Intense red light emission of Eu^3+-doped LiGd(PO_3)_4 for mercury-free lamps and plasma display panels application

Intense red light emission of Eu3+-doped LiGd(PO3)4 for mercury-free lamps and plasma

display panels application

Bing Han,1 Hongbin Liang,

1, * Haiyong Ni,

1 Qiang Su,

1,* Guangtao Yang,

2 Junyan Shi,

2

and Guobin Zhang 2

1MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and

Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China 2National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei,230026, China

Corresponding author: [email protected]

Abstract: In order to obtain a suitable red phosphor for mercury-free lamps and plasma display panels (PDPs), samples of trivalent europium-activated polyphosphate LiGd(PO3)4:Eu3+ (LGP:Eu3+) were prepared by a solid-state reaction technique at high temperature. The vacuum ultraviolet (VUV)–visible spectroscopic properties were investigated. Because the phosphor LiGd1-xEux(PO3)4 for x = 0.50 shows broad and strong absorption in VUV region and exhibits intensive emission under 147/172 nm excitation in comparison with the PDP commercial red phosphor (Y, Gd)BO3:Eu3+, it is considered to be a promising red phosphor for mercury-free lamps and plasma display panels application.

©2009 Optical Society of America

OCIS codes: (160.2540) Fluorescent and luminescent materials; (250.5230) Photoluminescence; (300.6280) Spectroscopy, fluorescence and luminescence.

References and links

1. X. Q. Zeng, S. J. Im, S. H. Jang, Y. M. Kim, H. B. Park, S. H. Son, H. Hatanaka, G. Y. Kim, and S. G. Kim, “Luminescent properties of (Y,Gd)BO3:Bi3+,RE3+ (RE = Eu, Tb) phosphor under VUV/UV excitation,” J. Lumin. 121, 1-6 (2006).

2. M. G. Kwak, J. I. Han, Y. H. Kim, S. K. Park, D. K. Lee, and S. H. Sohn, “Improvement of luminance efficiency in xenon dielectric barrier discharge flat lamp,” IEEE Trans. Plasma Sci. 31, 176-179 (2003).

3. J. S. Kim, J. H. Yang, T. J. Kim, and K. W. Whang, “Comparison of electric field and priming particle effects on address discharge time lag and addressing characteristics of high-Xe content AC PDP,” IEEE Trans. Plasma Sci. 31, 1083-1090 (2003).

4. C. H. Kim, I. E. Kwon, C. H. Park, Y. J. Hwang, H. S. Bae, B. Y. Yu, C. H. Pyun, and G. Y. Hong, “Phosphors for plasma display panels,” J. Alloys Compd. 311, 33-39 (2000).

5. T. Jüstel, J. C. Krupa, and D. U. Wiechert, “VUV spectroscopy of luminescent materials for plasma display panels and Xe discharge lamps,” J. Lumin. 93, 179-189 (2001).

6. S. X. Zhang, “Vacuum-ultraviolet/visible conversion phosphors for plasma display panels,” IEEE Trans. Plasma Sci. 34, 294-304 (2006).

7. H. Ettis, H. Naїli, and T. Mhiri, “The crystal structure, thermal behaviour and ionic conductivity of a novel lithium gadolinium polyphosphate LiGd(PO3)4,” J. Solid State Chem. 179, 3107-3113 (2006).

8. K. Jaouadi, H. Naїli, N. Zouari, T. Mhiri, and A. Daoud, “Synthesis and crystal structure of a new form of potassium-bismuth polyphosphate KBi(PO3)4,” J. Alloys Compd. 354, 104-114 (2003).

9. K. Jaouadi, N. Zouari, T. Mhiri, and M. Pierrot, “Synthesis and crystal structure of sodium-bismuth polyphosphate NaBi(PO3)4,” J. Cryst. Growth 273, 638-645 (2005).

10. H. Ettis, H. Naїli, and T. Mhiri, “Synthesis and crystal structure of a new potassium-gadolinium cyclotetraphosphate, KGdP4O12,” Cryst. Growth Des. 3, 599-602 (2003).

11. Parreu, R. Solé, J. Gavaldà, J. Massons, F. Díaz, and M. Aguiló, “Crystal growth, structural characterization, and linear thermal evolution of KGd(PO3)4,” Chem. Mater. 17, 822-828 (2005).

12. Parreu, M. C. Pujol, M. Aguiló, F. Díaz, X. Mateos, and V. Petrov, “Growth, spectroscopy and laser

#108560 - $15.00 USD Received 9 Mar 2009; revised 9 Apr 2009; accepted 13 Apr 2009; published 15 Apr 2009

(C) 2009 OSA 27 April 2009 / Vol. 17, No. 9 / OPTICS EXPRESS 7138

mailto:[email protected]

operation of Yb:KGd(PO3)4 single crystal,” Opt. Express 15, 2360-2368 (2007). 13. J. Zhu, W. D. Cheng, D. S. Wu, H. Zhang, Y. J. Gong, H. N. Tong, and D. Zhao, “A series of lithium rare

earth polyphosphates [LiLn(PO3)4] (Ln = La, Eu, Gd) and their structural, optical, and electronic properties,” Eur. J. Inorg. Chem. 2, 285-290 (2007).

14. Z. F. Tian, H. B. Liang, B. Han, Q. Su, Y. Tao, G. B. Zhang, and Y. B. Fu, “Photon cascade emission of Gd3+ in Na(Y,Gd)FPO4,” J. Phys. Chem. C 112, 12524-12529 (2008).

15. J. Laugier, and B. Bochu, POUDRIX, a suite of programs for the interpretation of X-ray experiments, ENSP/LMGP, BP 46, 38042 Sain Martin d’Hères, France, 2000, http://www.inpg.fr/LMPG and http://www.ccp14.ac.uk/tutorial/lmgp/.

16. R. D. Shannon, “Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Cryst. A 32, 751-767 (1976).

17. P. Dorenbos, “The Eu3+ charge transfer energy and the relation with the band gap of compounds,” J. Lumin. 111, 89-104 (2005).

18. P. Dorenbos, “Systematic behaviour in trivalent lanthanide charge transfer energies,” J. Phys. Condens. Matter 15, 8417-8434 (2003).

19. G. Blasse, and B. C. Grabmaier, Luminescent Materials (Springer-Verlag, Berlin, 1994). 20. J. P. Zhong, H. B. Liang, B. Han, Z. F. Tian, Q. Su, and Y. Tao, “Intensive emission of Dy3+ in

NaGd(PO3)4 for Hg-free lamps application,” Opt. Express 16, 7508-7515 (2008). 21. Q. Zeng, H. B. Liang, G. B. Zhang, M. D. Birowosuto, Z. F. Tian, H. H Lin, Y. B. Fu, P. Dorenbos, and Q.

Su, “Luminescence of Ce3+ activated fluoro-apatites M5(PO4)3F (M= Ca, Sr, Ba) under VUV–UV and x-ray excitation,” J. Phys. Condens. Matter 18, 9549-9560 (2006).

22. J. P. Zhong, H. B. Liang, H. H. Lin, B. Han, Q. Su, and G. B. Zhang, “Effects of crystal structure on the luminescence properties and energy transfer between Gd3+ and Ce3+ ions in MGd(PO3)4:Ce3+ (M = Li, Na, K, Cs),” J. Mater. Chem. 17, 4679-4684 (2007).

23. P. Dorenbos, “The 5d level positions of the trivalent lanthanides in inorganic compounds,” J. Lumin. 91, 155-176 (2000).

24. D. E. Henrie, R. L. Fellows, and G. R. Choppin, “Hypersensitivity in the electronic transitions of lanthanide and actinide complexs,” Coord. Chem. Rev. 18, 199-224 (1976).

25. D. J. Miller, and J. M. Lisy, “Entropic effects on hydrated alkali-metal cations: Infrared spectroscopy and ab initio calculations of M+(H2O)x=2-5 cluster ions for M = Li, Na, K, and Cs,” J. Am. Chem. Soc. 130, 15393-15404 (2008).

26. V. Jubera, J. P. Chaminade, A. Garcia, F. Guillen, and C. Fouassier, “Luminescent properties of Eu3+-activated lithium rare earth borates and oxyborates,” J. Lumin. 101, 1-10 (2003).

27. G. Blasse, and A. Bril, Luminescence of phosphors based on host lattices ABO4 (A is Sc, In; B is P, V, Nb),” J. Chem. Phys. 50, 2974-2980 (1969).

1. Introduction

Nowadays, tricolor luminescence lamps are widely used in lighting field. The main drawback of these lamps is that the electric discharge of mercury (Hg) atoms is used as excitation source, and the mercury is harmful to the environment when the lamps become broken or expired. In order to avoid the use of harmful mercury, mercury-free luminescence lamps are proposed, in which the phosphors convert the vacuum ultraviolet light (VUV, wavelength λ < 200 nm and energy E > 50,000 cm-1), that is generated by the discharge of Xe (with wavelength 147 nm) and Xe2 (172 nm), to blue, green, and red light. At the same time, plasma display panels (PDPs) have also been widely used as large flat panel displays (FPDs), whose luminescent process is the same as that of mercury-free luminescence lamps. Moreover, by increasing the content of Xe gas, the light intensity of Xe2 molecular emission band (172 nm) band becomes more intense while that of the Xe resonance emission line (147 nm) becomes much weaker [1]. There is a trend to apply high Xe content gas discharge in PDPs in order to get high VUV light conversion efficiency from Xe gas discharge, and high Xe content discharge has been applied in mercury-free fluorescent lamps [2-4]. So if Xe gas discharge is at high content level, the 172 nm band emission will dominate. The development of phosphors efficiently excited by such a 172nm band emission can be significant for these devices. The current phosphors for mercury-free luminescence lamps and PDPs are mainly (Y,Gd)BO3:Eu3+ (YGB) for red,



BaMgAl10O17:Eu2+ (BAM) for blue, and Zn2SiO4:Mn2+ (ZSM) for green [4-6], but there are some shortcomings for them. In detail, the CIE (Commission Internationale de l’Eclairage, International Commission on Illumination) chromaticity coordinates of YGB show larger differences compared to the NTSC (National Television Standard Committee) standard values, because of its dominant 5D0→7F1 transition of Eu3+; BAM exhibits serious degradation under heating and VUV irradiation; ZSM has a long decay time. So, it is urgent to search for highly efficient VUV phosphors for mercury-free lamps and PDPs.

The inorganic condensed polyphosphates with general formula MIREIII(PO3)4 (where MI are alkali metal ions and REIII rare earth ions) are relatively stable under normal conditions of temperature and humidity [7,8]. These compounds can be kept for many years in a perfect state of crystallinity and they are not water soluble [9]. They have been extensively investigated in the past years due to their interesting optical properties [10-13]. In this paper, we demonstrated that intense red light emission is obtained when the phosphor LiGd0.5Eu0.5(PO3)4 is under 147/172 nm excitation. Though the CIE color coordinates are not improved relatively to that of the commercial phosphor (Y,Gd)BO3:Eu3+, this phosphor is considered to be a potential red phosphor for mercury-free lamps and PDPs application.

2. Experimental

The phosphor LiGd(PO3)4:Eu3+ (LGP:Eu3+) was synthesized by using a high-temperature solid-state reaction technique. The doping concentration of Eu3+ were chosen in the range 5 to 100 at.％ of Gd3+ in LiGd(PO3)4. The starting materials were analytical reagent (A.R.) grade Li2CO3, NH4H2PO4, Gd2O3 (99.99 %) and Eu2O3 (99.99 %). The stoichiometric reactants were mixed and ground thoroughly in an agate mortar and then calcinated at 700 oC for 20 h in a corundum crucible under air atmosphere. After the reaction at 700 oC, the products were cooled down slowly to room temperature (RT) by switching off the muffle furnace and ground into white power.

The phase purity and structure of the final products were characterized by a powder X-ray diffraction (XRD) analysis with Cu Kα radiation (λ = 1.5405 Å) on a Rigaku D/max 2200 vpc X-Ray Diffractometer. The VUV excitation and corresponding emission spectra were measured at the VUV spectroscopy experimental station on beam line U24 of National Synchrotron Radiation Laboratory (NSRL). For all excitation and emission spectra, we normalized all parameters, which include (emission and excitation) slit width, integrating time, beam intensity, and relative intensity of energy at excitation wavelength. Further measurement details can be found in our previous work [14].

3. Results and discussion

Recently, single crystal of LiGd(PO3)4 was grown and its structure was reported [7]. On the basis of the published crystal structure, the reference XRD pattern was calculated by the program POUDRIX [15]. In Fig. 1, the XRD pattern of sample LGP:Eu3+ is plotted, indicating the expected compound was synthesized in this work. The synthesis temperature is much lower than that of (Y,Gd)BO3:Eu3+, as the commercial phosphor (Y,Gd)BO3:Eu3+ is usually prepared at about 1100 oC by solid-state reaction technique.



LGP:Eu3+

10 20 30 40 50 60 70

Rela

tive I

nte

nsity (

cps)

2 Theta (degree)

Calculated from ref [7]

Fig. 1. XRD pattern of sample LGP:Eu3+ (Cu Kα λ = 1.5405 Å) and the relative reference XRD data calculated by the program POUDRIX.

The compound LiGd(PO3)4 [7], crystallizing in the monoclinic system in the C2/c space group, can be described as a long polyphosphate chain containing alternating zigzag (PO3)n chains linked by distorted GdO8 dodecahedra. Because of the very small ionic radii difference between Eu3+ (106.6 pm) and Gd3+ (105.3 pm) ions in eightfold coordination [16], the compound LGP:Eu3+ is iso-structural with LiGd(PO3)4, and the XRD pattern of LGP:Eu3+ is the same as that of LiGd(PO3)4 although 50% of Gd3+ ions were substituted by Eu3+ ions.

Figure 2 shows the VUV excitation spectrum (curve a) of the sample LiGd0.5Eu0.5(PO3)4. As a comparison in the same conditions, the VUV excitation spectrum of a commercial PDP phosphor YGB:Eu3+ is also displayed in Fig. 2(b). Most of the absorption peaks in the range of 250-350 nm (curve a) correspond to the f-f transitions of Eu3+ ions in the host lattice. The sharp peak at about 273 nm is attributed to the 8S7/2-

6IJ transitions for Gd3+ ions, indicating the existence of the energy transfer process from Gd3+ to Eu3+ in this sample. The broad band between 130 nm and 250 nm, observed in curve a of Fig. 2, is mainly related to the host adsorption at relatively shorter wavelength side, and to the charge transfer (CT) transitions of ligand O2- atoms to Eu3+ in the host lattice at relatively longer wavelength side (part B) since the location of the O2-→Eu3+ CTB is in the range 200 ~ 250 nm in most phosphates [17,18]. At the same time, we can’t exclude the occurrence of the f-d transitions of Eu3+ in this spectral range. Of course, the line-like f-f absorption of Gd3+ may also be present in this region, but its intensity is weak because of the parity-forbidden characteristic [19], so we mainly discuss the former three broad bands (the host-related absorption, the CT band and f-d transitions of Eu3+) herein. Firstly, in our previous work, the host-related absorption bands of some phosphates and fluorophosphates were investigated [14,20,21]. Though the compositions and the structures of these phosphates, fluorophosphates and polyphosphates are different, they all show an absorption band in the range 150-170 nm. We may thus consider that the intrinsic absorption of PO3

- is located in this range. In our work on Ce3+ doped LiGd(PO3)4 [22], we found that the lowest 5d state for Ce3+ corresponded to an the energy about ECe = 3.413×104 cm-1 (~293 nm). According to Dorenbos’s viewpoint [23], the crystal field depression (D value) of the lowest 5d state in the host LiGd(PO3)4 relative to that in free gaseous state can be calculated with D = 49340 – ECe = 1.521×104 cm-1, with 4.934×104 cm-1 corresponding to the lowest 5d state energy of Ce3+ in free gaseous state. As stated in [23], this D value is also valid for Eu3+ ions in the host lattice and, consequently, the lowest 5d state energy of Eu3+ may be estimated thanks to equation (1) and data of table 1 of [22] by EEu = 7.003×104 cm-1 (~142 nm). This estimate position is in the range of part A of the excitation spectrum.



100 150 200 250 300 3500

1

2

3

4

5

a LGP:Eu3+

b commercial YGB:Eu3+

Re

lative

in

tensity (

arb

.un

its)

Wavelength (nm)

a

b

147 nm 172 nm

A

B

Fig. 2. The VUV excitation spectra of LGP:Eu3+ (a, under emission at 590 nm, RT), and of a commercial phosphor YGB:Eu3+ (b, under emission at 592 nm, RT).

According to the above consideration, we think that the PO3− ligand’s absorption, the f-d

transitions, and the Eu3+←O2− CTB of Eu3+ ions overlap one another in VUV excitation spectrum, resulting in a broad absorption band in the 130-250 nm spectral range. As a result, efficient energy transfer from host lattice to lanthanide ions is expected to occur with a strong absorption of the sample LGP:Eu3+ in the VUV region. In addition, we observe that LGP:Eu3+ shows much more intense absorption around 172 nm than YGB:Eu3+, so the sample LGP:Eu3+ could be greatly suitable for application in mercury-free luminescence lamps and PDPs in which the Xe gas discharge is at a high level content.

350 400 450 500 550 600 650 700 7500.0

0.5

1.0

1.5

2.0

c LGP:Eu3+

d commercial YGB:Eu3+

Rela

tive inte

nsity (

arb

.units)

Wavelength (nm)

λex

= 147 nm

0

1

2

3

4

5

λex

= 172 nma LGP:Eu3+

b commercial YGB:Eu3+

Fig. 3. The VUV excited emission spectra of LGP:Eu3+ (a, excitation at 172 nm, c, excitation at 147 nm, RT), and commercial phosphor YGB:Eu3+ (b, excitation at 172 nm, d, excitation at 147 nm, RT).

In Fig. 3, the emission spectra under 172 and 147 nm excitation for LGP:Eu3+ and for a commercial phosphor YGB:Eu3+ are compared. It can be seen that LGP:Eu3+ shows a main emission peak at 590 nm corresponding to the 5D0-

7F1 transition of Eu3+ under 172 and 147 nm excitations. At the same time, the similar emission wavelength is observed in commercial YGB:Eu3+. So it can be anticipated that the CIE color coordinates are not improved with LGP:Eu3+. The results of the comparison of the luminescent characteristics listed in Table 1, show that the emission of LGP:Eu3+ is stronger than that of commercial YGB:Eu3+ under 172



nm excitation, while under 147 nm excitation, both phosphors exhibit nearly the same luminescence intensity. According to the emission in the whole visible range (380-730 nm), the CIE color coordinates of LGP:Eu3+ and YGB:Eu3+ are calculated, and the results are also compiled in Table 1. The greater efficiency of LGP:Eu3+ compared to YGB:Eu3+ is probably related to the higher CTB energy for YGB:Eu3+ (see figure 2).

Table 1. Comparison of the luminescence of LGP:Eu3+ and of a commercial YGB:Eu3+ under VUV excitation

Phosphor LGP:Eu3+ YGB:Eu3+

Excitation wavelength (nm) 172 147 172 147 Position of main emission peak (nm) 590 590 592 592 FWHM* of main emission peak (nm) 6 6 4 4 Relative height of main emission peak 4.04 1.80 2.52 1.79 Integrated intensities in whole visible (380–730 nm) range

1.96 1 1.24 1

CIE color coordinates (λex =172 nm ) (0.65,0.37) (0.65,0.35) *FWHM = Full width at half maximum.

The luminescence of Eu3+ has been widely investigated. It is thought that the 5D0-7FJ

emissions can give information on the site symmetry of Eu3+ in a specific host lattice. When Eu3+ occupies a site with inversion symmetry, optical transitions between levels of the 4fn configuration are strictly forbidden as electric-dipole transition, and the 5D0-

7F1 magnetic-dipole transition dominates. In contrast, if there is no inversion symmetry at the Eu3+ site, the uneven crystal field components can mix opposite-parity states into the 4fn configurational levels, hence the electric-dipole transitions are no longer strictly forbidden and hypersensitive emission of 5D0-

7F2 transition dominates [17]. In the present case, LGP:Eu3+ shows a main 5D0-

7F1 transition, which seems to imply that the Eu3+ ion occupies sites with inversion symmetry [17]. However, in LiGd(PO3)4 crystalline structure, the Gd3+ ions form GdO8 dodecahedra and occupy a single crystallographic position with C2 point symmetry [7,20], which is not a inversion symmetry site. So there are some discrepancy between our result and most cases. In 1976, D. E. Henrie et al. [24] discussed the hypersensitive transitions of lanthanides, and proposed that the intensity of the hypersensitive transitions is high when the covalency of the Ln-O bond is intense. We think that Li, with the greatest electro-negativity and the minimum ionic radius among the alkali metals, may play an important role in this case. Li-O bond may show a certain covalency relatively to A-O bonds (A = Na, K, Cs) [25]. So the covalency of Li-O plays an important role in attracting electron pairs of Gd(Eu)-O in Gd(Eu)-O-Li terms, which increases the ionicity of Gd(Eu)-O and weakens the covalency of Gd(Eu)-O. So this may be the reason of the weak hypersensitive transitions of 5D0-

7F2 in the luminescence spectra of LGP:Eu3+. In fact, much more intense hypersensitive transitions of 5D0-

7F2 were found in AGd(PO3)4:Eu3+ (A = Na, K ,Cs) in our work (not reported in this paper), which confirm the role of Li+ ion in weakening the covalency of Gd(Eu)-O in LiGd(PO3)4 host. At the same time, we could not ignore the effect of high CTB energy (200 nm) on the parity-forbidden electric dipole transitions. The high CTB energy often leads to a low percentage of electric dipole transitions [26]. Such a correlation was also observed in other phosphate host-lattices by Blasse and Bril who suggested that the parity-forbidden 4f-4f transitions ‘‘borrow’’ intensity from the CTB to some extent [27].



0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

5

λex

= 172 nm

Rela

tive inte

nsity (

arb

.un

its)

Atomic concentration x

LiGd1-x

Eux(PO

3)4 RT

λex

= 147 nm

Fig. 4. The dependence of Eu3+ emission intensity of 5D0-7F1 transition with Eu3+ concentrations in LiGd1-xEux(PO3)4 under excitation at 147/172 nm.

Finally, the luminescence intensity of phosphor materials is known to be dependent on the doping concentration of luminescent ions. Fig. 4 shows the luminescent intensity of the 5D0-

7F1 transition (590 nm) versus Eu3+ concentration in LiGd1-xEux(PO3)4 powders under 147/172 nm excitation. The most efficient luminescence intensities occur for a Eu3+ content of x = 0.5. The drop in intensity as the Eu3+ content increase is due to the rise in nonradiative decay channels, which are promoted by the interaction with quenching centers during the energy transfer processes among Eu3+ ions (concentration quenching effect).

4. Conclusion

To summarize, the phosphor LiGd0.5Eu0.5(PO3)4 shows broad and strong absorption in the VUV range as well as intensive emission under 147/172 nm excitation. Though the CIE color coordinates are similar to that of the commercial phosphor YGB:Eu3+, the luminescence intensity of the phosphor LGP:Eu3+ is evaluated about 158% of the commercial YGB:Eu3+ under 172 nm excitation. Therefore, Eu3+-doped LiGd(PO3)4 is considered to be a potential red phosphor for mercury-free lamps and PDPs application.

Acknowledgments

The work is funded by the National Basic Research Program of China (973 Program) (Grant No. 2007CB935502), by the National Natural Science Foundation of China (Grants No. 20571088, and No. 20871121), and by Synchrotron Radiation Fund of Innovation Project of Ministry of Education (Grant No. 20080113S).



Documents

Intense red light emission of Eu^3+-doped LiGd(PO_3)_4 for mercury-free lamps and plasma display panels application