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Micro-structural study of the GeS2–In2S3–KCl glassy system by Raman scattering
Spectrochimica Acta Part A 64 (2006) 1039–1045
Micro-structural study of the GeS2–In2S3–KCl glassy system by Raman scattering Tao Haizheng ∗ , Zhao Xiujian, Tong Wei, Mao Shun Key Laboratory of Silicate Materials Science and Engineering (Wuhan University of Technology), Ministry of Education, 122 Luoshi Road, Hongshan-qu, Wuhan, Hubei 430070, PR China Received 4 May 2005; received in revised form 28 June 2005; accepted 13 September 2005
Abstract Room temperature Raman spectra of samples on three serials within the GeS2 –In2 S3 –KCl glassy system have been investigated systematically. According to XRD patterns and Raman spectra of several pseudo-binary systems, the Cl atoms, which was added into the GeS2 –In2 S3 glasses through KCl, was considered to be leading to the breaking of In–In bonds among the S3 In–InS3 ethane-like units and the forming of InS4−x Clx , InS6−x Clx mixed polyhedra. Considering the effect of K+ ions upon mixed anion units (InS4−x Clx and InS6−x Clx ) and the corresponding micro-structural model, the Raman spectral evolution of the GeS2 –In2 S3 –KCl glasses can be elucidated successfully. The microstructure of the GeS2 –In2 S3 –KCl glasses was considered to be that the potassium atoms, which exist in the form of chlorine atoms as its nearest neighbor, are homogeneously dispersed in the glassy net formed by the micro-structural units such as InS4 , InS6 , InS4−x Clx , InS6−x Clx , GeS4 polyhedra and S3 In(Ge)–In(Ge)S3 ethane-like units. © 2005 Elsevier B.V. All rights reserved. Keywords: Chalcohalide glasses; Raman Scattering; Microstructure
1. Introduction Recently Ga2 S3 -based chalcohalide glasses have attracted much attention because of their possible applications such as ultra-fast all optical switching, rare-earth doped 1.3 m fiber amplifier, and etc. [1–4]. Based on the similarity of chemical properties of Ga and In, it can be anticipated that In2 S3 -based chalcohalide glasses could also be the leading candidates in the above-mentioned fields. Considering the intimate relationship between the microstructure and the above-mentioned applications, understanding and utilization of the micro-structural study findings have practical importance. However, to our best knowledge, there were few reports about the micro-structural study of the In2 S3 -based chalcohalide glasses. Raman scattering method has been a valuable probing technique about the micro-structural analysis of chalcogenide glasses [5–8]. Nevertheless, clear ascription of the wide and usually overlapping Raman bands of these glasses has not always been possible, thus leading to diverse micro-structural modes.
∗
Corresponding author. Tel.: +86 27 87669729; fax: +86 27 87669729. E-mail address: [email protected] (H. Z. Tao).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.09.013
For example, about the Raman spectra of the GeS2 glass, the ascription of some bands and the corresponding micro-structural mode has also been one of the topics for debates and comments to this day [6,9]. The main objective of this work is to study the microstructure of the GeS2 –In2 S3 –KCl glasses through the Raman scattering technique.
2. Experimental Samples of the GeS2 –In2 S3 –KCl pseudo-ternary system were prepared by the conventional melt-quenching technique. Batches of Ge, In and S of 99.999% purity, and KCl of 99.99% purity were grounded and weighted in appropriate quantity into fused quartz ampoules within a N2 gas-filled glove box with <1 ppm H2 O and O2 concentrations. The fused quartz ampoules were washed in advance with deionized water, soaked for 15 min in 25% HF acid, re-washed with deionized water, dried at 150 ◦ C in an oven and then baked under vacuum at 1000 ◦ C for 5 h. The ampoules containing the raw materials were sealed under vacuum with 10−4 Pa, which were then inserted into a rocking furnace. The regime was as follows: soaking at 600 ◦ C (5 h) to assist the reaction while rocking, raising the temperature to
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950 ◦ C so as to completely melt the batches within 5 h, soaking at this temperature for 15 h, then decreasing to 700–850 ◦ C in 3 h depending on the glassy composition, and then stopping the rock and preserving the temperature for 3 h, lastly cooling the melts by quenching in the air or in the ice–water mixture. The chemical compositions of samples were analyzed using an energy-dispersive XRF analyzer. Homogeneity and amorphous characteristics of the prepared bulk materials were confirmed by optical and electron microscopy (Tesla BS 340) and X-ray diffraction (XRD) patterns with Cu K␣ radiation at an output power 2 KW (40 kV and 50 mA). Because of the poor water-resistance of samples having high KCl content, the Raman measurements were conducted by focusing the laser into the sample within the silica ampoule in a back (180◦ ) scattering configuration through the micro-Raman Spectrometer (Type: Renishaw RM-1000) using the 632.8 nm laser line. For the avoidance of local laser damage that could easily occur under the microscope and could locally crystallize the amorphous samples, a laser power not exceeding an approximate level of 2.2 mW was used. The resolution of the Raman spectra was 1 cm−1 . 3. Experimental results Fig. 1 shows the glass-forming region of the GeS2 – In2 S3 –KCl pseudo-ternary system obtained according to the method given above. It can be seen that this pseudo-ternary system has a relatively wide glass-forming region which is mainly situated in the GeS2 -rich domain and extends from the GeS2 apex to the situation around 50% KCl when the molar ratio of In2 S3 :KCl remains the constant 1:2. The results of XRF analysis revealed that the difference in composition between a batch and the glass sample was within
Fig. 1. The GeS2 –In2 S3 –KCl pseudo-ternary phase diagram showing the glassforming region (3 g sample within a 4 mm inner diameter silica tube cooling at 950 ◦ C) and the samples’ molar compositions investigated in this study. Serial 1: 0.7GeS2 –(0.7 − x)In2 S3 –xKCl (x = 0.015, 0.035, 0.05, 0.15, 0.2, 0.25); serial 2: (1 − 1.5x)GeS2 –0.5xIn2 S3 –xKCl (x = 0.1, 0.2, 0.3, 0.4, 0.45, 0.5); Serial 3: (1 − 2x)GeS2 –xIn2 S3 –xKCl (x = 0.1, 0.15, 0.2, 0.25).
Fig. 2. Raman spectra of the sample 0.95GeS2 –0.05In2 S3 and the crystal sulfur revealing the composition discrepancy of top, middle and bottom of the sample in the silica tube.
±0.5%. So, hereinafter, the sample composition was expressed by the batch composition. 3.1. GeS2 –In2 S3 pseudo-binary system To different from Ref. [10], which reported that 30% In2 S3 can be added to the GeS2 –In2 S3 pseudo-binary glassy system, under the present preparation procedure, the prepared sample was clearly three-phase separation to the eye with black in the bottom of the silica tube, red in the middle and yellow in the upper when only 5% In2 S3 was added. And this was further verified according to the Raman spectra of this sample (Fig. 2). The insertion of a relatively small quantity of In2 S3 into the GeS2 glass leads mainly to four spectral changes (see Fig. 3). Firstly, in the region 220–270 cm−1 , there is a gradual increase in intensity about the two small bands centered at about 255 and 235 cm−1 . The second change is the slight broadening about the strongest peak at 340 cm−1 . The third variance is the gradual prominence in the region 270–315 cm−1 . The last modification is about the decrease in the amplitude of the shoulder at about 370 cm−1 . 3.2. GeS2 –In2 S3 –KCl pseudo-ternary system To our surprised, only the addition of 1.5% KCl for the samples on serial 1 can lead to the homogeneous glass forming with the red color. That is to say, the addition of KCl has an
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Fig. 3. Raman Scattering spectra of (GeS2 )1−x –(In2 S3 )x glasses revealing the growth of scattering intensity in the region 210–315 cm−1 and the decrease of the shoulder at about 370 cm−1 . All the spectra were normalized according to the intensity of the strongest peak.
important effect on the phase separation suppression of the GeS2 –In2 S3 glasses. To probe the effect of the added KCl, the Raman spectra of samples on this serial are presented firstly (see Fig. 4). The transformation of the region 270–320 cm−1 from
Fig. 4. Raman spectra of samples on serial 1 within the GeS2 –In2 S3 –KCl glassy system.
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Fig. 5. Raman spectra of samples on serial 2 within the GeS2 –In2 S3 –KCl glassy system.
a broad shoulder into a single peak centered at about 302 cm−1 was observed for the first sight. Furthermore, the gradual shift toward the lower wave number about this peak with the increased KCl content x and the small prominence centered at about 280 cm−1 was also noted. Another noticeable change is about the gradual descending of the scattering strength in the region 220–260 cm−1 following the addition of KCl. But this evolution is not like the GeS2 –Ga2 S3 –CsCl glassy system studied by us before in which the two shoulders shrink to nothing when the ratio of Ga2 S3 :CsCl is 1:2 [11]. For the present glassy system, two small prominence still exist by the time the In2 S3 :CsCl ratio is equal to 1:2. The last variance noticed by us is about the increasingly narrowing of the strongest peak centered at approximately 342 cm−1 with the increased x. To affirm the phenomenon observed on serial 1 that the Raman scattering strength for the region 220–260 cm−1 does not shrink to nothing when the In2 S3 :CsCl ratio is equal to 1:2, the Raman spectra of samples on serial 2 were presented (see Fig. 5). Surely, with the descending of the GeS2 content, the two prominences centered at about 250 and 227 cm−1 come out more clearly. In addition, another two evolutions were observed following the descending of the GeS2 content. On the one hand, there is a fast intensity increasing about the peak centered at about 300 cm−1 going with the gradual shift toward the high-frequency aspect. On the other hand, there is a correspondingly rapid intensity dropping about the peak centered at about 340 cm−1 following a jot shift toward the low-frequency aspect. Fig. 6 shows the Raman spectra of samples on serial 3. Their component characteristic is that the ratio of In2 S3 :CsCl keeps
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Fig. 6. Raman spectra of samples on serial 3 within the GeS2 –In2 S3 –KCl glassy system. The insert figure is about the spectra amplifying in the region 220–280 cm−1 .
the constant 1:1, while the quantity of GeS2 drops gradually. Several clear evolutions of vibrational modes similar to that of samples on serial 2 can be seen in this serial. These modes will be shown to the signatures of several specific clusters, which we shall discuss in the next section. 4. Discussion In Fig. 3, the Raman spectrum of GeS2 is presented, that is similar to the previous works [6–9]. The strongest peak at about 340 cm−1 is due to the symmetric stretching vibrating of GeS4 tetrahedra undoubtedly. And the shoulder at about 370 cm−1 , usually designated as the companion A1 c band, has been attributed to the symmetric stretching vibration of the bridged S atoms of the edge-sharing tetrahedra Ge2 S6 based on its evolution with the temperature [6]. The band at 434 cm−1 is generally recognized to be due to S–S bonds or multi-sulfur bonds although some controversies exist till now [8,12]. Correspondingly, the appearance of the very weak Raman band at 255 cm−1 is originated from the S3 Ge–GeS3 ethane-like units because of the compositional fluctuations. And another weak shoulder at about 205 cm−1 , superimposed on the tail of the boson peak, is due to the stretching vibrations of Ge–Ge bonds in tetrahedra containing more than two Ge atoms [6,13]. Another little prominence at about 230 cm−1 that comes out gradually following the addition of In2 S3 can be ascribed to the ν3 mode of the S3 In–InS3 ethane-like unit [14]. Correspondingly, the gradual broadening of the strongest peak at bout 340 cm−1 , which most probably originated from the ν1 vibrat-
ing mode of the S3 Ge(In)–Ge(In)S3 ethane-like units located at about 360 and 320 cm−1 , respectively [14,15], further verified the existence of the ethane-like units. Now the forming of ethanelike units is induced not by the compositional fluctuations but most probably due to the sulfur deficiency brought in through the formation of InS4 and less InS6 polyhedra from In2 S3 where S/In ratio is less than 2. And the increasingly enhancement of intensity of the region 270–315 cm−1 indicates the formation of InS4 and InS6 polyhedra whose symmetric stretching vibrating modes located at about 305 and 280 cm−1 , respectively [16–18]. Within the glassy network, the indium exists mainly in the form of InS4 tetrahedra manifested by the far weaker intensity of the prominence at about 280 cm−1 compared with that of another one at about 305 cm−1 . Finally, the slowly dropping in the amplitude of the shoulder at about 370 cm−1 can be ascribed to the descending of the amount of the edge-sharing Ge2 S6 tetrahedra because of the increased quantity of other micro-structural units with the addition of In2 S3 . To correctively identify the assignments of the Raman modes, the melt-cooling products of several pseudo-binary systems were investigated firstly. According to our previous research [19], KCl does not interact with GeS2 at all. To understand the interaction between indium sulfur compounds and KCl, the pseudo-binary samples of (1 − x) In2 S3 –xKCl and 0.5InS–0.5KCl were probed. For the (1 − x) In2 S3 –xKCl pseudo-binary system, the melt-quenching products are very inhomogeneous to the eye and no new phases are found on their XRD patterns. In addition, almost no change about the Raman spectra occurs following the variance of x from 0 to 0.667 (mol) compared with the Raman spectra of In2 S3 crystalline [16]. Based on the above facts, it can be speculated that the added KCl has little influence on the microstructural units of In2 S3 crystalline under the present preparing procedure. But for the pseudo-binary sample 0.5InS–0.5KCl, the acute diffraction peaks of the new phase (crystalline InSCl) come out on the XRD pattern of this sample (Fig. 7). Furthermore, dramatic Raman spectroscopic changes occur when compared with that of the InS crystals (Fig. 8). Although there exist InS crystals within the melt-quenching product according to the XRD patterns, the two characteristic acute Raman peaks of InS crystals change into two broadened shoulder at the corresponding site. In addition, two acute Raman bands come out. According to the following facts, the peak centered at approximately 272 cm−1 can be ascribed to the symmetric stretching vibration of InS6−x Clx mixed octahedra. Firstly, according to Refs. [18,20], Indium is apt to six coordinations with chlorine, and the symmetric stretching Raman vibration mode of the InCl6 octahedron locates at about 280 cm−1 . Secondly, InS4 Cl2 mixed octahedra exist among the microstructure of the InSCl crystal that belongs to the hexagonal system. Considering the similarity of atomic weight of S and Cl, the structural differences of InS4 Cl2 mixed octahedra and InCl6 octahedra originates mainly from the presence of alkali metal cations K+ . Compared with the K–Cl bonds, the In–Cl bonds are more covalent, so
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as follows: f υ∝ µ
Fig. 7. XRD pattern of the melt-quenching product for the sample 0.5InS–0.5KCl.
the added K+ cations certainly make the In–Cl bonds harden and weaken and displace the chlorine atoms towards the K+ ions. In addition, according to the theory of molecular vibration [18], the oscillating frequency bears a relationship shown
Fig. 8. Raman spectra of the InS crystal and the melt-quenching product of the pseudo-binary sample 0.5InS–0.5KCl. The peaks at 272 and 296 cm−1 are ascribed to the ν1 mode of mixed tetrahedra InSx Cl6−x , InSx Cl4−x , respectively.
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(1)
where f is a constant related to the bond strength, and µ is the discount mass. So it is mainly due to the affection of K+ ions that the ν1 vibrating mode of InS6−x Clx mixed octahedra shifts about 8 cm−1 toward the lower wave number compared with that of sole InCl6 octahedra. Now let us see the ascription of the strongest peak sited at about 296 cm−1 . The InS crystals are composed of S3 In–InS3 ethane-like micro-structural units. The S3 In–InS3 ethane-like unit is easily substituted by two InS3 Cl tetrahedra when the Cl atoms are added because of the bigger affinity for In–Cl bonds compared with the In–In ones. According to Ref. [18], the symmetric stretching vibrating mode of InCl4 tetrahedron is sited at about 321 cm−1 . Similar to the above-mentioned ascription of the 272 cm−1 peak, the strongest peak at about 296 cm−1 is ascribed to the ν1 vibrating mode of InS4−x Clx mixed tetrahedra that shift 21 cm−1 toward the lower wave number due to the affection of K+ ions. And it is the affection of the added KCl that the two acute vibrating modes of the InS crystals degenerate into two widened shoulders when it comes to the 0.5InS–0.5KCl melt-quenching product due to the descending of the coupling ability of the corresponding vibrational mode. Based on the aforementioned results and the Raman spectral evolution of samples on three serials, it can be speculated that the KCl, which was added into the GeS2 –In2 S3 pseudo-binary system, mainly leads to the breaking of In–In bonds within the S3 In–InS3 ethane-like units and the forming of InS4−x Clx mixed tetrahedra and less InS6−x Clx mixed octahedra. Furthermore, based on our previous study on the GeS2 –Ga2 S3 –KCl pseudoternary system, the K+ ions are considered to be homogeneously dispersed in the glassy network in the form of the KCln sole shell. Later it will be confirmed that the Raman spectral evolution of the GeS2 –In2 S3 –KCl glasses can be successfully elucidated according to the above-mentioned model. To confirm the affection of the added KCl, the Raman spectra of the samples on serial 1 was discussed firstly. The content of GeS2 keeps constant 70% (mol%). The evolution of Raman spectra clearly reveals the micro-structural transformation within the GeS2 –In2 S3 –KCl glassy system following the ratio variance of KCl:In2 S3 . First of all, the evolution of the broad shoulder into the unimodal peak at about 302 cm−1 and the peak center shifting toward the lower wavenumber clearly confirmed the existence of InS4−x Clx mixed tetrahedra. When less KCl is added, the shoulder located at about 308 cm−1 originates from the vibrating of InS4 and InS4−x Clx mixed tetrahedra. Considering the affection of K+ , just like the ascription of the peak at about 296 cm−1 about the Raman spectra of the 0.5InS–0.5KCl melt-cooling product, the abovementioned peak center shifting toward the lower wavenumber can be elucidated reasonably considering the affection of K+ ions upon the InS4−x Clx mixed tetrahedra. In addition, according to the aforementioned model, the gradual decrease in intensity about the two
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shoulders located at about 225 and 250 cm−1 further reveals the interaction between the ethane-like units S3 In–InS3 and KCl and manifests itself as a sign of gradual substitution of ethane-like unit S3 In–InS3 by polyhedra InS4−x Clx and InS6−x Clx . And this was further confirmed by the slowly descending of the shoulder located at about 315 and 360 cm−1 ascribed to the ν1 mode of the S3 In(Ge)–In(Ge)S3 ethane-like units, respectively, which lead to the gradual narrowing of the strongest peak at about 340 cm−1 following the increased KCl content. Because of the existence of InS6−x Clx mixed octahedra, not like the GeS2 –Ga2 S3 –CsCl glassy system in which complete substitution of ethane-like unit S3 Ga–GaS3 by two GaS3 Cl tetrahedra occurs when the ratio of Ga2 S3 :CsCl is 1:2 [11], a few ethane-like units still exist for the sample 6 on this serial. And the further declining about the intensity in this region for the sample 7, in which the KCl crystals exist, gives another proof about the existence of the InS6−x Clx mixed octahedra. Now let us see the spectral transformation of the samples on serial 2. The increasingly strengthening about the two prominences in the region 210–260 cm−1 with the drop of GeS2 content is also an important proof about the existence of InS6−x Clx mixed octahedra. And the far stronger intensity about the peak at about 300 cm−1 compared with that of the shoulder located at about 270 cm−1 explained that the indium atoms exist mainly in the form of InS4−x Clx mixed tetrahedra within the glassy network. There is a gradual increasing about the quantity of the bonds for Ge–S–In within the glassy net following the descending of GeS2 content. According to the location of Ge and In among the element periodic table, the bonding of Ge–S–In within the glassy network leads to the shifting of Sulfur toward the Ge side, the strengthening of Ge–S bonds and the weakening of In–S bonds. Base on these analyses, the affection of K+ ions toward InS4−x Clx units and the Eq. (1), the shifting of the ν1 mode of the GeS4 tetrahedra toward the higher wave number and the InS4−x Clx mixed tetrahedra toward lower wavenumber can be explained successfully. The clear phase separation occurs about the sample 6 on serial 2, yellow at the top of the silica tube and red at the bottom. According to XRD patterns of this sample, the yellow phase is amorphous, while the red one contains crystal phase such as In2 S3 , KCl and InSCl. Fig. 9 presents the Raman spectral difference of the two phases. The peaks located at about 307 and 369 cm−1 and the distinct shoulder at about 247 cm−1 on the Raman spectra of the red phase was ascribed to the vibrating mode of the In2 S3 crystal [16]. On the other hand, on the Raman spectra of the yellow phase, the strongest peak sites at about 296 cm−1 that can be ascribed to the symmetric stretching vibrating mode of the InS4−x Clx units. This further confirmed the effect of K+ ions upon the InS4−x Clx mixed tetrahedra that leads to the 11 cm−1 shifting compared with the corresponding vibratinal mode of InS4 tetrahedra. For the Raman spectra of samples on serial 3, the clear gradual dropping about the two shoulder in the region 200–262 cm−1 (see the insert figure in Fig. 6) reveals the slowly decreasing of the ethane-like units following the increasing GeS2 content. The evolution about the two peaks at about 342 and 300 cm−1 can also be elucidated successfully similar to the explanation of
Fig. 9. Raman spectra of the sample 6 on serial 2 within the GeS2 –In2 S3 –KCl glassy system.
counterpart spectral variance of samples on serial 2 in terms of the proposed micro-structural model. 5. Conclusion Systematic measurements of Raman spectra of samples on three serials within the GeS2 –In2 S3 –KCl pseudo-ternary glassy system have been conducted at room temperature. Based on the proposed micro-structural model, all the Raman spectra can be reasonably ascribed and the spectral evolutions can be successfully elucidated. At the same time, the following conclusions can be deduced: 1. The added K+ ions are homogeneously dispersed in the glassy net in the form of Cl atoms as its nearest neighbor. 2. The added Cl atoms exist mainly in the form of InS4−x Clx mixed tetrahedra and less in the form of InS6−x Clx mixed octahedra. 3. Gradual substitution of InS4−x Clx and InS6−x Clx mixed polyhedra for the S3 In(Ge)–In (Ge)S3 ethane-like units occurs following the increased KCl content. Acknowledgements This work was partially funded by the National Natural Science Foundation of China (No. 50125205), the Opening Fund of Key Laboratory of Silicate Materials Science and Engineering (Wuhan University of Technology) Ministry of Education (No. SYSJJ2004-14) and Scientific Research Foundation of Wuhan University of Technology.
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Micro-structural study of the GeS2–In2S3–KCl glassy system by Raman scattering Tao Haizheng ∗ , Zhao Xiujian, Tong Wei, Mao Shun Key Laboratory of Silicate Materials Science and Engineering (Wuhan University of Technology), Ministry of Education, 122 Luoshi Road, Hongshan-qu, Wuhan, Hubei 430070, PR China Received 4 May 2005; received in revised form 28 June 2005; accepted 13 September 2005
Abstract Room temperature Raman spectra of samples on three serials within the GeS2 –In2 S3 –KCl glassy system have been investigated systematically. According to XRD patterns and Raman spectra of several pseudo-binary systems, the Cl atoms, which was added into the GeS2 –In2 S3 glasses through KCl, was considered to be leading to the breaking of In–In bonds among the S3 In–InS3 ethane-like units and the forming of InS4−x Clx , InS6−x Clx mixed polyhedra. Considering the effect of K+ ions upon mixed anion units (InS4−x Clx and InS6−x Clx ) and the corresponding micro-structural model, the Raman spectral evolution of the GeS2 –In2 S3 –KCl glasses can be elucidated successfully. The microstructure of the GeS2 –In2 S3 –KCl glasses was considered to be that the potassium atoms, which exist in the form of chlorine atoms as its nearest neighbor, are homogeneously dispersed in the glassy net formed by the micro-structural units such as InS4 , InS6 , InS4−x Clx , InS6−x Clx , GeS4 polyhedra and S3 In(Ge)–In(Ge)S3 ethane-like units. © 2005 Elsevier B.V. All rights reserved. Keywords: Chalcohalide glasses; Raman Scattering; Microstructure
1. Introduction Recently Ga2 S3 -based chalcohalide glasses have attracted much attention because of their possible applications such as ultra-fast all optical switching, rare-earth doped 1.3 m fiber amplifier, and etc. [1–4]. Based on the similarity of chemical properties of Ga and In, it can be anticipated that In2 S3 -based chalcohalide glasses could also be the leading candidates in the above-mentioned fields. Considering the intimate relationship between the microstructure and the above-mentioned applications, understanding and utilization of the micro-structural study findings have practical importance. However, to our best knowledge, there were few reports about the micro-structural study of the In2 S3 -based chalcohalide glasses. Raman scattering method has been a valuable probing technique about the micro-structural analysis of chalcogenide glasses [5–8]. Nevertheless, clear ascription of the wide and usually overlapping Raman bands of these glasses has not always been possible, thus leading to diverse micro-structural modes.
∗
Corresponding author. Tel.: +86 27 87669729; fax: +86 27 87669729. E-mail address: [email protected] (H. Z. Tao).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.09.013
For example, about the Raman spectra of the GeS2 glass, the ascription of some bands and the corresponding micro-structural mode has also been one of the topics for debates and comments to this day [6,9]. The main objective of this work is to study the microstructure of the GeS2 –In2 S3 –KCl glasses through the Raman scattering technique.
2. Experimental Samples of the GeS2 –In2 S3 –KCl pseudo-ternary system were prepared by the conventional melt-quenching technique. Batches of Ge, In and S of 99.999% purity, and KCl of 99.99% purity were grounded and weighted in appropriate quantity into fused quartz ampoules within a N2 gas-filled glove box with <1 ppm H2 O and O2 concentrations. The fused quartz ampoules were washed in advance with deionized water, soaked for 15 min in 25% HF acid, re-washed with deionized water, dried at 150 ◦ C in an oven and then baked under vacuum at 1000 ◦ C for 5 h. The ampoules containing the raw materials were sealed under vacuum with 10−4 Pa, which were then inserted into a rocking furnace. The regime was as follows: soaking at 600 ◦ C (5 h) to assist the reaction while rocking, raising the temperature to
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950 ◦ C so as to completely melt the batches within 5 h, soaking at this temperature for 15 h, then decreasing to 700–850 ◦ C in 3 h depending on the glassy composition, and then stopping the rock and preserving the temperature for 3 h, lastly cooling the melts by quenching in the air or in the ice–water mixture. The chemical compositions of samples were analyzed using an energy-dispersive XRF analyzer. Homogeneity and amorphous characteristics of the prepared bulk materials were confirmed by optical and electron microscopy (Tesla BS 340) and X-ray diffraction (XRD) patterns with Cu K␣ radiation at an output power 2 KW (40 kV and 50 mA). Because of the poor water-resistance of samples having high KCl content, the Raman measurements were conducted by focusing the laser into the sample within the silica ampoule in a back (180◦ ) scattering configuration through the micro-Raman Spectrometer (Type: Renishaw RM-1000) using the 632.8 nm laser line. For the avoidance of local laser damage that could easily occur under the microscope and could locally crystallize the amorphous samples, a laser power not exceeding an approximate level of 2.2 mW was used. The resolution of the Raman spectra was 1 cm−1 . 3. Experimental results Fig. 1 shows the glass-forming region of the GeS2 – In2 S3 –KCl pseudo-ternary system obtained according to the method given above. It can be seen that this pseudo-ternary system has a relatively wide glass-forming region which is mainly situated in the GeS2 -rich domain and extends from the GeS2 apex to the situation around 50% KCl when the molar ratio of In2 S3 :KCl remains the constant 1:2. The results of XRF analysis revealed that the difference in composition between a batch and the glass sample was within
Fig. 1. The GeS2 –In2 S3 –KCl pseudo-ternary phase diagram showing the glassforming region (3 g sample within a 4 mm inner diameter silica tube cooling at 950 ◦ C) and the samples’ molar compositions investigated in this study. Serial 1: 0.7GeS2 –(0.7 − x)In2 S3 –xKCl (x = 0.015, 0.035, 0.05, 0.15, 0.2, 0.25); serial 2: (1 − 1.5x)GeS2 –0.5xIn2 S3 –xKCl (x = 0.1, 0.2, 0.3, 0.4, 0.45, 0.5); Serial 3: (1 − 2x)GeS2 –xIn2 S3 –xKCl (x = 0.1, 0.15, 0.2, 0.25).
Fig. 2. Raman spectra of the sample 0.95GeS2 –0.05In2 S3 and the crystal sulfur revealing the composition discrepancy of top, middle and bottom of the sample in the silica tube.
±0.5%. So, hereinafter, the sample composition was expressed by the batch composition. 3.1. GeS2 –In2 S3 pseudo-binary system To different from Ref. [10], which reported that 30% In2 S3 can be added to the GeS2 –In2 S3 pseudo-binary glassy system, under the present preparation procedure, the prepared sample was clearly three-phase separation to the eye with black in the bottom of the silica tube, red in the middle and yellow in the upper when only 5% In2 S3 was added. And this was further verified according to the Raman spectra of this sample (Fig. 2). The insertion of a relatively small quantity of In2 S3 into the GeS2 glass leads mainly to four spectral changes (see Fig. 3). Firstly, in the region 220–270 cm−1 , there is a gradual increase in intensity about the two small bands centered at about 255 and 235 cm−1 . The second change is the slight broadening about the strongest peak at 340 cm−1 . The third variance is the gradual prominence in the region 270–315 cm−1 . The last modification is about the decrease in the amplitude of the shoulder at about 370 cm−1 . 3.2. GeS2 –In2 S3 –KCl pseudo-ternary system To our surprised, only the addition of 1.5% KCl for the samples on serial 1 can lead to the homogeneous glass forming with the red color. That is to say, the addition of KCl has an
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Fig. 3. Raman Scattering spectra of (GeS2 )1−x –(In2 S3 )x glasses revealing the growth of scattering intensity in the region 210–315 cm−1 and the decrease of the shoulder at about 370 cm−1 . All the spectra were normalized according to the intensity of the strongest peak.
important effect on the phase separation suppression of the GeS2 –In2 S3 glasses. To probe the effect of the added KCl, the Raman spectra of samples on this serial are presented firstly (see Fig. 4). The transformation of the region 270–320 cm−1 from
Fig. 4. Raman spectra of samples on serial 1 within the GeS2 –In2 S3 –KCl glassy system.
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Fig. 5. Raman spectra of samples on serial 2 within the GeS2 –In2 S3 –KCl glassy system.
a broad shoulder into a single peak centered at about 302 cm−1 was observed for the first sight. Furthermore, the gradual shift toward the lower wave number about this peak with the increased KCl content x and the small prominence centered at about 280 cm−1 was also noted. Another noticeable change is about the gradual descending of the scattering strength in the region 220–260 cm−1 following the addition of KCl. But this evolution is not like the GeS2 –Ga2 S3 –CsCl glassy system studied by us before in which the two shoulders shrink to nothing when the ratio of Ga2 S3 :CsCl is 1:2 [11]. For the present glassy system, two small prominence still exist by the time the In2 S3 :CsCl ratio is equal to 1:2. The last variance noticed by us is about the increasingly narrowing of the strongest peak centered at approximately 342 cm−1 with the increased x. To affirm the phenomenon observed on serial 1 that the Raman scattering strength for the region 220–260 cm−1 does not shrink to nothing when the In2 S3 :CsCl ratio is equal to 1:2, the Raman spectra of samples on serial 2 were presented (see Fig. 5). Surely, with the descending of the GeS2 content, the two prominences centered at about 250 and 227 cm−1 come out more clearly. In addition, another two evolutions were observed following the descending of the GeS2 content. On the one hand, there is a fast intensity increasing about the peak centered at about 300 cm−1 going with the gradual shift toward the high-frequency aspect. On the other hand, there is a correspondingly rapid intensity dropping about the peak centered at about 340 cm−1 following a jot shift toward the low-frequency aspect. Fig. 6 shows the Raman spectra of samples on serial 3. Their component characteristic is that the ratio of In2 S3 :CsCl keeps
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Fig. 6. Raman spectra of samples on serial 3 within the GeS2 –In2 S3 –KCl glassy system. The insert figure is about the spectra amplifying in the region 220–280 cm−1 .
the constant 1:1, while the quantity of GeS2 drops gradually. Several clear evolutions of vibrational modes similar to that of samples on serial 2 can be seen in this serial. These modes will be shown to the signatures of several specific clusters, which we shall discuss in the next section. 4. Discussion In Fig. 3, the Raman spectrum of GeS2 is presented, that is similar to the previous works [6–9]. The strongest peak at about 340 cm−1 is due to the symmetric stretching vibrating of GeS4 tetrahedra undoubtedly. And the shoulder at about 370 cm−1 , usually designated as the companion A1 c band, has been attributed to the symmetric stretching vibration of the bridged S atoms of the edge-sharing tetrahedra Ge2 S6 based on its evolution with the temperature [6]. The band at 434 cm−1 is generally recognized to be due to S–S bonds or multi-sulfur bonds although some controversies exist till now [8,12]. Correspondingly, the appearance of the very weak Raman band at 255 cm−1 is originated from the S3 Ge–GeS3 ethane-like units because of the compositional fluctuations. And another weak shoulder at about 205 cm−1 , superimposed on the tail of the boson peak, is due to the stretching vibrations of Ge–Ge bonds in tetrahedra containing more than two Ge atoms [6,13]. Another little prominence at about 230 cm−1 that comes out gradually following the addition of In2 S3 can be ascribed to the ν3 mode of the S3 In–InS3 ethane-like unit [14]. Correspondingly, the gradual broadening of the strongest peak at bout 340 cm−1 , which most probably originated from the ν1 vibrat-
ing mode of the S3 Ge(In)–Ge(In)S3 ethane-like units located at about 360 and 320 cm−1 , respectively [14,15], further verified the existence of the ethane-like units. Now the forming of ethanelike units is induced not by the compositional fluctuations but most probably due to the sulfur deficiency brought in through the formation of InS4 and less InS6 polyhedra from In2 S3 where S/In ratio is less than 2. And the increasingly enhancement of intensity of the region 270–315 cm−1 indicates the formation of InS4 and InS6 polyhedra whose symmetric stretching vibrating modes located at about 305 and 280 cm−1 , respectively [16–18]. Within the glassy network, the indium exists mainly in the form of InS4 tetrahedra manifested by the far weaker intensity of the prominence at about 280 cm−1 compared with that of another one at about 305 cm−1 . Finally, the slowly dropping in the amplitude of the shoulder at about 370 cm−1 can be ascribed to the descending of the amount of the edge-sharing Ge2 S6 tetrahedra because of the increased quantity of other micro-structural units with the addition of In2 S3 . To correctively identify the assignments of the Raman modes, the melt-cooling products of several pseudo-binary systems were investigated firstly. According to our previous research [19], KCl does not interact with GeS2 at all. To understand the interaction between indium sulfur compounds and KCl, the pseudo-binary samples of (1 − x) In2 S3 –xKCl and 0.5InS–0.5KCl were probed. For the (1 − x) In2 S3 –xKCl pseudo-binary system, the melt-quenching products are very inhomogeneous to the eye and no new phases are found on their XRD patterns. In addition, almost no change about the Raman spectra occurs following the variance of x from 0 to 0.667 (mol) compared with the Raman spectra of In2 S3 crystalline [16]. Based on the above facts, it can be speculated that the added KCl has little influence on the microstructural units of In2 S3 crystalline under the present preparing procedure. But for the pseudo-binary sample 0.5InS–0.5KCl, the acute diffraction peaks of the new phase (crystalline InSCl) come out on the XRD pattern of this sample (Fig. 7). Furthermore, dramatic Raman spectroscopic changes occur when compared with that of the InS crystals (Fig. 8). Although there exist InS crystals within the melt-quenching product according to the XRD patterns, the two characteristic acute Raman peaks of InS crystals change into two broadened shoulder at the corresponding site. In addition, two acute Raman bands come out. According to the following facts, the peak centered at approximately 272 cm−1 can be ascribed to the symmetric stretching vibration of InS6−x Clx mixed octahedra. Firstly, according to Refs. [18,20], Indium is apt to six coordinations with chlorine, and the symmetric stretching Raman vibration mode of the InCl6 octahedron locates at about 280 cm−1 . Secondly, InS4 Cl2 mixed octahedra exist among the microstructure of the InSCl crystal that belongs to the hexagonal system. Considering the similarity of atomic weight of S and Cl, the structural differences of InS4 Cl2 mixed octahedra and InCl6 octahedra originates mainly from the presence of alkali metal cations K+ . Compared with the K–Cl bonds, the In–Cl bonds are more covalent, so
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as follows: f υ∝ µ
Fig. 7. XRD pattern of the melt-quenching product for the sample 0.5InS–0.5KCl.
the added K+ cations certainly make the In–Cl bonds harden and weaken and displace the chlorine atoms towards the K+ ions. In addition, according to the theory of molecular vibration [18], the oscillating frequency bears a relationship shown
Fig. 8. Raman spectra of the InS crystal and the melt-quenching product of the pseudo-binary sample 0.5InS–0.5KCl. The peaks at 272 and 296 cm−1 are ascribed to the ν1 mode of mixed tetrahedra InSx Cl6−x , InSx Cl4−x , respectively.
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(1)
where f is a constant related to the bond strength, and µ is the discount mass. So it is mainly due to the affection of K+ ions that the ν1 vibrating mode of InS6−x Clx mixed octahedra shifts about 8 cm−1 toward the lower wave number compared with that of sole InCl6 octahedra. Now let us see the ascription of the strongest peak sited at about 296 cm−1 . The InS crystals are composed of S3 In–InS3 ethane-like micro-structural units. The S3 In–InS3 ethane-like unit is easily substituted by two InS3 Cl tetrahedra when the Cl atoms are added because of the bigger affinity for In–Cl bonds compared with the In–In ones. According to Ref. [18], the symmetric stretching vibrating mode of InCl4 tetrahedron is sited at about 321 cm−1 . Similar to the above-mentioned ascription of the 272 cm−1 peak, the strongest peak at about 296 cm−1 is ascribed to the ν1 vibrating mode of InS4−x Clx mixed tetrahedra that shift 21 cm−1 toward the lower wave number due to the affection of K+ ions. And it is the affection of the added KCl that the two acute vibrating modes of the InS crystals degenerate into two widened shoulders when it comes to the 0.5InS–0.5KCl melt-quenching product due to the descending of the coupling ability of the corresponding vibrational mode. Based on the aforementioned results and the Raman spectral evolution of samples on three serials, it can be speculated that the KCl, which was added into the GeS2 –In2 S3 pseudo-binary system, mainly leads to the breaking of In–In bonds within the S3 In–InS3 ethane-like units and the forming of InS4−x Clx mixed tetrahedra and less InS6−x Clx mixed octahedra. Furthermore, based on our previous study on the GeS2 –Ga2 S3 –KCl pseudoternary system, the K+ ions are considered to be homogeneously dispersed in the glassy network in the form of the KCln sole shell. Later it will be confirmed that the Raman spectral evolution of the GeS2 –In2 S3 –KCl glasses can be successfully elucidated according to the above-mentioned model. To confirm the affection of the added KCl, the Raman spectra of the samples on serial 1 was discussed firstly. The content of GeS2 keeps constant 70% (mol%). The evolution of Raman spectra clearly reveals the micro-structural transformation within the GeS2 –In2 S3 –KCl glassy system following the ratio variance of KCl:In2 S3 . First of all, the evolution of the broad shoulder into the unimodal peak at about 302 cm−1 and the peak center shifting toward the lower wavenumber clearly confirmed the existence of InS4−x Clx mixed tetrahedra. When less KCl is added, the shoulder located at about 308 cm−1 originates from the vibrating of InS4 and InS4−x Clx mixed tetrahedra. Considering the affection of K+ , just like the ascription of the peak at about 296 cm−1 about the Raman spectra of the 0.5InS–0.5KCl melt-cooling product, the abovementioned peak center shifting toward the lower wavenumber can be elucidated reasonably considering the affection of K+ ions upon the InS4−x Clx mixed tetrahedra. In addition, according to the aforementioned model, the gradual decrease in intensity about the two
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shoulders located at about 225 and 250 cm−1 further reveals the interaction between the ethane-like units S3 In–InS3 and KCl and manifests itself as a sign of gradual substitution of ethane-like unit S3 In–InS3 by polyhedra InS4−x Clx and InS6−x Clx . And this was further confirmed by the slowly descending of the shoulder located at about 315 and 360 cm−1 ascribed to the ν1 mode of the S3 In(Ge)–In(Ge)S3 ethane-like units, respectively, which lead to the gradual narrowing of the strongest peak at about 340 cm−1 following the increased KCl content. Because of the existence of InS6−x Clx mixed octahedra, not like the GeS2 –Ga2 S3 –CsCl glassy system in which complete substitution of ethane-like unit S3 Ga–GaS3 by two GaS3 Cl tetrahedra occurs when the ratio of Ga2 S3 :CsCl is 1:2 [11], a few ethane-like units still exist for the sample 6 on this serial. And the further declining about the intensity in this region for the sample 7, in which the KCl crystals exist, gives another proof about the existence of the InS6−x Clx mixed octahedra. Now let us see the spectral transformation of the samples on serial 2. The increasingly strengthening about the two prominences in the region 210–260 cm−1 with the drop of GeS2 content is also an important proof about the existence of InS6−x Clx mixed octahedra. And the far stronger intensity about the peak at about 300 cm−1 compared with that of the shoulder located at about 270 cm−1 explained that the indium atoms exist mainly in the form of InS4−x Clx mixed tetrahedra within the glassy network. There is a gradual increasing about the quantity of the bonds for Ge–S–In within the glassy net following the descending of GeS2 content. According to the location of Ge and In among the element periodic table, the bonding of Ge–S–In within the glassy network leads to the shifting of Sulfur toward the Ge side, the strengthening of Ge–S bonds and the weakening of In–S bonds. Base on these analyses, the affection of K+ ions toward InS4−x Clx units and the Eq. (1), the shifting of the ν1 mode of the GeS4 tetrahedra toward the higher wave number and the InS4−x Clx mixed tetrahedra toward lower wavenumber can be explained successfully. The clear phase separation occurs about the sample 6 on serial 2, yellow at the top of the silica tube and red at the bottom. According to XRD patterns of this sample, the yellow phase is amorphous, while the red one contains crystal phase such as In2 S3 , KCl and InSCl. Fig. 9 presents the Raman spectral difference of the two phases. The peaks located at about 307 and 369 cm−1 and the distinct shoulder at about 247 cm−1 on the Raman spectra of the red phase was ascribed to the vibrating mode of the In2 S3 crystal [16]. On the other hand, on the Raman spectra of the yellow phase, the strongest peak sites at about 296 cm−1 that can be ascribed to the symmetric stretching vibrating mode of the InS4−x Clx units. This further confirmed the effect of K+ ions upon the InS4−x Clx mixed tetrahedra that leads to the 11 cm−1 shifting compared with the corresponding vibratinal mode of InS4 tetrahedra. For the Raman spectra of samples on serial 3, the clear gradual dropping about the two shoulder in the region 200–262 cm−1 (see the insert figure in Fig. 6) reveals the slowly decreasing of the ethane-like units following the increasing GeS2 content. The evolution about the two peaks at about 342 and 300 cm−1 can also be elucidated successfully similar to the explanation of
Fig. 9. Raman spectra of the sample 6 on serial 2 within the GeS2 –In2 S3 –KCl glassy system.
counterpart spectral variance of samples on serial 2 in terms of the proposed micro-structural model. 5. Conclusion Systematic measurements of Raman spectra of samples on three serials within the GeS2 –In2 S3 –KCl pseudo-ternary glassy system have been conducted at room temperature. Based on the proposed micro-structural model, all the Raman spectra can be reasonably ascribed and the spectral evolutions can be successfully elucidated. At the same time, the following conclusions can be deduced: 1. The added K+ ions are homogeneously dispersed in the glassy net in the form of Cl atoms as its nearest neighbor. 2. The added Cl atoms exist mainly in the form of InS4−x Clx mixed tetrahedra and less in the form of InS6−x Clx mixed octahedra. 3. Gradual substitution of InS4−x Clx and InS6−x Clx mixed polyhedra for the S3 In(Ge)–In (Ge)S3 ethane-like units occurs following the increased KCl content. Acknowledgements This work was partially funded by the National Natural Science Foundation of China (No. 50125205), the Opening Fund of Key Laboratory of Silicate Materials Science and Engineering (Wuhan University of Technology) Ministry of Education (No. SYSJJ2004-14) and Scientific Research Foundation of Wuhan University of Technology.
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