Infrared and Raman studies on Sn_x–Sb₅–Se_95−x chalcogenide glasses

3.1 Infrared Spectra of Sn_x–Sb₅–Se_95−x system

Fig. 1 shows infrared spectra of Sn_x–Sb₅–Se_95−x system (where x = 0, 5, 10 and 12.5). Infrared transmission (%) versus the wave number (cm⁻¹) spectrum at the bottom shows at least five to six transmission bands. The band that extends from 100 to 131 cm⁻¹ is assigned to the Se₈-ring bending mode, whereas a 115-cm⁻¹ band was assigned to Se₈ chain in Ge–Se alloy (Goyal and Maan, 1995). The band situated between 131 and 190 cm⁻¹, with its two shoulders at 147 and 181 cm⁻¹, is attributed to the SbSe₃ stretching mode as reported in Kato et al. (1983). Furthermore, the band located around 190–210 cm⁻¹ is ascribed to the Se–Se stretching mode or to the Sb–Se bending mode, while the band around 210–239 cm⁻¹ with a shoulder at 220 showed SbSe₃ stretching mode and Se₈ chain mode, respectively. On the other hand, the band around 177–210 cm⁻¹ was assigned to Sb–Se bond in Sb–Ge–Se glass (Sharma et al., 1989). The transmission cut-off at 254 cm⁻¹ is clearly assigned to the Se₈ chain ring, as the absorption peaks around 227 and 247 cm⁻¹ were assigned to the Se polymeric chain and Se₈ ring modes, respectively (Zhenhua, 1991).

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Figure 1

Infrared transmission spectra of Sn_x–Sb₅–Se_95−x system.

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The second spectrum demonstrates the sample Sn = 5-mole%. A new band ranging from 117 to 145 cm⁻¹ is assigned to SnSe₄ tetrahedral mode. This band was not observed in the preceding spectrum. However, the fraction of Sn–Se bond shows the possibility of the first chance of forming Sn–Se covalent bond in this composition as reported in Adam et al. (2005). Further, support of this assignment is deduced from Raman analysis on binary Sn–Se, which shows a high intensity peak at 150 cm⁻¹ (Adam et al., 2002). Further change in the spectrum is observed when the intensity of the band between 147 and 181 cm⁻¹ in Sb₅Se₉₅ is reduced and shifted to the new band between 169 and 198 cm⁻¹, with a shoulder at 188 cm⁻¹. This band is ascribed to another pyramidal SbSe₃ mode. In contrast, the shoulder at 188 cm⁻¹ is assigned to SnSe₄ tetrahedral mode. Whereas Raman shift at 189 cm⁻¹ was assigned to SnSe₄ units in Ge_1−xSn_xSe₂ glasses (Stevens et al., 1985). The third band at 198–238 cm⁻¹ is attributed to Se–Se or SbSe₃ bending modes and its shoulders around 214 and 221 cm⁻¹ are assigned to Se₈ chains. It should be mentioned here that since the atomic masses of Sn and Sb are almost similar, their frequency vibrations are not clearly identified from IR analysis. Comparing and supporting the IR analysis with Raman study will clarify any sort of overlapping.

In Sn = 10-mole% spectrum reduction and shift of the band’s intensity from around 117–145 cm⁻¹ at x = 5 to 154–187 cm⁻¹ are detected. This new band is ascribed to SnSe₄ mode. The stretching pyramidal band of SbSe₃ is extended between 187 and 237 cm⁻¹ with a series of shoulders around 202, 214, 217 and 221. However, the shoulders around 217 and 237 cm⁻¹ are attributed to Se-chain mode, which can also be assigned to pyramidal SbSe₃ mode. Shift and reduction in the band location and intensity, respectively, are observed when the spectrum of Sn = 12.5 mole% is examined. The band between 132 and 151 cm⁻¹ shows a tetrahedral SnSe₄ bending mode. However, the band at 180–194 cm⁻¹ is ascribed to tetrahedral SnSe₄ stretching mode, while that extending between 194 and 216 cm⁻¹ is assigned to pyramidal SbSe₃ stretching mode. The last band between 216 and 238 cm⁻¹ could be assigned to a pyramidal SbSe₃ bending mode or to Se₈-chain mode.

3.2 Raman vibration spectra of Sn_x–Sb₅–Se_95−x system

Raman spectra for amorphous Se and Sn_x–Sb₅–Se_95−x system are shown in Fig. 2. A peak at 250.64 cm⁻¹ dominates the Raman spectrum of amorphous Se. This peak is also a characteristic of monoclinic selenium (Mort, 1973). The Raman peak at 250 cm⁻¹ is therefore assigned to totally symmetric vibration mode of Se₈-ring based on this evidence and the fact that this line is polarisation dependent. Weak shoulder at 239 cm⁻¹ was observed in amorphous Se, which coincided with a dominant peak of crystalline trigonal Se (Mort, 1973). Hence, the low frequency band at 239 cm⁻¹ is assigned to Se₈-bending mode. On the other hand, Raman vibrations at 235 and 255 cm⁻¹ are assigned to Se–Se chains in Ge_xSe_1−x glasses (Wong et al., 1998).

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Figure 2

Raman spectra of amorphous Se and Sn_x–Sb₅–Se_95−x system, where x = 0, 5, 10 and 12.5.

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As shown in Fig. 2, Raman spectra of Sn_x–Sb₅–Se_95−x system, many bands are identified around 250, 239, 190, 183 and 150 cm⁻¹. It is observed that for Sb₅Se₉₅ spectrum (x = 0), the peak at 190 cm⁻¹ is ascribed to SbSe₃ pyramidal vibration symmetrical stretching mode. The two short peaks at 250 and 239 cm⁻¹ are assigned to Se₈ stretching and bending modes, respectively. The forth peak at 150 cm⁻¹ is attributed to symmetrical bending pyramidal of SbSe₃. It is noticeable that changing of Sn, Sb and Se-mole% varies the intensity and width of the Raman peak. The spectrum of Sn = 5 mole% shows a shift and reduction of the peak at 190 cm⁻¹, which is assigned for a pyramidal SbSe₃ stretching mode. This change implies the occurrence of Sn sharing the bonds in Sn–Se–SbSe₃ net instead of Se–Se bonds. A little higher peak than that of Sb₅Se₉₅ is observed at 150 cm⁻¹ and assigned to symmetric bending mode of SnSe₄ tetrahedral. The peak intensities are increased at 250 and 239 cm⁻¹ and implies the existence of Se chains.

Extreme shift of Raman’s peak occurs in the spectrum of Sn = 10 mole%. The pyramidal SbSe₃ stretching is still dominant and approaching the symmetric stretching of SnSe₄ tetrahedral mode at 185 cm⁻¹. The peak at 184 cm⁻¹ was reported for SnSe₄ bending mode (Mikurt and McNell, 1890). The trend continues in Sn = 12.5-mole% spectrum while the peak at 184 cm⁻¹ is more reasonably attributed to symmetric stretching of SnSe₄ tetrahedral mode than stretching of SbSe₃ pyramidal mode.

Results of IR and Raman spectroscopes help us to explain the structure of Sn–Sb–Se system. Addition of Sn mole% to the system modifies the Sb–Se structure and incorporates co-ordination number 4 in glass region. Observation of Se stretching bond supports the glass formation in Se-rich region at which the Sn and Sb-mole% are less than their stoichiometric binary compounds with Se. The structure of Sn–Se and Sb–Se glasses can be envisaged as the local co-ordination that satisfies the 8-N rule of the classical valence bond theory (Lucovsky et al., 1977). The structure of Se glasses was believed to consist of long chains of selenium atoms, each having co-ordination number 2 (Aronovitz et al., 1983). To these selenium chains, small amounts of cross-linking atoms, such as Sn and Sb, were added. These amounts should be less than that required for SnSe₂ and Sb₂Se₃ stoichiometric compositions. Consequently, the glass structure can build up. Sn atoms are 4 coordinated when covalently bonded and each Sn atom having bonds to Se atoms, while Sb atoms are 3 coordinated and each Sb atom having bonds to Se atoms. It is deduced that the basic structural unit of SnSe₂ glass is made of SnSe₄ tetrahedral and that of Sb₂Se₃ glass is made of SbSe₃ pyramidal. These units spread out among Se chains and form a net of Se₃–Sn–Se–Se–Sb–Se₂ glass structure. The occurrence of a new IR transmission band around 125–145 cm⁻¹ and the shift of Raman peak from 190 to 183 cm⁻¹, which happens due to increasing Sn-mole% while Sb-mole% is fixed at 5, are strong evidences that support the configuration of this net structure.