Intensive emission of Dy 3+ in NaGd(PO 3 ) 4 for Hgfree lamps application

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1 Intensive emission of 3+ in NaGd(PO 3 ) 4 for Hgfree lamps application Jiuping Zhong, 1 Hongbin Liang, 1,* Bing Han, 1 Zifeng Tian, 1 and Qiang Su, 1,* and Ye Tao 2 1 MOE 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 , P. R. China 2 Laboratory of Beijing Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Science, Beijing , P. R. China * Corresponding author: cesbin@mail.sysu.edu.cn Abstract: The phosphor NaGd(PO 3 ) 4 : 3+ was synthesized by solid-state reaction technique at high temperature. The vacuum ultraviolet (VUV)-UV excitation spectra and visible emission spectra under VUV/UV excitation were investigated. The sample NaGd(PO 3 ) 4 : 3+ showed suitable spectroscopic characteristics such as broad and strong absorption around 172 nm, intensive emission with the chromaticity coordinates (0.33, 0.38) in warm-white light region. Additionally, this efficient white-emitting phosphor is activated by a single 3+ ion and with a lower preparation temperature, which tend to decrease the consumption of rare earth resource and energy. Therefore, the phosphor 3+ may be considered as a suitable candidate for Hg-free lamps application Optical Society of America OCIS codes: ( ) Fluorescent and luminescent materials; ( ) Photoluminescence; ( ) Spectroscopy, fluorescence and luminescence References and links 1. G. Blasse and B. C. Grabmaier, Luminescent Materials, (Springer-Verlag, Berlin, 1994). 2. S. Shionoga and W. M. Yen, Phosphor Handbook, (CRC Press, Boston, 1999). 3. T. Jüstel, H. Nikol, and C. Ronda, New development in the field of luminescent materials for lighting and displays, Angew. Chem. Int. Ed. 37, 3084 (1998). 4. 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 (2001). 5. H. Y. P. Hong, Crystal structure of NdLiP 4 O 12, Mater. Res. Bull. 10, 635 (1975). 6. K. Jaouadi, H. Naili, N. Zouari, T. Mhiri, and A. Daoud, Synthesis and crystal structure of a new form of potassium-bismuth polyphosphate KBi(PO 3 ) 4, J. Alloys Compd. 354, 104(2003). 7. K. Jaouadi, N. Zouari, T. Mhiri, and M. Pierrot, Synthesis and crystal structure of sodium-bismuth polyphosphate NaBi(PO 3 ) 4, J. Cryst. Growth 273, 638 (2005). 8. H. Ettis, H. Naili, and T. Mhiri, Synthesis and Crystal Structure of a New Potassium-Gadolinium Cyclotetraphosphate, KGdP 4 O 12, Cryst. Growth Des. 3, 599 (2003). 9. I. Parreu, R. Solé, J. Gavaldà, J. Massons, F. Díaz, and M. Aguiló, Crystal growth, structural characterization, and linear thermal evolution of KGd(PO 3 ) 4, Chem. Mater. 17, 822 (2005). 10. I. Parreu, J. M. C. Pujol, M. Aguiló, F. Díaz, X. Mateos, and V. Petrov, Growth, spectroscopy and laser operation of Yb:KGd(PO 3 ) 4 single crystal, Opt. Express 15, 2360(2007). 11. J. Amami, M. Ferid, and M. Trabelsi-Ayedi, Crystal structure and spectroscopic studies of NaGd(PO 3 ) 4, Mater. Res. Bull. 40, 2144 (2005). 12. R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Cryst. A 32, 751 (1976). 13. J. P. Zhong, H. B. Liang, B. Han, Q. Su, and G. B. Zhang, Effects of crystal structure on the luminescence properties and energy transfer between Gd 3+ and Ce 3+ ions in MGd(PO 3 ) 4 :Ce 3+ (M = Li, Na, K, Cs), J. Mater. Chem. 17, 4679 (2007). 14. H. B. Liang, Y. Tao, Q. Su, and S. B. Wang, VUV-UV photoluminescence spectra of strontium orthophosphate doped with rare earth ions, J. Solid State Chem. 167, 435 (2002). 15. H. B. Liang, Y. Tao, J. H. Xu, H. He, H. Wu, W. X. Chen, S. B. Wang, and Q. Su, Photoluminescence of Ce 3+, Pr 3+ and Tb 3+ activated Sr 3 Ln(PO 4 ) 3 under VUV-UV excitation, J. Solid State Chem. 177, 901 (2004). (C) 2008 OSA 12 May 2008 / Vol. 16, No. 10 / OPTICS EXPRESS 7508

2 16. Z. F. Tian, H. B. Liang, H. H. Lin, Q. Su, B. Guo, G. B. Zhang, and Y. B. Fu, Luminescence of NaGdFPO 4 :Ln 3+ after VUV excitation: A comparison with GdPO 4 :Ln 3+ (Ln= Ce, Tb), J. Solid State Chem. 179, 1291 (2006). 17. P. Dorenbos, The 5d level positions of the trivalent lanthanides in inorganic compounds, J. Lumin. 91, 155 (2000). 18. C. K. Jørgensen, Electron transfer spectra of lanthanide complexes, Mol. Phys. 5, 271 (1962). 19. Q. Su, in Proceedings of the 2 nd. International Conference on Rare Earth Development and Application, pp International Academic Publishers, Beijing (1991). 1. Introduction Nowadays, tricolor luminescence lamps are widely used in lighting field. Usually, the whiteemitting light is obtained by three phosphors in these lamps, that is, the blend of red-emitting phosphor Y 2 O 3 :Eu 3+ (YOE), the green-emitting Zn 2 SiO 4 :Mn 2+ (ZSM), CeMgAl 11 O 19 :Tb 3+ (CAT), (La,Ce)PO 4 :Tb 3+ (LAP), or (Ce,Gd)MgB 5 O 10 :Tb 3+ (CBT), and the blue-emitting BaMgAl 10 O17:Eu 2+ (BAM).[1,2] 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, Hg-free luminescence lamps are proposed, in which the phosphors convert the vacuum ultraviolet (VUV, wavelength λ < 200 nm and energy E > 50,000 cm -1 ) photons, that is generated by the discharge of Xe (with wavelength 147 nm) and Xe 2 (172 nm), to blue, green, and red light. The tricolor phosphors, (Y,Gd)BO 3 :Eu 3+ (YGB: Eu 3+ ) with red emission, Zn 2 SiO 4 :Mn 2+ (ZSM) with green emission, and BaMgAl 10 O 17 :Eu 2+ (BAM) with blue emission, are usually recommended to be used in Hg-free luminescence lamps.[3,4] It costs more rare earth consumption through blending tricolor phosphors to obtain white light, and the rare-earth europium is expensive. Moreover, the preparation temperature of the above three commercial tricolor phosphors is higher than 900 C by the solid-state reaction technique. The inorganic condensed polyphosphates with general formula M I RE III (PO 3 ) 4 (where M I are alkali metal ions and RE III rare earth metal ions) are relatively stable under normal conditions of temperature and humidity [5,6]. These compounds can be kept for many years in a perfect state of crystallinity and they are not water soluble [7]. They have been extensively investigated in the past years due to their interesting optical properties [8-10]. In this work, only one type of rare-earth 3+ ion was doped in NaGd(PO 3 ) 4, and intensive white emission was obtained when the sample NaGd(PO 3 ) 4 : 3+ ( 3+ ) was under 172 nm excitation. And dysprosium () is abundant in the ion adsorption type deposit of China and its price is cheap. 2. Experimental A series of polycrystalline samples of NaGd 1-x x (PO 3 ) 4 (x = 0 ~ 1.00) were prepared by a high temperature solid-state reaction of stoichiometric amounts (Na/RE/P = 1:1:4) of analytical reagent grade Na 2 CO 3, NH 4 H 2 PO 4, and 99.99% pure rare-earth oxides (Gd 2 O 3 and 2 O 3 ) using the following reactions: 973 8NH 4 H 2 PO 4 + (1-x)Gd 2 O 3 + x 2 O 3 + Na 2 CO 3 K / 40h 2NaGd 1-x x(po 3 ) 4 + 8NH H 2 O + CO 2 The pulverous mixtures were ground in an agate mortar and then calcinated at 973 K (700 o C) for 40 h in a corundum crucible under air atmosphere. The X-ray powder diffraction analyses were carried out with a Rigaku D/max 2200 vpc X- ray powder diffractometer (Cu Kα radiation, 40kV, 30mA) at room temperature (RT), and the data were collected with 2θ = 10 ~ 60 o, step size = 0.02 o. The UV luminescence spectra at RT were recorded on an Edinburgh FLS 920 combined fluorescence lifetime and steady state spectrometer. A 450 W xenon lamp was used as the excitation source for the UV excitation spectra and a blue-sensitive photomultiplier tube (R1527 PMT) was used for the emission spectra recording. (C) 2008 OSA 12 May 2008 / Vol. 16, No. 10 / OPTICS EXPRESS 7509

3 The VUV spectra were recorded at Beamline 4B8 in Beijing Synchrotron Radiation Facilities (BSRF) under dedicated synchrotron mode (2.5 GeV, mA). A 1 m Seya monochromator (1200 g/mm, nm, 1 nm bandwidth) was used for the synchrotron radiation excitation spectra measurement, and an Acton SP-308 monochromator (600 g/mm, nm) was used for the emission spectra measurement. The signal was detected with a Hamamatsu H photon counting unit. The vacuum in the sample chamber was about mbar. The effect of the experimental set-up response on the relative VUV excitation intensities of the samples were corrected by dividing the measured excitation intensities of the samples with the excitation intensities of sodium salicylate (o-c 6 H 4 OHCOONa) measured simultaneously in the same excitation conditions. 3. Results and discussion 3.1 X-ray Powder Diffraction In order to characterize the phase purity of the samples, X-ray powder diffraction (XRD) measurements were performed for all samples. As examples, the XRD patterns of samples NaGd(PO 3 ) 4, NaGd (PO 3 ) 4 and Na(PO 3 ) 4 were plotted in Fig. 1, indicating that all samples are of single phase and in good agreement with the reported powder patterns in JCPDS standard card numbered [NaGd(PO 3 ) 4 ]. These XRD patterns comparisons also show that the polyphosphate samples were synthesized successfully at 700 C. This temperature is much lower than the preparation temperature of current commercial tricolor phosphors by the solid-state reaction technique. Sample Na(PO 3 ) 4 Relative intensity (cps) Sample NaGd (PO 3 ) 4 Sample NaGd(PO 3 ) 4 JCPDS NaGd(PO 3 ) theta (degree) Fig. 1. XRD patterns of samples NaGd(PO 3 ) 4, NaGd (PO 3 ) 4 and Na(PO 3 ) 4. The compound NaGd(PO 3 ) 4, crystallizing in a monoclinic system with P2 1 /n space group, can be described as a long chain polyphosphate containing alternating zigzag (PO 3 ) n chains linked by distorted GdO 8 dodecahedra. It was testified there is only one site for Gd 3+ ions in NaGd(PO 3 ) 4 with Eu 3+ ions as a probe[11]. Because of the much small ionic radii difference between rare-earth ions 3+ (102.7 pm) and Gd 3+ (105.3 pm) in the eight-fold coordination environment [12], the compound Na(PO 3 ) 4 is iso-structure with NaGd(PO 3 ) 4, and the XRD patterns of NaGd (PO 3 ) 4 and Na(PO 3 ) 4 are the same with that of NaGd(PO 3 ) 4, although all Gd 3+ ions were substituted by 3+ ions in Na(PO 3 ) 4. (C) 2008 OSA 12 May 2008 / Vol. 16, No. 10 / OPTICS EXPRESS 7510

4 3.2 The UV-visible luminescence properties Trivalent dysprosium ( 3+ ) ion has two dominant bands in the emission spectrum. The yellow band (574 nm) corresponds with the hypersensitive transition 4 F 9/2 6 H 13/2 ( L = 2, J = 2), and the blue band (480 nm) corresponds with the 4 F 9/2 6 H 15/2 transition. Figure 2 shows the UV-visible excitation and emission spectra of the sample NaGd (PO 3 ) 4 at RT. A number of absorption peaks in the nm region can be seen in Fig. 2(a). These peaks at around 251 nm, 273 nm and 311 nm are attributed to the 8 S 7/2 6 D J, 6 I J, 6 P J transitions within Gd 3+ ions respectively [13], indicating the existence of the energy transfer process from Gd 3+ to 3+ in this sample. The other absorption peaks in the range of 280 ~ 500 nm, marked by the Arabic numerals 1-9 in Fig. 2(a), correspond to the f-f transitions of 3+ ions in the host lattice. The ground state of 3+ is 6 H 15/2, and peaks 1-9 are attributed to the transitions from this ground state to different excitation levels: 4 K 13/2 + 4 H 13/2 (1), 4 K 15/2 (2), 4 I 9/2 + 4 G 9/2 (3), 4 M 15/2 + 6 P 7/2 (4), 4 I 11/2 (5), 4 M 21/2 + 4 I 13/2 + 4 K 17/2 + 4 F 7/2 (6), 4 G 11/2 (7), 4 I 15/2 (8), and 4 F 9/2 (9), respectively. The emission spectra under the excitation of 349 nm UV radiation are exhibited in Fig. 2(b), in which blue emission at about 479 nm (peak 10) and the yellow emission at about 573 nm (peak 11) are strong. They correspond to the transitions from the 4 F 9/2 excited state to the 6 H 15/2 and 6 H 13/2 ground states, respectively. Relative Intensity(a.u.) (a) (b) Gd Excitation spectrum (λ em = 573nm) Emission spectrum 10 (λ ex = 349 nm) Wavelength (nm) Fig. 2. Excitation and emission spectra of NaGd (PO 3 ) 4 Figure 3 exhibits the emission spectra of the samples NaGd 1-x x (PO 3 ) 4 (x = 0.01 ~ 1.00) under 349 nm UV excitation in the same conditions. When the value of x exceeds 0.05, the blue and yellowish emission peaks become weaker and weaker due to the concentration quenching. The concentration quenching might be elucidated by the following two factors. (i) The excitation migration due to resonance between the activators is enhanced when the doping concentration is increased, and thus the excitation energy reaches quenching centers. (ii) The activators are paired or coagulated and are changed to a quenching center. (C) 2008 OSA 12 May 2008 / Vol. 16, No. 10 / OPTICS EXPRESS 7511

5 Relative intensity (a.u.) Wavelength (nm) x Fig. 3. Emission spectra of NaGd 1-x x (PO 3 ) 4 (λ ex = 349 nm) 3.2 The VUV excitation spectrum and emission spectra under VUV excitation Relative intensity (a.u.) (a) NGP : 3+ (b) D B C 6 185nmG J A 172 nm 8 S7/2 1 3 Gd 3+ 8 S7/2 NGP : 3+ (λ em = 479 nm) 8 S 7/ K 15/2 6 6 H15/2 PJ 2 YGB : Eu 3+ 3 ZSM (λ ex = 172 nm) Wavelength (nm) 6 IJ 6 D J 8 S7/2 Fig. 4. VUV spectra of NaGd (PO 3 ) 4 (labeled as 3+ ) and commercial phosphor (Y,Gd)BO 3 :Eu 3+ (YGB: Eu 3+ ) and Zn 2 SiO 4 :Mn 2+ (ZSM) The emission intensity of phosphor NaGd 1-x x (PO 3 ) 4 for x = 0.05 under 349 nm excitation is higher than that of other concentration samples, so the sample NaGd (PO 3 ) 4 (labeled as 3+ ) is chosen to measure the VUV excitation spectrum and emission spectra. Figure 4(a) shows the VUV excitation spectrum upon blue emission (479 nm). In order to describe the VUV absorption peaks clearly, the VUV spectra of undoped NaGd(PO 3 ) 4 are also showed in Fig. 5. After comparing the VUV excitation spectra of 3+ and that of undoped NaGd(PO 3 ) 4, it can be concluded that the peaks in the nm region (Fig. 4(a)) are mainly ascribed to the absorption of Gd 3+ ions, and the absorption peak at about 325 nm is due to the transition 6 H 15/2 4 K 15/2 of doped 3+ ions. Broad bands below the wavelength 190 nm in Fig. 4(a) are considered to include the host-related absorption, the f-d transitions of 3+ in the host lattice, and 3+ O 2 charge transfer band (CTB) from the following standpoints. (C) 2008 OSA 12 May 2008 / Vol. 16, No. 10 / OPTICS EXPRESS 7512

6 6 F J 8 S 7/2 8 S 7/2 1. In our previous work, the host-related absorption bands of some phosphates and fluorophosphates were investigated.[14-16] Though the compositions and the structure of these phosphates, fluorophosphates and polyphosphates are different, they all show absorption band around wavelength nm. We consider that the intrinsic absorption of PO 3 is located around this range nm (band D in Fig. 4(a)), which is confirmed by the inset spectrum in Fig. 5. undoped NaGd(PO 3 ) 4 6 I J 6 P 7/2 Relative Intensity (a.u.) PO 3 _ 4 H 8 J S 7/2 8 S 7/ G J x 3 8 S 8 7/2 S 7/2 Em. 311nm Ex. 273nm 6 D J Wavelength (nm) Fig. 5. The VUV spectra of undoped NaGd(PO 3 ) 4 at RT 2. For 3+ ions, when one electron is promoted from ground states 4f 9 to 4f 8 5d 1 excited levels, it can give rise to two groups of f-d transitions: spin-allowed (SA) transitions are stronger and with higher energies, while spin-forbidden (SF) are weaker and with lower energies. The energies of the lowest SA and the lowest SF f-d transitions can be evaluated according to the method proposed by Dorenbos and the spectroscopic data of Ce 3+ ions in the host lattice [17]. In our previous work[13], the decreasing of the lowest 5d state for Ce 3+ in the host lattice NaGd(PO 3 ) 4 (D value) is about cm 1 in comparison with free gaseous Ce 3+. Because the influence of the crystal field and covalency of the host lattice on the red shift of 5d levels are approximately equal for all rare-earth ions, we consider this D value is adopted by 3+ in NaGd(PO 3 ) 4. The energies of the lowest SA and the lowest SF f-d transitions for free 3+ ions are reported to be cm -1 and cm -1, respectively[20]. Then we predicate that the lowest SA transitions for 3+ ions in NaGd(PO 3 ) 4 is cm 1 (170 nm, strong), which are coincidence with the positions of band B in Fig. 4(a), and the lowest SF f-d is cm 1 (188 nm, weak), just because this band has overlap with the peak of 8 S 7/2 6 F J transition (183 nm) within Gd 3+ ions, band A has a little shift toward high energy region. 3. The energy of the 3+ O 2 CTB can be roughly estimated by the Jørgensen empirical formula[18]: E CT = [χ opt (X) χ opt (M)] cm 1 Here, E CT gives the energy of CTB in unit cm 1, while χ opt (X) and χ opt (M) are the optical electronegativities of the anion X and central metal cation M, respectively. Using χ opt (O 2 )=3.2 and χ opt ( 3+ )= 1.21[19], the CTB energy of 3+ in oxides can be approximately estimated to be cm 1 (168 nm), which is overlapped with the lowest SA f-d transitions of 3+ ions (band B in Fig. 4(a)). In addition, a sharp peak in band C (peaking at about 162 nm) can be observed in Fig. 4(a), which is probably caused by the high energy f-f transition of Gd 3+ or 3+ ions. (C) 2008 OSA 12 May 2008 / Vol. 16, No. 10 / OPTICS EXPRESS 7513

7 According to above considerations, we thought that the PO 3 ligand s absorption, the 3+ O 2 CTB, the f-d transitions of 3+, and part of f-f transitions within Gd 3+ or 3+ occur and overlap in VUV excitation spectrum, resulting in strong absorption of the sample 3+ at 172 nm wavelength. In Fig. 4(b), the emission spectrum of the sample 3+ under 172 nm excitation is exhibited (curve 1). The positions of the intensive blue and yellowish emission under VUV excitation are in agreement with that under UV excitation, but the relative intensities show some differences as the emission under UV excitation was not corrected by the instrumental response of blue-sensitive PMT. For the purpose of obtaining the relative emission intensity of the sample 3+ under 172 nm excitation, the intensity in whole visible ( nm) range was integrated, and compared with that of commercial phosphors YGB:Eu 3+ and ZSM measured in the same conditions (curves 2 and 3 in Fig. 4(b)). The results were determined through the ratios (K) as following: It was obtained that: 3+ ( Comm. K ) = + + I 3 ( λ) dλ 3 I ( 3+ YGB: Eu Comm. 3+ ( λ) dλ K ) =1.99, 3+ K ( ) =0.95. ZSM Comm. The integrated intensity of 3+ is almost twice than that of commercial red phosphor YGB:Eu 3+ and also almost equal to that of commercial green phosphor ZSM in the same conditions. These results show that the title sample has intensive emission when it is excited with the wavelength 172 nm VUV light, which is generated by the discharge of excimer Xe 2. The chromaticity coordinate (x, y) of the sample 3+ was calculated in term of the emission under 172 nm excitation and is showed in Fig. 6. The CIE color coordinate of 3+ is (0.33, 0.38), which is located in warm-white light region.. Fig. 6. The CIE color coordinates of 3+ together with that of commercial phosphor YGB:Eu 3+ and ZSM. (C) 2008 OSA 12 May 2008 / Vol. 16, No. 10 / OPTICS EXPRESS 7514

8 4. Conclusion The phosphor NaGd 1-x x (PO 3 ) 4 for x = 0.05 shows broad and strong absorption in the VUV range and high intensive emission under 172 nm excitation, and its CIE color coordinate enter the warm-white light region. Additionally, this efficient white emitting phosphor NaGd 1- x x (PO 3 ) 4 is activated by a single 3+ ion and with a lower preparation temperature than that of current commercial tricolor phosphors, which tend to decrease the consumption of rare earth resource and energy. Therefore, the phosphor 3+ may be considered as a suitable candidate for Hg-free lamps application. Acknowledgments This work was financial supported by the National Basic Research Program of China (973 Program) (Grant No. 2007CB935502), National Natural Science Foundation of China (Grant No ), Science and Technology Project of Guangdong Province (Grants No. 2005A and No. 2006B ). (C) 2008 OSA 12 May 2008 / Vol. 16, No. 10 / OPTICS EXPRESS 7515