 1329-1338.
[31] P. Van Do, V.P. Tuyen, V.X. Quang, N.T.
Thanh, V.T.T. Ha, H. Van Tuyen, N.M. Khaidukov, J. Marcazzó, Y.-I. Lee, B.T. Huy, Optical Materials, 35 (2013) 1636-1641.
[32] K. Binnemans, C. Gorller-Walrand, J.L. Adam, Chemical Physics Letters 280 (1997) 333-338.
[33] T.T. Hong, P.T. Dung, V.X. Quang, Energy Journal of Electronic Materials, 45 (2016) 2569- 2575.
[34] O. Viagin, A. Masalov, I. Ganina, Y. Malyukin, Optical Materials, 31 (2009) 1808-1810.
[35] M. Batentschuk, A. Osvet, G. Schierning, A.
Klier, J. Schneider, A. Winnacker, Radiation Measurements, 38 (2004) 539-543.
[36] C. Ma, S. Jiang, X. Zhou, Journal of Rare Earths, 28 (2010) 40-42.
[37] V. Uma, M. Vijayakumar, K. Marimuthu, G.
Muralidharan, Journal of Molecular Structure, 1151 (2018) 266-276.
