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Reduction and luminescence of europium ions in glass ceramics containing SrF2 nanocrystals
LETTER TO THE EDITOR
Journal of Non-Crystalline Solids 354 (2008) 4691–4694
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Reduction and luminescence of europium ions in glass ceramics containing SrF2 nanocrystals Qun Luo a, Xvsheng Qiao a, Xianping Fan a,*, Shiqi Liu a, Hui Yang a, Xianghua Zhang b a b
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China UMR-CNRS 6512 ‘‘Verres and Ceramiques”, Institut de Chimie de Rennes, Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France
a r t i c l e
i n f o
Article history: Received 17 March 2008 Received in revised form 17 July 2008 Available online 26 August 2008 PACS: 78.55.m 64.70.kj 42.70.Ce
a b s t r a c t Reduction of Eu3+ ? Eu2+ and luminescence of europium (Eu) ions in glass ceramics containing SrF2 nanocrystals have been investigated. The formation of SrF2 nanocrystals in glass ceramics was confirmed by Xray diffraction (XRD) and transmission electron microscopy (TEM). Blue luminescence of the Eu2+ ions was observed in the Eu doped glass ceramics which were prepared by the heat treatment of the glass in air atmosphere. The double-exponential decay curves of 5D0 state of Eu3+ in the Eu doped glass ceramics indicated that there were two different surroundings of the Eu ions in the glass ceramics. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Glass ceramics Luminescence Rare-earths in glasses
White light emitting diodes (W-LED) have attracted great attention as potential lighting to replace the conventional incandescent and fluorescent lamps [1]. W-LEDs can be fabricated by combining two or three different types of phosphors those can be excited by blue or ultraviolet LED chips [2]. The 4f–4f transition of Eu3+ ions gives sharp emission peaks of orange–red light and the 4f–5d transition of Eu2+ ions gives wide fluorescence of blue light which are greatly influenced by the composition and structure of materials [3–5]. By changing the ratio of Eu3+ to Eu2+ ions, it may be possible to regulate the CIE (International Commission on Illumination) and CRI (color rendering index) to reach warm white light [6]. The manipulation on the valence states of europium ions in solid state materials has extensively been investigated [7–9]. The reduction atmosphere, such as H2 or CO, is generally needed to reduce Eu3+ to Eu2+ in the synthesis of Eu2+ doped luminescent materials. The coexist of Eu3+ and Eu2+ ions have been found in some phosphors, glasses or films during melting or prepared under reduction atmosphere [10–15]. However, the preparation of Eu2+ doped materials in air atmosphere is more convenient than in reduction atmosphere. Some research works on reduction of Eu3+ to Eu2+ in air atmosphere have been reported, such as in Eu doped Al2O3–SiO2 * Corresponding author. Tel.: +86 571 87952334; fax: +86 571 87951234. E-mail address: [email protected] (X. Fan). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.07.019
glass [7] and Eu doped MO–B2O3 glasses (M = Ba, Sr, and Ca) [16]. In this letter, reduction of Eu3+ to Eu2+ in the glass ceramics containing SrF2 nanocrystals and their luminescence properties are described. The glass with composition of 50SiO2–22Al2O3–20SrF2–6NaF– 2EuF3 was prepared by melting the appropriate batch materials (SiO2, Al2O3, NaF, SrF2, EuF3) in covered corundum crucibles in normal atmosphere. After melted for 45 min at 1430 °C, the melt was poured on a brass mold and then pressed by another brass plate. The transparent glass samples were obtained. Then glass ceramics samples were obtained by heat treatment of precursor glasses at 590 °C (named as GC590) and 620 °C (named as GC620) for 1 h in air atmosphere, respectively. The glass ceramics were found to be transparent in visible light. X-ray diffraction measurements were performed in a XD-98 diffractometer with Cu Ka radiation at 4°/min scanning rate. Transmission electron microscope images were taken on a JEM-1230 microscope. Excitation and emission spectra were measured by using a Hitachi F-4500 fluorescence spectrophotometer. Luminescence decay curves were measured with an Edinburgh PLS920P spectrometer, using excitation of a microsecond flashlamp. All measurements were performed at room temperature. Fig. 1 shows the X-ray diffraction patterns of the Eu doped glass and glass ceramics. The glass as made was completely amorphous
LETTER TO THE EDITOR
Q. Luo et al. / Journal of Non-Crystalline Solids 354 (2008) 4691–4694
(111)
4692
7
F2 3+ 5
(311)
(200)
(220)
3+ 5
Eu : D0
7
Eu : D0
cps
3+ 5
Eu : D0
Intensity
GC620 GC590
3+ 5
Eu : D0 2+
6
Eu :4f 5d
4f
F3
7
F1 3+ 5
Eu : D0
7
F0
7
F4
Glass
7
Glass
GC590 10
20
30
40
50
60
70
80
GC620
2 theta (degree) Fig. 1. XRD patterns of the glass and glass ceramics.
400
450
500
550
600
650
700
750
Wavelength (nm) with no diffraction peaks. The XRD patterns of glass ceramics exhibited prominent peaks well accordant with JCPDS standard card (06-0262) of the SrF2 crystal with no second phase. From the obtained XRD patterns, the size of SrF2 nanocrystals could be calculated to be about 9 nm for GC590 and 22 nm for GC620 by using the Scherrer formula [17]. Fig. 2 shows the TEM and HRTEM (high resolution transmission electron microscope) images of the glass ceramic GC620. It can be found the dark spherical nanocrystals lying on the gray background corresponding to the glassy phase. The crystal size in the glass ceramic GC620 was about 20 nm, which was consistent with the Scherrer-calculated diameter. HRTEM image shows well-defined lattice images. The interplanar distance d value measured through separating two planes from the HRTEM image was 0.329 nm, which could be ascribed to the (1 1 1) plane of the SrF2 crystals (d(111) = 0.335 nm). Fig. 3 shows the luminescence spectra (excited at 394 nm) of the Eu doped glass and glass ceramics. The luminescence spectra of the glass consisted of some sharp emission peaks corresponding to the transitions of the excited 5D0 energy level to 7F0 (579 nm), 7 F1 (594 nm), 7F2 (614 nm), 7F3 (655 nm), and 7F4 (704 nm) levels of the Eu3+ ions. The broad emission band corresponding to Eu2+ ions could hardly be observed. However, the emission spectra of the glass ceramics consisted of not only some sharp emission peaks of the Eu3+ but also a broad band from 350 to 550 nm which could be ascribed to the 4f65d?4f7 transition of the Eu2+ ions. This could
Fig. 3. Emission (excited at 394 nm) spectra of the Eu doped glass and glass ceramics.
be confirmed with the corresponding excitation spectra of GC620 which was shown in Fig. 4. The excitation spectrum monitored at 614 nm consisted of sharp line rang from 300 to 500 nm which resulted from transitions between the 4f energy levels of Eu3+. When monitored wavelength was 440 nm, however, a broad excitation band corresponding to the transition from 8S7/2 to 4f65d level of Eu2+ could be observed. It was obvious that the some Eu3+ ions have been reduced to Eu2+ ions in the glass ceramics. To explain the reduction phenomenon of Eu3+ ? Eu2+, the substitution defect model or charge compensation modern has been proposed [10,13]. When trivalent Eu3+ ions were doped into those hosts containing M2+ (M2+ = Ca2+, Sr2+, Ba2+ and Zn2+), Eu3+ would replace the M2+ ions. In order to keep the charge balance, two Eu3+ ions would substitute for three M2+ ions to form EuM defect and one cation vacancy defect with two negative charges V 00M would be created. The vacancy acted as a donor of electron while the EuM defect became an acceptor of electron. Consequently, by thermal stimulation, the electrons in the vacancy defect V 00M were transferred to EuM and Eu3+ ions were reduced to Eu2+ ions. In the present case, such process could be described as following:
Fig. 2. TEM (a) and HRTEM (b) images of the glass ceramic GC620.
LETTER TO THE EDITOR
4693
Q. Luo et al. / Journal of Non-Crystalline Solids 354 (2008) 4691–4694
2000
1 2+ 8
6
Eu : S7/2
λem=440 nm
4f 5d
Normalized Intensity
λem=614 nm
Intensity (a.u.)
1500
5
3+ 7
Eu : F0
1000
L6
3+ 7
Eu : F0
D3
3+ 7
G3
Eu : F0
5
D2
GC590
GC620
0.1
5
3+ 7
Eu : F0
0 250
5
3+ 7
Eu : F0
500
5
Glass
D4
3+ 7
Eu : F0
5
D1 0
300
350
400
450
500
550
Fig. 4. Excitation (monitored at 440 nm and 614 nm) spectra of the Eu doped glass ceramic GC620.
3SrSr þ 2Eu ¼ 3Sr V 00Sr ¼ V Sr þ 2e0 ;
þ
2EuSr
þ
V 00Sr ;
ð1Þ ð2Þ
2EuSr þ 2e0 ¼ 2EuSr :
ð3Þ
By adding Eqs (1)–(3)
3SrSr þ 2Eu3þ ¼ 3Sr2þ þ 2EuSr þ V Sr :
ð4Þ
Furthermore, the concept of ‘optical basicity’ was also used to explain the reduction phenomenon [16,18–20]. According to this mechanism, divalent Eu2+ ions were preferred in glasses with low optical basicity. In this work, the luminescence of Eu2+ ions could not be observed in the 50SiO2–22Al2O3–20SrF2–6NaF– 2EuF3 glass, which indicated that the optical basicity of this glass was higher than the critical value for divalent Eu2+ ions to exist. In the glass ceramics, however, Eu3+ ions replaced the Sr2+ ions in the SrF2 nanocrystals and EuSr defects were formed. As a result, Eu3+ ions were reduced to Eu2+ ions according to charge compensation mechanism. The luminescence decay curves of the 5D0 state of Eu3+ were obtained by monitoring the 5D0 ? 7F2 emission line (614 nm) for the Eu doped glass and glass ceramics. Fig. 5 shows the observed luminescence decay curves and fitted curves of 5D0 state of Eu3+ in the glass, GC590 and GC620. The luminescence decay curve of 5D0 state of Eu3+ in the glass was best fitted to a single-exponential fitting function: IðtÞ ¼ 0:02 þ 0:98 expðt=2:74Þ, which indicated 2.74 ms lifetime. However, the luminescence decay curves of 5D0 state of Eu3+ in the glass ceramics were fitted to the double-exponential fitting functions with a long decay and a short decay. The fitted decay curves for GC590 and GC620 could be described as:
IðtÞ ¼ 0:62 expðt=2:56Þ þ 0:39 expðt=0:17Þ;
ð5Þ
IðtÞ ¼ 0:52 expðt=2:55Þ þ 0:47 expðt=0:11Þ:
ð6Þ 3+
Obviously, the luminescence intensity I(t) of the Eu in Eu doped glass ceramics could be described by the sum of two exponential decay components from
t t þ A2 exp ; IðtÞ ¼ A1 exp
s1
s2
3
4
5
6
Fig. 5. Decay curves (h, N and s) and fitted curves (solid line) of 5D0 state of Eu3+ in the glass and glass ceramics.
hsi ¼ 2þ
2
Time (ms)
Wavelength (nm)
3þ
1
600
ð7Þ
where s1 and s2 were long- and short-decay components, respectively. Parameters A1 and A2 were fitting constants. According to Eq. (7), the average lifetime hsi was given by:
A1 s21 þ A2 s22 ; A1 s 1 þ A2 s 2
ð8Þ
where weight factors A1 and A2 were introduced. According to Eq. (8), the average lifetime of 5D0 state of Eu3+ in the glass ceramics could be calculated to be about 2.46 ms and was slightly shorter than that of in the glass. It is well known that the luminescence of rare earth ions depended more or less efficiently on the nature of the host. The luminescence decay rate was the sum of the radiative decay and multiphonon relaxation (non-radiative relaxation) rates [21]. The non-radiative relaxation between various J states might occur by interaction of the electronic levels of rare earth ions with suitable vibrational modes of the environment. Therefore, the double-exponential decay curves indicated that there were two different surroundings of the europium ions in the glass ceramics: some in the glass and the others in the SrF2 nanocrystals. In conclusion, Eu doped glass ceramics containing SrF2 nanocrystals have been prepared. Blue luminescence of the Eu2+ ions was observed in such glass ceramics, which indicated the reduction of Eu3+ ? Eu2+. The luminescence decay curves of 5D0 state of Eu3+ in the glass ceramics showed a double-exponential function due to different surroundings of the Eu ions in the glass ceramics. Acknowledgements The authors gratefully acknowledge support for this research from the Science and Technology Department of Zhejiang Province (2006C14010), the Research Fund of the Doctoral Program of Higher Education of China (20070335012), the Chinese-French Cooperation Programs (MX07-01) and Program for Changjiang Scholars and Innovative Research Team in University. References [1] X.L. Liang, Y.X. Yang, C.F. Zhu, S.L. Yuan, G.R. Chen, A. Pring, F. Xia, Appl. Phys. Lett. 91 (2007) 091104. [2] J.S. Kim, K.T. Lim, Y. Jeong, P. Jeon, J. Choi, H. Park, Solid State Comm. 135 (2005) 21. [3] Z . Pei, Q. Su, J. Zhang, J. Alloys Compd. 198 (1993) 51. [4] K. Machida, D. Ueda, S. Inoue, G. Adachi, Chem. Lett. 18 (1999) 785. [5] K. Machida, D. Ueda, S. Inoue, G. Adachi, Electrochem. Solid-State Lett. 2 (1999) 597. [6] N.E. Jouhari, C. Prent, G.L. Flem, J. Solid Chem. 123 (1996) 398. [7] M. Nogami, T. Kawaguchi, A. Yasumori, Opt. Commun. 193 (2001) 237. [8] M.Y. Peng, Z.W. Pei, G.Y. Hong, Q. Su, J. Mater. Chem. 13 (2003) 1202. [9] H. You, M. Nogami, J. Phys. Chem. B 109 (2005) 13980. [10] Z. Lian, J. Wang, Y.H. Lv, S.B. Wang, Q. Su, J. Alloys Compd. 430 (2007) 257.
LETTER TO THE EDITOR
4694 [11] [12] [13] [14] [15] [16]
Q. Luo et al. / Journal of Non-Crystalline Solids 354 (2008) 4691–4694 J.H. Hao, J. Gao, Appl. Phys. Lett. 85 (2004) 3720. J.H. Hao, J. Gao, M. Cocivera, Appl. Phys. Lett. 82 (2003) 2778. M.Y. Peng, Z.W. Pei, G.Y. Hong, Q. Su, Chem. Phys. Lett. 371 (2003) 1. H.P. Xia, J.L. Zhang, J.H. Wang, Q.H. Nie, H.W. Song, Mater. Lett. 53 (2002) 273. Z.W. Pei, Q.H. Zeng, Q. Su, J. Phys. Chem. Solids 61 (2000) 9. C. Wang, M.Y. Peng, N. Jiang, X.W. Jiang, C.J. Zhao, J.R. Qiu, Mater. Lett. 61 (2007) 3608.
[17] [18] [19] [20] [21]
X.S. Qiao, X.P. Fan, J. Wang, M.Q. Wang, J. Appl. Phys. 99 (2006) 074302. J.A. Duffy, J. Non-Cryst. Solids 297 (2002) 275. J.A. Duffy, J. Non-Cryst. Solids 196 (1996) 45. J.A. Duffy, M.D. Ingram, J. Non-Cryst. Solids 21 (1976) 373. H.W. Moos, J. Lumin. 1 (2) (1970) 106.
Journal of Non-Crystalline Solids 354 (2008) 4691–4694
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Reduction and luminescence of europium ions in glass ceramics containing SrF2 nanocrystals Qun Luo a, Xvsheng Qiao a, Xianping Fan a,*, Shiqi Liu a, Hui Yang a, Xianghua Zhang b a b
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China UMR-CNRS 6512 ‘‘Verres and Ceramiques”, Institut de Chimie de Rennes, Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France
a r t i c l e
i n f o
Article history: Received 17 March 2008 Received in revised form 17 July 2008 Available online 26 August 2008 PACS: 78.55.m 64.70.kj 42.70.Ce
a b s t r a c t Reduction of Eu3+ ? Eu2+ and luminescence of europium (Eu) ions in glass ceramics containing SrF2 nanocrystals have been investigated. The formation of SrF2 nanocrystals in glass ceramics was confirmed by Xray diffraction (XRD) and transmission electron microscopy (TEM). Blue luminescence of the Eu2+ ions was observed in the Eu doped glass ceramics which were prepared by the heat treatment of the glass in air atmosphere. The double-exponential decay curves of 5D0 state of Eu3+ in the Eu doped glass ceramics indicated that there were two different surroundings of the Eu ions in the glass ceramics. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Glass ceramics Luminescence Rare-earths in glasses
White light emitting diodes (W-LED) have attracted great attention as potential lighting to replace the conventional incandescent and fluorescent lamps [1]. W-LEDs can be fabricated by combining two or three different types of phosphors those can be excited by blue or ultraviolet LED chips [2]. The 4f–4f transition of Eu3+ ions gives sharp emission peaks of orange–red light and the 4f–5d transition of Eu2+ ions gives wide fluorescence of blue light which are greatly influenced by the composition and structure of materials [3–5]. By changing the ratio of Eu3+ to Eu2+ ions, it may be possible to regulate the CIE (International Commission on Illumination) and CRI (color rendering index) to reach warm white light [6]. The manipulation on the valence states of europium ions in solid state materials has extensively been investigated [7–9]. The reduction atmosphere, such as H2 or CO, is generally needed to reduce Eu3+ to Eu2+ in the synthesis of Eu2+ doped luminescent materials. The coexist of Eu3+ and Eu2+ ions have been found in some phosphors, glasses or films during melting or prepared under reduction atmosphere [10–15]. However, the preparation of Eu2+ doped materials in air atmosphere is more convenient than in reduction atmosphere. Some research works on reduction of Eu3+ to Eu2+ in air atmosphere have been reported, such as in Eu doped Al2O3–SiO2 * Corresponding author. Tel.: +86 571 87952334; fax: +86 571 87951234. E-mail address: [email protected] (X. Fan). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.07.019
glass [7] and Eu doped MO–B2O3 glasses (M = Ba, Sr, and Ca) [16]. In this letter, reduction of Eu3+ to Eu2+ in the glass ceramics containing SrF2 nanocrystals and their luminescence properties are described. The glass with composition of 50SiO2–22Al2O3–20SrF2–6NaF– 2EuF3 was prepared by melting the appropriate batch materials (SiO2, Al2O3, NaF, SrF2, EuF3) in covered corundum crucibles in normal atmosphere. After melted for 45 min at 1430 °C, the melt was poured on a brass mold and then pressed by another brass plate. The transparent glass samples were obtained. Then glass ceramics samples were obtained by heat treatment of precursor glasses at 590 °C (named as GC590) and 620 °C (named as GC620) for 1 h in air atmosphere, respectively. The glass ceramics were found to be transparent in visible light. X-ray diffraction measurements were performed in a XD-98 diffractometer with Cu Ka radiation at 4°/min scanning rate. Transmission electron microscope images were taken on a JEM-1230 microscope. Excitation and emission spectra were measured by using a Hitachi F-4500 fluorescence spectrophotometer. Luminescence decay curves were measured with an Edinburgh PLS920P spectrometer, using excitation of a microsecond flashlamp. All measurements were performed at room temperature. Fig. 1 shows the X-ray diffraction patterns of the Eu doped glass and glass ceramics. The glass as made was completely amorphous
LETTER TO THE EDITOR
Q. Luo et al. / Journal of Non-Crystalline Solids 354 (2008) 4691–4694
(111)
4692
7
F2 3+ 5
(311)
(200)
(220)
3+ 5
Eu : D0
7
Eu : D0
cps
3+ 5
Eu : D0
Intensity
GC620 GC590
3+ 5
Eu : D0 2+
6
Eu :4f 5d
4f
F3
7
F1 3+ 5
Eu : D0
7
F0
7
F4
Glass
7
Glass
GC590 10
20
30
40
50
60
70
80
GC620
2 theta (degree) Fig. 1. XRD patterns of the glass and glass ceramics.
400
450
500
550
600
650
700
750
Wavelength (nm) with no diffraction peaks. The XRD patterns of glass ceramics exhibited prominent peaks well accordant with JCPDS standard card (06-0262) of the SrF2 crystal with no second phase. From the obtained XRD patterns, the size of SrF2 nanocrystals could be calculated to be about 9 nm for GC590 and 22 nm for GC620 by using the Scherrer formula [17]. Fig. 2 shows the TEM and HRTEM (high resolution transmission electron microscope) images of the glass ceramic GC620. It can be found the dark spherical nanocrystals lying on the gray background corresponding to the glassy phase. The crystal size in the glass ceramic GC620 was about 20 nm, which was consistent with the Scherrer-calculated diameter. HRTEM image shows well-defined lattice images. The interplanar distance d value measured through separating two planes from the HRTEM image was 0.329 nm, which could be ascribed to the (1 1 1) plane of the SrF2 crystals (d(111) = 0.335 nm). Fig. 3 shows the luminescence spectra (excited at 394 nm) of the Eu doped glass and glass ceramics. The luminescence spectra of the glass consisted of some sharp emission peaks corresponding to the transitions of the excited 5D0 energy level to 7F0 (579 nm), 7 F1 (594 nm), 7F2 (614 nm), 7F3 (655 nm), and 7F4 (704 nm) levels of the Eu3+ ions. The broad emission band corresponding to Eu2+ ions could hardly be observed. However, the emission spectra of the glass ceramics consisted of not only some sharp emission peaks of the Eu3+ but also a broad band from 350 to 550 nm which could be ascribed to the 4f65d?4f7 transition of the Eu2+ ions. This could
Fig. 3. Emission (excited at 394 nm) spectra of the Eu doped glass and glass ceramics.
be confirmed with the corresponding excitation spectra of GC620 which was shown in Fig. 4. The excitation spectrum monitored at 614 nm consisted of sharp line rang from 300 to 500 nm which resulted from transitions between the 4f energy levels of Eu3+. When monitored wavelength was 440 nm, however, a broad excitation band corresponding to the transition from 8S7/2 to 4f65d level of Eu2+ could be observed. It was obvious that the some Eu3+ ions have been reduced to Eu2+ ions in the glass ceramics. To explain the reduction phenomenon of Eu3+ ? Eu2+, the substitution defect model or charge compensation modern has been proposed [10,13]. When trivalent Eu3+ ions were doped into those hosts containing M2+ (M2+ = Ca2+, Sr2+, Ba2+ and Zn2+), Eu3+ would replace the M2+ ions. In order to keep the charge balance, two Eu3+ ions would substitute for three M2+ ions to form EuM defect and one cation vacancy defect with two negative charges V 00M would be created. The vacancy acted as a donor of electron while the EuM defect became an acceptor of electron. Consequently, by thermal stimulation, the electrons in the vacancy defect V 00M were transferred to EuM and Eu3+ ions were reduced to Eu2+ ions. In the present case, such process could be described as following:
Fig. 2. TEM (a) and HRTEM (b) images of the glass ceramic GC620.
LETTER TO THE EDITOR
4693
Q. Luo et al. / Journal of Non-Crystalline Solids 354 (2008) 4691–4694
2000
1 2+ 8
6
Eu : S7/2
λem=440 nm
4f 5d
Normalized Intensity
λem=614 nm
Intensity (a.u.)
1500
5
3+ 7
Eu : F0
1000
L6
3+ 7
Eu : F0
D3
3+ 7
G3
Eu : F0
5
D2
GC590
GC620
0.1
5
3+ 7
Eu : F0
0 250
5
3+ 7
Eu : F0
500
5
Glass
D4
3+ 7
Eu : F0
5
D1 0
300
350
400
450
500
550
Fig. 4. Excitation (monitored at 440 nm and 614 nm) spectra of the Eu doped glass ceramic GC620.
3SrSr þ 2Eu ¼ 3Sr V 00Sr ¼ V Sr þ 2e0 ;
þ
2EuSr
þ
V 00Sr ;
ð1Þ ð2Þ
2EuSr þ 2e0 ¼ 2EuSr :
ð3Þ
By adding Eqs (1)–(3)
3SrSr þ 2Eu3þ ¼ 3Sr2þ þ 2EuSr þ V Sr :
ð4Þ
Furthermore, the concept of ‘optical basicity’ was also used to explain the reduction phenomenon [16,18–20]. According to this mechanism, divalent Eu2+ ions were preferred in glasses with low optical basicity. In this work, the luminescence of Eu2+ ions could not be observed in the 50SiO2–22Al2O3–20SrF2–6NaF– 2EuF3 glass, which indicated that the optical basicity of this glass was higher than the critical value for divalent Eu2+ ions to exist. In the glass ceramics, however, Eu3+ ions replaced the Sr2+ ions in the SrF2 nanocrystals and EuSr defects were formed. As a result, Eu3+ ions were reduced to Eu2+ ions according to charge compensation mechanism. The luminescence decay curves of the 5D0 state of Eu3+ were obtained by monitoring the 5D0 ? 7F2 emission line (614 nm) for the Eu doped glass and glass ceramics. Fig. 5 shows the observed luminescence decay curves and fitted curves of 5D0 state of Eu3+ in the glass, GC590 and GC620. The luminescence decay curve of 5D0 state of Eu3+ in the glass was best fitted to a single-exponential fitting function: IðtÞ ¼ 0:02 þ 0:98 expðt=2:74Þ, which indicated 2.74 ms lifetime. However, the luminescence decay curves of 5D0 state of Eu3+ in the glass ceramics were fitted to the double-exponential fitting functions with a long decay and a short decay. The fitted decay curves for GC590 and GC620 could be described as:
IðtÞ ¼ 0:62 expðt=2:56Þ þ 0:39 expðt=0:17Þ;
ð5Þ
IðtÞ ¼ 0:52 expðt=2:55Þ þ 0:47 expðt=0:11Þ:
ð6Þ 3+
Obviously, the luminescence intensity I(t) of the Eu in Eu doped glass ceramics could be described by the sum of two exponential decay components from
t t þ A2 exp ; IðtÞ ¼ A1 exp
s1
s2
3
4
5
6
Fig. 5. Decay curves (h, N and s) and fitted curves (solid line) of 5D0 state of Eu3+ in the glass and glass ceramics.
hsi ¼ 2þ
2
Time (ms)
Wavelength (nm)
3þ
1
600
ð7Þ
where s1 and s2 were long- and short-decay components, respectively. Parameters A1 and A2 were fitting constants. According to Eq. (7), the average lifetime hsi was given by:
A1 s21 þ A2 s22 ; A1 s 1 þ A2 s 2
ð8Þ
where weight factors A1 and A2 were introduced. According to Eq. (8), the average lifetime of 5D0 state of Eu3+ in the glass ceramics could be calculated to be about 2.46 ms and was slightly shorter than that of in the glass. It is well known that the luminescence of rare earth ions depended more or less efficiently on the nature of the host. The luminescence decay rate was the sum of the radiative decay and multiphonon relaxation (non-radiative relaxation) rates [21]. The non-radiative relaxation between various J states might occur by interaction of the electronic levels of rare earth ions with suitable vibrational modes of the environment. Therefore, the double-exponential decay curves indicated that there were two different surroundings of the europium ions in the glass ceramics: some in the glass and the others in the SrF2 nanocrystals. In conclusion, Eu doped glass ceramics containing SrF2 nanocrystals have been prepared. Blue luminescence of the Eu2+ ions was observed in such glass ceramics, which indicated the reduction of Eu3+ ? Eu2+. The luminescence decay curves of 5D0 state of Eu3+ in the glass ceramics showed a double-exponential function due to different surroundings of the Eu ions in the glass ceramics. Acknowledgements The authors gratefully acknowledge support for this research from the Science and Technology Department of Zhejiang Province (2006C14010), the Research Fund of the Doctoral Program of Higher Education of China (20070335012), the Chinese-French Cooperation Programs (MX07-01) and Program for Changjiang Scholars and Innovative Research Team in University. References [1] X.L. Liang, Y.X. Yang, C.F. Zhu, S.L. Yuan, G.R. Chen, A. Pring, F. Xia, Appl. Phys. Lett. 91 (2007) 091104. [2] J.S. Kim, K.T. Lim, Y. Jeong, P. Jeon, J. Choi, H. Park, Solid State Comm. 135 (2005) 21. [3] Z . Pei, Q. Su, J. Zhang, J. Alloys Compd. 198 (1993) 51. [4] K. Machida, D. Ueda, S. Inoue, G. Adachi, Chem. Lett. 18 (1999) 785. [5] K. Machida, D. Ueda, S. Inoue, G. Adachi, Electrochem. Solid-State Lett. 2 (1999) 597. [6] N.E. Jouhari, C. Prent, G.L. Flem, J. Solid Chem. 123 (1996) 398. [7] M. Nogami, T. Kawaguchi, A. Yasumori, Opt. Commun. 193 (2001) 237. [8] M.Y. Peng, Z.W. Pei, G.Y. Hong, Q. Su, J. Mater. Chem. 13 (2003) 1202. [9] H. You, M. Nogami, J. Phys. Chem. B 109 (2005) 13980. [10] Z. Lian, J. Wang, Y.H. Lv, S.B. Wang, Q. Su, J. Alloys Compd. 430 (2007) 257.
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