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Luminescent properties of Ca2MgSi2O7 phosphor activated by Eu2+, Dy3+ and Nd3+
Optical Materials 27 (2004) 51–55 www.elsevier.com/locate/optmat
Luminescent properties of Ca2MgSi2O7 phosphor activated by Eu2þ, Dy3þ and Nd3þ Ling Jiang a, Chengkang Chang a
a,*
, Dali Mao a, Chuanli Feng
b
State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiaotong University, 1954 Huashan Road, Shanghai 200030, PR China b State Key Laboratory of Bio-Organic & Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fengling Road, Shanghai 200032, PR China Received 27 June 2003; accepted 17 February 2004 Available online 10 April 2004
Abstract Long lasting alkaline earth silicates, Ca2 MgSi2 O7 :Eu,Dy,Nd was prepared under a reduction atmosphere through solid state reaction. The obtained phosphor was characterized by means of X-ray diffraction (XRD) and photoluminescence spectrum (PLS). The crystal structure of Ca2 MgSi2 O7 :Eu,Dy,Nd phosphor was refined by Rietveld analysis. The obtained Ca2 MgSi2 O7 :Eu,Dy,Nd phosphor showed a yellow–green emission peaking at 518 nm, which is ascribed to the luminescent emission of the Eu2þ that occupied the octa-coordinated Ca2þ sites in the Ca2 MgSi2 O7 host. The electron affinity (ea) value for Eu2þ in [EuO8 ] was calculated to 1.9 eV. The decay profile and the emission spectrum indicated that when the value of Dy/Eu is increasing, there is a concentration quenching of Eu2þ . 2004 Elsevier B.V. All rights reserved. PACS: 71.20.Ps; 78.55.)m Keywords: Luminescence; Afterglow; Phosphor; Rietveld refinement
1. Introduction Phosphors containing Eu2þ ions have received increasing interest in recent years due to their technological important. Long lasting aluminate-based phosphors attracted more research because of their excellent properties, such as no radiation, high brightness and long afterglow. But the properties of these phosphors may be decreased greatly while soaked in the water after several hours, which limited their application [1,2]. Alkaline earth silicates are useful luminescent hosts with high physical and chemical stability. Poort et al. prepared Sr2 MgSi2 O8 :Eu and Ca2 MgSi2 O7 :Eu phosphors by firing at 800 C in a reducing atmosphere, and found that the main emission peaks located at 470 and 535 nm respectively [3]. But the long afterglow was not observed in these phosphors. Eu2þ , Dy3þ co-doped Sr2 MgSi2 O8 *
Corresponding author. Tel.: +86-21-6293-3344; fax: +86-21-62932522. E-mail address: [email protected] (C. Chang). 0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.02.019
phosphor was found to emit a blue–green light peaking at 476 nm upon UV illumination and show long afterglow [4]. In this study, a new long afterglow phosphor Ca2 MgSi2 O7 :Eu,Dy,Nd (thereafter denoted as C2 MS2 EDN) was synthesized by solid state reaction method. The lattice structure, photoluminescence and afterglow characteristics were reported. The effect of the Dy/Eu (molar ratio) on the luminescence properties of C2 MS2 EDN phosphor was also investigated.
2. Experimental The Photoluminescent phosphors were synthesized by solid state reaction method. CaCO3 , 4MgCO3 Æ Mg(OH)2 Æ 5H2 O, SiO2 , Eu2 O3 , Dy2 O3 , Nd2 O3 , all in analytical grade, were employed as the starting materials in the experiment. Small quantities of H3 BO3 were added as a flux. The starting materials were mixed homogeneously in a ball mill for 2 h, and the new long
52
L. Jiang et al. / Optical Materials 27 (2004) 51–55
afterglow C2 MS2 -EDN photoluminescent phosphors were prepared at 1200 C for 4 h and in a weak reduced atmosphere. All measurements were performed on powder samples. Phase identification of the synthesized phosphors was performed using the X-ray diffraction method. The experiments were conducted on a Rigaku D /max-3B Xray diffractometer, which was run at 40 KV and 35 mA, with a step-scan mode (0.022/2h) using Cu Ka X-rays The excitation and emission of wavelength 1.5405 A. spectra of the synthesized C2 MS2 -EDN phosphor were obtained by a fluorescent spectrometer (LS55B). The decay profiles were also generated using the same instrument after the samples were sufficiently excited under UV lights.
3. Results and discussion 3.1. Phase composition of the obtained phosphor The XRD pattern of C2 MS2 -EDN sample synthesized at 1200 C is shown in Fig. 1. Nearly all of the peaks can be indexed to the phase of Ca2 MgSi2 O7 :Eu, Dy,Nd, indicating that the doping ions, Eu2þ , Dy3þ , Nd3þ , did not form new phases in the synthesis process. That is to say, single phased Ca2 MgSi2 O7 phosphor was obtained by solid state reaction under 1200 C. 3.2. Structural analysis The crystal structure of Ca2 MgSi2 O7 :Eu,Dy,Nd phosphor was refined by Rietveld analysis [5] and the analyzed results are shown in Table 1. Where, RE ¼ Eu,
Dy and Nd, Rp and Rwp are the pattern R-factor and weighted pattern R-factor, respectively. When their values are less than 10%, the refinement results are commonly implied to be good and reasonable [6]; x=a, y=b, z=c are the fraction coordinates of the different atoms in the cell, which define crystal axes as coordinate axes and length of cell edges as unit length. NTotal represents the sum of equivalent atoms in the cell. In Table 2 are listed the bond lengths between oxygen atom and related atoms. From the above lattice parameters and symmetry codes, the lattice structure was figured out by computer software, B.S, which is shown in Fig. 2. From Fig. 2, it is viewed that the structure of Ca2 MgSi2 O7 consists of corrugated sheets of interconnected tetrahedral units held together electrostatically by interlayer Ca2þ cations having eight-fold oxygen coordination. There are two different types of tetrahedral in the sheet structure of Ca2 MgSi2 O7 . The Mg atoms are in tetrahedra of oxygens, all four of which are shared by adjacent [SiO4 ] tetrahedra, which themselves are linked in pairs to form [Si2 O7 ] groups. Consequently, in each silica tetrahedron, one oxygen is shared. The three sites available for incorporating Eu2þ , Dy3þ and Nd3þ in Ca2 MgSi2 O7 lattice are the Ca2þ sites, or and Si4þ the Mg2þ sites, or the Si4þ sites. Mg2þ (0.58 A) 2þ (0.26 A) are small, but Ca (1.12 A) is equal in size to and Nd3þ (1.12 A) and similar to Dy3þ Eu2þ (1.12 A) (see Table 3). So Eu2þ , Dy3þ and Nd3þ ions can (1.03 A) not incorporate into an tetrahedral [MgO4 ] and [SiO4 ], and only incorporate into an [CaO8 ] anion complexes in Ca2 MgSi2 O7 (see Table 2). As shown in Table 1, it is found that the incorporation of Eu2þ , Dy3þ and Nd3þ ions into the Ca2 MgSi2 O7 crystal lattice do not cause any significant lattice distortions. Eu2þ , Dy3þ and Nd3þ ions
Fig. 1. XRD pattern of Ca2 MgSi2 O7 :Eu,Dy,Nd phosphor.
L. Jiang et al. / Optical Materials 27 (2004) 51–55
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Table 1 The structure of (Ca1:955 MgSi2 O7 :Eu0:015 ,Dy0:02 , Nd0:01 ) phosphor from Rietveld refinement Composition phase
System and space group
Crystal cell parameters
Ca2 MgSi2 O7
Tetragonal
a (A) 7.8099
P-421m
b (A) 7.8099
R-factor c (A) 5.0090
V (A) 305.52
Rp 6.92%
Fraction coordinate and total number of equivalent atoms in unit cell Atom x=a y=b
z=c
NTotal
Ca1 RE1 Mg1 Si1 O1 O2 O3
0.5072 0.5072 0.0000 0.9366 0.1802 0.2617 0.7905
3.9120 0.0880 2.0000 4.0000 2.0000 4.0000 8.0000
0.3326 0.3326 0.0000 0.1409 0.5000 0.1439 0.0852
0.1674 0.1674 0.0000 0.3591 0.0000 0.3561 0.1839
Table 2 Bond lengths in the structure of Ca1:955 MgSi2 O7 :Eu0:015 ,Dy0:02 ,Nd0:01 phosphor from Rietveld refinement Composition phase Ca2 MgSi2 O7
Average distance (d)
Ca–O1ii Ca–O2ii Ca–O2i;iii Ca–O3ii;iv Ca–O3i;iii Si–O1vii Si–O2viii Si–O3ii;iv Mg–O3i;v;vi;vii
2.470 2.420 2.699 2.400 2.732 1.662 1.629 1.611 1.899
(3) (3) (4) (3) (4) (2) (3) (2) (2)
Figures in parentheses represent estimated standard deviations. Symmetry codes: (i) y, )x, 1)z; (ii) x, y, z; (iii) 1/2+x, 1/2)y, 1)z; (iv) 1/ 2-y, 1/2)x, z; (v) x, y, )1+z; (vi) )x, )y, )1+z; (vii) )y, x, 1)z; (viii) x, y, 1+z.
Rwp 8.84%
Table 3 Ion radius and coordination number of ions in Ca2 MgSi2 O7 Ion type Radius (A) Coordination Ligand number Ca2þ Mg2þ Si4þ
1.12 0.58 0.26
8 4 4
[CaO8 ] [MgO4 ] [SiO4 ]
3.3. Photoluminescence of Eu2þ in alkaline earth silicates Photoluminescent spectra of C2 MS2 -EDN are shown in Fig. 3. Curve (1) and curve (2) were the excitation spectrum and the emission spectrum respectively. The results showed that two excitation bands centered at 388 and 412 nm were observed. After excitation, the phosphor would emit yellow–green visible lights peaking at 518 nm. The emission peak of C2 MS2 -EDN phosphor is viewed as the typical emission of Eu2þ ascribed to the
Fig. 2. Structure map of Ca2 MgSi2 O7 along the [0 1 0] direction. Inset in the top right corner shows sketch map of [CaO8 ].
occupy the Ca2þ sites and totally substitute for 4.4% of the Ca2þ ions. The occupation ratio of Eu2þ , Dy3þ and Nd3þ in the lattice is approximately equal to the nominal doping amount (4.45%), which indicates that Eu2þ , Dy3þ and Nd3þ ions only replace Ca2þ ions.
Fig. 3. Excitation spectrum (1) and emission spectrum (2) of Ca2 MgSi2 O7 :Eu,Dy,Nd.
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L. Jiang et al. / Optical Materials 27 (2004) 51–55
4f–5d transitions. From the discussion in Section 3.2, Eu2þ ions only replace Ca2þ ions in the phosphor. So the emission peak of C2 MS2 -EDN phosphor is viewed as the emission of Eu2þ which sites in the Ca2þ sites with coordinate number of 8. The special Dy3þ and Nd3þ emission peaks are not present, indicating that the two kinds of ions maybe serve as the hole or electron traps and energy transporting media, not as the luminescent centers in the hosts [7].
and 2.45 eV for Eu2þ with coordinate of 8 such as in SrF2 [8]. O2 is similar to F . From the discussion in the Section 3.2, it is clear that Eu2þ ions incorporate into the Ca2þ sites with coordination number of 8. So we used the above-mentioned Eq. (1), and the calculated value of ea for O2 forming [CaO8 ] anion complexes in Ca2 MgSi2 O7 phosphor is given in Table 4. As can be seen, when the Eu2þ ions lie in the Ca2þ sites with coordination number of 8, the value of ea is 1.9 eV.
3.4. Electron affinity (ea) of Eu2þ in the Ca2 MgSi2 O7 host
3.5. The luminescent characteristic of Ca2 MgSi2 O7 : Eu,Dy,Nd phosphors with various Dy/Eu (ratio/molar)
According to earlier literature [8], the position in energy for the lower d-band edge for Eu2þ or Ce3þ in various compounds can be calculated as the following empirical relation h i 1=V E ¼ Q 1 ðV =4Þ 10ðn:ea:rÞ=8 ; ð1Þ
The luminescent decay curves of different phosphors co-doped with various Eu2þ excited under UV lights for 20 min at room temperature are shown in Fig. 4(1). And the emission spectra of Ca2 MgSi2 O7 :Eu,Dy,Nd with various Eu2þ were shown in Fig. 4(2). It is clear that all of the phosphors show a rapid decay with subsequent long-lasting phosphorescence. The emission intensity at 518 nm and the decay speed of the phosphors are different for phosphors with different Dy/Eu ratio. With increasing Eu2þ content slightly at fixed Dy3þ content (0.02 mol) and Nd3þ (0.01 mol) in Ca2 MgSi2 O7 :Eu, Dy,Nd phosphor, both the emission intensity and the afterglow increase. But when the value of Dy/Eu is over 20/7, the afterglow time and emission intensity decrease, which may be ascribed to the concentration quenching of Eu2þ . As already known, the luminescent decay time for the transitions between 4fn fi 4fn levels is typically in the range of ls to ms. The transitions between 4fn-1 5d fi 4fn levels are parity-allowed, and the luminescence decay times are about 10–50 ns [9]. Thus, the long afterglow observed in our phosphors could be attributed to energy exchange processes between traps or traps and emission states resulting from Eu, Dy and Nd doping. From Table 1, it can be seen that co-doped Dy3þ and Nd3þ can incorporate into the Ca2þ sites, but the special Dy3þ and Nd3þ emission peaks are not present, which may be ascribed to that Dy3þ and Nd3þ worked as the hole
which can provide a good fit to the emission peak for Eu2þ . Where Q is the position in energy for the lower dband edge for the free ion, here Q is 34000 cm1 for Eu2þ . V is the valence of the active cation, and here V is 2. n is the number of anions in the immediate shell about this ion, and ea is the electron affinity of the atoms that form anions. r is the radius of the host cation replaced by the active cation in the host crystal. The value of ea is different when Eu2þ ions are introduced into different anion complexes with various coordinate numbers. For example, in the fluorides, the ea for Eu2þ is 4.73 eV when Eu2þ gets 6 coordinate F ions to form an octahedral [EuF6 ] in the matrix of BeF2 , AlF3 and SiF4 . In comparison, the ea is 3.45 eV for Eu2þ with coordinate of 4
Table 4 The calculated value of ea in the Ca2 MgSi2 O7 phosphor n
r (nm)
Energy (cm1 )
Emitting wavelength (nm)
ea (eV)
8
0.112
19362.3
518
1.9
Fig. 4. The decay curves (1) and the emission spectra (2) of phosphors co-doped at various Dy/Eu ratios in Ca2 MgSi2 O7 host.
L. Jiang et al. / Optical Materials 27 (2004) 51–55
traps and prolonged the afterglow. So in the C2 MS2 EDN host, Dy3þ and Nd3þ ions may act as the role of trap centers, which capture the free holes, release the trapped holes and recombine with electrons accompanying with the luminescence, and the relative work is processing.
55
Acknowledgements This work was financially supported by Natural Science Foundation of Shanghai (contract no. 02ZE14055) and Shanghai Nanotechnology Promotion Center (contract no. 0252nm018).
4. Conclusions References Single phased Ca2 MgSi2 O7 :Eu,Dy,Nd with afterglow characteristics was prepared by solid state reaction in a reduced atmosphere. The Ca2 MgSi2 O7 :Eu,Dy,Nd phosphor produces a yellow–green emission peaking at 518 nm. The Rietveld refinement of the crystal structure of Ca1:955 MgSi2 O7 :Eu0:015 ,Dy0:02 ,Nd0:01 phosphor indicate that there is only one kind of Eu2þ emission center occupied in the octa-coordinated Ca2þ crystallographic sites in the Ca2 MgSi2 O7 host. The ea value for Eu2þ in [EuO8 ] was calculated to be 1.9 eV. When the value of Dy/Eu is over 20/7, there is a concentration quenching of Eu2þ .
[1] H. Takasaki, S. Tanabe, T. Hanada, J. Ceram. Soc. Jpn. 104 (1996) 322. [2] H. Yamamoto, T. Matsuzawa, J. Lumin. 73 (1997) 287. [3] S.H.M. Poort, H.M. Reijnhoudt, G. Blass, J. Alloy. Comp. 241 (1996) 75. [4] Y. Lin, Z. Tang, Z. Zhang, X. Wang, J. Zhang, J. Mater. Sci. Lett. 20 (2001) 1505. [5] H.M. Rietveld, Acta Crystallogr. 22 (1967) 151. [6] J. Liang, Jiegou Huaxue 4 (4) (1985) 241 (in Chinese). [7] Y. Lin, Z. Zhang, Z. Tang, X. Wang, J. Zhang, Z. Zheng, J. Euro. Ceram. Soc. 21 (2001) 683. [8] L.G.V. Uitert, J. Lumin. 29 (1984) 1. [9] P. Dorenbos, J. Lumin. 91 (2000) 156.
Luminescent properties of Ca2MgSi2O7 phosphor activated by Eu2þ, Dy3þ and Nd3þ Ling Jiang a, Chengkang Chang a
a,*
, Dali Mao a, Chuanli Feng
b
State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiaotong University, 1954 Huashan Road, Shanghai 200030, PR China b State Key Laboratory of Bio-Organic & Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fengling Road, Shanghai 200032, PR China Received 27 June 2003; accepted 17 February 2004 Available online 10 April 2004
Abstract Long lasting alkaline earth silicates, Ca2 MgSi2 O7 :Eu,Dy,Nd was prepared under a reduction atmosphere through solid state reaction. The obtained phosphor was characterized by means of X-ray diffraction (XRD) and photoluminescence spectrum (PLS). The crystal structure of Ca2 MgSi2 O7 :Eu,Dy,Nd phosphor was refined by Rietveld analysis. The obtained Ca2 MgSi2 O7 :Eu,Dy,Nd phosphor showed a yellow–green emission peaking at 518 nm, which is ascribed to the luminescent emission of the Eu2þ that occupied the octa-coordinated Ca2þ sites in the Ca2 MgSi2 O7 host. The electron affinity (ea) value for Eu2þ in [EuO8 ] was calculated to 1.9 eV. The decay profile and the emission spectrum indicated that when the value of Dy/Eu is increasing, there is a concentration quenching of Eu2þ . 2004 Elsevier B.V. All rights reserved. PACS: 71.20.Ps; 78.55.)m Keywords: Luminescence; Afterglow; Phosphor; Rietveld refinement
1. Introduction Phosphors containing Eu2þ ions have received increasing interest in recent years due to their technological important. Long lasting aluminate-based phosphors attracted more research because of their excellent properties, such as no radiation, high brightness and long afterglow. But the properties of these phosphors may be decreased greatly while soaked in the water after several hours, which limited their application [1,2]. Alkaline earth silicates are useful luminescent hosts with high physical and chemical stability. Poort et al. prepared Sr2 MgSi2 O8 :Eu and Ca2 MgSi2 O7 :Eu phosphors by firing at 800 C in a reducing atmosphere, and found that the main emission peaks located at 470 and 535 nm respectively [3]. But the long afterglow was not observed in these phosphors. Eu2þ , Dy3þ co-doped Sr2 MgSi2 O8 *
Corresponding author. Tel.: +86-21-6293-3344; fax: +86-21-62932522. E-mail address: [email protected] (C. Chang). 0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.02.019
phosphor was found to emit a blue–green light peaking at 476 nm upon UV illumination and show long afterglow [4]. In this study, a new long afterglow phosphor Ca2 MgSi2 O7 :Eu,Dy,Nd (thereafter denoted as C2 MS2 EDN) was synthesized by solid state reaction method. The lattice structure, photoluminescence and afterglow characteristics were reported. The effect of the Dy/Eu (molar ratio) on the luminescence properties of C2 MS2 EDN phosphor was also investigated.
2. Experimental The Photoluminescent phosphors were synthesized by solid state reaction method. CaCO3 , 4MgCO3 Æ Mg(OH)2 Æ 5H2 O, SiO2 , Eu2 O3 , Dy2 O3 , Nd2 O3 , all in analytical grade, were employed as the starting materials in the experiment. Small quantities of H3 BO3 were added as a flux. The starting materials were mixed homogeneously in a ball mill for 2 h, and the new long
52
L. Jiang et al. / Optical Materials 27 (2004) 51–55
afterglow C2 MS2 -EDN photoluminescent phosphors were prepared at 1200 C for 4 h and in a weak reduced atmosphere. All measurements were performed on powder samples. Phase identification of the synthesized phosphors was performed using the X-ray diffraction method. The experiments were conducted on a Rigaku D /max-3B Xray diffractometer, which was run at 40 KV and 35 mA, with a step-scan mode (0.022/2h) using Cu Ka X-rays The excitation and emission of wavelength 1.5405 A. spectra of the synthesized C2 MS2 -EDN phosphor were obtained by a fluorescent spectrometer (LS55B). The decay profiles were also generated using the same instrument after the samples were sufficiently excited under UV lights.
3. Results and discussion 3.1. Phase composition of the obtained phosphor The XRD pattern of C2 MS2 -EDN sample synthesized at 1200 C is shown in Fig. 1. Nearly all of the peaks can be indexed to the phase of Ca2 MgSi2 O7 :Eu, Dy,Nd, indicating that the doping ions, Eu2þ , Dy3þ , Nd3þ , did not form new phases in the synthesis process. That is to say, single phased Ca2 MgSi2 O7 phosphor was obtained by solid state reaction under 1200 C. 3.2. Structural analysis The crystal structure of Ca2 MgSi2 O7 :Eu,Dy,Nd phosphor was refined by Rietveld analysis [5] and the analyzed results are shown in Table 1. Where, RE ¼ Eu,
Dy and Nd, Rp and Rwp are the pattern R-factor and weighted pattern R-factor, respectively. When their values are less than 10%, the refinement results are commonly implied to be good and reasonable [6]; x=a, y=b, z=c are the fraction coordinates of the different atoms in the cell, which define crystal axes as coordinate axes and length of cell edges as unit length. NTotal represents the sum of equivalent atoms in the cell. In Table 2 are listed the bond lengths between oxygen atom and related atoms. From the above lattice parameters and symmetry codes, the lattice structure was figured out by computer software, B.S, which is shown in Fig. 2. From Fig. 2, it is viewed that the structure of Ca2 MgSi2 O7 consists of corrugated sheets of interconnected tetrahedral units held together electrostatically by interlayer Ca2þ cations having eight-fold oxygen coordination. There are two different types of tetrahedral in the sheet structure of Ca2 MgSi2 O7 . The Mg atoms are in tetrahedra of oxygens, all four of which are shared by adjacent [SiO4 ] tetrahedra, which themselves are linked in pairs to form [Si2 O7 ] groups. Consequently, in each silica tetrahedron, one oxygen is shared. The three sites available for incorporating Eu2þ , Dy3þ and Nd3þ in Ca2 MgSi2 O7 lattice are the Ca2þ sites, or and Si4þ the Mg2þ sites, or the Si4þ sites. Mg2þ (0.58 A) 2þ (0.26 A) are small, but Ca (1.12 A) is equal in size to and Nd3þ (1.12 A) and similar to Dy3þ Eu2þ (1.12 A) (see Table 3). So Eu2þ , Dy3þ and Nd3þ ions can (1.03 A) not incorporate into an tetrahedral [MgO4 ] and [SiO4 ], and only incorporate into an [CaO8 ] anion complexes in Ca2 MgSi2 O7 (see Table 2). As shown in Table 1, it is found that the incorporation of Eu2þ , Dy3þ and Nd3þ ions into the Ca2 MgSi2 O7 crystal lattice do not cause any significant lattice distortions. Eu2þ , Dy3þ and Nd3þ ions
Fig. 1. XRD pattern of Ca2 MgSi2 O7 :Eu,Dy,Nd phosphor.
L. Jiang et al. / Optical Materials 27 (2004) 51–55
53
Table 1 The structure of (Ca1:955 MgSi2 O7 :Eu0:015 ,Dy0:02 , Nd0:01 ) phosphor from Rietveld refinement Composition phase
System and space group
Crystal cell parameters
Ca2 MgSi2 O7
Tetragonal
a (A) 7.8099
P-421m
b (A) 7.8099
R-factor c (A) 5.0090
V (A) 305.52
Rp 6.92%
Fraction coordinate and total number of equivalent atoms in unit cell Atom x=a y=b
z=c
NTotal
Ca1 RE1 Mg1 Si1 O1 O2 O3
0.5072 0.5072 0.0000 0.9366 0.1802 0.2617 0.7905
3.9120 0.0880 2.0000 4.0000 2.0000 4.0000 8.0000
0.3326 0.3326 0.0000 0.1409 0.5000 0.1439 0.0852
0.1674 0.1674 0.0000 0.3591 0.0000 0.3561 0.1839
Table 2 Bond lengths in the structure of Ca1:955 MgSi2 O7 :Eu0:015 ,Dy0:02 ,Nd0:01 phosphor from Rietveld refinement Composition phase Ca2 MgSi2 O7
Average distance (d)
Ca–O1ii Ca–O2ii Ca–O2i;iii Ca–O3ii;iv Ca–O3i;iii Si–O1vii Si–O2viii Si–O3ii;iv Mg–O3i;v;vi;vii
2.470 2.420 2.699 2.400 2.732 1.662 1.629 1.611 1.899
(3) (3) (4) (3) (4) (2) (3) (2) (2)
Figures in parentheses represent estimated standard deviations. Symmetry codes: (i) y, )x, 1)z; (ii) x, y, z; (iii) 1/2+x, 1/2)y, 1)z; (iv) 1/ 2-y, 1/2)x, z; (v) x, y, )1+z; (vi) )x, )y, )1+z; (vii) )y, x, 1)z; (viii) x, y, 1+z.
Rwp 8.84%
Table 3 Ion radius and coordination number of ions in Ca2 MgSi2 O7 Ion type Radius (A) Coordination Ligand number Ca2þ Mg2þ Si4þ
1.12 0.58 0.26
8 4 4
[CaO8 ] [MgO4 ] [SiO4 ]
3.3. Photoluminescence of Eu2þ in alkaline earth silicates Photoluminescent spectra of C2 MS2 -EDN are shown in Fig. 3. Curve (1) and curve (2) were the excitation spectrum and the emission spectrum respectively. The results showed that two excitation bands centered at 388 and 412 nm were observed. After excitation, the phosphor would emit yellow–green visible lights peaking at 518 nm. The emission peak of C2 MS2 -EDN phosphor is viewed as the typical emission of Eu2þ ascribed to the
Fig. 2. Structure map of Ca2 MgSi2 O7 along the [0 1 0] direction. Inset in the top right corner shows sketch map of [CaO8 ].
occupy the Ca2þ sites and totally substitute for 4.4% of the Ca2þ ions. The occupation ratio of Eu2þ , Dy3þ and Nd3þ in the lattice is approximately equal to the nominal doping amount (4.45%), which indicates that Eu2þ , Dy3þ and Nd3þ ions only replace Ca2þ ions.
Fig. 3. Excitation spectrum (1) and emission spectrum (2) of Ca2 MgSi2 O7 :Eu,Dy,Nd.
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L. Jiang et al. / Optical Materials 27 (2004) 51–55
4f–5d transitions. From the discussion in Section 3.2, Eu2þ ions only replace Ca2þ ions in the phosphor. So the emission peak of C2 MS2 -EDN phosphor is viewed as the emission of Eu2þ which sites in the Ca2þ sites with coordinate number of 8. The special Dy3þ and Nd3þ emission peaks are not present, indicating that the two kinds of ions maybe serve as the hole or electron traps and energy transporting media, not as the luminescent centers in the hosts [7].
and 2.45 eV for Eu2þ with coordinate of 8 such as in SrF2 [8]. O2 is similar to F . From the discussion in the Section 3.2, it is clear that Eu2þ ions incorporate into the Ca2þ sites with coordination number of 8. So we used the above-mentioned Eq. (1), and the calculated value of ea for O2 forming [CaO8 ] anion complexes in Ca2 MgSi2 O7 phosphor is given in Table 4. As can be seen, when the Eu2þ ions lie in the Ca2þ sites with coordination number of 8, the value of ea is 1.9 eV.
3.4. Electron affinity (ea) of Eu2þ in the Ca2 MgSi2 O7 host
3.5. The luminescent characteristic of Ca2 MgSi2 O7 : Eu,Dy,Nd phosphors with various Dy/Eu (ratio/molar)
According to earlier literature [8], the position in energy for the lower d-band edge for Eu2þ or Ce3þ in various compounds can be calculated as the following empirical relation h i 1=V E ¼ Q 1 ðV =4Þ 10ðn:ea:rÞ=8 ; ð1Þ
The luminescent decay curves of different phosphors co-doped with various Eu2þ excited under UV lights for 20 min at room temperature are shown in Fig. 4(1). And the emission spectra of Ca2 MgSi2 O7 :Eu,Dy,Nd with various Eu2þ were shown in Fig. 4(2). It is clear that all of the phosphors show a rapid decay with subsequent long-lasting phosphorescence. The emission intensity at 518 nm and the decay speed of the phosphors are different for phosphors with different Dy/Eu ratio. With increasing Eu2þ content slightly at fixed Dy3þ content (0.02 mol) and Nd3þ (0.01 mol) in Ca2 MgSi2 O7 :Eu, Dy,Nd phosphor, both the emission intensity and the afterglow increase. But when the value of Dy/Eu is over 20/7, the afterglow time and emission intensity decrease, which may be ascribed to the concentration quenching of Eu2þ . As already known, the luminescent decay time for the transitions between 4fn fi 4fn levels is typically in the range of ls to ms. The transitions between 4fn-1 5d fi 4fn levels are parity-allowed, and the luminescence decay times are about 10–50 ns [9]. Thus, the long afterglow observed in our phosphors could be attributed to energy exchange processes between traps or traps and emission states resulting from Eu, Dy and Nd doping. From Table 1, it can be seen that co-doped Dy3þ and Nd3þ can incorporate into the Ca2þ sites, but the special Dy3þ and Nd3þ emission peaks are not present, which may be ascribed to that Dy3þ and Nd3þ worked as the hole
which can provide a good fit to the emission peak for Eu2þ . Where Q is the position in energy for the lower dband edge for the free ion, here Q is 34000 cm1 for Eu2þ . V is the valence of the active cation, and here V is 2. n is the number of anions in the immediate shell about this ion, and ea is the electron affinity of the atoms that form anions. r is the radius of the host cation replaced by the active cation in the host crystal. The value of ea is different when Eu2þ ions are introduced into different anion complexes with various coordinate numbers. For example, in the fluorides, the ea for Eu2þ is 4.73 eV when Eu2þ gets 6 coordinate F ions to form an octahedral [EuF6 ] in the matrix of BeF2 , AlF3 and SiF4 . In comparison, the ea is 3.45 eV for Eu2þ with coordinate of 4
Table 4 The calculated value of ea in the Ca2 MgSi2 O7 phosphor n
r (nm)
Energy (cm1 )
Emitting wavelength (nm)
ea (eV)
8
0.112
19362.3
518
1.9
Fig. 4. The decay curves (1) and the emission spectra (2) of phosphors co-doped at various Dy/Eu ratios in Ca2 MgSi2 O7 host.
L. Jiang et al. / Optical Materials 27 (2004) 51–55
traps and prolonged the afterglow. So in the C2 MS2 EDN host, Dy3þ and Nd3þ ions may act as the role of trap centers, which capture the free holes, release the trapped holes and recombine with electrons accompanying with the luminescence, and the relative work is processing.
55
Acknowledgements This work was financially supported by Natural Science Foundation of Shanghai (contract no. 02ZE14055) and Shanghai Nanotechnology Promotion Center (contract no. 0252nm018).
4. Conclusions References Single phased Ca2 MgSi2 O7 :Eu,Dy,Nd with afterglow characteristics was prepared by solid state reaction in a reduced atmosphere. The Ca2 MgSi2 O7 :Eu,Dy,Nd phosphor produces a yellow–green emission peaking at 518 nm. The Rietveld refinement of the crystal structure of Ca1:955 MgSi2 O7 :Eu0:015 ,Dy0:02 ,Nd0:01 phosphor indicate that there is only one kind of Eu2þ emission center occupied in the octa-coordinated Ca2þ crystallographic sites in the Ca2 MgSi2 O7 host. The ea value for Eu2þ in [EuO8 ] was calculated to be 1.9 eV. When the value of Dy/Eu is over 20/7, there is a concentration quenching of Eu2þ .
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