- Email: [email protected]
Preparation and properties of GeSe2-Ga2Se3-KBr new chalcohalide glasses
Journal of Alloys and Compounds 459 (2008) 472–476
Preparation and properties of GeSe2-Ga2Se3-KBr new chalcohalide glasses Gao Tang a,b , Zhiyong Yang a,b , Lan Luo a , Wei Chen a,∗ a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
Received 11 March 2007; received in revised form 23 April 2007; accepted 26 April 2007 Available online 29 April 2007
Abstract A series of novel GeSe2 -Ga2 Se3 -KBr chalcohalide glasses were prepared and the glass-forming region was determined. The evolution of glass structure was studied by Raman spectra. The optical and the thermal properties of glasses were investigated. The results show that these chalcohalide glasses have a broad transparent region in the entire 0.6–16 m spectral region. At least 35 mol% KBr can be dissolved into the glasses and the absorption edge of the glass shifts towards short-wavelength direction with increasing amount of KBr. The glasses have relatively high glass transition temperatures (Tg = 290–350 ◦ C) and good thermal stability. These properties make them promising candidate materials for infrared optics. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Optical materials; Microstructure; Thermal analysis; Optical properties
1. Introduction
2. Experimental
Selenium-based chalcogenide glasses have been widely studied during the past several decades due to their potential applications for infrared (IR) optics and optoelectronics [1–6]. These glasses are typically transparent from 1 to 16 m spectral region but opaque in the visible light region, which makes the quality control of IR system complicated and therefore limits their applications. Fortunately, recent work indicated that the transparent range of selenide glasses can be extended to the visible light region by incorporating sufficient cesium halides [4–6]. For getting some interesting chalcohalide glasses, in this paper, we introduced potassium bromide into Ge–Ga–Se glass matrix. The glass-forming region of studied GeSe2 -Ga2 Se3 -KBr system was determined. Their optical and thermal properties were investigated, and structures of several compositions were studied in terms of Raman spectroscopy.
A typical 5 g glass was synthesized by melting mixtures of the constituent elements (Ge, Ga and Se, all of 99.999% purity, and KBr of 99.9% purity) in evacuated (10−2 Pa) and flame-sealed silica ampoule in a rocking furnace. The mixtures were melted at 900–950 ◦ C for 12 h. After that, the ampoule was quenched in cold water then swiftly moved to a preheated furnace and annealed near corresponding glass transition temperature for 2 h. The bulk sample was obtained by taking it out from the ampoule. Glass rod was finally cut and polished for testing. The samples are about 2 mm in thickness. Compositions of studied glasses are listed as follows:
∗
Corresponding author. Tel.: +86 21 52414259; fax: +86 21 62405122. E-mail address: [email protected] (W. Chen).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.291
Series A: (1 − 2x)GeSe2 -xGa2 Se3 -xKBr glasses, x = 0.10, 0.15, 0.20, 0.25, 0.30, 0.35; Series B: (0.9 − y)GeSe2 -yGa2 Se3 -0.1KBr glasses, y = 0.05, 0.10, 0.20, 0.30. The differential thermal analysis (DTA) measurements of bulk glass pieces (40–50 mg) were carried out by a CDR-1P Thermal Analyzer (SBIF, Shanghai, PR China) with an accuracy of ±2 ◦ C at a heating rate of 10 ◦ C/min. Pure ␣Al2 O3 powder was used as reference material. Glass transition temperatures (Tg ) and the starting temperatures of crystallization (Tx ) were determined by the slope intercept method from measured DTA curves. The density was obtained using the Archimedes method and the accuracy was ±0.001g/cm3 . The Vis-NIR and IR transmission spectra were measured by a HITACHI U-3101 spectrophotometer with wavelength resolutions of 1 nm and a Shinmadzu IRPrestige-21 INFRARED spectrophotometer with the resolution of
G. Tang et al. / Journal of Alloys and Compounds 459 (2008) 472–476
473
4 cm−1 , respectively. And the precision of transmission is about 1%. The X-ray diffraction (XRD) patterns of samples were recorded on a Rigaku D/Max-2550V X-ray diffractometer with a Cu target (40 kV, 40 mA). The Raman spectra were measured by a Renishaw Invia Raman Microscope (UK) with a resolution of 0.5 cm−1 . The excited wavelength was 785 nm. All of the spectrums were measured at room temperature.
3. Result and discussion 3.1. The glass-forming region The obtained bulk glasses have dark-red to orange-red color depending on the composition. Fig. 1 shows the glass-forming region in the GeSe2 -Ga2 Se3 -KBr pseudo-ternary system. The region is determined by XRD pattern of samples and visual examination. GeSe2 -KBr pseudo-binary glasses cannot be obtained maybe due to the large difference in bond covalence and energies of Ge–Se and K–Br which increases the possible phase separation. A large glass-forming region is mainly centred at the Ga2 Se3 /KBr ratio of 1. And the maximum concentration of dissolved KBr is up to over 35 mol% (series A). The glassforming region of GeSe2 -Ga2 Se3 -KBr is different from other Ge–Ga–Se chalcohalide glasses with cesium halide addition which are mainly centred at the Ga2 Se3 /cesium halide ratio of 0.5 [4–6]. This situation may be associated with the fact that the radius of K+ is relatively small and the chemical bond strength in GeSe2 -Ga2 Se3 -KBr system is stronger than other chalcohalide glasses. While a large mount of KBr is added in series B, there is not enough Ga2 Se3 to form stable structure unit, and then crystallization may easily occur. 3.2. Raman spectroscopy For understanding properties of the glasses, it is necessary to investigate their structure. Similar to other chalcohalide glass, when KBr is added in Ge–Ga–Se glasses, [GaSe3 Br] and [Se3 Ga-Br-GaSe3 ] structural units may be formed because the halogens were preferentially bonded to Ga [6,7]. The luminescence properties of rare earth ions in other similar chalcohalide glass systems indicated the glass structure is sensitive to the
Fig. 1. Glass-forming region in the GeSe2 -Ga2 Se3 -KBr pseudo-ternary system.
Fig. 2. Raman spectra of glasses: Br/Ga = 1 (0.7GeSe2 -0.1Ga2 Se3 0.2KBr); Br/Ga = 0.5 (0.6GeSe2 -0.2Ga2 Se3 -0.2KBr); Br/Ga = 0.25 (0.7GeSe2 -0.2Ga2 Se3 -0.1KBr) and 0.85GeSe2 -0.15Ga2 Se3 glass.
Br/Ga ratio [3]. In our studied glasses, when Br/Ga ratio is smaller than 0.5, Br atoms mainly bonded to Ga in the form [Se3 Ga-Br-GaSe3 ]; as Br/Ga ratio is 0.5–1, with increasing Br/Ga ratio, relative amounts of [GaSe3 Br] to [Se3 Ga-BrGaSe3 ] units get larger; and as Br/Ga ratio reaches 1, [GaSe3 Br] units are predominant. Fig. 2 depicts the Raman spectra of GeSe2 -Ga2 Se3 -KBr glasses with different Br/Ga ratios and 0.85GeSe2 -0.15Ga2 Se3 glass. The Raman spectra are dominated by a band between 150 and 230 cm−1 which is composed of several overlapping bands. The strong band rising near 200 cm−1 is attributed to the overlap of ν1 (A1 ) symmetric stretching modes of corner-sharing [GeSe4 ] and [GaSe4 ] tetrahedral. The shoulder at 216 cm−1 called “companion peak” (c) is associated to the vibration ν1c (Ac1 ) of edge-sharing [GeSe4 ] and [GaSe4 ] tetrahedral. The other shoulder at lower wavenumber, 174 cm−1 , is associated with the [Se3 Ge-GeSe3 ] and [Se3 Ga-GaSe3 ] vibrations [8,9]. The introduction of KBr leads to three major changes in 0.7GeSe2 -0.1Ga2 Se3 -0.2KBr (Br/Ga = 1) glass. Firstly, a new low-phonon band at ∼158 cm−1 appears; secondly, the amplitude of 174 cm−1 band is reduced; thirdly, the 216 cm−1 band is enhanced. These phenomena indicate significant structural variations. The new 158 cm−1 in the first phenomenon band could be assigned to the vibration of [GaSe3 Br] unit. Similar new band is also observed in CsGaSe3/2 Cl glass [10]. The phonon energy of this unit is slightly higher than that of [GaSe3 I] unit [9]. For the second phenomenon, [Se3 Ga-GaSe3 ] unit may be broken by addition of KBr, and then [Se3 Ga-Br-GaSe3 ] and [GaSe3 Br] are possible to be formed. Theoretical evaluation implies that the Raman shift of [Se3 Ga-Br-GaSe3 ] is larger than that of [GaSe3 Br] and smaller than that of [GaSe4 ] [11]. This means the band of [Se3 Ga-Br-GaSe3 ] unit would be located between 158 and 200 cm−1 . Consequently, [Se3 GaGaSe3 ] unit and [Se3 Ga-Br-GaSe3 ] unit may overlap within the broad band around 174 cm−1 (Fig. 2). From the discussion in the last paragraph, [Se3 Ga-Br-GaSe3 ] units are in the minority when Ga/Br ratio reaches 1. Therefore, the 174 cm−1 broad band related to [Se3 Ga-GaSe3 ] unit and [Se3 Ga-Br-GaSe3 ] units
474
G. Tang et al. / Journal of Alloys and Compounds 459 (2008) 472–476
Table 1 Thermal and physical properties of GeSe2 -Ga2 Se3 -KBr glasses Tx (◦ C)
T (◦ C)
Density (g/cm3 )
Short-wavelength absorption edges, λs (nm)
Series A: (1 − 2x)GeSe2 -xGa2 Se3 -xKBr glasses x = 0.10 330 x = 0.15 324 x = 0.20 309 x = 0.25 298 x = 0.30 298 x = 0.35 293
445 453 521 442 421 393
115 129 212 144 123 100
4.185 4.153 4.100 4.079 4.020 3.976
668 663 643 622 608 593
Series B: (0.9 − y)GeSe2 -yGa2 Se3 -0.1KBr glasses y = 0.05 320 y = 0.10 330 y = 0.20 341 y = 0.30 354
440 445 463 430
120 115 122 76
4.134 4.185 4.233 4.289
685 668 677 689
Composition (in mol%)
Tg (◦ C)
decreases. The third phenomenon, the edge-shared GeSe4 and GaSe4 tetrahedral of 0.7GeSe2 -0.1Ga2 Se3 -0.2KBr (Br/Ga = 1) glass increase, in accordance with the results of its thermal stability T = 120 ◦ C lower than that of 0.6GeSe2 -0.2Ga2 Se3 -0.2KBr glass and 0.7GeSe2 -0.2Ga2 Se3 -0.1KBr glass which would be presented in Table 1 below. 3.3. Thermal properties Because the glass-forming region is mainly centred at the Ga2 Se3 /KBr ratio of 1, our study emphasis is put on the thermal properties of the series A. Fig. 3 shows their DTA curves with a heating rate of 10 ◦ C/min. The Tg , Tx , the thermal stability factors T (Tx − Tg ) and densities of studied samples are summarized in Table 1. The values of T for most glasses are more than 100 ◦ C, which indicates these glasses have good thermal stability and can easily be obtained in bulk forms. The most stable glass is obtained for sample 0.6GeSe2 -0.2Ga2 Se3 -0.2KBr (mol%). The thermal stability criterion T of this sample is even as large as 212 ◦ C. From Fig. 3 and Table 1, it is shown that when x increase in series A, crystallization curves are changed from dispersion profile to sharp peak, Tg and density decrease, Tx and T first increase then decrease. The results show that the structure of
glasses is changed when KBr and Ga2 Se3 are added. When x increase, the former [GeSe4 ] units of glasses will decrease. And the addition of KBr and Ga2 Se3 form [Se3 Ga-Br-GaSe3 ] unit or/and [GaSe3 Br] which loose the whole glass structure. As a result, Tg as well as density decrease. In addition, the Tg values still maintain a reasonable value indicating a high degree of glass connectivity. Meanwhile, the new glass units will accelerate the crystallization process when the process occurs. Therefore, the crystallization curves of DTA become sharp peaks when x increase. Due to strong complex-forming ability of Ga, the [GaSe3 ] triangles can easily transform into [GaSe4 ] tetrahedral after capturing the free lone pair electron of Se atoms [5,10]. And this mechanism intensifies the glass network connectivity [5]. While some Br− ions play the role of non-bridging terminator and decrease the network connectivity. So Tx and T accordingly first increase then decrease with the effect of two reciprocal factors mentioned above. When the amount of KBr and Ga2 Se3 is low, the first factor has more influence, and when the amount is larger than 20 mol%, the second factor is dominant. In series B, Tg and density increase with increasing content of Ga2 Se3 . The introduction of [GaSe4 ] tetrahedral makes the glass network compact. The different chemical bonds and microstructure units would enhance the glass network dimensionality and thermal stability criterion T increase [12]. When y ≥ 0.3, phase separation would easily happen due to the different bond energies of [GaSe4 ] and [GeSe4 ] tetrahedral units, then the thermal stability decreases. 3.4. Optical properties
Fig. 3. DTA curves of series A glasses: (1 − 2x)GeSe2 -xGa2 Se3 -xKBr (x = 0.10, 0.15, 0.20, 0.25, 0.30, 0.35).
Fig. 4 shows the IR spectrum of 0.6GeSe2 -0.2Ga2 Se3 0.2KBr glass. It is shown that the glass has a high transmittance (>70%) in 8–14 m atmosphere window and its longwavelength cut-off edges (defined half maximum of the glass transmission) locate at about 624 cm−1 (∼16 m). The absorption bands of impurities in IR region are attributed to O–H, Se–H, H2 O, Se–O, Ge–O, respectively. The impurities may be significantly reduced or eliminate by purifying starting materials or distilling the glasses by appropriate techniques [13]. For other glasses in GeSe2 -Ga2 Se3 -KBr system, there is a slight shift in glass transmission (%) and their long-wavelength cut-off
G. Tang et al. / Journal of Alloys and Compounds 459 (2008) 472–476
Fig. 4. IR transmission spectrum of 0.6GeSe2 -0.2Ga2 Se3 -0.2KBr glass (thickness = 2.0 mm).
edges remain changeless due to the multi-phonon vibrations of Ge–Se. The short-wavelength absorption edges λs (defined as the wavelength where the half maximum of glass transmittance), locate between 593 and 689 nm. The λs of studied samples are summarized in Table 1. It is known that the λs relate to the electron transition between valence band and conduction band. It is complicated to judge the λs in theory when KBr is added, and there is an empirical formula to evaluate the variation tendency of λs [4,6,14]: 1 λs ∝ χ+w−ϕ
(1)
where χ is the average electron affinity energy of anions, w the average energy of bond and ϕ is the average polarization energy of anions [14]. Glass transition temperature Tg relate to w average energy of bond and rigidity of the cross-linked glass [6,15]. Fig. 5 is the Vis-NIR spectra of series A glasses with 500–800 nm range. For series A, the λs of glasses has a remarkable blue shift with x increase. The λs of 0.3GeSe2 -
475
0.35Ga2 Se3 -0.35KBr glass is as small as 593 nm. This value can be compared to 0.42GeSe2 -0.18Ga2 Se3 -0.40CsCl glass [5]. The Ga2 Se3 and KBr additions change the average energy of bond w and the glass structure. As a result, Tg and λs accordingly varied. The value in Table 1 indicates that the Tg and λs have similar correlative variation trends in the glasses. The Tg decreases when λs decreases. This phenomenon is contrary to the Tg − Eg relation (Eg is the optical band gap of the glass and it is in inverse proportion to λs ) which is usually valid in chalcogenide glasses [15]. It is not surprising because Tg and λs have some different influencing factors. In most chalcogenide glasses, the average energy of bond w dominates dependences of Tg and λs . However, in our studied glasses, χ (Br− ) > χ (Se2− ) owing to the higher electronegativity Br− than Se2− . The addition of K+ and Br− with less polarizability compared to the cations (Ge2+ and Ga3+ ) and the anion (Se2− ) and the average energy of bond w decreases with Ga2 Se3 and KBr additions. This situation means the effect that parameters χ and ϕ play dominant roles in formula (1) in our GeSe2 -Ga2 Se3 -KBr chalcohalide glass system. For series A, the more KBr is added, the smaller is the λs . For series B, the amount of KBr is fixed, when Ga2 Se3 replace GeSe2 , there is red shift of λs because the average bond energy of Ga–Se is smaller than that of Ge–Se. This result is not conflict with the variation of Tg since the introduction of Ga2 Se3 increases the rigidity of the cross-linked glass. 4. Conclusion The GeSe2 -Ga2 Se3 -KBr chalcohalide glasses are systematically studied. The glass-forming region, which is mainly located at Ga2 Se3 /KBr = 1, is presented. The structure units of glasses are discussed. The glasses show good thermal stabilities against crystallization and the most stable glass with the composition of 0.6GeSe2 -0.2Ga2 Se3 -0.2KBr. The new chalcohalide glasses show excellent transparency in the entire 0.6–16 m spectral region and are attractive materials for infrared optics. Acknowledgement This work was financially supported by China’s manned space program (the 921-2.1 project). References
Fig. 5. Vis-NIR transmission spectra of series A: GeSe2 -Ga2 Se3 -KBr glasses (thickness = 2.0 mm).
[1] H.L. Ma, Y. Guimond, X.H. Zhang, J. Lucas, J. Non-Cryst. Solids 256–257 (1999) 165–169. [2] Z. Yang, W. Chen, L. Luo, J. Mater. Res. 20 (2005) 2597–2602. [3] Y.B. Shin, J. Heo, H.S. Kim, J. Mater. Res. 16 (2001) 1318–1324. [4] Z. Yang, L. Luo, W. Chen, J. Am. Ceram. Soc. 89 (7) (2006) 2327–2329. [5] L. Calvez, H.L. Ma, J. Lucas, X.H. Zhang, Adv. Mater. 19 (2007) 129– 132. [6] Z. Yang, G. Tang, L. Luo, W. Chen, J. Am. Ceram. Soc. 90 (2) (2007) 667–669. [7] J.H. Song, Y.G. Choi, J. Heo, J. Non-Cryst. Solids 352 (2006) 423–428. [8] P. Nˇemec, B. Frumarov´a, M. Frumar, J. Non-Cryst. Solids 270 (2000) 137–146. [9] Z. Yang, G. Tang, L. Luo, W. Chen, Appl. Phys. Lett. 89 (2006) 131117. [10] Yu.S. Tver’yanovich, M. Vlˇcek, A. Tverjanovich, J. Non-Cryst. Solids 333 (2004) 85–89.
476
G. Tang et al. / Journal of Alloys and Compounds 459 (2008) 472–476
[11] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, third ed., John Wiley & Sons, New York, 1978. [12] H. Guo, Y. Zhai, H. Tao, Y. Gong, X. Zhao, Mater. Res. Bull. 42 (2007) 1111–1118.
[13] V.S. Shiryaev, J.-L. Adam, X.H. Zhang, C. Boussard-Pl`edel, J. Lucas, M.F. Churbanov, J. Non-Cryst. Solids 336 (2004) 113–119. [14] M.L. Cui, Glass Technology, Light Industry Press, Beijing, China, 1987. [15] A. Giridhar, S. Mahadevan, J. Non-Cryst. Solids 126 (1990) 161–169.
Preparation and properties of GeSe2-Ga2Se3-KBr new chalcohalide glasses Gao Tang a,b , Zhiyong Yang a,b , Lan Luo a , Wei Chen a,∗ a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
Received 11 March 2007; received in revised form 23 April 2007; accepted 26 April 2007 Available online 29 April 2007
Abstract A series of novel GeSe2 -Ga2 Se3 -KBr chalcohalide glasses were prepared and the glass-forming region was determined. The evolution of glass structure was studied by Raman spectra. The optical and the thermal properties of glasses were investigated. The results show that these chalcohalide glasses have a broad transparent region in the entire 0.6–16 m spectral region. At least 35 mol% KBr can be dissolved into the glasses and the absorption edge of the glass shifts towards short-wavelength direction with increasing amount of KBr. The glasses have relatively high glass transition temperatures (Tg = 290–350 ◦ C) and good thermal stability. These properties make them promising candidate materials for infrared optics. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Optical materials; Microstructure; Thermal analysis; Optical properties
1. Introduction
2. Experimental
Selenium-based chalcogenide glasses have been widely studied during the past several decades due to their potential applications for infrared (IR) optics and optoelectronics [1–6]. These glasses are typically transparent from 1 to 16 m spectral region but opaque in the visible light region, which makes the quality control of IR system complicated and therefore limits their applications. Fortunately, recent work indicated that the transparent range of selenide glasses can be extended to the visible light region by incorporating sufficient cesium halides [4–6]. For getting some interesting chalcohalide glasses, in this paper, we introduced potassium bromide into Ge–Ga–Se glass matrix. The glass-forming region of studied GeSe2 -Ga2 Se3 -KBr system was determined. Their optical and thermal properties were investigated, and structures of several compositions were studied in terms of Raman spectroscopy.
A typical 5 g glass was synthesized by melting mixtures of the constituent elements (Ge, Ga and Se, all of 99.999% purity, and KBr of 99.9% purity) in evacuated (10−2 Pa) and flame-sealed silica ampoule in a rocking furnace. The mixtures were melted at 900–950 ◦ C for 12 h. After that, the ampoule was quenched in cold water then swiftly moved to a preheated furnace and annealed near corresponding glass transition temperature for 2 h. The bulk sample was obtained by taking it out from the ampoule. Glass rod was finally cut and polished for testing. The samples are about 2 mm in thickness. Compositions of studied glasses are listed as follows:
∗
Corresponding author. Tel.: +86 21 52414259; fax: +86 21 62405122. E-mail address: [email protected] (W. Chen).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.291
Series A: (1 − 2x)GeSe2 -xGa2 Se3 -xKBr glasses, x = 0.10, 0.15, 0.20, 0.25, 0.30, 0.35; Series B: (0.9 − y)GeSe2 -yGa2 Se3 -0.1KBr glasses, y = 0.05, 0.10, 0.20, 0.30. The differential thermal analysis (DTA) measurements of bulk glass pieces (40–50 mg) were carried out by a CDR-1P Thermal Analyzer (SBIF, Shanghai, PR China) with an accuracy of ±2 ◦ C at a heating rate of 10 ◦ C/min. Pure ␣Al2 O3 powder was used as reference material. Glass transition temperatures (Tg ) and the starting temperatures of crystallization (Tx ) were determined by the slope intercept method from measured DTA curves. The density was obtained using the Archimedes method and the accuracy was ±0.001g/cm3 . The Vis-NIR and IR transmission spectra were measured by a HITACHI U-3101 spectrophotometer with wavelength resolutions of 1 nm and a Shinmadzu IRPrestige-21 INFRARED spectrophotometer with the resolution of
G. Tang et al. / Journal of Alloys and Compounds 459 (2008) 472–476
473
4 cm−1 , respectively. And the precision of transmission is about 1%. The X-ray diffraction (XRD) patterns of samples were recorded on a Rigaku D/Max-2550V X-ray diffractometer with a Cu target (40 kV, 40 mA). The Raman spectra were measured by a Renishaw Invia Raman Microscope (UK) with a resolution of 0.5 cm−1 . The excited wavelength was 785 nm. All of the spectrums were measured at room temperature.
3. Result and discussion 3.1. The glass-forming region The obtained bulk glasses have dark-red to orange-red color depending on the composition. Fig. 1 shows the glass-forming region in the GeSe2 -Ga2 Se3 -KBr pseudo-ternary system. The region is determined by XRD pattern of samples and visual examination. GeSe2 -KBr pseudo-binary glasses cannot be obtained maybe due to the large difference in bond covalence and energies of Ge–Se and K–Br which increases the possible phase separation. A large glass-forming region is mainly centred at the Ga2 Se3 /KBr ratio of 1. And the maximum concentration of dissolved KBr is up to over 35 mol% (series A). The glassforming region of GeSe2 -Ga2 Se3 -KBr is different from other Ge–Ga–Se chalcohalide glasses with cesium halide addition which are mainly centred at the Ga2 Se3 /cesium halide ratio of 0.5 [4–6]. This situation may be associated with the fact that the radius of K+ is relatively small and the chemical bond strength in GeSe2 -Ga2 Se3 -KBr system is stronger than other chalcohalide glasses. While a large mount of KBr is added in series B, there is not enough Ga2 Se3 to form stable structure unit, and then crystallization may easily occur. 3.2. Raman spectroscopy For understanding properties of the glasses, it is necessary to investigate their structure. Similar to other chalcohalide glass, when KBr is added in Ge–Ga–Se glasses, [GaSe3 Br] and [Se3 Ga-Br-GaSe3 ] structural units may be formed because the halogens were preferentially bonded to Ga [6,7]. The luminescence properties of rare earth ions in other similar chalcohalide glass systems indicated the glass structure is sensitive to the
Fig. 1. Glass-forming region in the GeSe2 -Ga2 Se3 -KBr pseudo-ternary system.
Fig. 2. Raman spectra of glasses: Br/Ga = 1 (0.7GeSe2 -0.1Ga2 Se3 0.2KBr); Br/Ga = 0.5 (0.6GeSe2 -0.2Ga2 Se3 -0.2KBr); Br/Ga = 0.25 (0.7GeSe2 -0.2Ga2 Se3 -0.1KBr) and 0.85GeSe2 -0.15Ga2 Se3 glass.
Br/Ga ratio [3]. In our studied glasses, when Br/Ga ratio is smaller than 0.5, Br atoms mainly bonded to Ga in the form [Se3 Ga-Br-GaSe3 ]; as Br/Ga ratio is 0.5–1, with increasing Br/Ga ratio, relative amounts of [GaSe3 Br] to [Se3 Ga-BrGaSe3 ] units get larger; and as Br/Ga ratio reaches 1, [GaSe3 Br] units are predominant. Fig. 2 depicts the Raman spectra of GeSe2 -Ga2 Se3 -KBr glasses with different Br/Ga ratios and 0.85GeSe2 -0.15Ga2 Se3 glass. The Raman spectra are dominated by a band between 150 and 230 cm−1 which is composed of several overlapping bands. The strong band rising near 200 cm−1 is attributed to the overlap of ν1 (A1 ) symmetric stretching modes of corner-sharing [GeSe4 ] and [GaSe4 ] tetrahedral. The shoulder at 216 cm−1 called “companion peak” (c) is associated to the vibration ν1c (Ac1 ) of edge-sharing [GeSe4 ] and [GaSe4 ] tetrahedral. The other shoulder at lower wavenumber, 174 cm−1 , is associated with the [Se3 Ge-GeSe3 ] and [Se3 Ga-GaSe3 ] vibrations [8,9]. The introduction of KBr leads to three major changes in 0.7GeSe2 -0.1Ga2 Se3 -0.2KBr (Br/Ga = 1) glass. Firstly, a new low-phonon band at ∼158 cm−1 appears; secondly, the amplitude of 174 cm−1 band is reduced; thirdly, the 216 cm−1 band is enhanced. These phenomena indicate significant structural variations. The new 158 cm−1 in the first phenomenon band could be assigned to the vibration of [GaSe3 Br] unit. Similar new band is also observed in CsGaSe3/2 Cl glass [10]. The phonon energy of this unit is slightly higher than that of [GaSe3 I] unit [9]. For the second phenomenon, [Se3 Ga-GaSe3 ] unit may be broken by addition of KBr, and then [Se3 Ga-Br-GaSe3 ] and [GaSe3 Br] are possible to be formed. Theoretical evaluation implies that the Raman shift of [Se3 Ga-Br-GaSe3 ] is larger than that of [GaSe3 Br] and smaller than that of [GaSe4 ] [11]. This means the band of [Se3 Ga-Br-GaSe3 ] unit would be located between 158 and 200 cm−1 . Consequently, [Se3 GaGaSe3 ] unit and [Se3 Ga-Br-GaSe3 ] unit may overlap within the broad band around 174 cm−1 (Fig. 2). From the discussion in the last paragraph, [Se3 Ga-Br-GaSe3 ] units are in the minority when Ga/Br ratio reaches 1. Therefore, the 174 cm−1 broad band related to [Se3 Ga-GaSe3 ] unit and [Se3 Ga-Br-GaSe3 ] units
474
G. Tang et al. / Journal of Alloys and Compounds 459 (2008) 472–476
Table 1 Thermal and physical properties of GeSe2 -Ga2 Se3 -KBr glasses Tx (◦ C)
T (◦ C)
Density (g/cm3 )
Short-wavelength absorption edges, λs (nm)
Series A: (1 − 2x)GeSe2 -xGa2 Se3 -xKBr glasses x = 0.10 330 x = 0.15 324 x = 0.20 309 x = 0.25 298 x = 0.30 298 x = 0.35 293
445 453 521 442 421 393
115 129 212 144 123 100
4.185 4.153 4.100 4.079 4.020 3.976
668 663 643 622 608 593
Series B: (0.9 − y)GeSe2 -yGa2 Se3 -0.1KBr glasses y = 0.05 320 y = 0.10 330 y = 0.20 341 y = 0.30 354
440 445 463 430
120 115 122 76
4.134 4.185 4.233 4.289
685 668 677 689
Composition (in mol%)
Tg (◦ C)
decreases. The third phenomenon, the edge-shared GeSe4 and GaSe4 tetrahedral of 0.7GeSe2 -0.1Ga2 Se3 -0.2KBr (Br/Ga = 1) glass increase, in accordance with the results of its thermal stability T = 120 ◦ C lower than that of 0.6GeSe2 -0.2Ga2 Se3 -0.2KBr glass and 0.7GeSe2 -0.2Ga2 Se3 -0.1KBr glass which would be presented in Table 1 below. 3.3. Thermal properties Because the glass-forming region is mainly centred at the Ga2 Se3 /KBr ratio of 1, our study emphasis is put on the thermal properties of the series A. Fig. 3 shows their DTA curves with a heating rate of 10 ◦ C/min. The Tg , Tx , the thermal stability factors T (Tx − Tg ) and densities of studied samples are summarized in Table 1. The values of T for most glasses are more than 100 ◦ C, which indicates these glasses have good thermal stability and can easily be obtained in bulk forms. The most stable glass is obtained for sample 0.6GeSe2 -0.2Ga2 Se3 -0.2KBr (mol%). The thermal stability criterion T of this sample is even as large as 212 ◦ C. From Fig. 3 and Table 1, it is shown that when x increase in series A, crystallization curves are changed from dispersion profile to sharp peak, Tg and density decrease, Tx and T first increase then decrease. The results show that the structure of
glasses is changed when KBr and Ga2 Se3 are added. When x increase, the former [GeSe4 ] units of glasses will decrease. And the addition of KBr and Ga2 Se3 form [Se3 Ga-Br-GaSe3 ] unit or/and [GaSe3 Br] which loose the whole glass structure. As a result, Tg as well as density decrease. In addition, the Tg values still maintain a reasonable value indicating a high degree of glass connectivity. Meanwhile, the new glass units will accelerate the crystallization process when the process occurs. Therefore, the crystallization curves of DTA become sharp peaks when x increase. Due to strong complex-forming ability of Ga, the [GaSe3 ] triangles can easily transform into [GaSe4 ] tetrahedral after capturing the free lone pair electron of Se atoms [5,10]. And this mechanism intensifies the glass network connectivity [5]. While some Br− ions play the role of non-bridging terminator and decrease the network connectivity. So Tx and T accordingly first increase then decrease with the effect of two reciprocal factors mentioned above. When the amount of KBr and Ga2 Se3 is low, the first factor has more influence, and when the amount is larger than 20 mol%, the second factor is dominant. In series B, Tg and density increase with increasing content of Ga2 Se3 . The introduction of [GaSe4 ] tetrahedral makes the glass network compact. The different chemical bonds and microstructure units would enhance the glass network dimensionality and thermal stability criterion T increase [12]. When y ≥ 0.3, phase separation would easily happen due to the different bond energies of [GaSe4 ] and [GeSe4 ] tetrahedral units, then the thermal stability decreases. 3.4. Optical properties
Fig. 3. DTA curves of series A glasses: (1 − 2x)GeSe2 -xGa2 Se3 -xKBr (x = 0.10, 0.15, 0.20, 0.25, 0.30, 0.35).
Fig. 4 shows the IR spectrum of 0.6GeSe2 -0.2Ga2 Se3 0.2KBr glass. It is shown that the glass has a high transmittance (>70%) in 8–14 m atmosphere window and its longwavelength cut-off edges (defined half maximum of the glass transmission) locate at about 624 cm−1 (∼16 m). The absorption bands of impurities in IR region are attributed to O–H, Se–H, H2 O, Se–O, Ge–O, respectively. The impurities may be significantly reduced or eliminate by purifying starting materials or distilling the glasses by appropriate techniques [13]. For other glasses in GeSe2 -Ga2 Se3 -KBr system, there is a slight shift in glass transmission (%) and their long-wavelength cut-off
G. Tang et al. / Journal of Alloys and Compounds 459 (2008) 472–476
Fig. 4. IR transmission spectrum of 0.6GeSe2 -0.2Ga2 Se3 -0.2KBr glass (thickness = 2.0 mm).
edges remain changeless due to the multi-phonon vibrations of Ge–Se. The short-wavelength absorption edges λs (defined as the wavelength where the half maximum of glass transmittance), locate between 593 and 689 nm. The λs of studied samples are summarized in Table 1. It is known that the λs relate to the electron transition between valence band and conduction band. It is complicated to judge the λs in theory when KBr is added, and there is an empirical formula to evaluate the variation tendency of λs [4,6,14]: 1 λs ∝ χ+w−ϕ
(1)
where χ is the average electron affinity energy of anions, w the average energy of bond and ϕ is the average polarization energy of anions [14]. Glass transition temperature Tg relate to w average energy of bond and rigidity of the cross-linked glass [6,15]. Fig. 5 is the Vis-NIR spectra of series A glasses with 500–800 nm range. For series A, the λs of glasses has a remarkable blue shift with x increase. The λs of 0.3GeSe2 -
475
0.35Ga2 Se3 -0.35KBr glass is as small as 593 nm. This value can be compared to 0.42GeSe2 -0.18Ga2 Se3 -0.40CsCl glass [5]. The Ga2 Se3 and KBr additions change the average energy of bond w and the glass structure. As a result, Tg and λs accordingly varied. The value in Table 1 indicates that the Tg and λs have similar correlative variation trends in the glasses. The Tg decreases when λs decreases. This phenomenon is contrary to the Tg − Eg relation (Eg is the optical band gap of the glass and it is in inverse proportion to λs ) which is usually valid in chalcogenide glasses [15]. It is not surprising because Tg and λs have some different influencing factors. In most chalcogenide glasses, the average energy of bond w dominates dependences of Tg and λs . However, in our studied glasses, χ (Br− ) > χ (Se2− ) owing to the higher electronegativity Br− than Se2− . The addition of K+ and Br− with less polarizability compared to the cations (Ge2+ and Ga3+ ) and the anion (Se2− ) and the average energy of bond w decreases with Ga2 Se3 and KBr additions. This situation means the effect that parameters χ and ϕ play dominant roles in formula (1) in our GeSe2 -Ga2 Se3 -KBr chalcohalide glass system. For series A, the more KBr is added, the smaller is the λs . For series B, the amount of KBr is fixed, when Ga2 Se3 replace GeSe2 , there is red shift of λs because the average bond energy of Ga–Se is smaller than that of Ge–Se. This result is not conflict with the variation of Tg since the introduction of Ga2 Se3 increases the rigidity of the cross-linked glass. 4. Conclusion The GeSe2 -Ga2 Se3 -KBr chalcohalide glasses are systematically studied. The glass-forming region, which is mainly located at Ga2 Se3 /KBr = 1, is presented. The structure units of glasses are discussed. The glasses show good thermal stabilities against crystallization and the most stable glass with the composition of 0.6GeSe2 -0.2Ga2 Se3 -0.2KBr. The new chalcohalide glasses show excellent transparency in the entire 0.6–16 m spectral region and are attractive materials for infrared optics. Acknowledgement This work was financially supported by China’s manned space program (the 921-2.1 project). References
Fig. 5. Vis-NIR transmission spectra of series A: GeSe2 -Ga2 Se3 -KBr glasses (thickness = 2.0 mm).
[1] H.L. Ma, Y. Guimond, X.H. Zhang, J. Lucas, J. Non-Cryst. Solids 256–257 (1999) 165–169. [2] Z. Yang, W. Chen, L. Luo, J. Mater. Res. 20 (2005) 2597–2602. [3] Y.B. Shin, J. Heo, H.S. Kim, J. Mater. Res. 16 (2001) 1318–1324. [4] Z. Yang, L. Luo, W. Chen, J. Am. Ceram. Soc. 89 (7) (2006) 2327–2329. [5] L. Calvez, H.L. Ma, J. Lucas, X.H. Zhang, Adv. Mater. 19 (2007) 129– 132. [6] Z. Yang, G. Tang, L. Luo, W. Chen, J. Am. Ceram. Soc. 90 (2) (2007) 667–669. [7] J.H. Song, Y.G. Choi, J. Heo, J. Non-Cryst. Solids 352 (2006) 423–428. [8] P. Nˇemec, B. Frumarov´a, M. Frumar, J. Non-Cryst. Solids 270 (2000) 137–146. [9] Z. Yang, G. Tang, L. Luo, W. Chen, Appl. Phys. Lett. 89 (2006) 131117. [10] Yu.S. Tver’yanovich, M. Vlˇcek, A. Tverjanovich, J. Non-Cryst. Solids 333 (2004) 85–89.
476
G. Tang et al. / Journal of Alloys and Compounds 459 (2008) 472–476
[11] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, third ed., John Wiley & Sons, New York, 1978. [12] H. Guo, Y. Zhai, H. Tao, Y. Gong, X. Zhao, Mater. Res. Bull. 42 (2007) 1111–1118.
[13] V.S. Shiryaev, J.-L. Adam, X.H. Zhang, C. Boussard-Pl`edel, J. Lucas, M.F. Churbanov, J. Non-Cryst. Solids 336 (2004) 113–119. [14] M.L. Cui, Glass Technology, Light Industry Press, Beijing, China, 1987. [15] A. Giridhar, S. Mahadevan, J. Non-Cryst. Solids 126 (1990) 161–169.