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Crystal-field splitting of Ce3+ in narrow-band phosphor SrLiAl3N4
Journal of Rare Earths xxx (xxxx) xxx
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Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths
Crystal-field splitting of Ce3þ in narrow-band phosphor SrLiAl3N4* Zhen Song, Quanlin Liu* Beijing Key Laboratory for New Energy Materials and Technologies, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 January 2020 Received in revised form 15 February 2020 Accepted 20 February 2020 Available online xxx
As a promising narrow-band phosphor, SrLiAl3N4 has a seemingly ultra-small total crystal-field splitting of only 2400 cm1 with Ce3þ as dopant ions. This paper is devoted to unravel this anomalous phenomenon based on semi-quantitative crystal-field calculations. The results show that there may exist undetected excitation peaks immersed in the host excitation band, and the calibrated crystal-field splitting is 27000 cm1, comparable to those of other Ce3þ doped phosphors. In the end the effect of polyhedral deformation on energy level is briefly discussed. © 2020 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Keywords: Crystal-field splitting SrLiAl3N4 Narrow band Phosphor Rare earths
1. Introduction Narrow-band emission phosphor has aroused huge research interests in the area of back-light source for liquid crystal display (LCD).1e3 This kind of phosphor possesses high color purity originating from the small FWHM (full width at half maximum) of emission band. Therefore, the light source fabricated from those narrow-band phosphors could expand the color gamut, and more colors could be generated from the LCD panel.4e6 Although some rare-earth elements doped phosphors could emit sharp-line spectrum due to f4f transitions, their applications are severely limited by the low luminescence intensity due to the electric forbidden transitions and the mismatch between excitation wavelengths and visible blue light (~ 450 nm) of LED (light emitting diode) chips. In 2014, Pust et al. discovered the narrow-band red-emission phosphor SrLiAl3N4:Eu2þ,1 with FWHM as small as 1180 cm1 in contrast to 2050 - 2600 cm1 of (Ba,Sr)2Si5N8:Eu2þ and 2100 - 2500 cm1 of (Ca,Sr)AlSiN3:Eu2þ.7,8 Since then, much research effort has been devoted to SrLiAl3N4:Eu2þ on its crystal structure,9 electronic structure,10 luminescence properties,11,12 synthesis conditions13,14 and stability improvement.15e17
* Foundation item: Projects supported by the National Natural Science Foundation of China (51832005, 51602019). * Corresponding author. Tel.: þ86 10 62334705. E-mail address: [email protected] (Q.L. Liu).
Besides Eu2þ, Ce3þ is also doped into nitride hosts as luminescence center due to the parity-allowed f4d transition with high ~ o et al. reported the oscillator strength.18,19 Cui et al. and Lean luminescence of Ce3þ in SrLiAl3N4 host.20,21 The most distinctive characteristic is the site-selective emission/excitation spectra. There exist two crystallographic sites for Sr in SrLiAl3N4 host for substitution by Ce3þ, which are denoted as Ce(1) and Ce(2). Two photoluminescent excitation (PLE) peaks belonging to Ce(1) are located at 465 and 516 nm, respectively, while Ce(2) has a broad structureless PLE peak.21 However, Ce(1) in SrLiAl3N4 seems to have an ultra-small total crystal-field splitting of only 2397 cm1 on account of the two PLE peaks, i.e., 460 nm (21739 cm1) and 517 nm (19342 cm1) in the visible-light region.20 This value is surprisingly small compared with those in other Ce3þ doped phosphors. For example, oxide phosphors YAlO3:Ce3þ and Y3Al5O12:Ce3þ have crystal-field splitting of over 10000 and 27000 cm1, respectively.22,23 For nitride phosphors Sr2Si5N8:Ce3þ, La3Si6N11:Ce3þ and LaSi3N5:Ce3þ the values are 15000, 12000 and 10000, respectively.24e26 Since the site symmetry for Ce(1) is C1, the 5-fold degeneracy of 5d orbital will be totally removed. If the highest and lowest crystal-field levels are assigned to 460 nm (21739 cm1) and 517 nm (19342 cm1) PLE peaks, they give the ultra-small total crystal-field splitting of about 2400 cm1. This work provides a new perspective on the PLE peak assignments in Ce3þ doped SrLiAl3N4. By examining the possible schemes obtained from semi-quantitative crystal-field calculations, it is found that the undetected PLE peaks are possibly immersed in the
https://doi.org/10.1016/j.jre.2020.02.012 1002-0721/© 2020 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Please cite this article as: Song Z, Liu Q-i, Crystal-field splitting of Ce3þ in narrow-band phosphor SrLiAl3N4, Journal of Rare Earths, https:// doi.org/10.1016/j.jre.2020.02.012
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Z. Song, Q.L. Liu / Journal of Rare Earths xxx (xxxx) xxx
host excitation band, with the crystal-field splitting 27000 cm1. Meanwhile, the effect of polyhedral deformation on energy levels is also discussed.
D E rffiffiffiffiffiffiffiffiffiffiffiffiffiffi rk 4p q ð1Þq ReYk ðqLN ; 4LN Þ Bkq ¼ Ze2 kþ1 2k þ 1 R LN¼1 8 X
(4)
LN
q
Here, Z stands for the net charge of the nitrogen ligand, and Yk is the kth-rank spherical harmonic. The final expressions of Bkq are dependent on radial integrals hr 2 i and hr 4 i, which will be treated as adjusting parameters during the crystal-field calculation. They are listed in Table 1 for the real part and Table 2 for the imaginary part, respectively. Furthermore, the ideal cube, which best matches the real 8coordinated polyhedron surrounding Ce(1), was searched following a reported routine.30 By sharing the same radial integral values, the energy levels of Ce3þ in this artificial structure were readily obtained, from which the effect of polyhedral deformation on crystal-field splitting could be derived.
2. Methods SrLiAl3N4 host belongs to the triclinic system (space group P 1). All the atoms occupy Wyckoff position 2i, which has site symmetry C1. Therefore, with Bkq as the kth-rank crystal-field parameter, Uqk as the kth-rank unitary tensor operator and hl k C k k li as the kth-rank reduced matrix element (l ¼ 2 for d electron), the crystal-field potential for d electron can be expressed as:27
h 2 2 VðC1 Þ ¼ B20 U02 þ B21 U1 U12 þ iB021 U1 þ U12 iD E 2 2 l k C2 k l þB22 U2 þ U22 þ iB022 U2 U22 h 4 4 U14 þ iB041 U1 þ U14 þ B40 U04 þ B41 U1 4 4 þB42 U2 þ U24 þ iB042 U2 U24 4 4 4 þB43 U3 U34 þ iB043 U3 þ U34 þ B44 U44 þ U4 iD E 4 l k C4 k l þiB044 U4 þ U44
3. Results and discussion Under the site symmetry C1, there exists no orbital degeneracy for 5d electron of Ce3þ. Consequently, five distinct PLE peaks are expected, rather than degenerated ones like eg and t2g under Oh. As stated above, assigning the crystal-field splitting to the 465 and 516 nm PLE peaks gives unreasonable underestimated value. It is possible that those two PLE peaks only represent the lowest two 5d levels, while the remaining peaks are immersed in the host excitation band or located under 200 nm, beyond the detection ability of ordinary spectrometer. However, these kinds of peak assignment are not arbitrary. They have to comply with the crystal-field theory. With 465 and 516 nm peaks pined as the lowest two 5d levels, a series of peak assignment sets can be obtained. Then they are reordered in the ascent sequence according to the crystal-field splitting values. The first five sets are listed in detail in Table 3 and plotted using colored bars in Fig. 1 for further discussion. To show the validity of the crystal-field calculation method, the five crystal-field levels of Ce3þ in Y3Al5O12 are indicated by black bars on the bottom of Fig. 1. The largest difference in peak wavelength is smaller than 10 nm, which supports the reliability. First, those assignments with extremely small (15809 cm1) and large values (545979 cm1) can be excluded from consideration. In addition, the proposed peaks in (1) are located in the baseline region, which also supports the exclusion. The remaining (2), (3) and (4) assignments all have peaks overlapped with the band excitation zone. However, for (2) its 312 nm peak finds no evidence of existence near the root of band excitation peak. For (4) the crystal-field splitting of 32433 cm1 is unusually large, while for (3) it is comparable in magnitude with other Ce3þ doped phosphors. Besides, a
(1)
The crystal-field matrix element is usually calculated with basis function SLJMJ i, in which S, L and J stand for total spin, total orbital and the coupled angular-momentum quantum numbers, respectively. MJ is the magnetic quantum number. It has the form27
D E SLJMJ jVjS0 L0 J 0 MJ0 ¼
0 X 0 J ð1Þ2JMJ þSþL þk L kq
* 1
=
½J; J 0
2
l k Ck k l
+
k S
J L0
E D lN SL k U k k lN S0 L0 Bkq dSS0
(2)
where (), {} and <> denote 3-j, 6-j symbols and reduced matrix elements, respectively.28,29 The spin-orbit (SO) coupling effect is non-negligible for rareearth elements. Here the SO coupling constant xnl for Ce3þ 5d electron was chosen to be 991 cm1 in accordance with Tanner et al.23 Then the SO matrix could be constructed and added to the crystal-field matrix, with the expression
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D E L 0 lN SLJMJ jHSO jlN S0 L0 J 0 MJ0 ¼ xnl lðl þ 1Þð2l þ 1Þð1ÞLþS þJ S0
Under the assumption of point charge electrostatic model (PCEM), Bkq could be expressed from the crystallographic data. Once the spherical coordinates of polyhedral ligand ðRLN ; qLN ; 4LN Þ are known for all the 8 nitrogen atoms, they contribute to Bkq in the form
S L0
J 1
E D lN SLjjV 11 jjlN S0 L0 dJMJ ;J 0 MJ0
(3)
careful examination of the onset of the band excitation profile suggests two shoulders at wavelengths 229 and 214 nm. Finally, based on the above considerations, peak assignment (3) was selected to represent the crystal-field splitting of Ce(1) in SrLiAl3N4, which has a value over 27000 cm1 and several peaks overlapped with the band excitation.
Please cite this article as: Song Z, Liu Q-i, Crystal-field splitting of Ce3þ in narrow-band phosphor SrLiAl3N4, Journal of Rare Earths, https:// doi.org/10.1016/j.jre.2020.02.012
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Table 1 Expression of real part crystal-field parameters. Ligand #
N1 N2 N3 N4 N5 N6 N7 N8 Total
Bond length (nm)
0.26914 0.27973 0.29300 0.27275 0.28229 0.26895 0.29739 0.27414 N.A.
Crystal field parameters, Bkq B20 (10-2)
B21 (10-3)
B22 (10-3)
B40 (10-3)
B41 (10-3)
B42 (10-3)
B43 (10-3)
B44 (10-3)
ðe2 Zhr2 iÞ
ðe2 Zhr 2 iÞ
ðe2 Zhr2 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
1.442 0.4910 0.3893 4.700 2.194 4.881 1.772 1.421 1.873
12.67 24.56 21.46 7.608 1.726 8.932 3.269 11.17 0.6433
9.307 20.56 17.46 0 14.33 0.2537 12.63 10.43 8.360
0.5604 1.791 1.463 5.629 2.003 5.948 1.256 0.3856 10.64
1.580 1.674 1.282 1.766 0.2936 2.122 0.4793 1.379 3.112
0 1.410 1.151 0 1.126 0.1305 0.7755 0 4.633
3.057 2.772 2.121 0 0.5357 0 0.9205 2.806 12.27
2.051 1.632 1.194 0 1.276 0 0.8242 1.677 3.008
Table 2 Expression of imaginary part crystal-field parameters. Ligand #
N1 N2 N3 N4 N5 N6 N7 N8 Total
Crystal field parameters, Bkq
Bond length (nm)
0.26914 0.27973 0.29300 0.27275 0.28229 0.26895 0.29739 0.27414 N.A.
B021 (10-3)
B022 (10-3)
B041 (10-3)
B042 (10-3)
B043 (10-4)
B044 (10-3)
ðe2 Zhr 2 iÞ
ðe2 Zhr 2 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
18.20 1.203 2.159 7.158 3.108 6.994 6.111 17.21 11.08
25.16 2.018 3.550 0.9309 23.01 1.027 18.93 23.40 6.196
2.269 0 0.1290 1.662 0.5287 1.662 0.8961 2.124 6.245
0 0.1385 0.2339 0.4672 1.808 0.5282 1.162 0 2.053
7.934 4.099 6.580 0 0 0 0 4.431 10.07
1.758 0.3236 0.5066 0 2.595 0 1.983 1.865 1.136
Table 3 PLE peak assignments and the corresponding crystal-field splittings. Crystal-field splitting (cm1)
Crystal-field levels (nm) (1) (2) (3) (4) (5)
516 516 516 516 516
465 465 465 465 465
454 312 287 217 178
416 254 229 202 162
397 239 214 193 153
5809 22461 27349 32433 45979
Fig. 1. (a) PLE spectra of SrLiAl3N4:Ce3þ monitored at 545 nm measured at 300 K. Adopted from Fig. 4 of Ref.20 Copyright (2018), with permission from Elsevier; (b) Five sets of peak assignments (Table 3) plotted versus PLE spectra (dotted curve). Meanwhile, the five observed (Ref.31) and fitted crystal-field levels of Ce3þ in YAG (Y3Al5O12) are indicated as black bars.
Please cite this article as: Song Z, Liu Q-i, Crystal-field splitting of Ce3þ in narrow-band phosphor SrLiAl3N4, Journal of Rare Earths, https:// doi.org/10.1016/j.jre.2020.02.012
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From Fig. 1 it is obvious that the five PLE peaks belonging to (3) can be divided into two groups, i.e., the lower two and the upper three. This characteristic reminds us of the crystal-field splitting of 1 d electron in an 8-coordinated perfect cube, which gives lower doublet eg and higher triplet t2g orbitals.32 Meanwhile, in SrLiAl3N4, Ce(1) is located in an 8-coordinated distorted cube. Therefore, its luminescence could be thought as derived from the perfect cube environment. The fitting results of an ideal cube are listed in Table 4, and visualized in Fig. 2, in which the regular-packed blue atoms constitute an ideal cube. The d orbital population analysis (Table 5) sheds light on how the split orbitals are derived from the initially degenerated levels, as shown in Fig. 2. The lowest crystalfield level has leading terms of dxy and dx2y2, of which the latter originates from the high-lying levels in the perfect cube. Meanwhile, dz2 orbital contributes nearly half to the second-highest level, also with reversed order. Therefore, the results provide a new perspective on the effect of polyhedral deformation on crystal-
Table 4 Vertex coordinates of the ideal cube and real 8-coordinated polyhedron. Both have centers located at origin (0,0,0). The center-vertex distance of the ideal cube is R ¼ 0.27467 nm. Crystallographic data come from Ref. 1. Ligand Ideal cube vertex coordinates (nm) Real polyhedron coordinates (nm) 1 2 3 4 5 6 7 8
0.13658 0.24299 0.14602 0.03961 0.03961 0.14602 0.24299 0.13658
0.23463 0.01162 0.20997 0.03628 0.03628 0.20997 0.01162 0.23463
0.04167 0.12752 0.10017 0.26936 0.26936 0.10017 0.12752 0.04167
0.13868 0.24941 0.13857 0.03492 0.03881 0.14213 0.24007 0.13676
0.25924 0.02509 0.21346 0.03286 0.03039 0.20411 0.01176 0.24626
0.04477 0.15169 0.10191 0.26850 0.26439 0.10282 0.14310 0.01847
Fig. 2. Fitting of the real 8-coordinated polyhedron (red balls) by an ideal cube (blue balls). Population analysis of d orbitals in the real polyhedron and the schematic relationship with those under ideal cube, with spin-orbit coupling effect taken into account. The leading terms are denoted.
Table 5 Population analysis of d orbitals. 5d level
dxy
dxz
dyz
dx2y2
dz2
516 465 287 229 214
41.3% 34.3% 17.9% 5.9% 0.6%
8.1% 14.4% 65.1% 9.8% 2.6%
2.9% 8.1% 6.9% 21.5% 60.6%
38.7% 42.1% 5.7% 12.9% 0.6%
9.0% 1.2% 4.4% 49.9% 35.6%
nm nm nm nm nm
field splittings. The low and high-lying level sets under Oℎ symmetry will have cross point when the site symmetry is lowered, rather than keeping the original order. 4. Conclusions In this work, the crystal-field splitting of Ce3þ in SrLiAl3N4 was investigated for the explanation of its seemingly ultra-small crystal-field splitting. Semi-quantitative crystal-field calculations suggest that the three upper levels of Ce3þ may be immersed in the host excitation band, and the crystal-field splitting calibrated is comparable to those in other Ce3þ doped phosphors. This work is helpful in understanding luminescence of Ce3þ in narrow-band nitride phosphors. References 1. Pust P, Weiler V, Hecht C, Tücks A, Wochnik AS, Henß A-K, et al. Narrow-band red-emitting Sr[LiAl3N4]: Eu2þ as a next-generation LED-phosphor material. Nat Mater. 2014;13(9):891. 2. Pust P, Wochnik AS, Baumann E, Schmidt PJ, Wiechert D, Scheu C, et al. Ca [LiAl3N4]:Eu2þ-A narrow-band red-emitting nitridolithoaluminate. Chem Mater. 2014;26(11):3544. 3. Zhang X, Tsai Y-T, Wu S-M, Lin Y-C, Lee J-F, Sheu H-S, et al. Facile atmospheric pressure synthesis of high thermal stability and narrow-band red-emitting SrLiAl3N4:Eu2þ phosphor for high color rendering index white light-emitting diodes. ACS Appl Mater Interfaces. 2016;8(30):19612. 4. Fang M-H, Tsai Y-T, Sheu H-S, Lee J-F, Liu R-S. Pressure-controlled synthesis of high-performance SrLiAl3N4:Eu2þ narrow-band red phosphors. J Mater Chem C. 2018;6(38):10174. 5. Fang M-H, Meng S-Y, Majewska N, Lesniewski T, Mahlik S, Grinberg M, et al. Chemical control of SrLi(Al1exGax)3N4:Eu2þ red phosphors at extreme conditions for application in light-emitting diodes. Chem Mater. 2019;31(12):4614. 6. Zhang YP, Luo L, Chen GT, Liu YH, Liu RH, Chen XC. Green and red phosphor for LED backlight in wide color gamut LCD. J Rare Earths. 2020;38(1):1. 7. Liu XL, Song Z, Kong YW, Wang SX, Zhang SY, Xia ZG, et al. Effects of full-range Eu concentration on Sr2-2xEu2xSi5N8 phosphors: a deep-red emission and luminescent thermal quenching. J Alloys Compd. 2019;770:1069. 8. Liu XL, Song Z, Wang SX, Liu QL. The red persistent luminescence of (Sr,Ca) AlSiN3:Eu2þ and mechanism different to SrAl2O4:Eu2þ,Dy3þ. J Lumin. 2019;208: 313. 9. Fang M-H, Mahlik S, Lazarowska A, Grinberg M, Molokeev MS, Sheu H-S, et al. Structural evolution and effect of the neighboring cation on the photoluminescence of Sr(LiAl3)1x(SiMg3)xN4:Eu2þ phosphors. Angew Chem Int Ed. 2019;58(23):7767. 10. Tolhurst TM, Boyko TD, Pust P, Johnson NW, Schnick W, Moewes A. Investigations of the electronic structure and bandgap of the next-generation LED-phosphor Sr[LiAl3N4]:Eu2þdexperiment and calculations. Adv Opt Mater. 2015;3(4):546. 11. Hu W-W, Ji W-W, Khan SA, Hao L-Y, Xu X, Yin L-J, et al. Preparation of Sr1xCaxLiAl3N4:Eu2þ solid solutions and their photoluminescence properties. J Am Ceram Soc. 2016;99(10):3273. 12. Tian MM, Wang ZJ, Li ZL, Cheng JG, Wang C, Li XX, et al. Excellent human eye sensitivity matching, high luminous efficacy, high quantum yield and thermal stability of narrow-band red-emitting SrLiAl3N4:Eu2þ phosphor. J Lumin. 2019;207:545. 13. Cui DP, Xiang QC, Song Z, Xia ZG, Liu QL. The synthesis of narrow-band redemitting SrLiAl3N4:Eu2þ phosphor and improvement of its luminescence properties. J Mater Chem C. 2016;4(30):7332. 14. Kim SW, Hasegawa T, Hasegawa S, Yamanashi R, Nakagawa H, Toda K, et al. Improved synthesis of SrLiAl3N4:Eu2þ phosphor using complex nitride raw material. RSC Adv. 2016;6(66):61906. 15. Tsai Y-T, Nguyen H-D, Lazarowska A, Mahlik S, Grinberg M, Liu R-S. Improvement of the water resistance of a narrow-band red-emitting SrLiAl3N4: Eu2þ phosphor synthesized under high isostatic pressure through coating with an organosilica layer. Angew Chem Int Ed. 2016;55(33):9652. 16. Li WX, Song Z, Cui DP, Xia ZG, Liu QL. Moisture-induced degradation of the narrowband red-emitting SrLiAl3N4:Eu2þ phosphor. J Rare Earths. 2018;36(4):341. 17. Zhang Y, Zhang XJ, Zhang HR, Zhuang JL, Hu CF, Liu YL, et al. Improving moisture stability of SrLiAl3N4:Eu2þ through phosphor-in-glass approach to realize its application in plant growing LED device. J Colloid Interface Sci. 2019;545:195. 18. Wang XJ, Takeda T, Hirosaki N, Funahashi S, Xie R-J. Single-particle-diagnosis approach: an efficient strategy for discovering new nitride phosphors. J Rare Earths. 2018;36(1):42. 19. Wu WJ, Chao KF, Liu WQ, Wei LS, Hu DH, Wang TY, et al. Enhanced orange emission by doping CeB6 in CaAlSiN3:Ce3þ phosphor for application in white LEDs. J Rare Earths. 2018;36(12):1250. 20. Cui DP, Song Z, Xia ZG, Liu QL. Synthesis, structure and luminescence of SrLiAl3N4:Ce3þ phosphor. J Lumin. 2018;199:271.
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Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths
Crystal-field splitting of Ce3þ in narrow-band phosphor SrLiAl3N4* Zhen Song, Quanlin Liu* Beijing Key Laboratory for New Energy Materials and Technologies, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 January 2020 Received in revised form 15 February 2020 Accepted 20 February 2020 Available online xxx
As a promising narrow-band phosphor, SrLiAl3N4 has a seemingly ultra-small total crystal-field splitting of only 2400 cm1 with Ce3þ as dopant ions. This paper is devoted to unravel this anomalous phenomenon based on semi-quantitative crystal-field calculations. The results show that there may exist undetected excitation peaks immersed in the host excitation band, and the calibrated crystal-field splitting is 27000 cm1, comparable to those of other Ce3þ doped phosphors. In the end the effect of polyhedral deformation on energy level is briefly discussed. © 2020 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Keywords: Crystal-field splitting SrLiAl3N4 Narrow band Phosphor Rare earths
1. Introduction Narrow-band emission phosphor has aroused huge research interests in the area of back-light source for liquid crystal display (LCD).1e3 This kind of phosphor possesses high color purity originating from the small FWHM (full width at half maximum) of emission band. Therefore, the light source fabricated from those narrow-band phosphors could expand the color gamut, and more colors could be generated from the LCD panel.4e6 Although some rare-earth elements doped phosphors could emit sharp-line spectrum due to f4f transitions, their applications are severely limited by the low luminescence intensity due to the electric forbidden transitions and the mismatch between excitation wavelengths and visible blue light (~ 450 nm) of LED (light emitting diode) chips. In 2014, Pust et al. discovered the narrow-band red-emission phosphor SrLiAl3N4:Eu2þ,1 with FWHM as small as 1180 cm1 in contrast to 2050 - 2600 cm1 of (Ba,Sr)2Si5N8:Eu2þ and 2100 - 2500 cm1 of (Ca,Sr)AlSiN3:Eu2þ.7,8 Since then, much research effort has been devoted to SrLiAl3N4:Eu2þ on its crystal structure,9 electronic structure,10 luminescence properties,11,12 synthesis conditions13,14 and stability improvement.15e17
* Foundation item: Projects supported by the National Natural Science Foundation of China (51832005, 51602019). * Corresponding author. Tel.: þ86 10 62334705. E-mail address: [email protected] (Q.L. Liu).
Besides Eu2þ, Ce3þ is also doped into nitride hosts as luminescence center due to the parity-allowed f4d transition with high ~ o et al. reported the oscillator strength.18,19 Cui et al. and Lean luminescence of Ce3þ in SrLiAl3N4 host.20,21 The most distinctive characteristic is the site-selective emission/excitation spectra. There exist two crystallographic sites for Sr in SrLiAl3N4 host for substitution by Ce3þ, which are denoted as Ce(1) and Ce(2). Two photoluminescent excitation (PLE) peaks belonging to Ce(1) are located at 465 and 516 nm, respectively, while Ce(2) has a broad structureless PLE peak.21 However, Ce(1) in SrLiAl3N4 seems to have an ultra-small total crystal-field splitting of only 2397 cm1 on account of the two PLE peaks, i.e., 460 nm (21739 cm1) and 517 nm (19342 cm1) in the visible-light region.20 This value is surprisingly small compared with those in other Ce3þ doped phosphors. For example, oxide phosphors YAlO3:Ce3þ and Y3Al5O12:Ce3þ have crystal-field splitting of over 10000 and 27000 cm1, respectively.22,23 For nitride phosphors Sr2Si5N8:Ce3þ, La3Si6N11:Ce3þ and LaSi3N5:Ce3þ the values are 15000, 12000 and 10000, respectively.24e26 Since the site symmetry for Ce(1) is C1, the 5-fold degeneracy of 5d orbital will be totally removed. If the highest and lowest crystal-field levels are assigned to 460 nm (21739 cm1) and 517 nm (19342 cm1) PLE peaks, they give the ultra-small total crystal-field splitting of about 2400 cm1. This work provides a new perspective on the PLE peak assignments in Ce3þ doped SrLiAl3N4. By examining the possible schemes obtained from semi-quantitative crystal-field calculations, it is found that the undetected PLE peaks are possibly immersed in the
https://doi.org/10.1016/j.jre.2020.02.012 1002-0721/© 2020 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Please cite this article as: Song Z, Liu Q-i, Crystal-field splitting of Ce3þ in narrow-band phosphor SrLiAl3N4, Journal of Rare Earths, https:// doi.org/10.1016/j.jre.2020.02.012
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Z. Song, Q.L. Liu / Journal of Rare Earths xxx (xxxx) xxx
host excitation band, with the crystal-field splitting 27000 cm1. Meanwhile, the effect of polyhedral deformation on energy levels is also discussed.
D E rffiffiffiffiffiffiffiffiffiffiffiffiffiffi rk 4p q ð1Þq ReYk ðqLN ; 4LN Þ Bkq ¼ Ze2 kþ1 2k þ 1 R LN¼1 8 X
(4)
LN
q
Here, Z stands for the net charge of the nitrogen ligand, and Yk is the kth-rank spherical harmonic. The final expressions of Bkq are dependent on radial integrals hr 2 i and hr 4 i, which will be treated as adjusting parameters during the crystal-field calculation. They are listed in Table 1 for the real part and Table 2 for the imaginary part, respectively. Furthermore, the ideal cube, which best matches the real 8coordinated polyhedron surrounding Ce(1), was searched following a reported routine.30 By sharing the same radial integral values, the energy levels of Ce3þ in this artificial structure were readily obtained, from which the effect of polyhedral deformation on crystal-field splitting could be derived.
2. Methods SrLiAl3N4 host belongs to the triclinic system (space group P 1). All the atoms occupy Wyckoff position 2i, which has site symmetry C1. Therefore, with Bkq as the kth-rank crystal-field parameter, Uqk as the kth-rank unitary tensor operator and hl k C k k li as the kth-rank reduced matrix element (l ¼ 2 for d electron), the crystal-field potential for d electron can be expressed as:27
h 2 2 VðC1 Þ ¼ B20 U02 þ B21 U1 U12 þ iB021 U1 þ U12 iD E 2 2 l k C2 k l þB22 U2 þ U22 þ iB022 U2 U22 h 4 4 U14 þ iB041 U1 þ U14 þ B40 U04 þ B41 U1 4 4 þB42 U2 þ U24 þ iB042 U2 U24 4 4 4 þB43 U3 U34 þ iB043 U3 þ U34 þ B44 U44 þ U4 iD E 4 l k C4 k l þiB044 U4 þ U44
3. Results and discussion Under the site symmetry C1, there exists no orbital degeneracy for 5d electron of Ce3þ. Consequently, five distinct PLE peaks are expected, rather than degenerated ones like eg and t2g under Oh. As stated above, assigning the crystal-field splitting to the 465 and 516 nm PLE peaks gives unreasonable underestimated value. It is possible that those two PLE peaks only represent the lowest two 5d levels, while the remaining peaks are immersed in the host excitation band or located under 200 nm, beyond the detection ability of ordinary spectrometer. However, these kinds of peak assignment are not arbitrary. They have to comply with the crystal-field theory. With 465 and 516 nm peaks pined as the lowest two 5d levels, a series of peak assignment sets can be obtained. Then they are reordered in the ascent sequence according to the crystal-field splitting values. The first five sets are listed in detail in Table 3 and plotted using colored bars in Fig. 1 for further discussion. To show the validity of the crystal-field calculation method, the five crystal-field levels of Ce3þ in Y3Al5O12 are indicated by black bars on the bottom of Fig. 1. The largest difference in peak wavelength is smaller than 10 nm, which supports the reliability. First, those assignments with extremely small (15809 cm1) and large values (545979 cm1) can be excluded from consideration. In addition, the proposed peaks in (1) are located in the baseline region, which also supports the exclusion. The remaining (2), (3) and (4) assignments all have peaks overlapped with the band excitation zone. However, for (2) its 312 nm peak finds no evidence of existence near the root of band excitation peak. For (4) the crystal-field splitting of 32433 cm1 is unusually large, while for (3) it is comparable in magnitude with other Ce3þ doped phosphors. Besides, a
(1)
The crystal-field matrix element is usually calculated with basis function SLJMJ i, in which S, L and J stand for total spin, total orbital and the coupled angular-momentum quantum numbers, respectively. MJ is the magnetic quantum number. It has the form27
D E SLJMJ jVjS0 L0 J 0 MJ0 ¼
0 X 0 J ð1Þ2JMJ þSþL þk L kq
* 1
=
½J; J 0
2
l k Ck k l
+
k S
J L0
E D lN SL k U k k lN S0 L0 Bkq dSS0
(2)
where (), {} and <> denote 3-j, 6-j symbols and reduced matrix elements, respectively.28,29 The spin-orbit (SO) coupling effect is non-negligible for rareearth elements. Here the SO coupling constant xnl for Ce3þ 5d electron was chosen to be 991 cm1 in accordance with Tanner et al.23 Then the SO matrix could be constructed and added to the crystal-field matrix, with the expression
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D E L 0 lN SLJMJ jHSO jlN S0 L0 J 0 MJ0 ¼ xnl lðl þ 1Þð2l þ 1Þð1ÞLþS þJ S0
Under the assumption of point charge electrostatic model (PCEM), Bkq could be expressed from the crystallographic data. Once the spherical coordinates of polyhedral ligand ðRLN ; qLN ; 4LN Þ are known for all the 8 nitrogen atoms, they contribute to Bkq in the form
S L0
J 1
E D lN SLjjV 11 jjlN S0 L0 dJMJ ;J 0 MJ0
(3)
careful examination of the onset of the band excitation profile suggests two shoulders at wavelengths 229 and 214 nm. Finally, based on the above considerations, peak assignment (3) was selected to represent the crystal-field splitting of Ce(1) in SrLiAl3N4, which has a value over 27000 cm1 and several peaks overlapped with the band excitation.
Please cite this article as: Song Z, Liu Q-i, Crystal-field splitting of Ce3þ in narrow-band phosphor SrLiAl3N4, Journal of Rare Earths, https:// doi.org/10.1016/j.jre.2020.02.012
Z. Song, Q.L. Liu / Journal of Rare Earths xxx (xxxx) xxx
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Table 1 Expression of real part crystal-field parameters. Ligand #
N1 N2 N3 N4 N5 N6 N7 N8 Total
Bond length (nm)
0.26914 0.27973 0.29300 0.27275 0.28229 0.26895 0.29739 0.27414 N.A.
Crystal field parameters, Bkq B20 (10-2)
B21 (10-3)
B22 (10-3)
B40 (10-3)
B41 (10-3)
B42 (10-3)
B43 (10-3)
B44 (10-3)
ðe2 Zhr2 iÞ
ðe2 Zhr 2 iÞ
ðe2 Zhr2 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
1.442 0.4910 0.3893 4.700 2.194 4.881 1.772 1.421 1.873
12.67 24.56 21.46 7.608 1.726 8.932 3.269 11.17 0.6433
9.307 20.56 17.46 0 14.33 0.2537 12.63 10.43 8.360
0.5604 1.791 1.463 5.629 2.003 5.948 1.256 0.3856 10.64
1.580 1.674 1.282 1.766 0.2936 2.122 0.4793 1.379 3.112
0 1.410 1.151 0 1.126 0.1305 0.7755 0 4.633
3.057 2.772 2.121 0 0.5357 0 0.9205 2.806 12.27
2.051 1.632 1.194 0 1.276 0 0.8242 1.677 3.008
Table 2 Expression of imaginary part crystal-field parameters. Ligand #
N1 N2 N3 N4 N5 N6 N7 N8 Total
Crystal field parameters, Bkq
Bond length (nm)
0.26914 0.27973 0.29300 0.27275 0.28229 0.26895 0.29739 0.27414 N.A.
B021 (10-3)
B022 (10-3)
B041 (10-3)
B042 (10-3)
B043 (10-4)
B044 (10-3)
ðe2 Zhr 2 iÞ
ðe2 Zhr 2 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
ðe2 Zhr 4 iÞ
18.20 1.203 2.159 7.158 3.108 6.994 6.111 17.21 11.08
25.16 2.018 3.550 0.9309 23.01 1.027 18.93 23.40 6.196
2.269 0 0.1290 1.662 0.5287 1.662 0.8961 2.124 6.245
0 0.1385 0.2339 0.4672 1.808 0.5282 1.162 0 2.053
7.934 4.099 6.580 0 0 0 0 4.431 10.07
1.758 0.3236 0.5066 0 2.595 0 1.983 1.865 1.136
Table 3 PLE peak assignments and the corresponding crystal-field splittings. Crystal-field splitting (cm1)
Crystal-field levels (nm) (1) (2) (3) (4) (5)
516 516 516 516 516
465 465 465 465 465
454 312 287 217 178
416 254 229 202 162
397 239 214 193 153
5809 22461 27349 32433 45979
Fig. 1. (a) PLE spectra of SrLiAl3N4:Ce3þ monitored at 545 nm measured at 300 K. Adopted from Fig. 4 of Ref.20 Copyright (2018), with permission from Elsevier; (b) Five sets of peak assignments (Table 3) plotted versus PLE spectra (dotted curve). Meanwhile, the five observed (Ref.31) and fitted crystal-field levels of Ce3þ in YAG (Y3Al5O12) are indicated as black bars.
Please cite this article as: Song Z, Liu Q-i, Crystal-field splitting of Ce3þ in narrow-band phosphor SrLiAl3N4, Journal of Rare Earths, https:// doi.org/10.1016/j.jre.2020.02.012
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From Fig. 1 it is obvious that the five PLE peaks belonging to (3) can be divided into two groups, i.e., the lower two and the upper three. This characteristic reminds us of the crystal-field splitting of 1 d electron in an 8-coordinated perfect cube, which gives lower doublet eg and higher triplet t2g orbitals.32 Meanwhile, in SrLiAl3N4, Ce(1) is located in an 8-coordinated distorted cube. Therefore, its luminescence could be thought as derived from the perfect cube environment. The fitting results of an ideal cube are listed in Table 4, and visualized in Fig. 2, in which the regular-packed blue atoms constitute an ideal cube. The d orbital population analysis (Table 5) sheds light on how the split orbitals are derived from the initially degenerated levels, as shown in Fig. 2. The lowest crystalfield level has leading terms of dxy and dx2y2, of which the latter originates from the high-lying levels in the perfect cube. Meanwhile, dz2 orbital contributes nearly half to the second-highest level, also with reversed order. Therefore, the results provide a new perspective on the effect of polyhedral deformation on crystal-
Table 4 Vertex coordinates of the ideal cube and real 8-coordinated polyhedron. Both have centers located at origin (0,0,0). The center-vertex distance of the ideal cube is R ¼ 0.27467 nm. Crystallographic data come from Ref. 1. Ligand Ideal cube vertex coordinates (nm) Real polyhedron coordinates (nm) 1 2 3 4 5 6 7 8
0.13658 0.24299 0.14602 0.03961 0.03961 0.14602 0.24299 0.13658
0.23463 0.01162 0.20997 0.03628 0.03628 0.20997 0.01162 0.23463
0.04167 0.12752 0.10017 0.26936 0.26936 0.10017 0.12752 0.04167
0.13868 0.24941 0.13857 0.03492 0.03881 0.14213 0.24007 0.13676
0.25924 0.02509 0.21346 0.03286 0.03039 0.20411 0.01176 0.24626
0.04477 0.15169 0.10191 0.26850 0.26439 0.10282 0.14310 0.01847
Fig. 2. Fitting of the real 8-coordinated polyhedron (red balls) by an ideal cube (blue balls). Population analysis of d orbitals in the real polyhedron and the schematic relationship with those under ideal cube, with spin-orbit coupling effect taken into account. The leading terms are denoted.
Table 5 Population analysis of d orbitals. 5d level
dxy
dxz
dyz
dx2y2
dz2
516 465 287 229 214
41.3% 34.3% 17.9% 5.9% 0.6%
8.1% 14.4% 65.1% 9.8% 2.6%
2.9% 8.1% 6.9% 21.5% 60.6%
38.7% 42.1% 5.7% 12.9% 0.6%
9.0% 1.2% 4.4% 49.9% 35.6%
nm nm nm nm nm
field splittings. The low and high-lying level sets under Oℎ symmetry will have cross point when the site symmetry is lowered, rather than keeping the original order. 4. Conclusions In this work, the crystal-field splitting of Ce3þ in SrLiAl3N4 was investigated for the explanation of its seemingly ultra-small crystal-field splitting. Semi-quantitative crystal-field calculations suggest that the three upper levels of Ce3þ may be immersed in the host excitation band, and the crystal-field splitting calibrated is comparable to those in other Ce3þ doped phosphors. This work is helpful in understanding luminescence of Ce3þ in narrow-band nitride phosphors. References 1. Pust P, Weiler V, Hecht C, Tücks A, Wochnik AS, Henß A-K, et al. Narrow-band red-emitting Sr[LiAl3N4]: Eu2þ as a next-generation LED-phosphor material. Nat Mater. 2014;13(9):891. 2. Pust P, Wochnik AS, Baumann E, Schmidt PJ, Wiechert D, Scheu C, et al. Ca [LiAl3N4]:Eu2þ-A narrow-band red-emitting nitridolithoaluminate. Chem Mater. 2014;26(11):3544. 3. Zhang X, Tsai Y-T, Wu S-M, Lin Y-C, Lee J-F, Sheu H-S, et al. Facile atmospheric pressure synthesis of high thermal stability and narrow-band red-emitting SrLiAl3N4:Eu2þ phosphor for high color rendering index white light-emitting diodes. ACS Appl Mater Interfaces. 2016;8(30):19612. 4. Fang M-H, Tsai Y-T, Sheu H-S, Lee J-F, Liu R-S. Pressure-controlled synthesis of high-performance SrLiAl3N4:Eu2þ narrow-band red phosphors. J Mater Chem C. 2018;6(38):10174. 5. Fang M-H, Meng S-Y, Majewska N, Lesniewski T, Mahlik S, Grinberg M, et al. Chemical control of SrLi(Al1exGax)3N4:Eu2þ red phosphors at extreme conditions for application in light-emitting diodes. Chem Mater. 2019;31(12):4614. 6. Zhang YP, Luo L, Chen GT, Liu YH, Liu RH, Chen XC. Green and red phosphor for LED backlight in wide color gamut LCD. J Rare Earths. 2020;38(1):1. 7. Liu XL, Song Z, Kong YW, Wang SX, Zhang SY, Xia ZG, et al. Effects of full-range Eu concentration on Sr2-2xEu2xSi5N8 phosphors: a deep-red emission and luminescent thermal quenching. J Alloys Compd. 2019;770:1069. 8. Liu XL, Song Z, Wang SX, Liu QL. The red persistent luminescence of (Sr,Ca) AlSiN3:Eu2þ and mechanism different to SrAl2O4:Eu2þ,Dy3þ. J Lumin. 2019;208: 313. 9. Fang M-H, Mahlik S, Lazarowska A, Grinberg M, Molokeev MS, Sheu H-S, et al. Structural evolution and effect of the neighboring cation on the photoluminescence of Sr(LiAl3)1x(SiMg3)xN4:Eu2þ phosphors. Angew Chem Int Ed. 2019;58(23):7767. 10. Tolhurst TM, Boyko TD, Pust P, Johnson NW, Schnick W, Moewes A. Investigations of the electronic structure and bandgap of the next-generation LED-phosphor Sr[LiAl3N4]:Eu2þdexperiment and calculations. Adv Opt Mater. 2015;3(4):546. 11. Hu W-W, Ji W-W, Khan SA, Hao L-Y, Xu X, Yin L-J, et al. Preparation of Sr1xCaxLiAl3N4:Eu2þ solid solutions and their photoluminescence properties. J Am Ceram Soc. 2016;99(10):3273. 12. Tian MM, Wang ZJ, Li ZL, Cheng JG, Wang C, Li XX, et al. Excellent human eye sensitivity matching, high luminous efficacy, high quantum yield and thermal stability of narrow-band red-emitting SrLiAl3N4:Eu2þ phosphor. J Lumin. 2019;207:545. 13. Cui DP, Xiang QC, Song Z, Xia ZG, Liu QL. The synthesis of narrow-band redemitting SrLiAl3N4:Eu2þ phosphor and improvement of its luminescence properties. J Mater Chem C. 2016;4(30):7332. 14. Kim SW, Hasegawa T, Hasegawa S, Yamanashi R, Nakagawa H, Toda K, et al. Improved synthesis of SrLiAl3N4:Eu2þ phosphor using complex nitride raw material. RSC Adv. 2016;6(66):61906. 15. Tsai Y-T, Nguyen H-D, Lazarowska A, Mahlik S, Grinberg M, Liu R-S. Improvement of the water resistance of a narrow-band red-emitting SrLiAl3N4: Eu2þ phosphor synthesized under high isostatic pressure through coating with an organosilica layer. Angew Chem Int Ed. 2016;55(33):9652. 16. Li WX, Song Z, Cui DP, Xia ZG, Liu QL. Moisture-induced degradation of the narrowband red-emitting SrLiAl3N4:Eu2þ phosphor. J Rare Earths. 2018;36(4):341. 17. Zhang Y, Zhang XJ, Zhang HR, Zhuang JL, Hu CF, Liu YL, et al. Improving moisture stability of SrLiAl3N4:Eu2þ through phosphor-in-glass approach to realize its application in plant growing LED device. J Colloid Interface Sci. 2019;545:195. 18. Wang XJ, Takeda T, Hirosaki N, Funahashi S, Xie R-J. Single-particle-diagnosis approach: an efficient strategy for discovering new nitride phosphors. J Rare Earths. 2018;36(1):42. 19. Wu WJ, Chao KF, Liu WQ, Wei LS, Hu DH, Wang TY, et al. Enhanced orange emission by doping CeB6 in CaAlSiN3:Ce3þ phosphor for application in white LEDs. J Rare Earths. 2018;36(12):1250. 20. Cui DP, Song Z, Xia ZG, Liu QL. Synthesis, structure and luminescence of SrLiAl3N4:Ce3þ phosphor. J Lumin. 2018;199:271.
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Please cite this article as: Song Z, Liu Q-i, Crystal-field splitting of Ce3þ in narrow-band phosphor SrLiAl3N4, Journal of Rare Earths, https:// doi.org/10.1016/j.jre.2020.02.012