- Email: [email protected]
Pr3+ doped LaTiNbO6 as a single phosphor for white LEDs
Journal of Alloys and Compounds 492 (2010) L61–L63
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Pr3+ doped LaTiNbO6 as a single phosphor for white LEDs Xiaoding Qi ∗ , Chieh-Min Liu, Chung-Chiang Kuo Department of Materials Science and Engineering, National Cheng Kung University, Tainan City 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 11 September 2009 Available online 3 December 2009 Keywords: LaTiNbO6 Phosphor Luminescence Spectroscopy
a b s t r a c t A structurally disordered host crystal, LaTiNbO6 , has been doped with the Pr3+ ions emitting at two wavelength regions that could be mixed to give out white light. In agreement with the structural design, the samples showed inhomogeneously broadened transition lineshape arising from the disorder, which is not only efficient for the absorption of blue LED sources, but also insensitive to temperature variations. Aeschynite-structured LaTiNbO6 allowed doping of up to 100% Pr3+ without phase segregation. The samples gave out two clusters of dominant emissions peaked at 490 nm (3 P0 → 3 H4 ) and 609 nm (3 P0 → 3 H6 ), respectively, whose relative intensities could be tuned by the doping level to give a desired chromaticity.
1. Introduction Replacement of incandescent lighting source with white phosphor-converted light emitting diode (pc-LED) saves great amount of energy and is of particular importance at present time, as the energy supply and the greenhouse gas issues are becoming increasingly serious. The first pc-LEDs introduced by Nichia use a blue LED (∼460 nm) to excite a single phosphor, Y3 Al5 O12 :Ce3+ , whose yellow emission blends with the source to give out white light [1]. Although it is efficient and low cost, such a pc-LED shows insufficient color rendering properties and small amount of a red phosphor has to be added. There are currently large numbers of phosphors being studied for pc-LEDs, including various oxides [1–6], sulfides [7], oxysulfides [8], nitrides [9], oxynitrides [10], etc. The optically active ions doped in the phosphors are often rare-earth (RE) or transition metal ions. The electronic transitions between the 4fn states of trivalent RE ions (RE3+ ) sitting at low-symmetry crystallographic sites exhibit strong electric-dipole forced radiations in the visible range, which are ideal for phosphors. However, the spectral lines of RE3+ are sharp due to the weak interaction with the surrounding crystal field, which are not efficient for the absorptions of broad emissions of the LED sources. Here, we report a promising Pr3+ doped phosphor, La1−x Prx TiNbO6 , with inhomogeneously broadened spectral lines arising from a disordered crystal structure. Furthermore, two dominant transitions observed from the fluorescent 3 P0 level of Pr3+ to the lower 3 H6 and 3 H states can be mixed to give out white light. 4
∗ Corresponding author. Tel.: +886 6 2008060; fax: +886 6 2346290. E-mail address: [email protected] (X. Qi). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.188
© 2009 Elsevier B.V. All rights reserved.
La1−x Prx TiNbO6 has the aeschynite structure, which is orthorhombic with space group Pnma [11]. In this structure, Nb5+ and Ti4+ ions share a same crystallographic site, which is coordinated by 6 neighboring O2− to form a slightly distorted octahedron [11,12]. As illustrated in Fig. 1, the (Nb,Ti)O6 octahedra join in pairs by edge sharing with each pair being connected to six other pairs by corner sharing. The RE3+ ions occupy the interstices formed in the resulting three-dimensional network. Each RE3+ ion is surrounded by eight O2− ions, which form an irregular polyhedron having RE-O distances varying from 0.212 to 0.246 nm [11]. The REO8 polyhedra are apparently distorted, with distortions being of both even and odd parity. Even parity terms in the crystal field expansion cause splitting of the electronic energy levels, whereas odd parity terms result in the mixings of the opposite-parity wavefunctions to allow strong electric-dipole transitions between the RE3+ multiplets. Stoichiometric RETiNbO6 (RE = Nd, Pr and Er) has previously been grown in the single crystal form as the potential gain media in miniature lasers pumped by diodes [12–14]. 2. Experimental The La1−x Prx TiNbO6 (x = 0–1) compounds in current study were synthesized by standard solid-state reaction at 1300 ◦ C in air. For x = 0–0.20, a phase transition was found at the temperature over 1100 ◦ C and the samples had to be annealed subsequently at 1080 ◦ C for 12 h to obtain the desired aeschynite structure. The high temperature modification matched with a known monoclinic phase listed in the powder diffraction database [15]. The X-ray powder diffraction patterns of the samples are showed in Fig. 2. All the recorded reflection lines can be identified with the powder pattern of the aeschynite structure [14,16], indicating that the samples were pure. There were only slight shifts of reflections lines for samples with x = 0–1 as the consequence of very small difference of atomic radii between La3+ and Pr3+ . Under the same sintering conditions, an increase of grain size as doping level was observed in the scanning electron microscopy. The La1−x Prx TiNbO6 powders had extremely good chemical stability and were insoluble in water and nitric acid solution.
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X. Qi et al. / Journal of Alloys and Compounds 492 (2010) L61–L63
Fig. 1. Schematic drawing of the Aeschynite structure [11]. The shaded octahedra are (Ti, Nb)O6 , solid circles represent the RE ions, and the dotted lines connect a RE ion to its eight neighboring oxygen ions. The site symmetry of the RE ion is a horizontal mirror vertical to the b axis.
3. Results and discussion Although the La1−x Prx TiNbO6 solid solutions were prepared over entire range, spectroscopic characterization showed that only the samples with x ≤ 0.04 had the importance for the pc-LED applications and therefore, they will be discussed here. The photoluminescence (PL) spectra of the La1−x Prx TiNbO6 samples were recorded using the excitation light of wavelength 447 nm, which is in the output range of the commercial blue LEDs (420–470 nm). The PL spectra in the visible range showed that dominant luminescences of the samples were at two wavelength regions, i.e. 475–500 nm (blue-green) and 590–630 (orange-red), as shown in Fig. 3. The emissions peaked at 477 nm and 490 nm can be assigned, respectively, to the transitions 3 P1 → 3 H4 and 3 P0 → 3 H4 [13,14], as indicated in the energy level scheme in the inset of Fig. 3. From the excitation spectra shown in Fig. 4b, the energy difference between 3 P and 3 P was calculated to be about 650 cm−1 (0.08 eV), indi1 0 cating that 3 P1 was populated at room temperature and therefore, radiative transitions from this level were possible although they were generally weaker than those from 3 P0 .
Fig. 2. X-ray –2 scans of the La1−x Prx TiNbO6 samples with x = 0, 0.05, 0.1, 0.15, 0.4, 0.6, 0.8, and 1.
Fig. 3. Photoluminescence spectra of the La1−x Prx TiNbO6 samples (x = 0.008, 0.015, 0.02, 0.03, and 0.04), measured at 300 K with the excitation wavelength of 447 nm. Inset: the energy levels between which transitions have been observed.
The transitions from the 3 P0 state to the Stark levels of 3 H6 were the major contribution to the emission cluster centered at 610 nm in Fig. 3. For the Pr3+ ions in the aeschynite structure the site symmetry is C1h [11], low enough to fully remove the 13fold J-degeneracy of the 3 H6 level according to the group theory. However, only five peaks were resolved in Fig. 3 for the 3 P0 → 3 H6 transitions, due to the large linewidths which, in part, derived from the intrinsic disorder in the structure, where half of the Nb ions in NbO6 octahedra were replaced by the Ti ions. To shed some light on the structural broadening, the low-temperature absorption lineshape of 3 P0 was examined. The 3 P0 level of the Pr3+ ions cannot be split by the crystal field and this was confirmed by the single absorption line observed in the excitation spectra in Fig. 4b. At 77 K the thermal broadening was largely removed and the 3 P0 singlet was fitted with the Gaussian lineshape, 2
ln 2 1/2
exp
− 0 2 −
/2
ln 2
As shown in Fig. 4c, good agreement was obtained, confirming the inhomogeneous broadening effect from the disorder. The transition from 1 D2 to 3 H4 had also contributed to the luminescences centered at 610 nm. However, this contribution was relatively small because the 1 D2 → 3 H4 transition is, in principle, spin forbidden and only becomes partially allowed by the Judd–Ofelt mechanism [17,18]. This was confirmed in the 609 nm excitation spectrum shown in Fig. 4b, where the weak peak observed at 592 nm was the direct absorption of 1 D2 with the Stoke shift of 17 nm from the emission of the same level. In addition to the above three emissions, weak transitions from the fluorescent 3 P level to the second lowest level, 3 H , were also observed in the 5 0 wavelength region around 533 nm, as could be seen in Fig. 3. For the 4f2 configuration of Pr3+ ions (i.e. even number of electrons), transitions between J = 0 ↔ odd J values are known to be weak according to the Judd–Ofelt theory [17,18], whereas J = 0 ↔ J = 2, 4, 6 transitions are intense. Of most importance to the pc-LED applications of the La1−x Prx TiNbO6 powders were the emission clusters peaked at 490 and 609 nm, which could be blended to give out white light. The excitation spectra in Fig. 4 showed that these two luminescences could be excited by a same wavelength in the range 440–480 nm via the absorption transitions 3 H4 → 3 P2 + 1 I6 + 3 P1 , with 447 nm being the strongest peak. The large linewidths, derived from both structural disorder and thermal broadening at room temperature,
X. Qi et al. / Journal of Alloys and Compounds 492 (2010) L61–L63
L63
As shown in Fig. 3, the relative intensities of the two clusters could be tuned by different doping levels, which resulted in the changes of branch ratios between the 3 P0 → 3 H4 , 3 H6 transitions. The chromaticity of the mixed luminescences of La1−x Prx TiNbO6 (x = 0.008–0.04) was calculated using the International Commission on Illumination (CIE) standard observer color matching functions, and the resultant xy coordinates are marked in the CIE xy color space diagram [19], as shown in Fig. 5. The xy values of 1.5%, 2%, 3% and 4% doped samples are located within the conventional white area, with the 2% sample (0.35, 0.32) being closest to the pure white point (0.33, 0.33). Therefore, La1−x Prx TiNbO6 is a promising phosphor for pc-LED applications. Recently, attempts were also made to mix multiple luminescences of some Eu3+ doped compounds (e.g. BaY2 ZnO5 ) as single component phosphor for white pc-LED [2]. Such a Eu3+ phosphor used over 10 transitions to blend for a white light [2]. The large number of transition branches will limit the output intensity of the mixed light. With only two dominant transitions, La1−x Prx TiNbO6 is expected to be more efficient. 4. Conclusions
Fig. 4. Excitation spectra for the same samples in Fig. 3: (a) excitation for the 490 nm emission, (b) excitation for the 609 nm emission, and (c) Gaussian lineshape fitting for the absorption transition from the ground state 3 H4 to the singlet 3 P0 at 77 K, 0 = 20.51 × 103 cm−1 (487 nm). = 133 cm−1 (3.2 nm).
In conclusion, a structurally disordered host crystal, LaTiNbO6 , has been doped with the Pr3+ ions emitting at two wavelength regions that could be mixed to give out white light. In agreement with the structural design, the samples showed inhomogeneously broadened lineshape, which is not only efficient for the absorption of blue LED emissions, but also insensitive to temperature variations. LaTiNbO6 allowed doping of up to 100% Pr3+ without phase segregation. The samples gave out two clusters of dominant emissions peaked at 490 nm (3 P0 → 3 H4 ) and 609 nm (3 P0 → 3 H6 ), respectively, whose relative intensities could be tuned by the doping level to give a desired chromaticity. References
Fig. 5. CIE xy color space diagram adopted from Gage et al. [19] The coordinates for the La1−x Prx TiNbO6 samples with x = 0.8%, 1.5%, 2%, 3%, and 4% are marked as a, b, c, d, and e, respectively, in the diagram. ⊗ is for the pure white point (0.33, 0.33).
are ideal for effective absorption of blue LED emissions. Compared to other hosts in which the linewidths of RE3+ are mainly due to the contribution of thermal broadening, the transitions of the Pr3+ doped LaTiNbO6 are expected to be less sensitive to the temperature variations and therefore to have a better color stability.
[1] S. Nakamura, S.J. Pearton, G. Fasol, The Blue Laser Diode: The Complete Story, Springer Verlag, Berlin, 2000, pp. 230–235. [2] C.-H. Liang, Y.-C. Chang, Y.-S. Chang, Appl. Phys. Lett. 93 (2008) 211902. [3] Q. Ma, A. Zhang, M. Lu, Y. Zhou, Z. Qiu, G. Zhou, J. Phys. Chem. B 111 (2007) 12693–12699. [4] Y. Shimomura, T. Honma, M. Shigeiwa, T. Akai, K. Okamoto, N. Kijima, J. Electrochem. Soc. 154 (2007) J35–J38. [5] J.S. Kim, K.T. Lim, Y.S. Jeong, P.E. Jeon, J.C. Choi, H.L. Park, Solid State Commun. 135 (2005) 21–24. [6] C.F. Guo, F. Gao, Y. Xu, L.F. Liang, F.G. Shi, B.H. Yan, J. Phys. D: Appl. Phys. 42 (2009) 095407. [7] Y.D. Huh, J.H. Shim, Y. Kim, Y.R. Do, J. Electrochem. Soc. 150 (2003) H57–H60. [8] Y. Kawahara, V. Petrykin, T. Ichihara, N. Kijima, M. Kakihana, Chem. Mater. 16 (2006) 6303–6307. [9] R.J. Xie, N. Hirosaki, M. Mitomo, J. Electroceram. 21 (2008) 370–373. [10] Y.Q. Li, A.C.A. Delsing, G. de With, H.T. Hintzen, Chem. Mater. 17 (2005) 3242–3248. [11] V.B. Aleksandrov, Dokl. Akad. Nauk. SSSR 142 (1963) 181–184. [12] X. Qi, R. Illingworth, H.G. Gallagher, T.J.P. Han, B. Henderson, J. Cryst. Growth 160 (1996) 111–118. [13] X. Qi, T.J.P. Han, H.G. Gallagher, B. Henderson, R. Illingworth, I.S. Ruddock, J. Phys.: Cond. Mater. 8 (1996) 4837–4845. [14] X. Qi, H.G. Gallagher, T.J.P. Han, B. Henderson, J. Cryst. Growth 180 (1997) 73–80. [15] X-ray Powder Diffraction FileTM (PDF) #15-0872, the International Centre for Diffraction Data®. (ICDD). [16] PDF #20-1367, ICDD. [17] B.R. Judd, Phys. Rev. 127 (1962) 750–761. [18] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511–520. [19] S. Gage, D. Evans, M.W. Hodapp, H. Sorensen, Optoelectronics Applications Manual, first ed., MacGraw Hill, New York, 1977, p. 6.5.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Pr3+ doped LaTiNbO6 as a single phosphor for white LEDs Xiaoding Qi ∗ , Chieh-Min Liu, Chung-Chiang Kuo Department of Materials Science and Engineering, National Cheng Kung University, Tainan City 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 11 September 2009 Available online 3 December 2009 Keywords: LaTiNbO6 Phosphor Luminescence Spectroscopy
a b s t r a c t A structurally disordered host crystal, LaTiNbO6 , has been doped with the Pr3+ ions emitting at two wavelength regions that could be mixed to give out white light. In agreement with the structural design, the samples showed inhomogeneously broadened transition lineshape arising from the disorder, which is not only efficient for the absorption of blue LED sources, but also insensitive to temperature variations. Aeschynite-structured LaTiNbO6 allowed doping of up to 100% Pr3+ without phase segregation. The samples gave out two clusters of dominant emissions peaked at 490 nm (3 P0 → 3 H4 ) and 609 nm (3 P0 → 3 H6 ), respectively, whose relative intensities could be tuned by the doping level to give a desired chromaticity.
1. Introduction Replacement of incandescent lighting source with white phosphor-converted light emitting diode (pc-LED) saves great amount of energy and is of particular importance at present time, as the energy supply and the greenhouse gas issues are becoming increasingly serious. The first pc-LEDs introduced by Nichia use a blue LED (∼460 nm) to excite a single phosphor, Y3 Al5 O12 :Ce3+ , whose yellow emission blends with the source to give out white light [1]. Although it is efficient and low cost, such a pc-LED shows insufficient color rendering properties and small amount of a red phosphor has to be added. There are currently large numbers of phosphors being studied for pc-LEDs, including various oxides [1–6], sulfides [7], oxysulfides [8], nitrides [9], oxynitrides [10], etc. The optically active ions doped in the phosphors are often rare-earth (RE) or transition metal ions. The electronic transitions between the 4fn states of trivalent RE ions (RE3+ ) sitting at low-symmetry crystallographic sites exhibit strong electric-dipole forced radiations in the visible range, which are ideal for phosphors. However, the spectral lines of RE3+ are sharp due to the weak interaction with the surrounding crystal field, which are not efficient for the absorptions of broad emissions of the LED sources. Here, we report a promising Pr3+ doped phosphor, La1−x Prx TiNbO6 , with inhomogeneously broadened spectral lines arising from a disordered crystal structure. Furthermore, two dominant transitions observed from the fluorescent 3 P0 level of Pr3+ to the lower 3 H6 and 3 H states can be mixed to give out white light. 4
∗ Corresponding author. Tel.: +886 6 2008060; fax: +886 6 2346290. E-mail address: [email protected] (X. Qi). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.188
© 2009 Elsevier B.V. All rights reserved.
La1−x Prx TiNbO6 has the aeschynite structure, which is orthorhombic with space group Pnma [11]. In this structure, Nb5+ and Ti4+ ions share a same crystallographic site, which is coordinated by 6 neighboring O2− to form a slightly distorted octahedron [11,12]. As illustrated in Fig. 1, the (Nb,Ti)O6 octahedra join in pairs by edge sharing with each pair being connected to six other pairs by corner sharing. The RE3+ ions occupy the interstices formed in the resulting three-dimensional network. Each RE3+ ion is surrounded by eight O2− ions, which form an irregular polyhedron having RE-O distances varying from 0.212 to 0.246 nm [11]. The REO8 polyhedra are apparently distorted, with distortions being of both even and odd parity. Even parity terms in the crystal field expansion cause splitting of the electronic energy levels, whereas odd parity terms result in the mixings of the opposite-parity wavefunctions to allow strong electric-dipole transitions between the RE3+ multiplets. Stoichiometric RETiNbO6 (RE = Nd, Pr and Er) has previously been grown in the single crystal form as the potential gain media in miniature lasers pumped by diodes [12–14]. 2. Experimental The La1−x Prx TiNbO6 (x = 0–1) compounds in current study were synthesized by standard solid-state reaction at 1300 ◦ C in air. For x = 0–0.20, a phase transition was found at the temperature over 1100 ◦ C and the samples had to be annealed subsequently at 1080 ◦ C for 12 h to obtain the desired aeschynite structure. The high temperature modification matched with a known monoclinic phase listed in the powder diffraction database [15]. The X-ray powder diffraction patterns of the samples are showed in Fig. 2. All the recorded reflection lines can be identified with the powder pattern of the aeschynite structure [14,16], indicating that the samples were pure. There were only slight shifts of reflections lines for samples with x = 0–1 as the consequence of very small difference of atomic radii between La3+ and Pr3+ . Under the same sintering conditions, an increase of grain size as doping level was observed in the scanning electron microscopy. The La1−x Prx TiNbO6 powders had extremely good chemical stability and were insoluble in water and nitric acid solution.
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X. Qi et al. / Journal of Alloys and Compounds 492 (2010) L61–L63
Fig. 1. Schematic drawing of the Aeschynite structure [11]. The shaded octahedra are (Ti, Nb)O6 , solid circles represent the RE ions, and the dotted lines connect a RE ion to its eight neighboring oxygen ions. The site symmetry of the RE ion is a horizontal mirror vertical to the b axis.
3. Results and discussion Although the La1−x Prx TiNbO6 solid solutions were prepared over entire range, spectroscopic characterization showed that only the samples with x ≤ 0.04 had the importance for the pc-LED applications and therefore, they will be discussed here. The photoluminescence (PL) spectra of the La1−x Prx TiNbO6 samples were recorded using the excitation light of wavelength 447 nm, which is in the output range of the commercial blue LEDs (420–470 nm). The PL spectra in the visible range showed that dominant luminescences of the samples were at two wavelength regions, i.e. 475–500 nm (blue-green) and 590–630 (orange-red), as shown in Fig. 3. The emissions peaked at 477 nm and 490 nm can be assigned, respectively, to the transitions 3 P1 → 3 H4 and 3 P0 → 3 H4 [13,14], as indicated in the energy level scheme in the inset of Fig. 3. From the excitation spectra shown in Fig. 4b, the energy difference between 3 P and 3 P was calculated to be about 650 cm−1 (0.08 eV), indi1 0 cating that 3 P1 was populated at room temperature and therefore, radiative transitions from this level were possible although they were generally weaker than those from 3 P0 .
Fig. 2. X-ray –2 scans of the La1−x Prx TiNbO6 samples with x = 0, 0.05, 0.1, 0.15, 0.4, 0.6, 0.8, and 1.
Fig. 3. Photoluminescence spectra of the La1−x Prx TiNbO6 samples (x = 0.008, 0.015, 0.02, 0.03, and 0.04), measured at 300 K with the excitation wavelength of 447 nm. Inset: the energy levels between which transitions have been observed.
The transitions from the 3 P0 state to the Stark levels of 3 H6 were the major contribution to the emission cluster centered at 610 nm in Fig. 3. For the Pr3+ ions in the aeschynite structure the site symmetry is C1h [11], low enough to fully remove the 13fold J-degeneracy of the 3 H6 level according to the group theory. However, only five peaks were resolved in Fig. 3 for the 3 P0 → 3 H6 transitions, due to the large linewidths which, in part, derived from the intrinsic disorder in the structure, where half of the Nb ions in NbO6 octahedra were replaced by the Ti ions. To shed some light on the structural broadening, the low-temperature absorption lineshape of 3 P0 was examined. The 3 P0 level of the Pr3+ ions cannot be split by the crystal field and this was confirmed by the single absorption line observed in the excitation spectra in Fig. 4b. At 77 K the thermal broadening was largely removed and the 3 P0 singlet was fitted with the Gaussian lineshape, 2
ln 2 1/2
exp
− 0 2 −
/2
ln 2
As shown in Fig. 4c, good agreement was obtained, confirming the inhomogeneous broadening effect from the disorder. The transition from 1 D2 to 3 H4 had also contributed to the luminescences centered at 610 nm. However, this contribution was relatively small because the 1 D2 → 3 H4 transition is, in principle, spin forbidden and only becomes partially allowed by the Judd–Ofelt mechanism [17,18]. This was confirmed in the 609 nm excitation spectrum shown in Fig. 4b, where the weak peak observed at 592 nm was the direct absorption of 1 D2 with the Stoke shift of 17 nm from the emission of the same level. In addition to the above three emissions, weak transitions from the fluorescent 3 P level to the second lowest level, 3 H , were also observed in the 5 0 wavelength region around 533 nm, as could be seen in Fig. 3. For the 4f2 configuration of Pr3+ ions (i.e. even number of electrons), transitions between J = 0 ↔ odd J values are known to be weak according to the Judd–Ofelt theory [17,18], whereas J = 0 ↔ J = 2, 4, 6 transitions are intense. Of most importance to the pc-LED applications of the La1−x Prx TiNbO6 powders were the emission clusters peaked at 490 and 609 nm, which could be blended to give out white light. The excitation spectra in Fig. 4 showed that these two luminescences could be excited by a same wavelength in the range 440–480 nm via the absorption transitions 3 H4 → 3 P2 + 1 I6 + 3 P1 , with 447 nm being the strongest peak. The large linewidths, derived from both structural disorder and thermal broadening at room temperature,
X. Qi et al. / Journal of Alloys and Compounds 492 (2010) L61–L63
L63
As shown in Fig. 3, the relative intensities of the two clusters could be tuned by different doping levels, which resulted in the changes of branch ratios between the 3 P0 → 3 H4 , 3 H6 transitions. The chromaticity of the mixed luminescences of La1−x Prx TiNbO6 (x = 0.008–0.04) was calculated using the International Commission on Illumination (CIE) standard observer color matching functions, and the resultant xy coordinates are marked in the CIE xy color space diagram [19], as shown in Fig. 5. The xy values of 1.5%, 2%, 3% and 4% doped samples are located within the conventional white area, with the 2% sample (0.35, 0.32) being closest to the pure white point (0.33, 0.33). Therefore, La1−x Prx TiNbO6 is a promising phosphor for pc-LED applications. Recently, attempts were also made to mix multiple luminescences of some Eu3+ doped compounds (e.g. BaY2 ZnO5 ) as single component phosphor for white pc-LED [2]. Such a Eu3+ phosphor used over 10 transitions to blend for a white light [2]. The large number of transition branches will limit the output intensity of the mixed light. With only two dominant transitions, La1−x Prx TiNbO6 is expected to be more efficient. 4. Conclusions
Fig. 4. Excitation spectra for the same samples in Fig. 3: (a) excitation for the 490 nm emission, (b) excitation for the 609 nm emission, and (c) Gaussian lineshape fitting for the absorption transition from the ground state 3 H4 to the singlet 3 P0 at 77 K, 0 = 20.51 × 103 cm−1 (487 nm). = 133 cm−1 (3.2 nm).
In conclusion, a structurally disordered host crystal, LaTiNbO6 , has been doped with the Pr3+ ions emitting at two wavelength regions that could be mixed to give out white light. In agreement with the structural design, the samples showed inhomogeneously broadened lineshape, which is not only efficient for the absorption of blue LED emissions, but also insensitive to temperature variations. LaTiNbO6 allowed doping of up to 100% Pr3+ without phase segregation. The samples gave out two clusters of dominant emissions peaked at 490 nm (3 P0 → 3 H4 ) and 609 nm (3 P0 → 3 H6 ), respectively, whose relative intensities could be tuned by the doping level to give a desired chromaticity. References
Fig. 5. CIE xy color space diagram adopted from Gage et al. [19] The coordinates for the La1−x Prx TiNbO6 samples with x = 0.8%, 1.5%, 2%, 3%, and 4% are marked as a, b, c, d, and e, respectively, in the diagram. ⊗ is for the pure white point (0.33, 0.33).
are ideal for effective absorption of blue LED emissions. Compared to other hosts in which the linewidths of RE3+ are mainly due to the contribution of thermal broadening, the transitions of the Pr3+ doped LaTiNbO6 are expected to be less sensitive to the temperature variations and therefore to have a better color stability.
[1] S. Nakamura, S.J. Pearton, G. Fasol, The Blue Laser Diode: The Complete Story, Springer Verlag, Berlin, 2000, pp. 230–235. [2] C.-H. Liang, Y.-C. Chang, Y.-S. Chang, Appl. Phys. Lett. 93 (2008) 211902. [3] Q. Ma, A. Zhang, M. Lu, Y. Zhou, Z. Qiu, G. Zhou, J. Phys. Chem. B 111 (2007) 12693–12699. [4] Y. Shimomura, T. Honma, M. Shigeiwa, T. Akai, K. Okamoto, N. Kijima, J. Electrochem. Soc. 154 (2007) J35–J38. [5] J.S. Kim, K.T. Lim, Y.S. Jeong, P.E. Jeon, J.C. Choi, H.L. Park, Solid State Commun. 135 (2005) 21–24. [6] C.F. Guo, F. Gao, Y. Xu, L.F. Liang, F.G. Shi, B.H. Yan, J. Phys. D: Appl. Phys. 42 (2009) 095407. [7] Y.D. Huh, J.H. Shim, Y. Kim, Y.R. Do, J. Electrochem. Soc. 150 (2003) H57–H60. [8] Y. Kawahara, V. Petrykin, T. Ichihara, N. Kijima, M. Kakihana, Chem. Mater. 16 (2006) 6303–6307. [9] R.J. Xie, N. Hirosaki, M. Mitomo, J. Electroceram. 21 (2008) 370–373. [10] Y.Q. Li, A.C.A. Delsing, G. de With, H.T. Hintzen, Chem. Mater. 17 (2005) 3242–3248. [11] V.B. Aleksandrov, Dokl. Akad. Nauk. SSSR 142 (1963) 181–184. [12] X. Qi, R. Illingworth, H.G. Gallagher, T.J.P. Han, B. Henderson, J. Cryst. Growth 160 (1996) 111–118. [13] X. Qi, T.J.P. Han, H.G. Gallagher, B. Henderson, R. Illingworth, I.S. Ruddock, J. Phys.: Cond. Mater. 8 (1996) 4837–4845. [14] X. Qi, H.G. Gallagher, T.J.P. Han, B. Henderson, J. Cryst. Growth 180 (1997) 73–80. [15] X-ray Powder Diffraction FileTM (PDF) #15-0872, the International Centre for Diffraction Data®. (ICDD). [16] PDF #20-1367, ICDD. [17] B.R. Judd, Phys. Rev. 127 (1962) 750–761. [18] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511–520. [19] S. Gage, D. Evans, M.W. Hodapp, H. Sorensen, Optoelectronics Applications Manual, first ed., MacGraw Hill, New York, 1977, p. 6.5.