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The effects of ZnCl2 and ErCl3 on the vibrational spectra and structure of tellurite glasses
Journal of Non-Crystalline Solids 352 (2006) 690–694 www.elsevier.com/locate/jnoncrysol
The effects of ZnCl2 and ErCl3 on the vibrational spectra and structure of tellurite glasses Luı´s Fortes, Luı´s F. Santos *, M. Clara Gonc¸alves, Rui M. Almeida Departamento de Engenharia de Materiais/ICEMS Instituto Superior Te´cnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Available online 17 February 2006
Abstract A series of zinc tellurite glasses, containing up to 40 mol% ZnCl2 and doped with 1–10 mol% ErCl3, was prepared by melting and casting and their structure was analyzed by polarized Raman and variable incidence infrared reflection spectroscopies. Kramers–Kronig analysis of the infrared reflectivity led to the identification of the vibrational mode components. The Raman spectra are dominated by an intense, depolarized boson peak at 45 cm1 and a high frequency, polarized peak at 767 cm1. The introduction of ZnCl2 and ErCl3 modifiers led to a blue shift of the high frequency peak, while the intensity of the boson peak was found to increase continuously with the Er3+ content. It is shown that the erbium and zinc compounds both break TeAOATe bonds, introducing non-bridging chlorine species, connected mostly to the zinc atoms. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.70.a; 78.20.e; 78.30.j; 81.05.kf Keywords: Reflectivity; FTIR measurements; Raman spectroscopy; Tellurites
1. Introduction Tellurium oxide-based glasses are candidates for optical applications such as fast optical switches and broadband amplifiers [1,2]. These glasses exhibit wide infrared transparency, high refractive index and high non-linear refractive index; they also have good stability and corrosion resistance and they constitute an excellent matrix for active element doping, justifying a continuous technological interest [3]. The addition of halides to tellurium oxide-based glasses reduces the concentration of residual OH groups [4,5] and it has also been shown to increase the measured lifetime of Er3+ dopant ions at 1.5 lm [6]. The structure of TeO2-rich glasses consists of TeO4 trigonal bipyramidal building units, where one equatorial site is occupied by a pair of electrons and the equatorial oxygen in one unit is the axial site of the next, in analogy with a-TeO2 *
Corresponding author. Tel.: +351 218418113; fax: +351 218418132. E-mail address: [email protected] (L.F. Santos).
0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.052
[7,8]. The introduction of modifier ions destroys the threedimensional network, creating non-bridging oxygen (NBO) species and gradually transforming the TeO4 units into TeO3+1 and TeO3 [7,8]. As an intermediate, ZnO is believed to be incorporated into a chain-like structure of TeO3, TeO3+1 and TeO4 groups, connected by six-coordinated Zn2+ ions, where the relative concentrations of the tellurite units are a function of the actual composition [9–11]. This assortment of species provides a significant variety of structural sites for the incorporation of dopant species, such as erbium ions [2], leading to a large Er3+ solubility, for example, approximately up to 10 mol% in the TeO2–ZnO–ZnCl2 system [5]. Therefore, higher Er3+ content can be incorporated with reduced ion clustering and stronger photoluminescence efficiency [5]. On the other hand, erbium doping has been shown to have an effect on the structure of purely oxide zinc tellurite glasses [12,13]. Among the compositions in the TeO2–ZnO–ZnCl2 system, those with 20 ZnO mol% presented the highest glass-forming ability [5], therefore, the present study analyzes the vibrational spectra of oxychloride tellurite glasses
L. Fortes et al. / Journal of Non-Crystalline Solids 352 (2006) 690–694
[(80 x)TeO2–20ZnO–xZnCl2] containing up to 40 mol% ZnCl2, doped with 1–10 mol% ErCl3 and the structural changes which occur in these glasses with increasing zinc and erbium chloride contents. 2. Experimental A series of oxychloride tellurite glass samples (3 mm thick) were prepared, with the compositions given in Table 1. Mixtures of commercial powders (10 g batches) were melted in lidded vitreous silica crucibles, inside a nitrogen-filled glove box, for about 1 h, at 800 °C, after a first dehydration step of 2 h at 200 °C and poured into a stainless steel mould. Annealing of the samples was carried out for about 12 h at 240 °C. Further details are given elsewhere [5]. Although lidded crucibles were used chlorine losses are expected during melting, especially for higher zinc chloride content and glass compositions are likely to be somewhat different from the nominal ones. Infrared (IR) reflectance spectra (Nicolet 5DXC and 20F spectrometers, for medium and far IR, respectively) were recorded for some of the glasses, at 4 cm1 resolution with 400 scans per spectrum, at 10° (near-normal), 30° and
Table 1 Batch compositions of the tellurite glasses (in mol%) Glass a
820 + X 721 + Xa 622 + Xa 523 + Xa 424 + Xa a
TeO2
ZnO
ZnCl2
ErCl3
80 70 60 50 40
20 20 20 20 20
– 10 20 30 40
Xa Xa Xa Xa Xa
X = 0, 1, 2, 3, 5 and 10 mol%, on a 100 + X mol% basis.
691
50° off-normal incidence, for polished samples, by means of a variable angle specular reflectance attachment (Specac). Polarized Raman spectra, in the HH (k) and HV (?) configurations, were collected with a 90° scattering geometry, at room temperature and a resolution of 4 cm1. A double monochromator (Spex 1403), together with an argon ion laser (Spectra-Physics, mod. 2016) and a photomultiplier detector (Hamamatsu R928) were used, with one second of integration time, between 10 cm1 and 1500 cm1. The green and blue excitation lines produced fluorescence bands due to the presence of erbium; therefore, the 476.5 nm excitation was used, for which no fluorescence was observed. 3. Results 3.1. Infrared measurements Fig. 1 presents the IR specular reflectivity spectra obtained for the tellurite glass with 40 mol% ZnCl2 and 5 mol% ErCl3, at different angles of incidence. Although few changes occurred in the region below 500 cm1, the higher frequency band peaking at 663 cm1, observed at 10° and 30° off-normal, partly splits into 670 cm1 and 630 cm1 components, at 50° off-normal. Fig. 2 shows the TO (transverse optical ‘phonon’, with the atomic motion perpendicular to the ‘wavevector’) and LO (longitudinal optical ‘phonon’, with the atomic motion parallel to the ‘wavevector’) [14] spectral components for the same composition, calculated by Kramers–Kronig analysis, based on the 10° off-normal spectrum [14]. The high frequency TO peak occurred at 688 cm1, while the LO peak was at 763 cm1.
TO
LO
10˚
1000
Extinction (a.u.)
Reflectivity (a.u.)
30˚
50˚
800
600
Wavenumber
400
200
(cm-1)
Fig. 1. IR specular reflectivity spectra of 40TeO2–20ZnO–40ZnCl2 (mol%) glass, doped with 5 mol% ErCl3, at different off-normal incidence angles.
1000
900
800
700
600
500
400
300
200
Wavenumber (cm-1) Fig. 2. Calculated LO and TO spectra of 40TeO2–20ZnO–40ZnCl2 (mol%) glass, doped with 5 mol% ErCl3.
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622 622+1 622+3 622+5
0
200
400
600
Raman shift
820+2 820+5 820+10
(b)
Intensity (a.u.)
Intensity (a.u.)
(a)
800
1000
0
200
400
600
800
1000
Raman shift (cm-1)
(cm-1)
Fig. 3. Polarized (HH) Raman spectra normalized at the high frequency peak (767 cm1): (a) 60TeO2–20ZnO–20ZnCl2 glasses, containing 0–5 ErCl3 (mol%); (b) 80TeO2–20ZnO glasses, containing 2–10 ErCl3 (mol%).
3.2. Raman measurements The Raman spectra were dominated by a low frequency band at 45 cm1, the boson peak, which was weakly polarized (depolarization ratio DR = I?/Ik, 0.65) and did not shift significantly with the incorporation of zinc or erbium compounds. Small differences of up to 4 cm1 are within the instrumental errors (±2 cm1) and were not considered. Fig. 3 presents the polarized (HH) Raman spectra normalized at the 767 cm1 peak, for the 622 and the 820 compositions, where one can see an increase of the boson peak with increasing erbium content, with the exception of the 622 + 3 ErCl3 (mol%) composition. The introduction of zinc chloride did not lead to any consistent changes in the boson peak and its DR did not change with the ErCl3 or ZnCl2 contents. The polarized (HH) Raman spectra of several compositions, normalized at the boson peak (45 cm1), are shown in Figs. 4 and 5. The spectra of the undoped compositions
(Fig. 4) are dominated, in the high frequency region, by the band at 767 cm1, with a shoulder at 675 cm1, plus an intermediate frequency band at 420 cm1. An increase in the ZnCl2 content resulted in a decrease of the shoulder intensity, with respect to the 767 cm1 band, while the intensity of the intermediate band simultaneously decreased (apart from the 721 composition). A shift toward higher frequencies is observed for the main high frequency band while the intermediate band shifts toward lower frequencies (405 cm1, for the 40 mol% ZnCl2 composition), with increasing zinc chloride content. Two low frequency bands are also observed, at 120 cm1 and 280 cm1, as shown in detail in Fig. 6(a). The compositions doped with different Er concentrations presented somewhat different spectra (Fig. 5), with the highest frequency band decreasing and shifting toward higher frequencies (785 cm1, for the 10 mol% ErCl3 composition) with increasing erbium content. The 675 cm1 shoulder and the 420 cm1 band also decreased with
721 622 523 424
Intensity (a.u.)
Intensity (a.u.)
820+2 820+5 820+10
0
200
400
Raman shift
600
800
1000
(cm-1)
Fig. 4. Polarized (HH) Raman spectra of TeO2–ZnO–ZnCl2 glasses with varying TeO2 and ZnCl2 contents, normalized at the boson peak (45 cm1).
0
2 00
400
Raman shift
600
800
1000
(cm-1)
Fig. 5. Polarized (HH) Raman spectra of the 80TeO2–20ZnO glass composition (mol%), doped with 2, 5 and 10 mol% ErCl3, normalized at the boson peak (45 cm1).
L. Fortes et al. / Journal of Non-Crystalline Solids 352 (2006) 690–694
(a)
(b)
0
100
200
424+10 424+5 424+3 424+1 424
Intensity (a.u.)
Intensity (a.u.)
721 622 523 424
300
693
0
100
200
300
Raman shift (cm-1)
Raman shift (cm-1)
Fig. 6. Polarized (HH) low frequency Raman spectra of different oxychloride tellurite glasses: (a) TeO2–ZnO–ZnCl2 glasses; (b) 40TeO2–20ZnO–40ZnCl2 glasses, containing 0–10 ErCl3 (mol%).
1.0
Depolarisation ratio
0.8
0.6
0.4
622
0.2
622+10 0.0 0
200
400
600
800
1000
Raman shift (cm-1) Fig. 7. Depolarization ratio of 622 and 622 + 10 compositions.
increasing erbium content, but the 420 cm1 band presented a red shift of 30 cm1 between 2 and 10 mol% of ErCl3. The intensity of the 120 cm1 low frequency band simultaneously increased, as seen in Fig. 6(b). Apart from the boson peak, the Raman spectra of all samples were strongly polarized (DR < 0.4), with maximum degree of polarization (minima in DR) at 270 cm1 (DR 0.3), 420 cm1 (DR 0.2) and 830 cm1 (DR 0.15), for the undoped compositions (Fig. 7). Erbium doped compositions presented lower DR, with maximum degree of polarization at 270 cm1 (DR 0.2), 420 cm1 (DR 0.15) and 800 cm1 (DR 0.06). 4. Discussion The position of the highest frequency TO and LO components (Fig. 2) were not obtainable from the oblique incidence spectra (Fig. 1) with good accuracy, as opposed to what has been found in the case of other glasses [14]. On the other hand, the large calculated TO–LO splitting
(75 cm1) and the fact that the positions of the TO and LO components almost coincide with the two highest frequency Raman modes, suggests a significant degree of ionic character for these glasses and also of intermediate range order [14]. It is well established that the high frequency Raman bands at 420 and 767, plus the shoulder at 675 cm1 are all related to the presence of TeOx units. The 675 cm1 shoulder corresponds, in TeO2-rich glass compositions, to the higher intensity, high frequency band and has been assigned to the stretching vibrations of the TeO4 trigonal bypiramidal groups [7]. Introduction of glass modifiers, such as the zinc compounds, promotes the formation of NBO species and the TeO4 units are gradually transformed into TeO3+1 and TeO3 trigonal pyramid units, leading to an increase in the intensity of the 767 cm1 band [7–10]. This band is believed to be a superposition of TeO3+1 and TeO3 peaks, since, in crystalline ZnTeO3 and Zn2Te3O8, the TeO3+1 and TeO3 peaks appear at 740 cm1 and 790 cm1, respectively [7]. The 420 cm1 band is ascribed to TeAOATe bending modes [7–10]. An increase in the ZnCl2 content (Fig. 4) leads to a clear decrease in the 675 cm1 shoulder with respect to the 767 cm1 peak, revealing the change of the TeO4 backbone to TeO3+1 and TeO3 units, as expected. The same behavior is observed with increasing ErCl3 content in Fig. 5. Figs. 4 and 5 show that the addition of ZnCl2 and ErCl3 modifiers led to a blue shift of the high frequency peak, due to an increase in frequency in the TeO3 vibration, motivated by the increasing concentration of NBO species. This shift has also been observed [12] for erbium oxide doped glasses. One should notice, however, that this shift is continuous with the introduction of ErCl3 (at least up to 10 mol%, Fig. 5), while, after the addition of 20 mol% of ZnCl2 (Fig. 4), the shift was minimum, perhaps revealing an incorporation of the zinc chloride as modifier up to 20 mol% and as a network former beyond 20 mol%. The intensity of the 420 cm1 band generally decreased with an increase of either the zinc chloride or
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L. Fortes et al. / Journal of Non-Crystalline Solids 352 (2006) 690–694
erbium chloride contents (Figs. 4 and 5). Such an effect has also been observed in compositions without chlorine, where small additions of erbium oxide caused the same behavior [12,13]. This general trend shows that both the erbium and zinc compounds break the TeAOATe bonds. However, it is observed that erbium chloride has a stronger effect than zinc chloride in this respect, probably due to the fact that zinc chloride can be partially incorporated as a network former. The observed red shift is also more notorious for compositions with erbium chloride, than for TeO2–ZnO–ZnCl2 glasses. The two low frequency bands increased with the ErCl3 and ZnCl2 contents (Fig. 6), but they have distinct origins: the lower frequency shoulder 120 cm1 is also observed in purely oxide glasses [8–10,12], with or without erbium oxide and can tentatively be assigned to cage-like modes (cation site vibrations), involving erbium (or zinc) cations. On the other hand, the 270 cm1 band is dependent on the chloride content, since it increases with both the ZnCl2 and ErCl3 contents and is absent in purely erbium doped oxide compositions [12]. The minimum depolarization ratio for the 270 cm1 band (Fig. 7) suggests that it involves symmetric stretching vibrations of non-bridging anions about fixed network forming cations [15]. Since the totally symmetric mode (A1) of ZnCl2 tetrahedra is 4 known to occur at 276 cm1 [15], this band can be tentatively attributed to symmetric stretching vibrations of nonbridging chlorine ions about zinc cations. An increase of the boson peak with increasing erbium content, except for the 622 + 3 composition, is observed in Fig. 3(a). It was reported in the literature that such an increase is expected up to 1 mol% Er in purely oxide glasses, corresponding to the limit of rare-earth ion solubility without clustering [12,16], which should be followed by a decrease, corresponding to the occurrence of clustering. In order to compare our results with the previously reported data of Jaba et al. [12], we tested the same normalization procedure as they use (at the 675 cm1 shoulder), but a continuous increase of the boson peak with erbium concentration was still found. The present vibrational spectroscopy data is consistent with the model where TeO4 bipyramid units, containing both bridging and non-bridging oxygen species, are linked by ZnO groups to TeO3 trigonal pyramid units [9–11]. The ZnCl2 and ErCl3 additions break oxygen bridges and introduce non-bridging chlorine species, preferably connected to the zinc atoms, while erbium is always surrounded by oxygen atoms, with tellurium as a possible second near neighbor [17]. The addition of up to 10 mol% of ErCl3 to the tellurite compositions does not appear to cause the formation of Er3+-rich clusters, as it has been corroborated by Er3+ photoluminescence data at 1.5 lm and the corresponding 4I13/2 lifetime measurements [5,18] suggesting that the dispersion of Er3+ ions in the present oxychloride glasses (up to 10 mol%) is superior to that usually observed in purely oxide tellurite glasses (up to 4 mol%) [12,19].
5. Conclusions Increasing ErCl3 and ZnCl2 contents caused a changing of the TeO4 backbone of tellurite glasses to TeO3 pyramidal units, with non-bridging chlorine species preferably connected to the zinc atoms. The 420 cm1 band decreased with increasing ErCl3 and ZnCl2 contents, showing that both erbium and zinc ions break TeAOATe bonds, but ErCl3 had a stronger effect on this band than ZnCl2, probably due to the fact that ZnCl2 may also have a glass network forming role above 20 mol%. The lower frequency band 120 cm1 is related to cation vibrational modes, while the 280 cm1 band is due to Zn–Cl vibrations. The boson peak intensity generally increases with the amount of erbium chloride. Acknowledgements This work was supported by the Fundac¸a˜o para a Cieˆncia e a Tecnologia (FCT, Portugal), under contract POCTI/CTM/33307 and also by FSE/FEDER. We would also like to acknowledge the support of NSF’s International Materials Institute for New Functionality in Glass (IMI-NFG). References [1] M.E. Lines, J. Appl. Phys. 69 (1991) 6876. [2] A. Jha, S. Shen, M. Naftaly, Phys. Rev. B 62 (2000) 6215. [3] R.A.H. El-Mallawany, Tellurite Glasses Handbook – Physical Properties and Data, CRC, Boca Raton, USA, 2001. [4] B. Bridge, T.E. Bavins, D. Woods, T. Woolven, J. Non-Cryst. Solids 88 (1986) 262. [5] L.M. Fortes, L.F. Santos, M.C. Gonc¸alves, R.M. Almeida, J. NonCryst. Solids 324 (2003) 150. [6] D.L. Sidebottom, M.A. Hruschka, B.G. Potter, R.K. Brow, J. NonCryst. Solids 222 (1997) 282. [7] H. Burger, K. Kneipp, H. Hobert, W. Vogel, V. Kozhukharov, S. Neov, J. Non-Cryst. Solids 151 (1992) 134. [8] T. Sekiya, N. Mochida, A. Ohtsuka, J. Non-Cryst. Solids 168 (1994) 106. [9] M. Mazzuca, J. Portier, B. Tanguy, F. Romain, A. Fadli, S. Turrell, J. Mol. Struct. 349 (1995) 413. [10] C. Duverger, M. Bouazaoui, S. Turrell, J. Non-Cryst. Solids 220 (1997) 169. [11] S. Sakida, J. Jin, T. Yoko, Phys. Chem. Glasses 41 (2000) 65. [12] N. Jaba, A. Mermet, E. Duval, B. Champagnon, J. Non-Cryst. Solids 351 (2005) 833. [13] M.R. Sahar, Sulhadi, M.S. Rohani, in: Proceedings of the 2005 International Symposium on Glass, Shangai, 2005, p. SA1-4. [14] R.M. Almeida, Phys. Rev. B 45 (1992) 161. [15] R.M. Almeida, L.F. Santos, J. Am. Ceram. Soc. 72 (1989) 2065. [16] V.K. Tikhomorov, A. Jha, A. Perakis, E. Sarantopoulou, M. Naftaly, V. Krasteva, R. Li, A.B. Seddon, J. Non-Cryst. Solids 256&257 (1999) 98. [17] L.M. Fortes, L.F. Santos, M.C. Gonc¸alves, R.M. Almeida, F. D’Acapito, J. Non-Cryst. Solids 348 (2004) 11. [18] L.M. Fortes, L.F. Santos, M.C. Gonc¸alves, R.M. Almeida, M. Mattarelli, M. Montagna, A. Chiasera, M. Ferrari, A. Monteil, S. Chaussedent, G.C. Righini, Opt. Mater., submitted for publication. [19] M.R. Sahar, Sulhadi, M.S. Rohani, in: Proceedings of the XXth ICG in Kyoto, September 27th–October 1st, 2004.
The effects of ZnCl2 and ErCl3 on the vibrational spectra and structure of tellurite glasses Luı´s Fortes, Luı´s F. Santos *, M. Clara Gonc¸alves, Rui M. Almeida Departamento de Engenharia de Materiais/ICEMS Instituto Superior Te´cnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Available online 17 February 2006
Abstract A series of zinc tellurite glasses, containing up to 40 mol% ZnCl2 and doped with 1–10 mol% ErCl3, was prepared by melting and casting and their structure was analyzed by polarized Raman and variable incidence infrared reflection spectroscopies. Kramers–Kronig analysis of the infrared reflectivity led to the identification of the vibrational mode components. The Raman spectra are dominated by an intense, depolarized boson peak at 45 cm1 and a high frequency, polarized peak at 767 cm1. The introduction of ZnCl2 and ErCl3 modifiers led to a blue shift of the high frequency peak, while the intensity of the boson peak was found to increase continuously with the Er3+ content. It is shown that the erbium and zinc compounds both break TeAOATe bonds, introducing non-bridging chlorine species, connected mostly to the zinc atoms. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.70.a; 78.20.e; 78.30.j; 81.05.kf Keywords: Reflectivity; FTIR measurements; Raman spectroscopy; Tellurites
1. Introduction Tellurium oxide-based glasses are candidates for optical applications such as fast optical switches and broadband amplifiers [1,2]. These glasses exhibit wide infrared transparency, high refractive index and high non-linear refractive index; they also have good stability and corrosion resistance and they constitute an excellent matrix for active element doping, justifying a continuous technological interest [3]. The addition of halides to tellurium oxide-based glasses reduces the concentration of residual OH groups [4,5] and it has also been shown to increase the measured lifetime of Er3+ dopant ions at 1.5 lm [6]. The structure of TeO2-rich glasses consists of TeO4 trigonal bipyramidal building units, where one equatorial site is occupied by a pair of electrons and the equatorial oxygen in one unit is the axial site of the next, in analogy with a-TeO2 *
Corresponding author. Tel.: +351 218418113; fax: +351 218418132. E-mail address: [email protected] (L.F. Santos).
0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.052
[7,8]. The introduction of modifier ions destroys the threedimensional network, creating non-bridging oxygen (NBO) species and gradually transforming the TeO4 units into TeO3+1 and TeO3 [7,8]. As an intermediate, ZnO is believed to be incorporated into a chain-like structure of TeO3, TeO3+1 and TeO4 groups, connected by six-coordinated Zn2+ ions, where the relative concentrations of the tellurite units are a function of the actual composition [9–11]. This assortment of species provides a significant variety of structural sites for the incorporation of dopant species, such as erbium ions [2], leading to a large Er3+ solubility, for example, approximately up to 10 mol% in the TeO2–ZnO–ZnCl2 system [5]. Therefore, higher Er3+ content can be incorporated with reduced ion clustering and stronger photoluminescence efficiency [5]. On the other hand, erbium doping has been shown to have an effect on the structure of purely oxide zinc tellurite glasses [12,13]. Among the compositions in the TeO2–ZnO–ZnCl2 system, those with 20 ZnO mol% presented the highest glass-forming ability [5], therefore, the present study analyzes the vibrational spectra of oxychloride tellurite glasses
L. Fortes et al. / Journal of Non-Crystalline Solids 352 (2006) 690–694
[(80 x)TeO2–20ZnO–xZnCl2] containing up to 40 mol% ZnCl2, doped with 1–10 mol% ErCl3 and the structural changes which occur in these glasses with increasing zinc and erbium chloride contents. 2. Experimental A series of oxychloride tellurite glass samples (3 mm thick) were prepared, with the compositions given in Table 1. Mixtures of commercial powders (10 g batches) were melted in lidded vitreous silica crucibles, inside a nitrogen-filled glove box, for about 1 h, at 800 °C, after a first dehydration step of 2 h at 200 °C and poured into a stainless steel mould. Annealing of the samples was carried out for about 12 h at 240 °C. Further details are given elsewhere [5]. Although lidded crucibles were used chlorine losses are expected during melting, especially for higher zinc chloride content and glass compositions are likely to be somewhat different from the nominal ones. Infrared (IR) reflectance spectra (Nicolet 5DXC and 20F spectrometers, for medium and far IR, respectively) were recorded for some of the glasses, at 4 cm1 resolution with 400 scans per spectrum, at 10° (near-normal), 30° and
Table 1 Batch compositions of the tellurite glasses (in mol%) Glass a
820 + X 721 + Xa 622 + Xa 523 + Xa 424 + Xa a
TeO2
ZnO
ZnCl2
ErCl3
80 70 60 50 40
20 20 20 20 20
– 10 20 30 40
Xa Xa Xa Xa Xa
X = 0, 1, 2, 3, 5 and 10 mol%, on a 100 + X mol% basis.
691
50° off-normal incidence, for polished samples, by means of a variable angle specular reflectance attachment (Specac). Polarized Raman spectra, in the HH (k) and HV (?) configurations, were collected with a 90° scattering geometry, at room temperature and a resolution of 4 cm1. A double monochromator (Spex 1403), together with an argon ion laser (Spectra-Physics, mod. 2016) and a photomultiplier detector (Hamamatsu R928) were used, with one second of integration time, between 10 cm1 and 1500 cm1. The green and blue excitation lines produced fluorescence bands due to the presence of erbium; therefore, the 476.5 nm excitation was used, for which no fluorescence was observed. 3. Results 3.1. Infrared measurements Fig. 1 presents the IR specular reflectivity spectra obtained for the tellurite glass with 40 mol% ZnCl2 and 5 mol% ErCl3, at different angles of incidence. Although few changes occurred in the region below 500 cm1, the higher frequency band peaking at 663 cm1, observed at 10° and 30° off-normal, partly splits into 670 cm1 and 630 cm1 components, at 50° off-normal. Fig. 2 shows the TO (transverse optical ‘phonon’, with the atomic motion perpendicular to the ‘wavevector’) and LO (longitudinal optical ‘phonon’, with the atomic motion parallel to the ‘wavevector’) [14] spectral components for the same composition, calculated by Kramers–Kronig analysis, based on the 10° off-normal spectrum [14]. The high frequency TO peak occurred at 688 cm1, while the LO peak was at 763 cm1.
TO
LO
10˚
1000
Extinction (a.u.)
Reflectivity (a.u.)
30˚
50˚
800
600
Wavenumber
400
200
(cm-1)
Fig. 1. IR specular reflectivity spectra of 40TeO2–20ZnO–40ZnCl2 (mol%) glass, doped with 5 mol% ErCl3, at different off-normal incidence angles.
1000
900
800
700
600
500
400
300
200
Wavenumber (cm-1) Fig. 2. Calculated LO and TO spectra of 40TeO2–20ZnO–40ZnCl2 (mol%) glass, doped with 5 mol% ErCl3.
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L. Fortes et al. / Journal of Non-Crystalline Solids 352 (2006) 690–694
622 622+1 622+3 622+5
0
200
400
600
Raman shift
820+2 820+5 820+10
(b)
Intensity (a.u.)
Intensity (a.u.)
(a)
800
1000
0
200
400
600
800
1000
Raman shift (cm-1)
(cm-1)
Fig. 3. Polarized (HH) Raman spectra normalized at the high frequency peak (767 cm1): (a) 60TeO2–20ZnO–20ZnCl2 glasses, containing 0–5 ErCl3 (mol%); (b) 80TeO2–20ZnO glasses, containing 2–10 ErCl3 (mol%).
3.2. Raman measurements The Raman spectra were dominated by a low frequency band at 45 cm1, the boson peak, which was weakly polarized (depolarization ratio DR = I?/Ik, 0.65) and did not shift significantly with the incorporation of zinc or erbium compounds. Small differences of up to 4 cm1 are within the instrumental errors (±2 cm1) and were not considered. Fig. 3 presents the polarized (HH) Raman spectra normalized at the 767 cm1 peak, for the 622 and the 820 compositions, where one can see an increase of the boson peak with increasing erbium content, with the exception of the 622 + 3 ErCl3 (mol%) composition. The introduction of zinc chloride did not lead to any consistent changes in the boson peak and its DR did not change with the ErCl3 or ZnCl2 contents. The polarized (HH) Raman spectra of several compositions, normalized at the boson peak (45 cm1), are shown in Figs. 4 and 5. The spectra of the undoped compositions
(Fig. 4) are dominated, in the high frequency region, by the band at 767 cm1, with a shoulder at 675 cm1, plus an intermediate frequency band at 420 cm1. An increase in the ZnCl2 content resulted in a decrease of the shoulder intensity, with respect to the 767 cm1 band, while the intensity of the intermediate band simultaneously decreased (apart from the 721 composition). A shift toward higher frequencies is observed for the main high frequency band while the intermediate band shifts toward lower frequencies (405 cm1, for the 40 mol% ZnCl2 composition), with increasing zinc chloride content. Two low frequency bands are also observed, at 120 cm1 and 280 cm1, as shown in detail in Fig. 6(a). The compositions doped with different Er concentrations presented somewhat different spectra (Fig. 5), with the highest frequency band decreasing and shifting toward higher frequencies (785 cm1, for the 10 mol% ErCl3 composition) with increasing erbium content. The 675 cm1 shoulder and the 420 cm1 band also decreased with
721 622 523 424
Intensity (a.u.)
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Fig. 4. Polarized (HH) Raman spectra of TeO2–ZnO–ZnCl2 glasses with varying TeO2 and ZnCl2 contents, normalized at the boson peak (45 cm1).
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2 00
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Fig. 5. Polarized (HH) Raman spectra of the 80TeO2–20ZnO glass composition (mol%), doped with 2, 5 and 10 mol% ErCl3, normalized at the boson peak (45 cm1).
L. Fortes et al. / Journal of Non-Crystalline Solids 352 (2006) 690–694
(a)
(b)
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Intensity (a.u.)
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693
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Raman shift (cm-1)
Fig. 6. Polarized (HH) low frequency Raman spectra of different oxychloride tellurite glasses: (a) TeO2–ZnO–ZnCl2 glasses; (b) 40TeO2–20ZnO–40ZnCl2 glasses, containing 0–10 ErCl3 (mol%).
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622+10 0.0 0
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Raman shift (cm-1) Fig. 7. Depolarization ratio of 622 and 622 + 10 compositions.
increasing erbium content, but the 420 cm1 band presented a red shift of 30 cm1 between 2 and 10 mol% of ErCl3. The intensity of the 120 cm1 low frequency band simultaneously increased, as seen in Fig. 6(b). Apart from the boson peak, the Raman spectra of all samples were strongly polarized (DR < 0.4), with maximum degree of polarization (minima in DR) at 270 cm1 (DR 0.3), 420 cm1 (DR 0.2) and 830 cm1 (DR 0.15), for the undoped compositions (Fig. 7). Erbium doped compositions presented lower DR, with maximum degree of polarization at 270 cm1 (DR 0.2), 420 cm1 (DR 0.15) and 800 cm1 (DR 0.06). 4. Discussion The position of the highest frequency TO and LO components (Fig. 2) were not obtainable from the oblique incidence spectra (Fig. 1) with good accuracy, as opposed to what has been found in the case of other glasses [14]. On the other hand, the large calculated TO–LO splitting
(75 cm1) and the fact that the positions of the TO and LO components almost coincide with the two highest frequency Raman modes, suggests a significant degree of ionic character for these glasses and also of intermediate range order [14]. It is well established that the high frequency Raman bands at 420 and 767, plus the shoulder at 675 cm1 are all related to the presence of TeOx units. The 675 cm1 shoulder corresponds, in TeO2-rich glass compositions, to the higher intensity, high frequency band and has been assigned to the stretching vibrations of the TeO4 trigonal bypiramidal groups [7]. Introduction of glass modifiers, such as the zinc compounds, promotes the formation of NBO species and the TeO4 units are gradually transformed into TeO3+1 and TeO3 trigonal pyramid units, leading to an increase in the intensity of the 767 cm1 band [7–10]. This band is believed to be a superposition of TeO3+1 and TeO3 peaks, since, in crystalline ZnTeO3 and Zn2Te3O8, the TeO3+1 and TeO3 peaks appear at 740 cm1 and 790 cm1, respectively [7]. The 420 cm1 band is ascribed to TeAOATe bending modes [7–10]. An increase in the ZnCl2 content (Fig. 4) leads to a clear decrease in the 675 cm1 shoulder with respect to the 767 cm1 peak, revealing the change of the TeO4 backbone to TeO3+1 and TeO3 units, as expected. The same behavior is observed with increasing ErCl3 content in Fig. 5. Figs. 4 and 5 show that the addition of ZnCl2 and ErCl3 modifiers led to a blue shift of the high frequency peak, due to an increase in frequency in the TeO3 vibration, motivated by the increasing concentration of NBO species. This shift has also been observed [12] for erbium oxide doped glasses. One should notice, however, that this shift is continuous with the introduction of ErCl3 (at least up to 10 mol%, Fig. 5), while, after the addition of 20 mol% of ZnCl2 (Fig. 4), the shift was minimum, perhaps revealing an incorporation of the zinc chloride as modifier up to 20 mol% and as a network former beyond 20 mol%. The intensity of the 420 cm1 band generally decreased with an increase of either the zinc chloride or
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erbium chloride contents (Figs. 4 and 5). Such an effect has also been observed in compositions without chlorine, where small additions of erbium oxide caused the same behavior [12,13]. This general trend shows that both the erbium and zinc compounds break the TeAOATe bonds. However, it is observed that erbium chloride has a stronger effect than zinc chloride in this respect, probably due to the fact that zinc chloride can be partially incorporated as a network former. The observed red shift is also more notorious for compositions with erbium chloride, than for TeO2–ZnO–ZnCl2 glasses. The two low frequency bands increased with the ErCl3 and ZnCl2 contents (Fig. 6), but they have distinct origins: the lower frequency shoulder 120 cm1 is also observed in purely oxide glasses [8–10,12], with or without erbium oxide and can tentatively be assigned to cage-like modes (cation site vibrations), involving erbium (or zinc) cations. On the other hand, the 270 cm1 band is dependent on the chloride content, since it increases with both the ZnCl2 and ErCl3 contents and is absent in purely erbium doped oxide compositions [12]. The minimum depolarization ratio for the 270 cm1 band (Fig. 7) suggests that it involves symmetric stretching vibrations of non-bridging anions about fixed network forming cations [15]. Since the totally symmetric mode (A1) of ZnCl2 tetrahedra is 4 known to occur at 276 cm1 [15], this band can be tentatively attributed to symmetric stretching vibrations of nonbridging chlorine ions about zinc cations. An increase of the boson peak with increasing erbium content, except for the 622 + 3 composition, is observed in Fig. 3(a). It was reported in the literature that such an increase is expected up to 1 mol% Er in purely oxide glasses, corresponding to the limit of rare-earth ion solubility without clustering [12,16], which should be followed by a decrease, corresponding to the occurrence of clustering. In order to compare our results with the previously reported data of Jaba et al. [12], we tested the same normalization procedure as they use (at the 675 cm1 shoulder), but a continuous increase of the boson peak with erbium concentration was still found. The present vibrational spectroscopy data is consistent with the model where TeO4 bipyramid units, containing both bridging and non-bridging oxygen species, are linked by ZnO groups to TeO3 trigonal pyramid units [9–11]. The ZnCl2 and ErCl3 additions break oxygen bridges and introduce non-bridging chlorine species, preferably connected to the zinc atoms, while erbium is always surrounded by oxygen atoms, with tellurium as a possible second near neighbor [17]. The addition of up to 10 mol% of ErCl3 to the tellurite compositions does not appear to cause the formation of Er3+-rich clusters, as it has been corroborated by Er3+ photoluminescence data at 1.5 lm and the corresponding 4I13/2 lifetime measurements [5,18] suggesting that the dispersion of Er3+ ions in the present oxychloride glasses (up to 10 mol%) is superior to that usually observed in purely oxide tellurite glasses (up to 4 mol%) [12,19].
5. Conclusions Increasing ErCl3 and ZnCl2 contents caused a changing of the TeO4 backbone of tellurite glasses to TeO3 pyramidal units, with non-bridging chlorine species preferably connected to the zinc atoms. The 420 cm1 band decreased with increasing ErCl3 and ZnCl2 contents, showing that both erbium and zinc ions break TeAOATe bonds, but ErCl3 had a stronger effect on this band than ZnCl2, probably due to the fact that ZnCl2 may also have a glass network forming role above 20 mol%. The lower frequency band 120 cm1 is related to cation vibrational modes, while the 280 cm1 band is due to Zn–Cl vibrations. The boson peak intensity generally increases with the amount of erbium chloride. Acknowledgements This work was supported by the Fundac¸a˜o para a Cieˆncia e a Tecnologia (FCT, Portugal), under contract POCTI/CTM/33307 and also by FSE/FEDER. We would also like to acknowledge the support of NSF’s International Materials Institute for New Functionality in Glass (IMI-NFG). References [1] M.E. Lines, J. Appl. Phys. 69 (1991) 6876. [2] A. Jha, S. Shen, M. Naftaly, Phys. Rev. B 62 (2000) 6215. [3] R.A.H. El-Mallawany, Tellurite Glasses Handbook – Physical Properties and Data, CRC, Boca Raton, USA, 2001. [4] B. Bridge, T.E. Bavins, D. Woods, T. Woolven, J. Non-Cryst. Solids 88 (1986) 262. [5] L.M. Fortes, L.F. Santos, M.C. Gonc¸alves, R.M. Almeida, J. NonCryst. Solids 324 (2003) 150. [6] D.L. Sidebottom, M.A. Hruschka, B.G. Potter, R.K. Brow, J. NonCryst. Solids 222 (1997) 282. [7] H. Burger, K. Kneipp, H. Hobert, W. Vogel, V. Kozhukharov, S. Neov, J. Non-Cryst. Solids 151 (1992) 134. [8] T. Sekiya, N. Mochida, A. Ohtsuka, J. Non-Cryst. Solids 168 (1994) 106. [9] M. Mazzuca, J. Portier, B. Tanguy, F. Romain, A. Fadli, S. Turrell, J. Mol. Struct. 349 (1995) 413. [10] C. Duverger, M. Bouazaoui, S. Turrell, J. Non-Cryst. Solids 220 (1997) 169. [11] S. Sakida, J. Jin, T. Yoko, Phys. Chem. Glasses 41 (2000) 65. [12] N. Jaba, A. Mermet, E. Duval, B. Champagnon, J. Non-Cryst. Solids 351 (2005) 833. [13] M.R. Sahar, Sulhadi, M.S. Rohani, in: Proceedings of the 2005 International Symposium on Glass, Shangai, 2005, p. SA1-4. [14] R.M. Almeida, Phys. Rev. B 45 (1992) 161. [15] R.M. Almeida, L.F. Santos, J. Am. Ceram. Soc. 72 (1989) 2065. [16] V.K. Tikhomorov, A. Jha, A. Perakis, E. Sarantopoulou, M. Naftaly, V. Krasteva, R. Li, A.B. Seddon, J. Non-Cryst. Solids 256&257 (1999) 98. [17] L.M. Fortes, L.F. Santos, M.C. Gonc¸alves, R.M. Almeida, F. D’Acapito, J. Non-Cryst. Solids 348 (2004) 11. [18] L.M. Fortes, L.F. Santos, M.C. Gonc¸alves, R.M. Almeida, M. Mattarelli, M. Montagna, A. Chiasera, M. Ferrari, A. Monteil, S. Chaussedent, G.C. Righini, Opt. Mater., submitted for publication. [19] M.R. Sahar, Sulhadi, M.S. Rohani, in: Proceedings of the XXth ICG in Kyoto, September 27th–October 1st, 2004.