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Hydrothermal synthesis and luminescent properties of LnPO4:Tb (Ln = La, Gd) phosphors under VUV excitation
Journal of Alloys and Compounds 436 (2007) 383–386
Hydrothermal synthesis and luminescent properties of LnPO4:Tb (Ln = La, Gd) phosphors under VUV excitation Chunfang Wu a , Yuhua Wang a,∗ , Wei Jie b a
Department of Materials Science, School of Physical Science and Technology, Lanzhou University, Lanzhou, Gansu Province 730000, PR China b Key Lab for Magnetism and Magnetic Materials of MOE, School of Physical Science and Technology, Lanzhou University, Lanzhou, Gansu Province 730000, PR China Received 23 May 2006; received in revised form 13 July 2006; accepted 13 July 2006 Available online 12 September 2006
Abstract Tb doped LnPO4 (Ln = La, Gd) phosphors with monoclinic system were successfully prepared by mild hydrothermal reaction at 240 ◦ C. The 543 nm emission of Tb3+ in GdPO4 is higher than in LaPO4 . By comparison, it is found that the intensity of host absorption band of GdPO4 :Tb is higher than that of LaPO4 :Tb which is ascribed to the energy transfer efficiency between the PO4 3− molecule and Tb3+ is higher. The emission intensity of optimal composition of GdPO4 :0.3Tb is comparable with that of commercial Zn2 SiO4 :0.04Mn phosphor. These results suggest that GdPO4 :0.3Tb is a potential candidate for plasma display panels (PDPs) application. © 2006 Elsevier B.V. All rights reserved. PACS: 78.55. −m Keywords: VUV; Luminescence; Hydrothermal synthesis; Phosphate
1. Introduction Tricolor phosphors for plasma display panels (PDPs) are hot research points in recent years [1–3]. Been excited by vacuum ultraviolet ray (VUV) in PDPs, few phosphors applied in lamp are feasible to be applied in PDPs. Zn2 SiO4 :Mn is well known as a good green-emitting phosphor for PDP [4,5]. However, the decay time of Zn2 SiO4 :Mn is long. Using the extended Huckel method, Saito et al. [6] calculated energy of the tetrahedral PO4 3− molecule. According to their results, the lowest energies of the transitions of 2t2 → 2a, 3t2 are found to exist at 7–10 eV (177–124 nm). The PDPs excitation source (147 nm) is in the range. Therefore, it can be concluded that orthophosphate would be a potential phosphor host. Rao and Devine [7] have realized red, blue and green emissions in rare-earth orthophosphate by doping Eu3+ , Tm3+ and Tb3+ , respectively. Jung and Lee [8] tried to modify the luminance property of LaPO4 :Tb particles by adding some manganese as a co-activator. But under the excitation of VUV (147 nm), the doping Mn is not help-
∗
Corresponding author. Tel.: +86 931 8912079; fax: +86 931 8913554. E-mail address: [email protected] (Y. Wang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.07.056
ful to improve the photoluminescence intensity of LaPO4 :Tb particles. So, Tb doped orthophosphate can be a new candidate to replace Zn2 SiO4 :Mn. Usually, the rare-earth orthophosphate are prepared by solid state reaction [9], precipitation method [10], spray pyrolysis [11] and hydrothermal means [12]. Up to now, the excitation and emission properties in VUV range of Tb doped rare-earth phosphate prepared by hydrothermal method is scarcely reported. So in this work, we choose the hydrothermal method to synthesis Tb doped LaPO4 and GdPO4 phosphors. Meanwhile, the excitation and the emission properties of the resulted phosphors and the dependence of emission properties on the Tb3+ concentration in LaPO4 and GdPO4 are investigated. 2. Experimental procedure The starting materials are Gd2 O3 (99.99%), La2 O3 (99.99%), Tb4 O7 (99.99%) and (NH4 )2 HPO4 (A.R.). Stoichiometric amount of each oxides are dissolved in diluted nitric by heating. (NH4 )2 HPO4 is dissolved and added into the former clear solution. Then the mixture is transferred into a Teflon-lined stainless steel autoclave with a filling capacity of 40%. The hydrothermal reaction lasts 6 h at 240 ◦ C. The crystal structure of samples is characterized by X-ray powder diffraction operating at 40 kV/60 mA, using monochromatized Cu K␣ radiation. The excitation and emission spectra are measured by FLS920T spectrophotometer
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C. Wu et al. / Journal of Alloys and Compounds 436 (2007) 383–386
Fig. 1. XRD patterns of LT15 (a) and GT15 (b).
Fig. 2. SEM photographs of LT15 (a) and GT15 (b). equipped with VM504 vacuum monochromator, and the VUV excitation spectra are corrected by the sodium salicylate at the same measurement conditions.
3. Results and discussion The XRD patterns of La0.85 Tb0.15 PO4 (LT15) and Gd0.85 Tb0.15 PO4 (GT15) are shown in Fig. 1. The LT15 and GT15 phosphors prepared by hydrothermal reaction at a relative low temperature in our work are monoclinc and they need not further high temperature calcinations as solid state reaction, precipitation method and spray pyrolysis do. It suggests that the hydrothermal reaction is an effective and cost-saving method to prepare phosphors. But the XRD pattern of GT15 is somewhat different from that of LT15. The peaks’ positions of the two samples are similar other than the intensity ratios of the strongest peak (which is indexed as (1 2 0) surface) to other peaks. It indicates the preferential growth along the surface which is vertical with (1 2 0) surface in GT15 occurred. But the preferential growth in LT15 is not as serious as in GT. From this, we conclude that the morphology of the two phosphors is different. To confirm it, the morphology of LT15 and GT15 were examined by SEM. The morphology of LT15 and GT15 is shown in Fig. 2. The shape of GT15 and LT15 particles indeed is different. The GT15 particles grow with preference in some crystal face and finally form a column-like shape with the maximum length about 2 m. But in LT15, the preferential growth is not so evident compared with GT15. There are much more little particles than the columnlike particle, and the particles’ size of LT15 is smaller than that of GT15.
The fluorescence intensity of 543nm emission of Tb3+ in LaPO4 :Tb (0.05 ≤ X ≤ 0.3) (LT) and GdPO4 :Tb (0.05 ≤ X ≤ 0.5) (GT), are displayed as a function of Tb3 concentration and represented in Fig. 3. The emission intensity of Tb3+ in the two phosphors increases proportionately with Tb concentration and then decreases, going through a maximum at 0.15 and 0.3 concentrations, respectively. The emission intensity decreases at high Tb concentration is referred to as concentration quenching [13] which involves the resonance transfer of electronic excitation energy from the initially absorbing atom to another identi-
Fig. 3. Effect of Tb3+ content on 543 nm emission intensity in LT and GT.
C. Wu et al. / Journal of Alloys and Compounds 436 (2007) 383–386
Fig. 4. Excitation spectra of GT15 and LT15.
cal atom, and finally to some as yet unspecified quenching site. As an example, the excitation of Gd0.85 Tb0.15 PO4 (GT15) and La0.85 Tb0.15 PO4 (LT15) is shown in Fig. 4. The bands in the region from 120 to 160 nm may be due to the overlap of two bands peaking at about 145 and 155 nm. The position of the charge transfer band (CTB) of Tb3+ can be calculated with an empirical formula given by Jørgensen [14]: ECT = [(X) − (M)] × 30, 000 cm−1 Here, (X) is the optical electronegativity of the anion and (M) is that of the central metal ion. Using (O) = 3.2 [15], (Tb) = 0.95 [16], the CTB of Tb3+ in oxide can be calculated as 148 nm. So, the former is attributed to the CTB of Tb3+ . The latter may be assigned to the absorption band of PO4 3− anion molecules (host absorption band). Similar host absorption bands are observed in other rare-earth orthophosphates which are shown in Table 1. As for GT15, the VUV excitation spectra are somewhat different from that of LT15. Firstly, a small peak at 274 nm corresponding to 8 S7/2 → 6 IJ (J = 11/2, 9/2 and 7/2) transitions is present, indicating the occurrence of the energy transfer process from Gd3+ to Tb3+ . Secondly, the intensity of host absorption band is higher than that of LT15 which is consistent with that of emission spectra. Similar phenomenon has also been observed in lanthanum metaborate host lattice [19] and BaLnB9 O16 :Tb (Ln = La, Gd) [20]. It cannot be explained expressly. But the result indicated that Gd3+ plays an important intermediate role in the energy transfer from PO4 3− to the activator, such as Tb3+ . Under 147 nm excitation, the luminescence of Gd0.7 Tb0.3 PO4 is compared with that of commercial Zn2 SiO4 :0.4Mn in Fig. 5. The brightness of Gd0.3 Tb0.3 PO4 is 121% of Table 1 The position of some orthophosphates host absorption Compound
Absorption band (nm)
LaPO4 :Tm (La, Gd)PO4 :Tb, Eu LaPO4 :Eu, GdPO4 :Eu, YPO4:Eu, LuPO4 :Eu YPO4 :Tb
170 [9] 120–160 [10] 159, 160, 152, 145 [17] 125–160 nm [18]
Fig. 5. Photoluminescence Gd0.7 Tb0.3 PO4 .
385
spectra
of
Zn2 SiO4 :Mn
and
prepared
Zn2 SiO4 :0.4Mn. The decay time (τ e ) of prepared particle is 3.8 ms which is shorter than Zn2 SiO4 :Mn. 4. Conclusion (1) Mild hydrothermal reaction is an effective method to prepare phosphate phosphors for PDPs. (2) By comparison, as for Tb doped rare earth orthophosphate phosphor for PDPs, GdPO4 is a better host matrix than LaPO4 because the quenching concentration and the green emission intensity of Tb in GdPO4 are higher than that of LaPO4 . (3) The brightness of Gd0.7 Tb0.3 PO4 is higher than that of commercial Zn2 SiO4 :0.4Mn phosphor. Therefore, Gd0.7 Tb0.3 PO4 can be used as a green-emitting phosphor for PDPs. Acknowledgements This work was supported by Program for New Century Excellent Talents in University of China (NCET, 04-0978), the Key Science Research Project of Ministry of Education of China (105170) and Specialized Research Fund for the Doctoral Program of Higher Education of China (SRFDP, 20040730019). References [1] H.-C. Lu, H.-K. Chen, T.-Y. Tseng, W.-L. Kuo, M.S. Alam, Y.C. Kang, J. Electron Spectrosc. Relat. Phenom. 144–147 (2005) 983. [2] K.Y. Jung, D.Y. Lee, Y.C. Kang, J. Lumin. 115 (2005) 91. [3] C.-H. Kim, E. Kwon, C.-H. Park, Y.-J. Hwang, H.-S. Bae, B.-Y. Yu, C.-H. Pyun, G.-Y. Hong, J. Alloy. Compd. 311 (2000) 33. [4] Y.C. Kang, H.D. Park, Appl. Phys. A77 (2003) 529. [5] C. Barthou, J. Benoit, P. Benalloul, J. Electrochem. Soc. 141 (2) (1994) 524. [6] S. Saito, K. Wada, R. Onaka, J. Phys. Soc. Jpn. 37 (3) (1974) 711. [7] R.P. Rao, D.J. Devine, J. Lumin. 87–89 (2000) 1260. [8] K.Y. Jung, K.K. Lee, Y.C. Kang, H.D. Park, J. Mater. Sci. Lett. 22 (2003) 1527. [9] R.P. Rao, J. Lumin. 113 (2005) 271.
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C. Wu et al. / Journal of Alloys and Compounds 436 (2007) 383–386
[10] X. Wu, H. You, H. Cui, X. Zeng, G. Hong, C.-H. Kim, C.-H. Pyun, B.-Y. Yu, C.-H. Park, Mater. Res. Bull. 37 (2002) 1531. [11] K.K. Lee, Y.C. Kang, K.Y. Jung, H.D. Park, Jpn. J. Appl. Phys. 41 (2002) 5590. [12] L.X. Yu, H.W. Song, Z.X. Liu, et al., Solid Stat. Commun. 134 (2005) 753. [13] D.L. Dexter, J. Chem. Phys. 22 (1954) 1063. [14] C.K. Jørgensen, Mol. Phys. 5 (1962) 271. [15] L. van Piterson, S. Soverna, et al., J. Electrochem. Soc. 147 (2000) 4688.
[16] Q. Su, Proceeding of the Second International conference on Rare Earth Development and Application, vol. 1, International Academic Publishers, Beijing, China, 1991, p. 765. [17] E. Nakazawa, F. Shiga, J. Lumin. 15 (1977) 255–259. [18] W. Di, X. Wang, B. Chen, H. Lai, X. Zhao, Opt. Mater. 27 (2005) 1386–1390. [19] A. Mayolet, W. Zhang, et al., J. Electrochem. Soc. 143 (1) (1996) 330. [20] H. You, X. Wu, X. Zeng, G. Hong, C.-H. Kim, C.-H. Pyun, C.H. Park, Mater. Sci. Eng. B86 (2001) 11.
Hydrothermal synthesis and luminescent properties of LnPO4:Tb (Ln = La, Gd) phosphors under VUV excitation Chunfang Wu a , Yuhua Wang a,∗ , Wei Jie b a
Department of Materials Science, School of Physical Science and Technology, Lanzhou University, Lanzhou, Gansu Province 730000, PR China b Key Lab for Magnetism and Magnetic Materials of MOE, School of Physical Science and Technology, Lanzhou University, Lanzhou, Gansu Province 730000, PR China Received 23 May 2006; received in revised form 13 July 2006; accepted 13 July 2006 Available online 12 September 2006
Abstract Tb doped LnPO4 (Ln = La, Gd) phosphors with monoclinic system were successfully prepared by mild hydrothermal reaction at 240 ◦ C. The 543 nm emission of Tb3+ in GdPO4 is higher than in LaPO4 . By comparison, it is found that the intensity of host absorption band of GdPO4 :Tb is higher than that of LaPO4 :Tb which is ascribed to the energy transfer efficiency between the PO4 3− molecule and Tb3+ is higher. The emission intensity of optimal composition of GdPO4 :0.3Tb is comparable with that of commercial Zn2 SiO4 :0.04Mn phosphor. These results suggest that GdPO4 :0.3Tb is a potential candidate for plasma display panels (PDPs) application. © 2006 Elsevier B.V. All rights reserved. PACS: 78.55. −m Keywords: VUV; Luminescence; Hydrothermal synthesis; Phosphate
1. Introduction Tricolor phosphors for plasma display panels (PDPs) are hot research points in recent years [1–3]. Been excited by vacuum ultraviolet ray (VUV) in PDPs, few phosphors applied in lamp are feasible to be applied in PDPs. Zn2 SiO4 :Mn is well known as a good green-emitting phosphor for PDP [4,5]. However, the decay time of Zn2 SiO4 :Mn is long. Using the extended Huckel method, Saito et al. [6] calculated energy of the tetrahedral PO4 3− molecule. According to their results, the lowest energies of the transitions of 2t2 → 2a, 3t2 are found to exist at 7–10 eV (177–124 nm). The PDPs excitation source (147 nm) is in the range. Therefore, it can be concluded that orthophosphate would be a potential phosphor host. Rao and Devine [7] have realized red, blue and green emissions in rare-earth orthophosphate by doping Eu3+ , Tm3+ and Tb3+ , respectively. Jung and Lee [8] tried to modify the luminance property of LaPO4 :Tb particles by adding some manganese as a co-activator. But under the excitation of VUV (147 nm), the doping Mn is not help-
∗
Corresponding author. Tel.: +86 931 8912079; fax: +86 931 8913554. E-mail address: [email protected] (Y. Wang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.07.056
ful to improve the photoluminescence intensity of LaPO4 :Tb particles. So, Tb doped orthophosphate can be a new candidate to replace Zn2 SiO4 :Mn. Usually, the rare-earth orthophosphate are prepared by solid state reaction [9], precipitation method [10], spray pyrolysis [11] and hydrothermal means [12]. Up to now, the excitation and emission properties in VUV range of Tb doped rare-earth phosphate prepared by hydrothermal method is scarcely reported. So in this work, we choose the hydrothermal method to synthesis Tb doped LaPO4 and GdPO4 phosphors. Meanwhile, the excitation and the emission properties of the resulted phosphors and the dependence of emission properties on the Tb3+ concentration in LaPO4 and GdPO4 are investigated. 2. Experimental procedure The starting materials are Gd2 O3 (99.99%), La2 O3 (99.99%), Tb4 O7 (99.99%) and (NH4 )2 HPO4 (A.R.). Stoichiometric amount of each oxides are dissolved in diluted nitric by heating. (NH4 )2 HPO4 is dissolved and added into the former clear solution. Then the mixture is transferred into a Teflon-lined stainless steel autoclave with a filling capacity of 40%. The hydrothermal reaction lasts 6 h at 240 ◦ C. The crystal structure of samples is characterized by X-ray powder diffraction operating at 40 kV/60 mA, using monochromatized Cu K␣ radiation. The excitation and emission spectra are measured by FLS920T spectrophotometer
384
C. Wu et al. / Journal of Alloys and Compounds 436 (2007) 383–386
Fig. 1. XRD patterns of LT15 (a) and GT15 (b).
Fig. 2. SEM photographs of LT15 (a) and GT15 (b). equipped with VM504 vacuum monochromator, and the VUV excitation spectra are corrected by the sodium salicylate at the same measurement conditions.
3. Results and discussion The XRD patterns of La0.85 Tb0.15 PO4 (LT15) and Gd0.85 Tb0.15 PO4 (GT15) are shown in Fig. 1. The LT15 and GT15 phosphors prepared by hydrothermal reaction at a relative low temperature in our work are monoclinc and they need not further high temperature calcinations as solid state reaction, precipitation method and spray pyrolysis do. It suggests that the hydrothermal reaction is an effective and cost-saving method to prepare phosphors. But the XRD pattern of GT15 is somewhat different from that of LT15. The peaks’ positions of the two samples are similar other than the intensity ratios of the strongest peak (which is indexed as (1 2 0) surface) to other peaks. It indicates the preferential growth along the surface which is vertical with (1 2 0) surface in GT15 occurred. But the preferential growth in LT15 is not as serious as in GT. From this, we conclude that the morphology of the two phosphors is different. To confirm it, the morphology of LT15 and GT15 were examined by SEM. The morphology of LT15 and GT15 is shown in Fig. 2. The shape of GT15 and LT15 particles indeed is different. The GT15 particles grow with preference in some crystal face and finally form a column-like shape with the maximum length about 2 m. But in LT15, the preferential growth is not so evident compared with GT15. There are much more little particles than the columnlike particle, and the particles’ size of LT15 is smaller than that of GT15.
The fluorescence intensity of 543nm emission of Tb3+ in LaPO4 :Tb (0.05 ≤ X ≤ 0.3) (LT) and GdPO4 :Tb (0.05 ≤ X ≤ 0.5) (GT), are displayed as a function of Tb3 concentration and represented in Fig. 3. The emission intensity of Tb3+ in the two phosphors increases proportionately with Tb concentration and then decreases, going through a maximum at 0.15 and 0.3 concentrations, respectively. The emission intensity decreases at high Tb concentration is referred to as concentration quenching [13] which involves the resonance transfer of electronic excitation energy from the initially absorbing atom to another identi-
Fig. 3. Effect of Tb3+ content on 543 nm emission intensity in LT and GT.
C. Wu et al. / Journal of Alloys and Compounds 436 (2007) 383–386
Fig. 4. Excitation spectra of GT15 and LT15.
cal atom, and finally to some as yet unspecified quenching site. As an example, the excitation of Gd0.85 Tb0.15 PO4 (GT15) and La0.85 Tb0.15 PO4 (LT15) is shown in Fig. 4. The bands in the region from 120 to 160 nm may be due to the overlap of two bands peaking at about 145 and 155 nm. The position of the charge transfer band (CTB) of Tb3+ can be calculated with an empirical formula given by Jørgensen [14]: ECT = [(X) − (M)] × 30, 000 cm−1 Here, (X) is the optical electronegativity of the anion and (M) is that of the central metal ion. Using (O) = 3.2 [15], (Tb) = 0.95 [16], the CTB of Tb3+ in oxide can be calculated as 148 nm. So, the former is attributed to the CTB of Tb3+ . The latter may be assigned to the absorption band of PO4 3− anion molecules (host absorption band). Similar host absorption bands are observed in other rare-earth orthophosphates which are shown in Table 1. As for GT15, the VUV excitation spectra are somewhat different from that of LT15. Firstly, a small peak at 274 nm corresponding to 8 S7/2 → 6 IJ (J = 11/2, 9/2 and 7/2) transitions is present, indicating the occurrence of the energy transfer process from Gd3+ to Tb3+ . Secondly, the intensity of host absorption band is higher than that of LT15 which is consistent with that of emission spectra. Similar phenomenon has also been observed in lanthanum metaborate host lattice [19] and BaLnB9 O16 :Tb (Ln = La, Gd) [20]. It cannot be explained expressly. But the result indicated that Gd3+ plays an important intermediate role in the energy transfer from PO4 3− to the activator, such as Tb3+ . Under 147 nm excitation, the luminescence of Gd0.7 Tb0.3 PO4 is compared with that of commercial Zn2 SiO4 :0.4Mn in Fig. 5. The brightness of Gd0.3 Tb0.3 PO4 is 121% of Table 1 The position of some orthophosphates host absorption Compound
Absorption band (nm)
LaPO4 :Tm (La, Gd)PO4 :Tb, Eu LaPO4 :Eu, GdPO4 :Eu, YPO4:Eu, LuPO4 :Eu YPO4 :Tb
170 [9] 120–160 [10] 159, 160, 152, 145 [17] 125–160 nm [18]
Fig. 5. Photoluminescence Gd0.7 Tb0.3 PO4 .
385
spectra
of
Zn2 SiO4 :Mn
and
prepared
Zn2 SiO4 :0.4Mn. The decay time (τ e ) of prepared particle is 3.8 ms which is shorter than Zn2 SiO4 :Mn. 4. Conclusion (1) Mild hydrothermal reaction is an effective method to prepare phosphate phosphors for PDPs. (2) By comparison, as for Tb doped rare earth orthophosphate phosphor for PDPs, GdPO4 is a better host matrix than LaPO4 because the quenching concentration and the green emission intensity of Tb in GdPO4 are higher than that of LaPO4 . (3) The brightness of Gd0.7 Tb0.3 PO4 is higher than that of commercial Zn2 SiO4 :0.4Mn phosphor. Therefore, Gd0.7 Tb0.3 PO4 can be used as a green-emitting phosphor for PDPs. Acknowledgements This work was supported by Program for New Century Excellent Talents in University of China (NCET, 04-0978), the Key Science Research Project of Ministry of Education of China (105170) and Specialized Research Fund for the Doctoral Program of Higher Education of China (SRFDP, 20040730019). References [1] H.-C. Lu, H.-K. Chen, T.-Y. Tseng, W.-L. Kuo, M.S. Alam, Y.C. Kang, J. Electron Spectrosc. Relat. Phenom. 144–147 (2005) 983. [2] K.Y. Jung, D.Y. Lee, Y.C. Kang, J. Lumin. 115 (2005) 91. [3] C.-H. Kim, E. Kwon, C.-H. Park, Y.-J. Hwang, H.-S. Bae, B.-Y. Yu, C.-H. Pyun, G.-Y. Hong, J. Alloy. Compd. 311 (2000) 33. [4] Y.C. Kang, H.D. Park, Appl. Phys. A77 (2003) 529. [5] C. Barthou, J. Benoit, P. Benalloul, J. Electrochem. Soc. 141 (2) (1994) 524. [6] S. Saito, K. Wada, R. Onaka, J. Phys. Soc. Jpn. 37 (3) (1974) 711. [7] R.P. Rao, D.J. Devine, J. Lumin. 87–89 (2000) 1260. [8] K.Y. Jung, K.K. Lee, Y.C. Kang, H.D. Park, J. Mater. Sci. Lett. 22 (2003) 1527. [9] R.P. Rao, J. Lumin. 113 (2005) 271.
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C. Wu et al. / Journal of Alloys and Compounds 436 (2007) 383–386
[10] X. Wu, H. You, H. Cui, X. Zeng, G. Hong, C.-H. Kim, C.-H. Pyun, B.-Y. Yu, C.-H. Park, Mater. Res. Bull. 37 (2002) 1531. [11] K.K. Lee, Y.C. Kang, K.Y. Jung, H.D. Park, Jpn. J. Appl. Phys. 41 (2002) 5590. [12] L.X. Yu, H.W. Song, Z.X. Liu, et al., Solid Stat. Commun. 134 (2005) 753. [13] D.L. Dexter, J. Chem. Phys. 22 (1954) 1063. [14] C.K. Jørgensen, Mol. Phys. 5 (1962) 271. [15] L. van Piterson, S. Soverna, et al., J. Electrochem. Soc. 147 (2000) 4688.
[16] Q. Su, Proceeding of the Second International conference on Rare Earth Development and Application, vol. 1, International Academic Publishers, Beijing, China, 1991, p. 765. [17] E. Nakazawa, F. Shiga, J. Lumin. 15 (1977) 255–259. [18] W. Di, X. Wang, B. Chen, H. Lai, X. Zhao, Opt. Mater. 27 (2005) 1386–1390. [19] A. Mayolet, W. Zhang, et al., J. Electrochem. Soc. 143 (1) (1996) 330. [20] H. You, X. Wu, X. Zeng, G. Hong, C.-H. Kim, C.-H. Pyun, C.H. Park, Mater. Sci. Eng. B86 (2001) 11.