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Red luminescence of Lu2O2S:Ce and Y2O2S:Ce at room temperature
Journal of Alloys and Compounds 436 (2007) 174–177
Red luminescence of Lu2O2S:Ce and Y2O2S:Ce at room temperature Jun-Jing Zhao, Chang-Xin Guo ∗ , Ru-Wang Guo, Jun-Tao Hu Department of Physics, University of Science and Technology of China, Hefei 230026, China Received 14 May 2006; received in revised form 29 June 2006; accepted 29 June 2006 Available online 22 August 2006
Abstract Lu2 O2 S:Ce (8.90 g/cm3 ) and Y2 O2 S:Ce (4.92 g/cm3 ) powder crystals have been prepared by oxalate co-precipitation first and then solid state reaction at 1150 ◦ C in reducing atmosphere. X-ray diffraction (XRD) patterns show that they are of hexagonal structures. Photoluminescence (PL) of them is red and can be observed at room temperature. The emission spectrum of Lu2 O2 S:Ce has one broad band peaking at 650 nm; Y2 O2 S:Ce has two peaking at 346 and 600 nm. Gaussian components of these peaks suggest that they originate from the 5d–4f transition of Ce3+ . Photoluminescence, photoluminescence excitation (PLE) and diffuse reflectance spectra of Lu2 O2 S:Ce and Y2 O2 S:Ce as well as their luminescent mechanism have been studied. © 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Mc; 71.20.Eh; 78.60−b Keywords: Rare earth luminescent material; Luminescence; Scintillator
1. Introduction Current applications in high energy physics strongly stimulate the search for promising high density, fast and radiation hard scintillation materials. For some applications, the readout of scintillation crystals with photomultiplier tubes (PMT’s) presents problems: e.g. in applications where a high voltage or strong magnetic fields is unwanted. The detection of scintillation light with silicon photodiodes is then an alternative. Commonly used silicon photodiodes exhibit an efficient sensitivity in the range from 500 to 1000 nm, which generates a need for long wavelength scintillators (LWS) [1]. All Eu- and Tb-doped compounds show long decay time constants (τ) in the range of 102 –104 s [2]. So the problem is to find materials with a short decay time among the long wavelength phosphors. The luminescence of Ce3+ , due to the 5d–4f electric dipole allowed transition, has relatively shorter decay time (20–40 ns) among trivalent lanthanide ions [3]; the density of Lu2 O2 S is 8.90 g/cm3 , higher than that of the recently widespread used scintillators, such as Bi4 Ge3 O12 (7.13 g/cm3 ) and PbWO4 ∗
Corresponding author. Tel.: +86 551 3601097; fax: +86 551 3601073. E-mail addresses: [email protected] (J.-J. Zhao), [email protected] (C.-X. Guo). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.06.103
(8.28 g/cm3 ). Thereby, the research on Lu2 O2 S:Ce may be of great significance in the exploration of promising scintillators. In dozens of host materials, the emission of Ce3+ is efficient, but in some rare earth oxide and oxysulfide hosts, it is completely quenched, such as Y2 O3 , La2 O3 [4], La2 O2 S [4,5], Gd2 O2 S [5] and Lu2 O3 [6]. On the other hand, information about the luminescence of Y2 O2 S:Ce and Lu2 O2 S:Ce has barely been found in one article, describing them as red broad bands, though the 5d–4f emission of Ce3+ in most hosts is in the blue range or shorter [7], and such red luminescence is too weak to be observed unless at low temperature below 170 K [5]. In the present work, we have developed the synthesis method of Lu2 O2 S:Ce and Y2 O2 S:Ce and detected their red luminescence at room temperature; we also studied their crystal structures, photoluminescence (PL), photoluminescence excitation (PLE) spectra, diffuse reflectance spectra as well as their luminescent mechanism. 2. Experiments Lu2 O2 S:Ce and Y2 O2 S:Ce powder phosphors were synthesized in the same method by oxalate co-precipitation [8] first and then solid state reaction in reducing atmosphere. The precursor (R1−x ,Cex )2 O3 (R = Lu or Y) was obtained by sintering the oxalate co-precipitate (R1−x ,Cex )2 (C2 O4 )3 (R = Lu or Y) at 900 ◦ C in reducing atmosphere for 40 min. After the mixture of precursor, S, Na2 CO3
J.-J. Zhao et al. / Journal of Alloys and Compounds 436 (2007) 174–177
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and Na3 PO4 at a weight ratio of 100:30:30:5 was fully milled together, it was filled into a small ceramic crucible and pressed tightly, with 2–3 mm sublimed sulfur on the surface. The small crucible (50 ml) was then put into a bigger one (100 ml) with a certain amount of active carbon in it to create a reducing atmosphere when sintering. The double-crucible was sintered in a muffle at 1150 ◦ C for 25 min. The reducing atmosphere was created to protect Ce3+ from being oxidated to Ce4+ . After washed with dilute nitric acid and dried at 100 ◦ C for 2 h, the samples (Y2 O2 S:Ce or Lu2 O2 S:Ce) turned out to be light-yellow powders and the body color darkens with the increase of doping concentration. The X-ray diffraction (XRD) was carried out by a Rigaku D/MAX-rA diffractometer with Cu K␣ (λ = 0.15418 nm) as the incident radiation. Photoluminescence spectra were performed on a Hitachi 850-type fluorescence spectrophotometer with a Xe lamp as the excitation light source. The diffuse reflectance spectra were recorded by a Schimadzu model UV-240 spectrophotometer. All of the measurements were performed at room temperature.
3. Results and discussion XRD patterns of the Lu2 O2 S:Ce (1.5 mol%) and Y2 O2 S:Ce (1 mol%) phosphors have been presented in Fig. 1a and c. Compared with the JCPDS cards, it is obvious that both samples ¯ are of hexagonal structures, with space group P 3m1. The lattice constant of them are a = 0.371 and 0.378 nm, c = 0.649 and 0.659 nm, respectively. Figs. 2 and 3 exhibit the PL and PLE spectra of Lu2 O2 S:Ce (1.5 mol%) and Y2 O2 S:Ce (1.0 mol%) as well as the Gaussian components of their emission peaks. From Fig. 2, we can see that the emission spectrum of Lu2 O2 S:Ce (1.5 mol%) appears to be one broad band with peak at about 650 nm, no matter the irradiation wavelength is 250 or 460 nm; while for Y2 O2 S:Ce (1.0 mol%), under the irradiation of 267 nm, there are two obvious emission bands with peaks at about 346 and 600 nm. In Fig. 3, curves a1 (peak at 650 nm) is the emission band of Lu2 O2 S:Ce (1.5 mol%), with excitation wavelength 460 nm; a2 (peak at 346 nm) and a3 (peak at 600 nm) are those of Y2 O2 S:Ce (1 mol%), with excitation wavelength 267 nm. The fitted peaks are at about 619 and 706 nm for a1 , 336 and 361 nm for a2 (see the inserted figure in Fig. 3), 577 and 652 nm for a3 . Trans-
Fig. 1. XRD patterns of hexagonal Lu2 O2 S:Ce (1.5 mol%) (a), Y2 O2 S:Ce (1.0 mol%) (c), synthesized by oxalate co-precipitation and solid state reaction two-step method and pure Lu2 O2 S (b) and Y2 O2 S (d) from JCPDS card.
forming into wavenumber, the differences of the fitted peaks are about 1991 cm−1 for b1 and c1 , 2061 cm−1 for b2 and c2 , and 1994 cm−1 for b3 and c3 , basically consistent with the energy difference of the split 4f state of Ce3+ ion (about 2000 cm−1 ), and therefore curves b1 –b3 and c1 –c3 correspond to the transition of 5d state to 4f 2 F5/2 and 4f 2 F7/2 of Ce3+ , respectively. Diffuse reflectance spectra taken from the undoped and Cedoped Lu2 O2 S and Y2 O2 S samples are given in Fig. 4. Comparing the difference between host and Ce-doped samples, we can see that a relative strong absorption peak at 460 nm appears only in Ce-doped materials. The schematic diagrams of energy levels of Lu2 O2 S:Ce and Y2 O2 S:Ce deduced from the PL, PLE and diffuse reflectance spectra are showed in Fig. 5. The relative energy levels of Ce3+ compared with energy gap are also denoted. Emissions from the lowest 5d level (D1 for Lu2 O2 S:Ce and D1 for Y2 O2 S:Ce) to 4f 2 F7/2 and 4f 2 F5/2 can be seen in both
Fig. 2. PL and PLE spectra of Lu2 O2 S:Ce (1.5 mol%) (left) and Y2 O2 S:Ce (1.0 mol%) (right) at room temperature.
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Fig. 3. Gaussian components of the emission spectrum of Lu2 O2 S:Ce (1.5 mol%) excited by 460 nm (a1 ) and that of Y2 O2 S:Ce (1.0 mol%) excited by 267 nm (a2 and a3 ).
Fig. 4. Diffuse reflectance spectra of Lu2 O2 S host, Lu2 O2 S:Ce (0.5 mol%), Y2 O2 S host and Y2 O2 S:Ce (1.0 mol%).
Fig. 5. Schematic diagrams of energy levels of Ce3+ ion in Lu2 O2 S:Ce (left) and Y2 O2 S:Ce (right).
J.-J. Zhao et al. / Journal of Alloys and Compounds 436 (2007) 174–177
Ce-doped samples, while from D2 (second detected 5d level) to 4f 2 F7/2 and 4f 2 F5/2 can only been seen in Y2 O2 S:Ce. Levels B (4.96 eV above the top of valence band) for Lu2 O2 S:Ce and B (4.64 eV) for Y2 O2 S:Ce originate from hosts. The lowest energy position of conduction band is above levels B or B , but the exact position is undetermined (denoted by dashed lines in Fig. 5). In our study region, the excitation of electrons from valence band to B or B levels is most efficient for Ce3+ . Levels A (2.7 eV) and A (2.7 eV) are associated with the doping of Ce3+ . The position of A is lower than D1 in Y2 O2 S:Ce, so the excitation of A will not induce the 5d–4f emission of Ce3+ ; while in Lu2 O2 S:Ce, A is higher than D1 , and therefore its excitation can effectively yield 5d–4f emission. The PL spectra at room temperature of Lu2 O2 S:Ce and Y2 O2 S:Ce varying with doping concentration (not depicted) were also performed. The PL intensity of them suggests that the optimum concentration is 0.5 mol% for both samples. 4. Conclusions In summary, high density Lu2 O2 S:Ce and Y2 O2 S:Ce have been prepared by oxalate co-precipitation and solid state reaction two-step method. XRD patterns show that they are of hexagonal structures. The red photoluminescence of these samples can be observed at room temperature. The energy level diagrams of these two materials have been determined by PL, PLE and diffuse reflectance spectra. The luminescent mechanism has been discussed. Under UV (250–270 nm) excitation, one broad emission band with peak at 650 nm can be seen for Lu2 O2 S:Ce and two peaks at 346 and 600 nm for Y2 O2 S:Ce. Gaussian com-
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ponents of these emission spectra suggest that these emissions originate from the transition of 5d state to 4f 2 F5/2 and 2 F7/2 of Ce3+ . Acknowledgments This work is supported by National High Technology Research and Development Program of China (863 Program, Grant No. 2002AA324060) and the National Natural Science Foundation of China (Grant No. 50332050). References [1] P. Schotanus, P. Dorenbos, V.D. Ryzhikov, Detection of CdS(Te) and ZnSe(Te) scintillation light with silicon photodiodes, IEEE Trans. Nucl. Sci. 39 (1992) 546. [2] P.A. Rodnyi, E.I. Gorohova, S.B. Mikhrin, A.N. Mishin, A.S. Potapov, Quest and investigation of long wavelength scintillators, Nucl. Instrum. Methods A 486 (2002) 244. [3] S.E. Derenzo, M.J. Weber, E. Bourret-Courchesne, M.K. Klintenberg, The quest for the ideal inorganic scintillator, Nucl. Instrum. Methods A 505 (2003) 111. [4] G. Blasse, W. Schipper, J.J. Hamelink, On the quenching of the luminescence of the trivalent cerium ion, Inorg. Chim. Acta 189 (1991) 77. [5] S. Yokono, T. Abe, T. Hoshina, Red luminescence of Ce3+ due to the large stokes shifts in Y2 O2 S and Lu2 O2 S, J. Lumin. 24–25 (1981) 309. [6] U. Happek, S.A. Basum, J. Choi, J.K. Krebs, M. Raukas, Electron transfer processes in rare earth doped insulators, J. Alloys Compd. 303–304 (2000) 198. [7] K. Narita, A. Taya, Tech. Digest, Phosphor Res. Soc. 147th Meeting, 1979. [8] J. Ma, T.S. Zhang, L.B. Kong, P. Hing, Y.J. Leng, S.H. Chan, Preparation and characterization of dense Ce0.85 Y0.15 O2−␦ ceramics, J. Eur. Ceram. Soc. 24 (2004) 2641.
Red luminescence of Lu2O2S:Ce and Y2O2S:Ce at room temperature Jun-Jing Zhao, Chang-Xin Guo ∗ , Ru-Wang Guo, Jun-Tao Hu Department of Physics, University of Science and Technology of China, Hefei 230026, China Received 14 May 2006; received in revised form 29 June 2006; accepted 29 June 2006 Available online 22 August 2006
Abstract Lu2 O2 S:Ce (8.90 g/cm3 ) and Y2 O2 S:Ce (4.92 g/cm3 ) powder crystals have been prepared by oxalate co-precipitation first and then solid state reaction at 1150 ◦ C in reducing atmosphere. X-ray diffraction (XRD) patterns show that they are of hexagonal structures. Photoluminescence (PL) of them is red and can be observed at room temperature. The emission spectrum of Lu2 O2 S:Ce has one broad band peaking at 650 nm; Y2 O2 S:Ce has two peaking at 346 and 600 nm. Gaussian components of these peaks suggest that they originate from the 5d–4f transition of Ce3+ . Photoluminescence, photoluminescence excitation (PLE) and diffuse reflectance spectra of Lu2 O2 S:Ce and Y2 O2 S:Ce as well as their luminescent mechanism have been studied. © 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Mc; 71.20.Eh; 78.60−b Keywords: Rare earth luminescent material; Luminescence; Scintillator
1. Introduction Current applications in high energy physics strongly stimulate the search for promising high density, fast and radiation hard scintillation materials. For some applications, the readout of scintillation crystals with photomultiplier tubes (PMT’s) presents problems: e.g. in applications where a high voltage or strong magnetic fields is unwanted. The detection of scintillation light with silicon photodiodes is then an alternative. Commonly used silicon photodiodes exhibit an efficient sensitivity in the range from 500 to 1000 nm, which generates a need for long wavelength scintillators (LWS) [1]. All Eu- and Tb-doped compounds show long decay time constants (τ) in the range of 102 –104 s [2]. So the problem is to find materials with a short decay time among the long wavelength phosphors. The luminescence of Ce3+ , due to the 5d–4f electric dipole allowed transition, has relatively shorter decay time (20–40 ns) among trivalent lanthanide ions [3]; the density of Lu2 O2 S is 8.90 g/cm3 , higher than that of the recently widespread used scintillators, such as Bi4 Ge3 O12 (7.13 g/cm3 ) and PbWO4 ∗
Corresponding author. Tel.: +86 551 3601097; fax: +86 551 3601073. E-mail addresses: [email protected] (J.-J. Zhao), [email protected] (C.-X. Guo). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.06.103
(8.28 g/cm3 ). Thereby, the research on Lu2 O2 S:Ce may be of great significance in the exploration of promising scintillators. In dozens of host materials, the emission of Ce3+ is efficient, but in some rare earth oxide and oxysulfide hosts, it is completely quenched, such as Y2 O3 , La2 O3 [4], La2 O2 S [4,5], Gd2 O2 S [5] and Lu2 O3 [6]. On the other hand, information about the luminescence of Y2 O2 S:Ce and Lu2 O2 S:Ce has barely been found in one article, describing them as red broad bands, though the 5d–4f emission of Ce3+ in most hosts is in the blue range or shorter [7], and such red luminescence is too weak to be observed unless at low temperature below 170 K [5]. In the present work, we have developed the synthesis method of Lu2 O2 S:Ce and Y2 O2 S:Ce and detected their red luminescence at room temperature; we also studied their crystal structures, photoluminescence (PL), photoluminescence excitation (PLE) spectra, diffuse reflectance spectra as well as their luminescent mechanism. 2. Experiments Lu2 O2 S:Ce and Y2 O2 S:Ce powder phosphors were synthesized in the same method by oxalate co-precipitation [8] first and then solid state reaction in reducing atmosphere. The precursor (R1−x ,Cex )2 O3 (R = Lu or Y) was obtained by sintering the oxalate co-precipitate (R1−x ,Cex )2 (C2 O4 )3 (R = Lu or Y) at 900 ◦ C in reducing atmosphere for 40 min. After the mixture of precursor, S, Na2 CO3
J.-J. Zhao et al. / Journal of Alloys and Compounds 436 (2007) 174–177
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and Na3 PO4 at a weight ratio of 100:30:30:5 was fully milled together, it was filled into a small ceramic crucible and pressed tightly, with 2–3 mm sublimed sulfur on the surface. The small crucible (50 ml) was then put into a bigger one (100 ml) with a certain amount of active carbon in it to create a reducing atmosphere when sintering. The double-crucible was sintered in a muffle at 1150 ◦ C for 25 min. The reducing atmosphere was created to protect Ce3+ from being oxidated to Ce4+ . After washed with dilute nitric acid and dried at 100 ◦ C for 2 h, the samples (Y2 O2 S:Ce or Lu2 O2 S:Ce) turned out to be light-yellow powders and the body color darkens with the increase of doping concentration. The X-ray diffraction (XRD) was carried out by a Rigaku D/MAX-rA diffractometer with Cu K␣ (λ = 0.15418 nm) as the incident radiation. Photoluminescence spectra were performed on a Hitachi 850-type fluorescence spectrophotometer with a Xe lamp as the excitation light source. The diffuse reflectance spectra were recorded by a Schimadzu model UV-240 spectrophotometer. All of the measurements were performed at room temperature.
3. Results and discussion XRD patterns of the Lu2 O2 S:Ce (1.5 mol%) and Y2 O2 S:Ce (1 mol%) phosphors have been presented in Fig. 1a and c. Compared with the JCPDS cards, it is obvious that both samples ¯ are of hexagonal structures, with space group P 3m1. The lattice constant of them are a = 0.371 and 0.378 nm, c = 0.649 and 0.659 nm, respectively. Figs. 2 and 3 exhibit the PL and PLE spectra of Lu2 O2 S:Ce (1.5 mol%) and Y2 O2 S:Ce (1.0 mol%) as well as the Gaussian components of their emission peaks. From Fig. 2, we can see that the emission spectrum of Lu2 O2 S:Ce (1.5 mol%) appears to be one broad band with peak at about 650 nm, no matter the irradiation wavelength is 250 or 460 nm; while for Y2 O2 S:Ce (1.0 mol%), under the irradiation of 267 nm, there are two obvious emission bands with peaks at about 346 and 600 nm. In Fig. 3, curves a1 (peak at 650 nm) is the emission band of Lu2 O2 S:Ce (1.5 mol%), with excitation wavelength 460 nm; a2 (peak at 346 nm) and a3 (peak at 600 nm) are those of Y2 O2 S:Ce (1 mol%), with excitation wavelength 267 nm. The fitted peaks are at about 619 and 706 nm for a1 , 336 and 361 nm for a2 (see the inserted figure in Fig. 3), 577 and 652 nm for a3 . Trans-
Fig. 1. XRD patterns of hexagonal Lu2 O2 S:Ce (1.5 mol%) (a), Y2 O2 S:Ce (1.0 mol%) (c), synthesized by oxalate co-precipitation and solid state reaction two-step method and pure Lu2 O2 S (b) and Y2 O2 S (d) from JCPDS card.
forming into wavenumber, the differences of the fitted peaks are about 1991 cm−1 for b1 and c1 , 2061 cm−1 for b2 and c2 , and 1994 cm−1 for b3 and c3 , basically consistent with the energy difference of the split 4f state of Ce3+ ion (about 2000 cm−1 ), and therefore curves b1 –b3 and c1 –c3 correspond to the transition of 5d state to 4f 2 F5/2 and 4f 2 F7/2 of Ce3+ , respectively. Diffuse reflectance spectra taken from the undoped and Cedoped Lu2 O2 S and Y2 O2 S samples are given in Fig. 4. Comparing the difference between host and Ce-doped samples, we can see that a relative strong absorption peak at 460 nm appears only in Ce-doped materials. The schematic diagrams of energy levels of Lu2 O2 S:Ce and Y2 O2 S:Ce deduced from the PL, PLE and diffuse reflectance spectra are showed in Fig. 5. The relative energy levels of Ce3+ compared with energy gap are also denoted. Emissions from the lowest 5d level (D1 for Lu2 O2 S:Ce and D1 for Y2 O2 S:Ce) to 4f 2 F7/2 and 4f 2 F5/2 can be seen in both
Fig. 2. PL and PLE spectra of Lu2 O2 S:Ce (1.5 mol%) (left) and Y2 O2 S:Ce (1.0 mol%) (right) at room temperature.
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Fig. 3. Gaussian components of the emission spectrum of Lu2 O2 S:Ce (1.5 mol%) excited by 460 nm (a1 ) and that of Y2 O2 S:Ce (1.0 mol%) excited by 267 nm (a2 and a3 ).
Fig. 4. Diffuse reflectance spectra of Lu2 O2 S host, Lu2 O2 S:Ce (0.5 mol%), Y2 O2 S host and Y2 O2 S:Ce (1.0 mol%).
Fig. 5. Schematic diagrams of energy levels of Ce3+ ion in Lu2 O2 S:Ce (left) and Y2 O2 S:Ce (right).
J.-J. Zhao et al. / Journal of Alloys and Compounds 436 (2007) 174–177
Ce-doped samples, while from D2 (second detected 5d level) to 4f 2 F7/2 and 4f 2 F5/2 can only been seen in Y2 O2 S:Ce. Levels B (4.96 eV above the top of valence band) for Lu2 O2 S:Ce and B (4.64 eV) for Y2 O2 S:Ce originate from hosts. The lowest energy position of conduction band is above levels B or B , but the exact position is undetermined (denoted by dashed lines in Fig. 5). In our study region, the excitation of electrons from valence band to B or B levels is most efficient for Ce3+ . Levels A (2.7 eV) and A (2.7 eV) are associated with the doping of Ce3+ . The position of A is lower than D1 in Y2 O2 S:Ce, so the excitation of A will not induce the 5d–4f emission of Ce3+ ; while in Lu2 O2 S:Ce, A is higher than D1 , and therefore its excitation can effectively yield 5d–4f emission. The PL spectra at room temperature of Lu2 O2 S:Ce and Y2 O2 S:Ce varying with doping concentration (not depicted) were also performed. The PL intensity of them suggests that the optimum concentration is 0.5 mol% for both samples. 4. Conclusions In summary, high density Lu2 O2 S:Ce and Y2 O2 S:Ce have been prepared by oxalate co-precipitation and solid state reaction two-step method. XRD patterns show that they are of hexagonal structures. The red photoluminescence of these samples can be observed at room temperature. The energy level diagrams of these two materials have been determined by PL, PLE and diffuse reflectance spectra. The luminescent mechanism has been discussed. Under UV (250–270 nm) excitation, one broad emission band with peak at 650 nm can be seen for Lu2 O2 S:Ce and two peaks at 346 and 600 nm for Y2 O2 S:Ce. Gaussian com-
177
ponents of these emission spectra suggest that these emissions originate from the transition of 5d state to 4f 2 F5/2 and 2 F7/2 of Ce3+ . Acknowledgments This work is supported by National High Technology Research and Development Program of China (863 Program, Grant No. 2002AA324060) and the National Natural Science Foundation of China (Grant No. 50332050). References [1] P. Schotanus, P. Dorenbos, V.D. Ryzhikov, Detection of CdS(Te) and ZnSe(Te) scintillation light with silicon photodiodes, IEEE Trans. Nucl. Sci. 39 (1992) 546. [2] P.A. Rodnyi, E.I. Gorohova, S.B. Mikhrin, A.N. Mishin, A.S. Potapov, Quest and investigation of long wavelength scintillators, Nucl. Instrum. Methods A 486 (2002) 244. [3] S.E. Derenzo, M.J. Weber, E. Bourret-Courchesne, M.K. Klintenberg, The quest for the ideal inorganic scintillator, Nucl. Instrum. Methods A 505 (2003) 111. [4] G. Blasse, W. Schipper, J.J. Hamelink, On the quenching of the luminescence of the trivalent cerium ion, Inorg. Chim. Acta 189 (1991) 77. [5] S. Yokono, T. Abe, T. Hoshina, Red luminescence of Ce3+ due to the large stokes shifts in Y2 O2 S and Lu2 O2 S, J. Lumin. 24–25 (1981) 309. [6] U. Happek, S.A. Basum, J. Choi, J.K. Krebs, M. Raukas, Electron transfer processes in rare earth doped insulators, J. Alloys Compd. 303–304 (2000) 198. [7] K. Narita, A. Taya, Tech. Digest, Phosphor Res. Soc. 147th Meeting, 1979. [8] J. Ma, T.S. Zhang, L.B. Kong, P. Hing, Y.J. Leng, S.H. Chan, Preparation and characterization of dense Ce0.85 Y0.15 O2−␦ ceramics, J. Eur. Ceram. Soc. 24 (2004) 2641.