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Crystal growth and optical properties of Gd1.99−xYxCe0.01SiO5 single crystals
ARTICLE IN PRESS
Journal of Crystal Growth 277 (2005) 175–180 www.elsevier.com/locate/jcrysgro
Crystal growth and optical properties of Gd1.99xYxCe0.01SiO5 single crystals Mingyin Jiea,b,, Guangjun Zhaoa, Xionghui Zenga,b, Liangbi Sua,b, Huiyong Panga,b, Xiaoming Hea, Jun Xua a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Qinghe Road, 390, P. O. Box 800-211, Shanghai 201800, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China Received 14 October 2004; accepted 14 December 2004 Communicated by M. Roth Available online 9 February 2005
Abstract Gd1.99xYxCe0.01SiO5 (Ce:GYSO) crystals (x ¼ 0; 0.0995, 0.199) have been grown by the Czochralski (Cz) method. Crystal structure and the distribution coefficients of Ce have been determined for all three crystals. Spectroscopic measurements indicate that optical transmittance and luminescence intensity of Gd1.99xYxCe0.01SiO5 (x ¼ 0:0995; 0.199) crystals are substantially higher than those of Ce:Gd2SiO5 (Ce:GSO), especially at x ¼ 0:0995; which makes them good candidate materials for scintillation applications. The particularly important result is that the alloyed Ce:GYSO crystals can be grown easily by the Cz method and, unlike Ce:GSO, they do not undergo cleavage during the growth process or subsequent mechanical treatment. r 2005 Elsevier B.V. All rights reserved. PACS: 74.25.Gz; 64.75.+g; 62.20.Mk; 61.72.Ww Keywords: A1. Ce distribution coefficient; A1. Crystal structure; A1. Optical spectra; A2. Czochralski method; B1. Ce:GYSO
1. Introduction Corresponding author. Shanghai Institute of Optics and
Fine Mechanics, Chinese Academy of Sciences, Qinghe Road, 390, P. O. Box 800-211, shanghai 201800, PR China. Tel.: +86 21 69915174; fax: +86 21 69918607. E-mail addresses: [email protected] (M. Jie), [email protected] (G. Zhao).
Cerium-doped rare earth oxyorthosilicate crystals are newly found to be excellent scintillators with high light yield and fast decay time. Ce-doped gadolinium oxyorthosilicate, Ce:Gd2SiO5 (Ce:GSO), and yttrium oxyorthosilicate, Ce:Y2SiO5 (Ce:YSO), are typical representatives of this family
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.12.160
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Table 1 Physical parameters and scintillation properties of blue-emitting scintillators Ce:GSOCe:YSOBGONal:Tl Effective atomic number 59 6.71 Density (g/cm3) Emission peak (nm) 430 Light output (%Nal:Tl ) 20 Decay constant (ns) 60 Energy resolution, FWHM (137Cs)10% Refractive index 1.85 Radiation hardiness (rad) 4108 Melting point (1C) 1950 Cleavage plane (1 0 0)
34 4.54 420 25 37 7–8% 1.80 — 1980 no
74 51 7.13 3.67 480 415 7–10 100 300 230 9.5%7.0% 2.15 1.85 1056103 1050 651 no (1 0 0)
of crystals [1]. The structure of GSO belongs to the monoclinic space group P21/c and is composed of a two-dimensional network of corner linked (OGd4) tetrahedra into which the (SiO4) tetrahedra are packed. The relatively weak bonding between these layers results in a strong tendency to cleave along the (1 0 0) plane. In comparison, the crystal structure of YSO belongs to the monoclinic C2/c space group. Hereby, the (SiO4) and (OY4) tetrahedra share edges and form chains interconnected by isolated (SiO4) tetrahedra, and this arrangement results in a more rigid and isotropic structure [2]. The main physical parameters and scintillation properties of Ce:GSO and Ce:YSO, along with such conventional scintillators as NaI (Tl) and BGO, are summarized in Table 1. In comparison with the traditional BGO scintillation crystal, the light output of Ce:GSO is twice as high, and the decay time is 1/5 of that of BGO. Moreover, irradiation hardness of the Ce:GSO crystal is quite prominent, so it can be widely used in a variety of applications of highenergy physics and nuclear physics, nuclear medical imaging (PET ), oil well survey, etc. [3,4]. Although the Ce:GSO crystal has excellent scintillation properties and high density (6.71 g/ cm3), the severe cleavage along the (1 0 0) plane during crystal growth, cooling, and further mechanical processing makes practical applications of the crystal very difficult. In contrast, the
Ce:YSO crystal exhibits good mechanical properties. We can speculate that Ce-doped GSO alloyed with YSO may exhibit no cleavage, unlike the pure Ce:GSO crystal. To our knowledge, similar alloyed crystals, such as Ce:LGSO (Ce:Lu1xGdxSiO5) and Ce:LYSO (Ce:Lu1xYxSiO5) [5,6], have been grown successfully by the Czochralski (Cz) method without exhibiting significant cleavage. They show good scintillation properties and have been protected by a patent [7]. Yet, there is no report on growing Ce:GYSO crystals until now. In this paper, we aim at investigating the growth process of the alloyed Gd1.99xYxCe0.01SiO5 (Ce:GYSO) crystals (x ¼ 0; 0.0995, 0.199) by the Cz method and analyzing their crystalline structure, the distribution coefficients of Ce and optical properties of Gd1.99xYxCe0.01SiO5 (x ¼ 0; 0.0995, 0.199).
2. Experimental procedure Single crystals of Gd1.99xYxCe0.01SiO5 crystal (x ¼ 0; 0.0995, 0.199) were grown by the Cz method in inductively heated iridium crucibles under nitrogen ambient atmosphere. The charges were prepared from Gd2O3 (4N), Y2O3 (4N), Ce2O3 (4N) and SiO2 (5N). The starting materials were fired at 1000 1C for more than 10 h prior to weighing and mixing to remove moisture, then pressed into pellets and sintered at 1200 1C before loading into the iridium crucible. The seed crystals were oriented at 601 relatively to the [0 1 0] axis. Crystal structure was investigated using X-ray powder diffraction (Rigaku Corporation, D/max 2550 V X-ray diffractometer with Cu Ka1 radiation). Gd, Y and Ce concentrations were measured by ICP (Inductively coupled plasma atomic emission spectrometer). Samples of identical size were cut from the top parts of crystals and carefully polished into 0.5 mm thickness wafers. Optical absorption spectra were taken using JASCO model V-570 UV/VIS/NIR Spectrophotometer V-570, and luminescence spectra were measured by JASCO model FP-6500 Spectrophotometer. The wavelength ranges of measurements were 190–1000 and 220–750 nm, respectively. For
ARTICLE IN PRESS M. Jie et al. / Journal of Crystal Growth 277 (2005) 175–180
different crystal samples, the conditions of measurements were kept constant. All measurements were performed at room temperature.
3. Results and discussion Fig. 1 shows the photographs of Ce:GSO (Gd1.99Ce0.01SiO5) and Ce:GYSO (Gd1.99xYxCe0.01SiO5, x ¼ 0:0995 and 0.199) crystals grown by the Cz method under similar conditions. As shown in Fig. 1a, the surface of Ce:GSO is rough and heavily cracked along the (1 0 0) cleavage plane. On the contrary, the Ce:GYSO crystals shown in Figs. 1b and c are smooth and crystallographically perfect. These results indicate that Ce:GYSO with appropriate content of Y2O3 can be easily grown to have a higher degree of crystalline perfection than Ce:GSO. Fig. 2 shows the XRD patterns of Ce:GSO and Gd1.99xYxCe0.01SiO5 (x ¼ 0:199) crystals, which are nearly identical. This result confirms that the Gd1.99xYxCe0.01SiO5 (x ¼ 0:199) crystal still maintains the monoclinic crystallographic structure belonging to the space group P21/c.
177
The measured lattice parameters and densities of Gd1.99xYxCe0.01SiO5 (x ¼ 0; 0.0995, 0.199) crystals are listed in Table 2. With the increased content of yttrium, the lattice parameters a; b; c and b vary in the range of 0–0.3% compared with those of Ce:GSO and the density is reduced by about 3%. Therefore, the changes of lattice parameters and densities are insignificant. The reported distribution coefficient of Ce in GSO is about 0.6, while that in YSO is about 0.34 F
B
25
30
35
40
45
50
55
2 Theta / ˚ Fig. 2. XRD curves of Ce:GSO (B) and Gd1.99xYxCe0.01SiO5 (x ¼ 0:199) (F).
Fig. 1. Photographs of crystals: Ce:GSO (a), Gd1.99xYxCe0.01SiO5 (x ¼ 0:0995) (b) and Gd1.99-xYxCe0.01SiO5 (x ¼ 0:199) (c) grown by the Cz method under similar conditions.
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Table 2 Lattice parameters and densities of Gd1.99xYxCe0.01SiO5 (x ¼ 0, 0.0995, 0.199) and YSO crystals grown by the Cz method Crystal
Ce:GSO [2] Ce:GSO (this work) Ce:GYSO (x ¼ 0:0995) Ce:GYSO (x ¼ 0:199) YSO [2]
Density (g/cm3)
Unit cell parameters a(A˚)
b(A˚)
c(A˚)
b(deg)
9.12 9.14005370.001577 9.13226970.001135 9.13464770.001693 10.41
7.06 7.05597370.000756 7.04982170.000716 7.03944470.001053 6.721
6.73 6.74903770.000818 6.74585970.000674 6.74390370.000977 12.49
107.5 107.523470.012501 107.496970.011207 107.469470.016612 102.65
6.71 6.71 6.60 6.49 4.54
Table 3 Composition and distribution coefficients of Ce in GYSO crystals (measured by ICP) Crystal
Charge
Top of the crystal
Distribution coefficient
Ce:GSO Ce: GYSO (x ¼ 0:0995) Ce: GYSO (x ¼ 0:199)
Gd1.99Ce0.01SiO5 Gd1.8905Y0.0995Ce0.01SiO5 Gd1.791Y0.199Ce0.01SiO5
Gd1.613Ce0.0047SiO5 Gd1.436Y0.082Ce0.0044SiO5 Gd1.736Y0.204Ce0.0056SiO5
0.581 0.578 0.576
[10]. Because of the difference between the ionic radii of the dopant Ce3+ and the substituted host crystal cation (the ratio becoming larger when Y3+ ions replace Gd3+ ions in Ce:GYSO [8]), it may be speculated that the distribution coefficient of Ce in GYSO must be lower than in the GSO crystal. Our experimental results listed in Table 3 show that the distribution coefficient of Ce decreases slightly indeed with the increasing amount of Y content in GSO. The above results indicate cumulatively that addition of Y into Ce:GYSO at concentrations up to about 10 at% (i.e., x ¼ 0:199) does not appreciably change the crystallographic structure. However, the crystalline quality of Ce:GYSO can be significantly improved. Optical Emission and absorption spectra of Ce:GYSO have been measured at room temperature, and they are shown in Figs. 3a and b respectively together with the Ce:GSO spectra. Apparently, the positions and shapes of the Ce:GYSO emission bands do not change relatively to the Ce:GSO crystal, but their integrated emission intensity almost doubles. According to the previous work [9], Ce:GSO and Ce:YSO both belonging to the monoclinic system, have two kinds of luminescence centers. For Ce:GSO, one
luminescence center quenches severely at room temperature, but it does not hold for Ce:YSO, Therefore, the luminescence quantum efficiency of Ce:GSO is lower than that of Ce:YSO at room temperature. As a result, the light yield of YSO is higher than that of GSO, as listed in Table 1. Therefore, it can be inferred that the stronger luminescence intensity of the Ce:GYSO crystal in comparison with Ce:GSO may result from the larger amount of active luminescence centers. The optical absorption edges of Ce-doped GSO and GYSO samples below about 400 nm are shown in Fig. 3b. They are associated with transitions from the 4f ground state to the lowest 5d sublevel of the Ce3+ ion, subjected to the crystal field splitting of the host. With the increase of Y content, the crystal field becomes stronger and, consequently, the absorption edge shifts to shorter wavelengths [10]. As shown in Fig. 4, the optical transmission edges of Ce:GYSO also shift to shorter wavelengths accordingly. In the wavelength range longer than 400 nm, the optical transmission of Ce-doped GYSO crystals increases to 82%, 6% higher than that of Ce:GSO. We recall from Fig. 3a, that the emission peak of Ce:GYSO peaks at 437 nm, namely in the high transparency wavelength range.
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It is well known that the increase of the optical transmission will correspondingly improve the light output. Therefore, the higher transmission in the luminescence spectra region is also one of the causes for the integral emission intensity of Ce:GYSO being larger than that of Ce:GSO. Concurrently, it should be noted that both the transmission above 400 nm and the integrated emission intensity around 437 nm of Gd1.99xYxCe0.01SiO5 (x ¼ 0:0995) are higher than those of Gd1.99xYxCe0.01SiO5 (x ¼ 0:199). This indicates that the content of Y in Ce:GYSO must not exceed a certain value for higher light output to be achieved. Our experiments imply that the smaller Y content corresponding to x ¼ 0:0995 in Gd1.99xYxCe0.01SiO5 is more appropriate for scintillation applications.
Emission intensity /a.u.
800 0.95GSO/0.05YSO 0.9GSO/0.1YSO 600 GSO
400
200
0 400
500
Optical density / a.u.
(a)
600
700
Wavelength /nm
2.0
0.95GSO/0.05YSO
1.5
0.9GSO/0.1YSO
1.0
4. Conclusions
GSO 0.5 0.0 350
400
(b)
450
500
Wavelength /nm
Fig. 3. (a) Emission spectra of Ce-doped GSO and GYSO crystals peaking at 437 nm under 345 nm, excitation. (b) Absorption spectra of Ce-doped GSO and GYSO crystals (0.5 mm thickness).
0.95GSO/0.05YSO 80 Transmittance /%
179
0.9GSO/0.1YSO Ce:GSO
70
Cz growth has yielded high quality Gd1.99xYxCe0.01SiO5 (Ce:GYSO) single crystals (x ¼ 0:0995; 0.199) exhibiting no cleavage phenomena as compared with the simple oxyorthosilicate Gd1.99Ce0.01SiO5 (Ce:GSO) crystal. Substitution of Y for Gd in Ce:GYSO up to about 10 at% (x ¼ 0:199) does not appreciably change the lattice parameters of the monoclinic crystallographic structure and the distribution coefficient of Ce. Crystals of Gd1.99xYxCe0.01SiO5 with x ¼ 0:0995 have been found to exhibit the highest transmittance and integrated emission intensity, the latter about twice as high as that of the Gd1.99Ce0.01SiO5 (Ce:GSO) crystal. The combination of structural, chemical and optical characterization results obtained in this work indicate that the Gd1.99xYxCe0.01SiO5 (x ¼ 0:0995) crystal is a potentially better material than Ce:GSO for scintillation applications.
60
References 380
400
420
440
460
Wavelength /nm Fig. 4. Transmission spectra of Ce-doped GSO and GYSO crystals (0.5 mm thickness).
[1] K. Takagi, T. Fukazawa, Appl. Phys. Lett. 42 (1983) 43. [2] J. Felsche, in: Structure and Bonding, vol. 13, Springer, Berlin, 1973, p. 99. [3] P. Lecoq, J. of Luminescence 60 and 61 (1994) 948.
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[4] M. Tanaka, K. Hara, K. Kondo, Nucl. Instr. and Meth. A 404 (1998) 283. [5] G.B. Loutts, A.I. Zagumennyi, S.V. Lavrishchev, Yu.D. Zavartsev, P.A. Studenikin, J. Crystal Growth 174 (1997) 331. [6] D.W. Cooke, K.J. McClellan, B.L. Bennett, J.M. Roper, M.T. Whittaker, J. Appl. Phys. 88 (2000) 7360.
[7] S. Akiyama, H. Ishibashi, T. Utsu, C.L. Melcher, J.S. Schweitzer, US Patent 5 264 154 1993. [8] C.D. Brandle, A.J. Valentino, G.W. Berkstresser, J. Crystal Growth 79 (1986) 308. [9] H. Suzuki, T.A. Tombrello, C.L. Melcher, J.S. Schweitzer, Nucl. Instr. and Meth. A 320 (1992) 263. [10] C.L. Melcher, R.A. Manente, C.A. Perterson, J.S. Schweitzer, J. Crystal Growth 128 (1993) 1001.
Journal of Crystal Growth 277 (2005) 175–180 www.elsevier.com/locate/jcrysgro
Crystal growth and optical properties of Gd1.99xYxCe0.01SiO5 single crystals Mingyin Jiea,b,, Guangjun Zhaoa, Xionghui Zenga,b, Liangbi Sua,b, Huiyong Panga,b, Xiaoming Hea, Jun Xua a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Qinghe Road, 390, P. O. Box 800-211, Shanghai 201800, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China Received 14 October 2004; accepted 14 December 2004 Communicated by M. Roth Available online 9 February 2005
Abstract Gd1.99xYxCe0.01SiO5 (Ce:GYSO) crystals (x ¼ 0; 0.0995, 0.199) have been grown by the Czochralski (Cz) method. Crystal structure and the distribution coefficients of Ce have been determined for all three crystals. Spectroscopic measurements indicate that optical transmittance and luminescence intensity of Gd1.99xYxCe0.01SiO5 (x ¼ 0:0995; 0.199) crystals are substantially higher than those of Ce:Gd2SiO5 (Ce:GSO), especially at x ¼ 0:0995; which makes them good candidate materials for scintillation applications. The particularly important result is that the alloyed Ce:GYSO crystals can be grown easily by the Cz method and, unlike Ce:GSO, they do not undergo cleavage during the growth process or subsequent mechanical treatment. r 2005 Elsevier B.V. All rights reserved. PACS: 74.25.Gz; 64.75.+g; 62.20.Mk; 61.72.Ww Keywords: A1. Ce distribution coefficient; A1. Crystal structure; A1. Optical spectra; A2. Czochralski method; B1. Ce:GYSO
1. Introduction Corresponding author. Shanghai Institute of Optics and
Fine Mechanics, Chinese Academy of Sciences, Qinghe Road, 390, P. O. Box 800-211, shanghai 201800, PR China. Tel.: +86 21 69915174; fax: +86 21 69918607. E-mail addresses: [email protected] (M. Jie), [email protected] (G. Zhao).
Cerium-doped rare earth oxyorthosilicate crystals are newly found to be excellent scintillators with high light yield and fast decay time. Ce-doped gadolinium oxyorthosilicate, Ce:Gd2SiO5 (Ce:GSO), and yttrium oxyorthosilicate, Ce:Y2SiO5 (Ce:YSO), are typical representatives of this family
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.12.160
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Table 1 Physical parameters and scintillation properties of blue-emitting scintillators Ce:GSOCe:YSOBGONal:Tl Effective atomic number 59 6.71 Density (g/cm3) Emission peak (nm) 430 Light output (%Nal:Tl ) 20 Decay constant (ns) 60 Energy resolution, FWHM (137Cs)10% Refractive index 1.85 Radiation hardiness (rad) 4108 Melting point (1C) 1950 Cleavage plane (1 0 0)
34 4.54 420 25 37 7–8% 1.80 — 1980 no
74 51 7.13 3.67 480 415 7–10 100 300 230 9.5%7.0% 2.15 1.85 1056103 1050 651 no (1 0 0)
of crystals [1]. The structure of GSO belongs to the monoclinic space group P21/c and is composed of a two-dimensional network of corner linked (OGd4) tetrahedra into which the (SiO4) tetrahedra are packed. The relatively weak bonding between these layers results in a strong tendency to cleave along the (1 0 0) plane. In comparison, the crystal structure of YSO belongs to the monoclinic C2/c space group. Hereby, the (SiO4) and (OY4) tetrahedra share edges and form chains interconnected by isolated (SiO4) tetrahedra, and this arrangement results in a more rigid and isotropic structure [2]. The main physical parameters and scintillation properties of Ce:GSO and Ce:YSO, along with such conventional scintillators as NaI (Tl) and BGO, are summarized in Table 1. In comparison with the traditional BGO scintillation crystal, the light output of Ce:GSO is twice as high, and the decay time is 1/5 of that of BGO. Moreover, irradiation hardness of the Ce:GSO crystal is quite prominent, so it can be widely used in a variety of applications of highenergy physics and nuclear physics, nuclear medical imaging (PET ), oil well survey, etc. [3,4]. Although the Ce:GSO crystal has excellent scintillation properties and high density (6.71 g/ cm3), the severe cleavage along the (1 0 0) plane during crystal growth, cooling, and further mechanical processing makes practical applications of the crystal very difficult. In contrast, the
Ce:YSO crystal exhibits good mechanical properties. We can speculate that Ce-doped GSO alloyed with YSO may exhibit no cleavage, unlike the pure Ce:GSO crystal. To our knowledge, similar alloyed crystals, such as Ce:LGSO (Ce:Lu1xGdxSiO5) and Ce:LYSO (Ce:Lu1xYxSiO5) [5,6], have been grown successfully by the Czochralski (Cz) method without exhibiting significant cleavage. They show good scintillation properties and have been protected by a patent [7]. Yet, there is no report on growing Ce:GYSO crystals until now. In this paper, we aim at investigating the growth process of the alloyed Gd1.99xYxCe0.01SiO5 (Ce:GYSO) crystals (x ¼ 0; 0.0995, 0.199) by the Cz method and analyzing their crystalline structure, the distribution coefficients of Ce and optical properties of Gd1.99xYxCe0.01SiO5 (x ¼ 0; 0.0995, 0.199).
2. Experimental procedure Single crystals of Gd1.99xYxCe0.01SiO5 crystal (x ¼ 0; 0.0995, 0.199) were grown by the Cz method in inductively heated iridium crucibles under nitrogen ambient atmosphere. The charges were prepared from Gd2O3 (4N), Y2O3 (4N), Ce2O3 (4N) and SiO2 (5N). The starting materials were fired at 1000 1C for more than 10 h prior to weighing and mixing to remove moisture, then pressed into pellets and sintered at 1200 1C before loading into the iridium crucible. The seed crystals were oriented at 601 relatively to the [0 1 0] axis. Crystal structure was investigated using X-ray powder diffraction (Rigaku Corporation, D/max 2550 V X-ray diffractometer with Cu Ka1 radiation). Gd, Y and Ce concentrations were measured by ICP (Inductively coupled plasma atomic emission spectrometer). Samples of identical size were cut from the top parts of crystals and carefully polished into 0.5 mm thickness wafers. Optical absorption spectra were taken using JASCO model V-570 UV/VIS/NIR Spectrophotometer V-570, and luminescence spectra were measured by JASCO model FP-6500 Spectrophotometer. The wavelength ranges of measurements were 190–1000 and 220–750 nm, respectively. For
ARTICLE IN PRESS M. Jie et al. / Journal of Crystal Growth 277 (2005) 175–180
different crystal samples, the conditions of measurements were kept constant. All measurements were performed at room temperature.
3. Results and discussion Fig. 1 shows the photographs of Ce:GSO (Gd1.99Ce0.01SiO5) and Ce:GYSO (Gd1.99xYxCe0.01SiO5, x ¼ 0:0995 and 0.199) crystals grown by the Cz method under similar conditions. As shown in Fig. 1a, the surface of Ce:GSO is rough and heavily cracked along the (1 0 0) cleavage plane. On the contrary, the Ce:GYSO crystals shown in Figs. 1b and c are smooth and crystallographically perfect. These results indicate that Ce:GYSO with appropriate content of Y2O3 can be easily grown to have a higher degree of crystalline perfection than Ce:GSO. Fig. 2 shows the XRD patterns of Ce:GSO and Gd1.99xYxCe0.01SiO5 (x ¼ 0:199) crystals, which are nearly identical. This result confirms that the Gd1.99xYxCe0.01SiO5 (x ¼ 0:199) crystal still maintains the monoclinic crystallographic structure belonging to the space group P21/c.
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The measured lattice parameters and densities of Gd1.99xYxCe0.01SiO5 (x ¼ 0; 0.0995, 0.199) crystals are listed in Table 2. With the increased content of yttrium, the lattice parameters a; b; c and b vary in the range of 0–0.3% compared with those of Ce:GSO and the density is reduced by about 3%. Therefore, the changes of lattice parameters and densities are insignificant. The reported distribution coefficient of Ce in GSO is about 0.6, while that in YSO is about 0.34 F
B
25
30
35
40
45
50
55
2 Theta / ˚ Fig. 2. XRD curves of Ce:GSO (B) and Gd1.99xYxCe0.01SiO5 (x ¼ 0:199) (F).
Fig. 1. Photographs of crystals: Ce:GSO (a), Gd1.99xYxCe0.01SiO5 (x ¼ 0:0995) (b) and Gd1.99-xYxCe0.01SiO5 (x ¼ 0:199) (c) grown by the Cz method under similar conditions.
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Table 2 Lattice parameters and densities of Gd1.99xYxCe0.01SiO5 (x ¼ 0, 0.0995, 0.199) and YSO crystals grown by the Cz method Crystal
Ce:GSO [2] Ce:GSO (this work) Ce:GYSO (x ¼ 0:0995) Ce:GYSO (x ¼ 0:199) YSO [2]
Density (g/cm3)
Unit cell parameters a(A˚)
b(A˚)
c(A˚)
b(deg)
9.12 9.14005370.001577 9.13226970.001135 9.13464770.001693 10.41
7.06 7.05597370.000756 7.04982170.000716 7.03944470.001053 6.721
6.73 6.74903770.000818 6.74585970.000674 6.74390370.000977 12.49
107.5 107.523470.012501 107.496970.011207 107.469470.016612 102.65
6.71 6.71 6.60 6.49 4.54
Table 3 Composition and distribution coefficients of Ce in GYSO crystals (measured by ICP) Crystal
Charge
Top of the crystal
Distribution coefficient
Ce:GSO Ce: GYSO (x ¼ 0:0995) Ce: GYSO (x ¼ 0:199)
Gd1.99Ce0.01SiO5 Gd1.8905Y0.0995Ce0.01SiO5 Gd1.791Y0.199Ce0.01SiO5
Gd1.613Ce0.0047SiO5 Gd1.436Y0.082Ce0.0044SiO5 Gd1.736Y0.204Ce0.0056SiO5
0.581 0.578 0.576
[10]. Because of the difference between the ionic radii of the dopant Ce3+ and the substituted host crystal cation (the ratio becoming larger when Y3+ ions replace Gd3+ ions in Ce:GYSO [8]), it may be speculated that the distribution coefficient of Ce in GYSO must be lower than in the GSO crystal. Our experimental results listed in Table 3 show that the distribution coefficient of Ce decreases slightly indeed with the increasing amount of Y content in GSO. The above results indicate cumulatively that addition of Y into Ce:GYSO at concentrations up to about 10 at% (i.e., x ¼ 0:199) does not appreciably change the crystallographic structure. However, the crystalline quality of Ce:GYSO can be significantly improved. Optical Emission and absorption spectra of Ce:GYSO have been measured at room temperature, and they are shown in Figs. 3a and b respectively together with the Ce:GSO spectra. Apparently, the positions and shapes of the Ce:GYSO emission bands do not change relatively to the Ce:GSO crystal, but their integrated emission intensity almost doubles. According to the previous work [9], Ce:GSO and Ce:YSO both belonging to the monoclinic system, have two kinds of luminescence centers. For Ce:GSO, one
luminescence center quenches severely at room temperature, but it does not hold for Ce:YSO, Therefore, the luminescence quantum efficiency of Ce:GSO is lower than that of Ce:YSO at room temperature. As a result, the light yield of YSO is higher than that of GSO, as listed in Table 1. Therefore, it can be inferred that the stronger luminescence intensity of the Ce:GYSO crystal in comparison with Ce:GSO may result from the larger amount of active luminescence centers. The optical absorption edges of Ce-doped GSO and GYSO samples below about 400 nm are shown in Fig. 3b. They are associated with transitions from the 4f ground state to the lowest 5d sublevel of the Ce3+ ion, subjected to the crystal field splitting of the host. With the increase of Y content, the crystal field becomes stronger and, consequently, the absorption edge shifts to shorter wavelengths [10]. As shown in Fig. 4, the optical transmission edges of Ce:GYSO also shift to shorter wavelengths accordingly. In the wavelength range longer than 400 nm, the optical transmission of Ce-doped GYSO crystals increases to 82%, 6% higher than that of Ce:GSO. We recall from Fig. 3a, that the emission peak of Ce:GYSO peaks at 437 nm, namely in the high transparency wavelength range.
ARTICLE IN PRESS M. Jie et al. / Journal of Crystal Growth 277 (2005) 175–180
It is well known that the increase of the optical transmission will correspondingly improve the light output. Therefore, the higher transmission in the luminescence spectra region is also one of the causes for the integral emission intensity of Ce:GYSO being larger than that of Ce:GSO. Concurrently, it should be noted that both the transmission above 400 nm and the integrated emission intensity around 437 nm of Gd1.99xYxCe0.01SiO5 (x ¼ 0:0995) are higher than those of Gd1.99xYxCe0.01SiO5 (x ¼ 0:199). This indicates that the content of Y in Ce:GYSO must not exceed a certain value for higher light output to be achieved. Our experiments imply that the smaller Y content corresponding to x ¼ 0:0995 in Gd1.99xYxCe0.01SiO5 is more appropriate for scintillation applications.
Emission intensity /a.u.
800 0.95GSO/0.05YSO 0.9GSO/0.1YSO 600 GSO
400
200
0 400
500
Optical density / a.u.
(a)
600
700
Wavelength /nm
2.0
0.95GSO/0.05YSO
1.5
0.9GSO/0.1YSO
1.0
4. Conclusions
GSO 0.5 0.0 350
400
(b)
450
500
Wavelength /nm
Fig. 3. (a) Emission spectra of Ce-doped GSO and GYSO crystals peaking at 437 nm under 345 nm, excitation. (b) Absorption spectra of Ce-doped GSO and GYSO crystals (0.5 mm thickness).
0.95GSO/0.05YSO 80 Transmittance /%
179
0.9GSO/0.1YSO Ce:GSO
70
Cz growth has yielded high quality Gd1.99xYxCe0.01SiO5 (Ce:GYSO) single crystals (x ¼ 0:0995; 0.199) exhibiting no cleavage phenomena as compared with the simple oxyorthosilicate Gd1.99Ce0.01SiO5 (Ce:GSO) crystal. Substitution of Y for Gd in Ce:GYSO up to about 10 at% (x ¼ 0:199) does not appreciably change the lattice parameters of the monoclinic crystallographic structure and the distribution coefficient of Ce. Crystals of Gd1.99xYxCe0.01SiO5 with x ¼ 0:0995 have been found to exhibit the highest transmittance and integrated emission intensity, the latter about twice as high as that of the Gd1.99Ce0.01SiO5 (Ce:GSO) crystal. The combination of structural, chemical and optical characterization results obtained in this work indicate that the Gd1.99xYxCe0.01SiO5 (x ¼ 0:0995) crystal is a potentially better material than Ce:GSO for scintillation applications.
60
References 380
400
420
440
460
Wavelength /nm Fig. 4. Transmission spectra of Ce-doped GSO and GYSO crystals (0.5 mm thickness).
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