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EXAFS investigation on the structural environment of Tm3+ in Ge–Ga–S–CsBr glasses
Journal of Non-Crystalline Solids 353 (2007) 1251–1254 www.elsevier.com/locate/jnoncrysol
EXAFS investigation on the structural environment of Tm3+ in Ge–Ga–S–CsBr glasses Jay Hyok Song a, Yong Gyu Choi b, Kohei Kadono c, Kohei Fukumi c, Hiroyuki Kageyama c, Jong Heo a,* a
Center for Information Materials and Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea b Department of Materials Science and Engineering, Hankuk Aviation University, Goyang, Gyeonggi 412-791, Republic of Korea c National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka, Ikeda, Osaka 563-8577, Japan Available online 23 March 2007
Abstract The local structures around Tm3+ in Ge0.25Ga0.10S0.65 and 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glasses were investigated using Extended X-ray absorption fine structure (EXAFS) spectroscopy. In Ge0.25Ga0.10S0.65 glass, Tm3+ ions are surrounded by approximately seven S ions. Addition of 10 mol% CsBr resulted in significant changes in the EXAFS spectrum of Tm3+ ions due to the changes in the local structure surrounding Tm3+ ions. The first-nearest coordination shell around Tm3+ ion is predominantly composed of about six Br ions in 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.70.a; 76.30.Kg; 78.55.Qr; 78.70.Dm Keywords: Chalcogenides; Chalcohalides; Rare-earths in glasses; X-ray absorption
1. Introduction Tm3+-doped sulfide glasses have been investigated to develop fiber-optic amplifiers for the S-band (1460– 1530 nm) communication window [1–3]. The radiative properties of the 1.48 lm stimulated emission from the Tm3+:3H4 ! 3H6 transition in Ge–Ga–As–S glasses were significantly improved upon the addition of CsBr [2,3]. The lifetime of the Tm3+:3H4 level increased to 1.18 from 0.20 ms with the incorporation of CsBr. Intensity of the 1.48 lm emission became higher than the 1.82 lm emission (3F4 ! 3H6) and the similar changes in optical properties of Dy3+ ions were also reported [4–6]. According to previous reports, the CsBr addition on Ge–Ga–S glasses signifi-
*
Corresponding author. Tel.: +82 54 279 2147; fax: +82 54 279 5872. E-mail address: [email protected] (J. Heo).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.09.045
cantly decreased multiphonon relaxation rates of rareearth ions [4–7]. This decrease became possible due to the change in the effective phonon energy from 375 to 245 cm1 via the formation of Ga–Br bond. Phonon side band (PSB) analyses on the local structure around Eu3+ ions in Ge–Ga–S–CsBr (or CsCl) glasses also suggested the existence of Ga–Br bonds next to rare-earth ions [8]. However, there has been no report on direct observation of the local environment of rare-earth ions in sulfide as well as Ge–Ga–S–CsBr glasses. This work is concerned with the local environment of Tm3+ ions in sulfide and chalcohalide glasses. The local environment of Tm3+ in (1 x) (Ge0.25Ga0.10S0.65) xCsBr (in mole fraction, x = 0.00 and 0.10) glasses was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. Analysis of the Tm L3-edge EXAFS spectra showed that Tm3+ ion was coordinated with approximately seven S ions in Ge0.25Ga0.10S0.65
J.H. Song et al. / Journal of Non-Crystalline Solids 353 (2007) 1251–1254
glasses. Upon the addition of 10 mol% CsBr, Tm3+ ion environment was changed to Br ions. Relation between the local structure and emission properties of Tm3+ is discussed. 2. Experimental procedures Nominal compositions of host glasses were (1 x) (Ge0.25Ga0.10S0.65) xCsBr in mole fraction, where x = 0.00 and 0.10. All samples were prepared from highpurity powders in their elemental forms (>99.99%). Tm metal (>99.9%) was used as a source of Tm3+ ions. The concentrations of Tm3+ in Ge0.25Ga0.10S0.65 and 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glasses were 1.00 and 0.25 mol%, respectively. Starting materials were melted at 950 °C for 12 h and the glasses thus obtained were annealed for one hour at near glass-transition temperature. Detailed fabrication procedures can be found elsewhere [2]. EXAFS spectra of Tm L3-edge (8648 eV) were recorded at the Beam Line 9C and 12C of Photon Factory, the National Laboratory for High Energy Physics (KEK, Tsukuba, Japan). Spectra of Tm2S3 and TmBr3 were measured with the transmission mode to prevent distortion of the EXAFS spectra. Glasses containing Tm3+ ions were recorded by the fluorescence method using the Lytle detector with a Co filter. The spectra were analyzed following the standard method of UWXAFS software [9]. Scattering amplitudes and phase shifts of Tm–S and Tm–Br pairs were calculated using the FEFF program [10]. Non-linear least-squares fittings were carried out in R-space using the FEFFIT program [11]. 3. Results Fig. 1 shows the k3-weighted v(k) spectra of Tm3+ ions in both crystals and glasses. Background of measured EXAFS spectra was subtracted using fourth-order splines
10
10
TmBr3
k3-weighted χ (k) (a.u.)
Tm2S3 5
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GeGaS-CsBr glass
GeGaS glass
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Photoelectron momentum (A-1) Fig. 1. k3-weighted v(k) spectra of Tm L3-edge EXAFS spectra.
12
to minimize the magnitude of the Fourier transform of ˚ . All parameters used in EXAFS spectrum below 1.4 A the data reduction processes were kept constant for both glasses and crystals. As shown in Fig. 1, the spectrum recorded from the Ge0.25Ga0.10S0.65 glass was similar to that of Tm2S3. However, an addition of 10 mol% CsBr resulted in significant changes in shape of k3-weighted v(k) spectrum of Tm3+ in glasses, indicating modifications of local structure around Tm3+ ions. The shape of Tm3+ EXAFS spectrum of CsBr-glass was similar to the spectrum from TmBr3. The partial radial distribution function (RDF) spectra of crystals and glasses are shown in Fig. 2. Fourier transformations of v(k) with a k3-weighting factor were performed ˚ 1. The main for the range of 2.86 ± 0.08–11.21 ± 0.12 A ˚ , after phase peaks in RDF curves of Tm2S3 (2.74 ± 0.01 A ˚ correction) and TmBr3 (2.79 ± 0.01 A, after phase correction) represent the first neighbor shells around Tm in each crystals, respectively [12–15]. As shown in Fig. 2, the peak position in the RDF curves of glasses changed with CsBr addition and became similar to that of TmBr3. These results indicate that Tm3+ ions are mainly surrounded by Br ions in 0.90 (Ge0.25Ga0.10S0.65) 0.1CsBr glass. Table 1 shows results of the fitting on the first coordination shell around Tm3+ ions. In Tm2S3 and Ge0.25Ga0.10S0.65 glass, the fitting was performed by assuming the presence of Tm-S pairs for the interatomic distance of ˚ . For the TmBr3 and glass con1.52 ± 0.15–2.80 ± 0.05 A taining CsBr, based on the similarities between EXAFS spectra of TmBr3 and 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass, Tm-Br pairs were used to analyze the first coordination shell of Tm3+ ions. The fitting range was 1.88 ± 0.05– ˚ (in Fig. 2). Result of the least-square fitting 2.84 ± 0.06 A showed that Tm3+ ions were surrounded by 6.77 (±0.85) number of S ions in Ge0.25Ga0.10S0.65 glass. Upon the addition of CsBr, the first coordination shell around Tm3+ ions were consisted with 5.86 (±1.58) Br ions. A fit-
Magnitude of Fourier transform (a.u.)
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Tm2S3
10
10
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data fit
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GGS glass
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Distance (A) Fig. 2. Radial distribution function (RDF) curves of Tm3+ ions. Note that the phase shifts were not corrected.
J.H. Song et al. / Journal of Non-Crystalline Solids 353 (2007) 1251–1254
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Table 1 Coordination numbers (N), bond distances (R) and Debye–Waller factors (r2) of Tm–S and Tm–Br bonds in crystals and glasses, along with R-factors showing the validity of fitting ˚) ˚ 2) Bond Composition R (A N r2 (A R-factor Tm2S3 crystal Ge0.25Ga0.10S0.65 glass TmBr3 crystal 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass
Tm–S Tm–Br * Values
2.74 2.77 2.79 2.79
(0.01) (0.01) (0.01) (0.01)
6.50 6.77 6.00 5.86
(fixed) (0.85) (fixed) (1.58)
0.0115 0.0107 0.0066 0.0083
(0.0015) (0.0014) (0.0003) (0.0018)
0.013 0.015 0.001 0.020
in the parentheses are the estimated uncertainties.
ting using two subshells of Tm–S and Tm–Br pairs were also attempted for the glass containing CsBr. Bond distances and Debye-Waller factors were fixed for each pair using the results in Table 1 to decrease the number of free variables. In this case, coordination numbers of S and Br thus obtained were 0.49 ± 0.65 and 6.17 ± 0.32, respectively. This result again supports that Tm3+ ions are predominantly surrounded by Br ions in 0.90 (Ge0.25Ga0.10S0.65) 0.1CsBr glass. 4. Discussion The Tm L3-edge EXAFS results showed that the firstshell around Tm3+ ions is composed of S ions in Ge0.25Ga0.10S0.65 glass. It is known that rare-earth solubility of GeS2 glass increases significantly with Ga2S3 addition [16]. According to the previous report, increase in the rare-earth solubility is related to the presence of edge-sharing GaS4 tetrahedra with Ga2S3 addition [16]. Rare-earths can be dissolved into glass matrix by breaking these edgesharing mode and accepting the role of charge compensators for non-bridging S ions. It was also suggested that, in Ga–La–S glasses, rare-earth ions are located next to the non-bridging S ions of GaS4 tetrahedra similar to those of Ge–Ga–S glasses [17]. Our current EXAFS analysis on Ge0.25Ga0.10S0.65 glass also indicated that Tm3+ ions are coordinated with sulfur ions, in accordance with the previous reports. Upon CsBr addition, the coordination environment of Tm3+ showed pronounced changes and Tm3+ ions were mainly surrounded by Br ions in 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass. It should be noted that the number of S ions are approximately 6 times larger than Br ions in 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass. The preferred coordination of Tm3+ ions with Br instead of S are most probably related to the formation of [GaS3/2Br] structural units in the glass matrix. It was reported that Ga ions make bonds with Br and form [GaS3/2Br] structural units in Ge0.25Ga0.10S0.65 glass upon CsBr addition [6,18]. In 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass, where the ratio between Ga and CsBr is close to unity, most Ga ions form [GaS3/2Br] tetrahedra. In this case, [GaS3/2Br] tetrahedral units need charge compensator and rare-earths ions can be dissolved by assuming the role of charge compensators for these tetrahedral units. Therefore, Tm3+ ions (or
rare-earth ions in general) experience the reduced multiphonon relaxation and local refractive index compared to the glasses without CsBr. As a result, the emission properties showed significant improvement as reported [4,5]. 5. Conclusions Tm L3-edge EXAFS spectra were analyzed for (1 x) (Ge25Ga0.10S0.65) xCsBr glasses (x = 0.00 and 0.10, in mole fraction) to investigate changes in the local structure around Tm3+ ions upon CsBr addition. The k3-weighted v(k) spectrum and the RDF curve of Tm3+ ions in Ge25Ga0.10S0.65 glass were significantly modified upon CsBr addition. Tm3+ ions in the sulfide glass were mainly surrounded by approximately seven S ions while they were coordinated by 6 Br ions in 0.90 (Ge25Ga0.10S0.65) 0.10CsBr glass. Acknowledgements This work has been performed under the approval of the Photon Factory Program Advisory Committee. (Proposal No. 2005G246) This work was financially supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-042-D00160 and KRF-2005-005-J13101) and SRC/ERC program of MOST/KOSEF (R11-2003-006). The authors would like to thank Dr. Katsumi Handa and Ms. Junko Ide of Synchrotron Radiation Center at Ritsumeikan University who provided valuable assistance during the EXAFS experiments. References [1] J.Y. Allain, M. Monerie, H. Poignant, Electron. Lett. 25 (1989) 1660. [2] J.H. Song, J. Heo, S.H. Park, J. Appl. Phys. 97 (2005) 083542. [3] D.J. Lee, J. Heo, S.H. Park, J. Non-Cryst. Solids 331 (2003) 184. [4] Y.B. Shin, J. Heo, Chem. Phys. Lett. 317 (2000) 637. [5] Y.B. Shin, J. Heo, H.S. Kim, J. Mat. Res. 16 (2001) 1318. [6] Y.G. Choi, R.J. Curry, B.J. Park, K.H. Kim, J. Heo, D.W. Hewak, Chem. Phys. Lett. 403 (2005) 29. [7] Y.S. Han, D.J. Lee, J. Heo, J. Non-Cryst. Solids 321 (2003) 210. [8] W.J. Chung, J. Heo, J. Am. Ceram. Soc. 86 (2003) 286. [9] E.A. Stern, M. Newville, B. Ravel, Y. Yacoby, D. Haskel, Physica B 209 (1995) 117.
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[10] A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson, Phys. Rev. B 58 (1998) 7565. [11] M. Newville, B. Ravel, D. Haskel, J.J. Rehr, E.A. Stern, Y. Yacoby, Physica B 208& 209 (1995) 154. [12] J.G. White, P.N. Yocom, S. Lerner, Inorg. Chem 6 (1967) 1872. [13] T. Schleid, F. Lissner, J. Alloy. Compd. 189 (1992) 69.
[14] [15] [16] [17]
D. Brown, S. Fletcher, D.G. Holah, J. Chem. Soc. A 8 (1968) 889. N.E. Brese, M. O’Keeffe, Acta Crystallogr. B 47 (1991) 92. J. Heo, J.M. Yoon, S.-Y. Ryou, J. Non-Cryst. Solids 238 (1998) 115. S. Benazeth, M.H. Tulier, A.M. Loireau-Lozac’H, H. Dexpert, P. Lagarde, J. Flahaut, J. Non-Cryst. Solids 110 (1989) 89. [18] J.H. Song, Y.G. Choi, J. Heo, J. Non-Cryst. Solids 352 (2006) 423.
EXAFS investigation on the structural environment of Tm3+ in Ge–Ga–S–CsBr glasses Jay Hyok Song a, Yong Gyu Choi b, Kohei Kadono c, Kohei Fukumi c, Hiroyuki Kageyama c, Jong Heo a,* a
Center for Information Materials and Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea b Department of Materials Science and Engineering, Hankuk Aviation University, Goyang, Gyeonggi 412-791, Republic of Korea c National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka, Ikeda, Osaka 563-8577, Japan Available online 23 March 2007
Abstract The local structures around Tm3+ in Ge0.25Ga0.10S0.65 and 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glasses were investigated using Extended X-ray absorption fine structure (EXAFS) spectroscopy. In Ge0.25Ga0.10S0.65 glass, Tm3+ ions are surrounded by approximately seven S ions. Addition of 10 mol% CsBr resulted in significant changes in the EXAFS spectrum of Tm3+ ions due to the changes in the local structure surrounding Tm3+ ions. The first-nearest coordination shell around Tm3+ ion is predominantly composed of about six Br ions in 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.70.a; 76.30.Kg; 78.55.Qr; 78.70.Dm Keywords: Chalcogenides; Chalcohalides; Rare-earths in glasses; X-ray absorption
1. Introduction Tm3+-doped sulfide glasses have been investigated to develop fiber-optic amplifiers for the S-band (1460– 1530 nm) communication window [1–3]. The radiative properties of the 1.48 lm stimulated emission from the Tm3+:3H4 ! 3H6 transition in Ge–Ga–As–S glasses were significantly improved upon the addition of CsBr [2,3]. The lifetime of the Tm3+:3H4 level increased to 1.18 from 0.20 ms with the incorporation of CsBr. Intensity of the 1.48 lm emission became higher than the 1.82 lm emission (3F4 ! 3H6) and the similar changes in optical properties of Dy3+ ions were also reported [4–6]. According to previous reports, the CsBr addition on Ge–Ga–S glasses signifi-
*
Corresponding author. Tel.: +82 54 279 2147; fax: +82 54 279 5872. E-mail address: [email protected] (J. Heo).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.09.045
cantly decreased multiphonon relaxation rates of rareearth ions [4–7]. This decrease became possible due to the change in the effective phonon energy from 375 to 245 cm1 via the formation of Ga–Br bond. Phonon side band (PSB) analyses on the local structure around Eu3+ ions in Ge–Ga–S–CsBr (or CsCl) glasses also suggested the existence of Ga–Br bonds next to rare-earth ions [8]. However, there has been no report on direct observation of the local environment of rare-earth ions in sulfide as well as Ge–Ga–S–CsBr glasses. This work is concerned with the local environment of Tm3+ ions in sulfide and chalcohalide glasses. The local environment of Tm3+ in (1 x) (Ge0.25Ga0.10S0.65) xCsBr (in mole fraction, x = 0.00 and 0.10) glasses was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. Analysis of the Tm L3-edge EXAFS spectra showed that Tm3+ ion was coordinated with approximately seven S ions in Ge0.25Ga0.10S0.65
J.H. Song et al. / Journal of Non-Crystalline Solids 353 (2007) 1251–1254
glasses. Upon the addition of 10 mol% CsBr, Tm3+ ion environment was changed to Br ions. Relation between the local structure and emission properties of Tm3+ is discussed. 2. Experimental procedures Nominal compositions of host glasses were (1 x) (Ge0.25Ga0.10S0.65) xCsBr in mole fraction, where x = 0.00 and 0.10. All samples were prepared from highpurity powders in their elemental forms (>99.99%). Tm metal (>99.9%) was used as a source of Tm3+ ions. The concentrations of Tm3+ in Ge0.25Ga0.10S0.65 and 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glasses were 1.00 and 0.25 mol%, respectively. Starting materials were melted at 950 °C for 12 h and the glasses thus obtained were annealed for one hour at near glass-transition temperature. Detailed fabrication procedures can be found elsewhere [2]. EXAFS spectra of Tm L3-edge (8648 eV) were recorded at the Beam Line 9C and 12C of Photon Factory, the National Laboratory for High Energy Physics (KEK, Tsukuba, Japan). Spectra of Tm2S3 and TmBr3 were measured with the transmission mode to prevent distortion of the EXAFS spectra. Glasses containing Tm3+ ions were recorded by the fluorescence method using the Lytle detector with a Co filter. The spectra were analyzed following the standard method of UWXAFS software [9]. Scattering amplitudes and phase shifts of Tm–S and Tm–Br pairs were calculated using the FEFF program [10]. Non-linear least-squares fittings were carried out in R-space using the FEFFIT program [11]. 3. Results Fig. 1 shows the k3-weighted v(k) spectra of Tm3+ ions in both crystals and glasses. Background of measured EXAFS spectra was subtracted using fourth-order splines
10
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TmBr3
k3-weighted χ (k) (a.u.)
Tm2S3 5
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GeGaS glass
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Photoelectron momentum (A-1) Fig. 1. k3-weighted v(k) spectra of Tm L3-edge EXAFS spectra.
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to minimize the magnitude of the Fourier transform of ˚ . All parameters used in EXAFS spectrum below 1.4 A the data reduction processes were kept constant for both glasses and crystals. As shown in Fig. 1, the spectrum recorded from the Ge0.25Ga0.10S0.65 glass was similar to that of Tm2S3. However, an addition of 10 mol% CsBr resulted in significant changes in shape of k3-weighted v(k) spectrum of Tm3+ in glasses, indicating modifications of local structure around Tm3+ ions. The shape of Tm3+ EXAFS spectrum of CsBr-glass was similar to the spectrum from TmBr3. The partial radial distribution function (RDF) spectra of crystals and glasses are shown in Fig. 2. Fourier transformations of v(k) with a k3-weighting factor were performed ˚ 1. The main for the range of 2.86 ± 0.08–11.21 ± 0.12 A ˚ , after phase peaks in RDF curves of Tm2S3 (2.74 ± 0.01 A ˚ correction) and TmBr3 (2.79 ± 0.01 A, after phase correction) represent the first neighbor shells around Tm in each crystals, respectively [12–15]. As shown in Fig. 2, the peak position in the RDF curves of glasses changed with CsBr addition and became similar to that of TmBr3. These results indicate that Tm3+ ions are mainly surrounded by Br ions in 0.90 (Ge0.25Ga0.10S0.65) 0.1CsBr glass. Table 1 shows results of the fitting on the first coordination shell around Tm3+ ions. In Tm2S3 and Ge0.25Ga0.10S0.65 glass, the fitting was performed by assuming the presence of Tm-S pairs for the interatomic distance of ˚ . For the TmBr3 and glass con1.52 ± 0.15–2.80 ± 0.05 A taining CsBr, based on the similarities between EXAFS spectra of TmBr3 and 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass, Tm-Br pairs were used to analyze the first coordination shell of Tm3+ ions. The fitting range was 1.88 ± 0.05– ˚ (in Fig. 2). Result of the least-square fitting 2.84 ± 0.06 A showed that Tm3+ ions were surrounded by 6.77 (±0.85) number of S ions in Ge0.25Ga0.10S0.65 glass. Upon the addition of CsBr, the first coordination shell around Tm3+ ions were consisted with 5.86 (±1.58) Br ions. A fit-
Magnitude of Fourier transform (a.u.)
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Tm2S3
10
10
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data fit
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Distance (A) Fig. 2. Radial distribution function (RDF) curves of Tm3+ ions. Note that the phase shifts were not corrected.
J.H. Song et al. / Journal of Non-Crystalline Solids 353 (2007) 1251–1254
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Table 1 Coordination numbers (N), bond distances (R) and Debye–Waller factors (r2) of Tm–S and Tm–Br bonds in crystals and glasses, along with R-factors showing the validity of fitting ˚) ˚ 2) Bond Composition R (A N r2 (A R-factor Tm2S3 crystal Ge0.25Ga0.10S0.65 glass TmBr3 crystal 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass
Tm–S Tm–Br * Values
2.74 2.77 2.79 2.79
(0.01) (0.01) (0.01) (0.01)
6.50 6.77 6.00 5.86
(fixed) (0.85) (fixed) (1.58)
0.0115 0.0107 0.0066 0.0083
(0.0015) (0.0014) (0.0003) (0.0018)
0.013 0.015 0.001 0.020
in the parentheses are the estimated uncertainties.
ting using two subshells of Tm–S and Tm–Br pairs were also attempted for the glass containing CsBr. Bond distances and Debye-Waller factors were fixed for each pair using the results in Table 1 to decrease the number of free variables. In this case, coordination numbers of S and Br thus obtained were 0.49 ± 0.65 and 6.17 ± 0.32, respectively. This result again supports that Tm3+ ions are predominantly surrounded by Br ions in 0.90 (Ge0.25Ga0.10S0.65) 0.1CsBr glass. 4. Discussion The Tm L3-edge EXAFS results showed that the firstshell around Tm3+ ions is composed of S ions in Ge0.25Ga0.10S0.65 glass. It is known that rare-earth solubility of GeS2 glass increases significantly with Ga2S3 addition [16]. According to the previous report, increase in the rare-earth solubility is related to the presence of edge-sharing GaS4 tetrahedra with Ga2S3 addition [16]. Rare-earths can be dissolved into glass matrix by breaking these edgesharing mode and accepting the role of charge compensators for non-bridging S ions. It was also suggested that, in Ga–La–S glasses, rare-earth ions are located next to the non-bridging S ions of GaS4 tetrahedra similar to those of Ge–Ga–S glasses [17]. Our current EXAFS analysis on Ge0.25Ga0.10S0.65 glass also indicated that Tm3+ ions are coordinated with sulfur ions, in accordance with the previous reports. Upon CsBr addition, the coordination environment of Tm3+ showed pronounced changes and Tm3+ ions were mainly surrounded by Br ions in 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass. It should be noted that the number of S ions are approximately 6 times larger than Br ions in 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass. The preferred coordination of Tm3+ ions with Br instead of S are most probably related to the formation of [GaS3/2Br] structural units in the glass matrix. It was reported that Ga ions make bonds with Br and form [GaS3/2Br] structural units in Ge0.25Ga0.10S0.65 glass upon CsBr addition [6,18]. In 0.90 (Ge0.25Ga0.10S0.65) 0.10CsBr glass, where the ratio between Ga and CsBr is close to unity, most Ga ions form [GaS3/2Br] tetrahedra. In this case, [GaS3/2Br] tetrahedral units need charge compensator and rare-earths ions can be dissolved by assuming the role of charge compensators for these tetrahedral units. Therefore, Tm3+ ions (or
rare-earth ions in general) experience the reduced multiphonon relaxation and local refractive index compared to the glasses without CsBr. As a result, the emission properties showed significant improvement as reported [4,5]. 5. Conclusions Tm L3-edge EXAFS spectra were analyzed for (1 x) (Ge25Ga0.10S0.65) xCsBr glasses (x = 0.00 and 0.10, in mole fraction) to investigate changes in the local structure around Tm3+ ions upon CsBr addition. The k3-weighted v(k) spectrum and the RDF curve of Tm3+ ions in Ge25Ga0.10S0.65 glass were significantly modified upon CsBr addition. Tm3+ ions in the sulfide glass were mainly surrounded by approximately seven S ions while they were coordinated by 6 Br ions in 0.90 (Ge25Ga0.10S0.65) 0.10CsBr glass. Acknowledgements This work has been performed under the approval of the Photon Factory Program Advisory Committee. (Proposal No. 2005G246) This work was financially supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-042-D00160 and KRF-2005-005-J13101) and SRC/ERC program of MOST/KOSEF (R11-2003-006). The authors would like to thank Dr. Katsumi Handa and Ms. Junko Ide of Synchrotron Radiation Center at Ritsumeikan University who provided valuable assistance during the EXAFS experiments. References [1] J.Y. Allain, M. Monerie, H. Poignant, Electron. Lett. 25 (1989) 1660. [2] J.H. Song, J. Heo, S.H. Park, J. Appl. Phys. 97 (2005) 083542. [3] D.J. Lee, J. Heo, S.H. Park, J. Non-Cryst. Solids 331 (2003) 184. [4] Y.B. Shin, J. Heo, Chem. Phys. Lett. 317 (2000) 637. [5] Y.B. Shin, J. Heo, H.S. Kim, J. Mat. Res. 16 (2001) 1318. [6] Y.G. Choi, R.J. Curry, B.J. Park, K.H. Kim, J. Heo, D.W. Hewak, Chem. Phys. Lett. 403 (2005) 29. [7] Y.S. Han, D.J. Lee, J. Heo, J. Non-Cryst. Solids 321 (2003) 210. [8] W.J. Chung, J. Heo, J. Am. Ceram. Soc. 86 (2003) 286. [9] E.A. Stern, M. Newville, B. Ravel, Y. Yacoby, D. Haskel, Physica B 209 (1995) 117.
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[10] A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson, Phys. Rev. B 58 (1998) 7565. [11] M. Newville, B. Ravel, D. Haskel, J.J. Rehr, E.A. Stern, Y. Yacoby, Physica B 208& 209 (1995) 154. [12] J.G. White, P.N. Yocom, S. Lerner, Inorg. Chem 6 (1967) 1872. [13] T. Schleid, F. Lissner, J. Alloy. Compd. 189 (1992) 69.
[14] [15] [16] [17]
D. Brown, S. Fletcher, D.G. Holah, J. Chem. Soc. A 8 (1968) 889. N.E. Brese, M. O’Keeffe, Acta Crystallogr. B 47 (1991) 92. J. Heo, J.M. Yoon, S.-Y. Ryou, J. Non-Cryst. Solids 238 (1998) 115. S. Benazeth, M.H. Tulier, A.M. Loireau-Lozac’H, H. Dexpert, P. Lagarde, J. Flahaut, J. Non-Cryst. Solids 110 (1989) 89. [18] J.H. Song, Y.G. Choi, J. Heo, J. Non-Cryst. Solids 352 (2006) 423.