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Glassy state and structure of Sn–Sb–Se chalcogenide alloy
LETTER TO THE EDITOR
Journal of Non-Crystalline Solids 352 (2006) 2288–2291 www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Glassy state and structure of Sn–Sb–Se chalcogenide alloy Praveen Kumar, R. Thangaraj
*
Semiconductors Laboratory, Department of Applied Physics, Guru Nanak Dev University, Amritsar 143005, India Received 22 October 2005; received in revised form 3 January 2006 Available online 5 May 2006
Abstract The effect of Sn addition on the glass transition and structure of c-Sb20Se80 chalcogenide alloy have been studied by X-ray diffraction and differential scanning calorimetric studies. The increase in the glass forming region and the glass transition temperature with the addition of Sn is discussed by considering the formation of [SnSe4] tetrahedra, another type of network former, which inhibits the crystallization. The differential scanning calorimetric studies on SnxSb20Se80x (8 6 x 6 18) glassy samples reveal a single glass transition temperature for all values of x while a single crystallization peak was obtained only for 10 6 x < 12. The X-ray diffraction studies reveal that the glass crystallizes to Sb2Se3 and SnSe2 phases upon annealing. The glass formation and composition dependence of glass transition temperature in the Sn–Sb–Se chalcogenide alloy could be understood by considering the topological phase transitions and a chemically ordered network model. Ó 2006 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 64.70.Pf; 61.10.Nz Keywords: Glass formation; Chalcogenides; Structure; X-ray diffraction
1. Introduction The composition dependence of various physical properties of ternary chalcogenide glasses have been extensively studied due to their potential technological applications in solid-state devices [1–3]. The glass formation region will be larger if group III, IV and V elements, which have light atomic masses [4], short atomic radii [5], high degree of covalent bonds, larger number of lone pairs etc., [6,7] were added to the chalcogen elements [8]. On the other hand, the region of glass formed by the addition of heavy mass elements is small with the reduction of optical band gaps of the chalcogen [9]. The interest in Sn–Sb–Se chalcogenide arises to investigate the glass formation region and to study topological phase transitions in the narrow glass forming region in this alloy. Recently, the composition dependence of properties of chalcogenide glasses indicates the presence *
Corresponding author. Tel.: +91 183 2258802 3165; fax: +91 183 2258819. E-mail address: rthangaraj@rediffmail.com (R. Thangaraj). 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.02.041
of two critical thresholds [10–19], namely, the rigidity percolation threshold (RPT) [15–17] and the chemical threshold (CT) [18,19]. The rigidity percolation threshold deals with dimensionality and rigidity of the glass network. According to Phillip’s constraint theory for a covalent network glass [15–17], the rigidity percolation threshold occurs at an average coordination number, Z = 2.40. If the medium range order is taken into account in the constraint theory the percolation threshold is likely to be shifted to higher Z values [20]. On the other hand, at the chemical threshold, the chemical ordering is maximized with the bonding being fully heteropolar [18]. Therefore, the investigation of RPT and CT by measuring the different physical properties has been of recent interest in different glass systems [19]. In the present work, differential scanning calorimetric (DSC) and X-ray diffraction (XRD) studies were performed on the as-prepared samples to investigate the glassy state in the Sn–Sb–Se chalcogenide alloy. Further the effect of annealing on the structure of the glass was discussed by considering the formation of different structural units (viz., Sb2Se3 and SnSe2) for the considered compositions.
LETTER TO THE EDITOR
P. Kumar, R. Thangaraj / Journal of Non-Crystalline Solids 352 (2006) 2288–2291
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2. Experimental Bulk samples of SnxSb20Se80x (0 6 x 6 22) were prepared by conventional melt quenching technique. The appropriate amounts of constituent elements of 4 N purity were weighed and sealed in a quartz ampoule (length = 120 mm, id = 6 mm) under a vacuum of 104 mbar. The sealed ampoule was then placed and heated in a vertical furnace to about 1123 K, at a heating rate of 200 K/h. The ampoule was inverted for nearly 45 h at regular intervals of time in order to ensure homogeneous mixing of the constituents. Then the melt was quenched in ice-water to obtain glassy samples. The material was separated from the ampoule by dissolving it in a solution of HF + H2O2 for 48 h. A DSC instrument, Mettler Toledo Stare System was used to measure the glass transition temperature of the samples with a heating rate of 10 K/min. About 10 mg of the powdered sample was taken in an Al pan with an empty reference pan. All the samples were scanned in the temperature range between 305 and 773 K under flowing N2 atmosphere. The glass transition temperature (Tg) was taken as the temperature corresponding to the intersection of the two linear portions adjoining the transition elbow. The values of Tg were determined with an accuracy of better than ±1 K using the microprocessor of thermal analyzer. The annealing of the as-prepared and powdered samples was done in vacuum (105 mbar) for 1 h. The XRD studies were performed to confirm the amorphous nature of the as prepared samples and crystallized phases of the annealed samples. 3. Results and discussions
Fig. 1. X-ray diffraction patterns for x = 0 (a), 4 (b), 8 (c), 18 (d), 20 (e), and 22 (f) in the as-prepared SnxSb20Se80x chalcogenide alloy.
glasses at a threshold value of Z = 2.40, called the rigidity percolation threshold [15–17]. Glass forming ability of many alloys is generally found to be high at the composition corresponding to Z = 2.40 in the chalcogenide glass system. For the present chalcogenide system SnxSbySez, the average coordination number is given by Z ¼ ð4x þ 3y þ 2zÞ=ðx þ y þ zÞ;
Fig. 1 shows the X-ray diffraction patterns of the as-prepared SnxSb20Se80x chalcogenide alloy. The general feature of these patterns confirms that for 8 6 x 6 18, amorphous samples were obtained. For compositions with x < 8, the alloy consists of c-Sb2Se3 phase embedded in some amorphous matrix. The addition of Sn to c-Sb20Se80 will lead to the formation of [SnSe4] tetrahedral, another type of network former in the Se-based glass network. Due to the homogeneous mixing of the constituent phases and higher average bond energy of [SnSe4] tetrahedra as compared to [Sb2Se3] pyramidal units, glassy samples are obtained for x P 8. This transformation from crystalline to amorphous phase can also be understood on the basis of Philips constraints theory proposed for covalent network glasses [15–17]. The constraint theory suggests that covalent network glasses consist of under cross-linked floppy and over constrained rigid networks. It also predicts a critical composition corresponding to average coordination number, Z = 2.40, at which the number of constraints acting on the network are balanced by the degrees of freedom available for the atoms in the network. So the glasses with Z < 2.40 are under cross-linked and glasses with Z > 2.40 are rigidly connected. Thus a transformation from a floppy to rigid network structure occurs in network
ð1Þ
where x, y and z are the atomic percentages of Sn, Sb and Se while 4, 3 and 2 their coordination numbers in amorphous state, respectively. The calculated values of Z are summarized in Table 1. The Z values in Table 1 indicate that the transformation from crystalline to amorphous state in this chalcogenide alloy can be treated as the approaching of the RPT. Fig. 2 shows a typical DSC curve for SnxSb20Se80x (x = 10, 12) chalcogenide glasses. Similar curves were also obtained for other glassy samples and the corresponding glass transition temperatures (Tg’s) are summarized in Table 1. A sharp change in the Tg’s values around
Table 1 Values of average coordination number (Z), R-value and glass transition temperature (Tg) with Sn concentration x (at.%)
Z
R-value
Tg (K)
8 10 12 14 16 18
2.36 2.40 2.44 2.48 2.52 2.56
1.56 1.40 1.26 1.14 1.03 0.94
409.0 426.5 434.9 448.9 460.2 465.0
LETTER TO THE EDITOR
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P. Kumar, R. Thangaraj / Journal of Non-Crystalline Solids 352 (2006) 2288–2291
considering the chemical bond energies [22]. The heteronuclear bond energies UA–B between unlike atoms A and B are calculated using homonuclear bond energies, namely UA–A and UB–B and the difference in the Pauling’s electronegativities of the two atoms (xA xB) U A–B ¼ 0:5½U A–A þ U B–B þ 23ðxA xB Þ2 ;
Fig. 2. DSC thermograms at a heating rate of 10 K/min for SnxSb20Se80x (x = 10, 12) chalcogenide glasses.
ð3Þ
where Pauling’s electronegativities of Sn, Sb and Se are 1.96, 2.05 and 2.55 respectively. Using the values of 187.0, 299.2 and 332.6 kJ/mol for homonuclear bond energies of Sn–Sn, Sb–Sb and Se–Se respectively [23] one obtains the value of 401.2, 346.18 kJ/mol respectively for Sn–Se and Sb–Se bonds. Since the amount of Sb is fixed in the present chalcogenide alloy, the percentage Sb bonds are fixed. With the addition of Sn up to 18 at.%, the concentration of SnSe4/2 tetrahedral structural units build up at the expense of Se–Se bonds and the replacement of Se–Se weak bonds with Sn–Se strong bonds (USn–Se USe–Se = 68.6 kJ/mol), which leads to an increase in Tg. At the critical composition with 18 at.% of Sn, the glass becomes chemically ordered and contains only the strong heteronuclear bonds and consequent stagnation in glass
x = 10, confirms the existence of RPT in the present system under investigation. The glass transition temperature increases with the addition of Sn, while the peak crystallization temperature decreases with the appearance of additional exothermic peak. The single glass transition temperature in the DSC thermograms shows that these glasses are homogeneous while the existence of two exothermic peaks for x P 12, indicates the phase separation for the considered glass compositions. The second peak may be due to the crystallization of Sn-rich phase in x = 12 composition. The magnitude of change in Tg is small for x changing from 16 to 18. This may be due to the occurrence of the chemical threshold (CT) in between these two compositions [18,19]. This could be understood by calculating the R-value for the present chalcogenide alloy. The R-value is obtained using the equation R ¼ 2z=ð4x þ 3yÞ;
ð2Þ
where x, y and z are the atomic fractions of Sn, Sb and Se, respectively. The R-value represents the ratio of the covalent bonding possibilities of chalcogen atoms to the covalent bonding possibilities of the non-chalcogen atoms [21]. Thus the value of R = 1 represents the case of the existence of only heteronuclear bonds in a given system which unequivocally indicates that the occurrence of the chemical threshold [14]. The R-values shown in Table 1 indicate that in the present system R = 1 occurs for x values between 16 and 18. Further addition of Sn leads to the formation of Sn-tetrahedra with the nucleation of Sb-rich backbone to deliver the requisite amount of Se as shown in Fig. 1 with the presence of some new peaks for x > 18 chalcogenide alloys. The observed behavior in Tg can also be interpreted using the chemically ordered network (CON) model by
Fig. 3. X-ray diffraction patterns for thermally annealed glasses at different temperatures, i.e., (1) 390 K, (2) 483 K for x = 10 and (3) 438 K, (4) 503 K, (5) 558 K for x = 12 in SnxSb20Se80x chalcogenide glasses.
LETTER TO THE EDITOR
P. Kumar, R. Thangaraj / Journal of Non-Crystalline Solids 352 (2006) 2288–2291
transition temperatures. With further increase in Sn concentration the high energy Sn–Se bonds (USn–Se USb–Se = 55.02 kJ/mol) are formed with the decrease of weak Sb–Se bonds to nucleate Sb–Sb homopolar bonds. Thus, the increase in Sn concentration leads to the formation of [SnSe4] tetrahedra at the expense of [Sb2Se3] pyramidal structural units to nucleate ethane-like Sb2(Se1/2)4 cluster units for x > 18. Since stoichiometric glass compositions are easy to crystallize, more addition of Sn does not yield the glassy alloys. The Sn addition inhibits crystallization [2] but higher content of Sn yields crystalline form of chalcogenide alloys in the present system. The glass compositions in the intermediate phase display rather exceptional mechanical, thermal and optical properties [19]. It is generally believed that the formation of the intermediate phase is driven by free energy considerations. To compare the change brought by adding Sn, we have compared the XRD of two compositions namely: x = 10 and x = 12, where the second one belongs to the intermediate phase regime. The XRD of the annealed samples are shown in Fig. 3(a) and (b). Fig. 3(a) shows that no Sn phase crystallizes with annealing at peak crystallization temperature, while Fig. 3(b) shows the presence of c-SnSe2 peaks along with others viz., c-Sb2Se3, c-Se and c-Sb. This indicates that phase separation of SnSe2 occurs in the present system. 4. Conclusions The glassy state and the effect of Sn addition on c-Sb20Se80 have been studied for Sn–Sb–Se chalcogenide. The addition of Sn leads to the emergence of [SnSe4] tetrahedra, which inhibits crystallization of the parent chalcogenide alloy, whereas higher concentrations yields (x > 18) yield crystalline samples. The glass transition temperature increases with Sn addition with a sharp increase at x = 10, where rigidity percolation threshold (RPT) occurs in the present chalcogenide alloy. A comparison of the annealed samples gives an indication of phase separation for x P 12 in this chalcogenide glass.
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Acknowledgments The authors are thankful to Mr Jagtar Singh, SAIF, Punjab University, Chandigarh and Mr Nripendra Singh, Central Instrumentation Centre, NIPER, Chandigarh for their timely help in taking the XRD and DSC studies. References [1] R.M. Mehra, R. Kumar, P.C. Mathur, Thin Solid Films 170 (1989) 15. [2] Z. Wang, T. Chengjiang, L. Yuunmei, C. Quanging, J. Non-Cryst. Solids 191 (1995) 132. [3] R.M. Mehra, S. Kohli, A. Pundir, V.K. Schdev, P.C. Mathur, J. Appl. Phys. 81 (1997) 7842. [4] A.R. Hilton, C.E. Jones, M. Brau, Phys. Chem. Glasses 7 (4) (1966) 105. [5] J.R. Chelikowsky, J.C. Philips, Phys. Rev. B 17 (6) (1978) 2453. [6] L. Zhenhua, J. Non-Cryst. Solids 127 (1999) 298. [7] S.S. Fouad, S.A. Fayek, M.H. Ali, Vacuum 49 (1998) 25. [8] D. Lezal, J. Optoelect. Adv. Mater. 5 (1) (2003) 23. [9] S.R. Elliott, Physics of Amorphous Materials, second ed., Longmann Scientific and Technical Press, New York, 1990. [10] A.S. Soltan, A.A. Abu Sehly, M.A. Abdel-Rahim, J. Phys. Chem. Solids 63 (2002) 801. [11] V. Pamukchleva, Z. Levi, E. Savova, Semicond. Sci. Technol. 13 (1998) 1309. [12] Evgenia R. Skordeva, Darina D. Arsova, J. Non-Cryst. Solids 192&193 (1995) 665. [13] D. Nesheva, E. Skordeva, Phys. Stat. Solidi (a) 172 (1999) 149. [14] G. Saffarini, Appl. Phys. A 74 (2002) 283. [15] J.C. Philips, M.F. Thorpe, Solid Stat. Commun. 53 (1985) 699. [16] P. Boolchand, M.F. Thorpe, Phys. Rev. B 50 (1994) 10366. [17] J.C. Philips, in: M.F. Thorpe, P.M. Duxbury (Eds.), Rigidity Theory and Applications, Kluwer Academic/Planum, New York, 1999. [18] G. Lucovesky, T.M. Hayes, in: M.H. Brodsky (Ed.), Amorphous Semiconductors, Springer, Berlin, 1979. [19] P. Boolchand (Ed.), Insulating and Semiconducting Glasses, World Scientific, Singapore, 2000. [20] K. Tanaka, J. Non-Cryst. Solids 103 (1988) 149. [21] L. Tichy, H. Ticha, Mater. Lett. 21 (1994) 313. [22] G. Lucovsky, R. Nemonich, F.L. Galeener, in: W. Spear (Ed.), Proceedings of International Conference Amorphous and Liquid Semiconductors, Edinburg, CICL, University of Edinburg, 1977, p. 130. [23] L. Pauling, The Chemical bond, Cornell University, New York, 1976.
Journal of Non-Crystalline Solids 352 (2006) 2288–2291 www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Glassy state and structure of Sn–Sb–Se chalcogenide alloy Praveen Kumar, R. Thangaraj
*
Semiconductors Laboratory, Department of Applied Physics, Guru Nanak Dev University, Amritsar 143005, India Received 22 October 2005; received in revised form 3 January 2006 Available online 5 May 2006
Abstract The effect of Sn addition on the glass transition and structure of c-Sb20Se80 chalcogenide alloy have been studied by X-ray diffraction and differential scanning calorimetric studies. The increase in the glass forming region and the glass transition temperature with the addition of Sn is discussed by considering the formation of [SnSe4] tetrahedra, another type of network former, which inhibits the crystallization. The differential scanning calorimetric studies on SnxSb20Se80x (8 6 x 6 18) glassy samples reveal a single glass transition temperature for all values of x while a single crystallization peak was obtained only for 10 6 x < 12. The X-ray diffraction studies reveal that the glass crystallizes to Sb2Se3 and SnSe2 phases upon annealing. The glass formation and composition dependence of glass transition temperature in the Sn–Sb–Se chalcogenide alloy could be understood by considering the topological phase transitions and a chemically ordered network model. Ó 2006 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 64.70.Pf; 61.10.Nz Keywords: Glass formation; Chalcogenides; Structure; X-ray diffraction
1. Introduction The composition dependence of various physical properties of ternary chalcogenide glasses have been extensively studied due to their potential technological applications in solid-state devices [1–3]. The glass formation region will be larger if group III, IV and V elements, which have light atomic masses [4], short atomic radii [5], high degree of covalent bonds, larger number of lone pairs etc., [6,7] were added to the chalcogen elements [8]. On the other hand, the region of glass formed by the addition of heavy mass elements is small with the reduction of optical band gaps of the chalcogen [9]. The interest in Sn–Sb–Se chalcogenide arises to investigate the glass formation region and to study topological phase transitions in the narrow glass forming region in this alloy. Recently, the composition dependence of properties of chalcogenide glasses indicates the presence *
Corresponding author. Tel.: +91 183 2258802 3165; fax: +91 183 2258819. E-mail address: rthangaraj@rediffmail.com (R. Thangaraj). 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.02.041
of two critical thresholds [10–19], namely, the rigidity percolation threshold (RPT) [15–17] and the chemical threshold (CT) [18,19]. The rigidity percolation threshold deals with dimensionality and rigidity of the glass network. According to Phillip’s constraint theory for a covalent network glass [15–17], the rigidity percolation threshold occurs at an average coordination number, Z = 2.40. If the medium range order is taken into account in the constraint theory the percolation threshold is likely to be shifted to higher Z values [20]. On the other hand, at the chemical threshold, the chemical ordering is maximized with the bonding being fully heteropolar [18]. Therefore, the investigation of RPT and CT by measuring the different physical properties has been of recent interest in different glass systems [19]. In the present work, differential scanning calorimetric (DSC) and X-ray diffraction (XRD) studies were performed on the as-prepared samples to investigate the glassy state in the Sn–Sb–Se chalcogenide alloy. Further the effect of annealing on the structure of the glass was discussed by considering the formation of different structural units (viz., Sb2Se3 and SnSe2) for the considered compositions.
LETTER TO THE EDITOR
P. Kumar, R. Thangaraj / Journal of Non-Crystalline Solids 352 (2006) 2288–2291
2289
2. Experimental Bulk samples of SnxSb20Se80x (0 6 x 6 22) were prepared by conventional melt quenching technique. The appropriate amounts of constituent elements of 4 N purity were weighed and sealed in a quartz ampoule (length = 120 mm, id = 6 mm) under a vacuum of 104 mbar. The sealed ampoule was then placed and heated in a vertical furnace to about 1123 K, at a heating rate of 200 K/h. The ampoule was inverted for nearly 45 h at regular intervals of time in order to ensure homogeneous mixing of the constituents. Then the melt was quenched in ice-water to obtain glassy samples. The material was separated from the ampoule by dissolving it in a solution of HF + H2O2 for 48 h. A DSC instrument, Mettler Toledo Stare System was used to measure the glass transition temperature of the samples with a heating rate of 10 K/min. About 10 mg of the powdered sample was taken in an Al pan with an empty reference pan. All the samples were scanned in the temperature range between 305 and 773 K under flowing N2 atmosphere. The glass transition temperature (Tg) was taken as the temperature corresponding to the intersection of the two linear portions adjoining the transition elbow. The values of Tg were determined with an accuracy of better than ±1 K using the microprocessor of thermal analyzer. The annealing of the as-prepared and powdered samples was done in vacuum (105 mbar) for 1 h. The XRD studies were performed to confirm the amorphous nature of the as prepared samples and crystallized phases of the annealed samples. 3. Results and discussions
Fig. 1. X-ray diffraction patterns for x = 0 (a), 4 (b), 8 (c), 18 (d), 20 (e), and 22 (f) in the as-prepared SnxSb20Se80x chalcogenide alloy.
glasses at a threshold value of Z = 2.40, called the rigidity percolation threshold [15–17]. Glass forming ability of many alloys is generally found to be high at the composition corresponding to Z = 2.40 in the chalcogenide glass system. For the present chalcogenide system SnxSbySez, the average coordination number is given by Z ¼ ð4x þ 3y þ 2zÞ=ðx þ y þ zÞ;
Fig. 1 shows the X-ray diffraction patterns of the as-prepared SnxSb20Se80x chalcogenide alloy. The general feature of these patterns confirms that for 8 6 x 6 18, amorphous samples were obtained. For compositions with x < 8, the alloy consists of c-Sb2Se3 phase embedded in some amorphous matrix. The addition of Sn to c-Sb20Se80 will lead to the formation of [SnSe4] tetrahedral, another type of network former in the Se-based glass network. Due to the homogeneous mixing of the constituent phases and higher average bond energy of [SnSe4] tetrahedra as compared to [Sb2Se3] pyramidal units, glassy samples are obtained for x P 8. This transformation from crystalline to amorphous phase can also be understood on the basis of Philips constraints theory proposed for covalent network glasses [15–17]. The constraint theory suggests that covalent network glasses consist of under cross-linked floppy and over constrained rigid networks. It also predicts a critical composition corresponding to average coordination number, Z = 2.40, at which the number of constraints acting on the network are balanced by the degrees of freedom available for the atoms in the network. So the glasses with Z < 2.40 are under cross-linked and glasses with Z > 2.40 are rigidly connected. Thus a transformation from a floppy to rigid network structure occurs in network
ð1Þ
where x, y and z are the atomic percentages of Sn, Sb and Se while 4, 3 and 2 their coordination numbers in amorphous state, respectively. The calculated values of Z are summarized in Table 1. The Z values in Table 1 indicate that the transformation from crystalline to amorphous state in this chalcogenide alloy can be treated as the approaching of the RPT. Fig. 2 shows a typical DSC curve for SnxSb20Se80x (x = 10, 12) chalcogenide glasses. Similar curves were also obtained for other glassy samples and the corresponding glass transition temperatures (Tg’s) are summarized in Table 1. A sharp change in the Tg’s values around
Table 1 Values of average coordination number (Z), R-value and glass transition temperature (Tg) with Sn concentration x (at.%)
Z
R-value
Tg (K)
8 10 12 14 16 18
2.36 2.40 2.44 2.48 2.52 2.56
1.56 1.40 1.26 1.14 1.03 0.94
409.0 426.5 434.9 448.9 460.2 465.0
LETTER TO THE EDITOR
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P. Kumar, R. Thangaraj / Journal of Non-Crystalline Solids 352 (2006) 2288–2291
considering the chemical bond energies [22]. The heteronuclear bond energies UA–B between unlike atoms A and B are calculated using homonuclear bond energies, namely UA–A and UB–B and the difference in the Pauling’s electronegativities of the two atoms (xA xB) U A–B ¼ 0:5½U A–A þ U B–B þ 23ðxA xB Þ2 ;
Fig. 2. DSC thermograms at a heating rate of 10 K/min for SnxSb20Se80x (x = 10, 12) chalcogenide glasses.
ð3Þ
where Pauling’s electronegativities of Sn, Sb and Se are 1.96, 2.05 and 2.55 respectively. Using the values of 187.0, 299.2 and 332.6 kJ/mol for homonuclear bond energies of Sn–Sn, Sb–Sb and Se–Se respectively [23] one obtains the value of 401.2, 346.18 kJ/mol respectively for Sn–Se and Sb–Se bonds. Since the amount of Sb is fixed in the present chalcogenide alloy, the percentage Sb bonds are fixed. With the addition of Sn up to 18 at.%, the concentration of SnSe4/2 tetrahedral structural units build up at the expense of Se–Se bonds and the replacement of Se–Se weak bonds with Sn–Se strong bonds (USn–Se USe–Se = 68.6 kJ/mol), which leads to an increase in Tg. At the critical composition with 18 at.% of Sn, the glass becomes chemically ordered and contains only the strong heteronuclear bonds and consequent stagnation in glass
x = 10, confirms the existence of RPT in the present system under investigation. The glass transition temperature increases with the addition of Sn, while the peak crystallization temperature decreases with the appearance of additional exothermic peak. The single glass transition temperature in the DSC thermograms shows that these glasses are homogeneous while the existence of two exothermic peaks for x P 12, indicates the phase separation for the considered glass compositions. The second peak may be due to the crystallization of Sn-rich phase in x = 12 composition. The magnitude of change in Tg is small for x changing from 16 to 18. This may be due to the occurrence of the chemical threshold (CT) in between these two compositions [18,19]. This could be understood by calculating the R-value for the present chalcogenide alloy. The R-value is obtained using the equation R ¼ 2z=ð4x þ 3yÞ;
ð2Þ
where x, y and z are the atomic fractions of Sn, Sb and Se, respectively. The R-value represents the ratio of the covalent bonding possibilities of chalcogen atoms to the covalent bonding possibilities of the non-chalcogen atoms [21]. Thus the value of R = 1 represents the case of the existence of only heteronuclear bonds in a given system which unequivocally indicates that the occurrence of the chemical threshold [14]. The R-values shown in Table 1 indicate that in the present system R = 1 occurs for x values between 16 and 18. Further addition of Sn leads to the formation of Sn-tetrahedra with the nucleation of Sb-rich backbone to deliver the requisite amount of Se as shown in Fig. 1 with the presence of some new peaks for x > 18 chalcogenide alloys. The observed behavior in Tg can also be interpreted using the chemically ordered network (CON) model by
Fig. 3. X-ray diffraction patterns for thermally annealed glasses at different temperatures, i.e., (1) 390 K, (2) 483 K for x = 10 and (3) 438 K, (4) 503 K, (5) 558 K for x = 12 in SnxSb20Se80x chalcogenide glasses.
LETTER TO THE EDITOR
P. Kumar, R. Thangaraj / Journal of Non-Crystalline Solids 352 (2006) 2288–2291
transition temperatures. With further increase in Sn concentration the high energy Sn–Se bonds (USn–Se USb–Se = 55.02 kJ/mol) are formed with the decrease of weak Sb–Se bonds to nucleate Sb–Sb homopolar bonds. Thus, the increase in Sn concentration leads to the formation of [SnSe4] tetrahedra at the expense of [Sb2Se3] pyramidal structural units to nucleate ethane-like Sb2(Se1/2)4 cluster units for x > 18. Since stoichiometric glass compositions are easy to crystallize, more addition of Sn does not yield the glassy alloys. The Sn addition inhibits crystallization [2] but higher content of Sn yields crystalline form of chalcogenide alloys in the present system. The glass compositions in the intermediate phase display rather exceptional mechanical, thermal and optical properties [19]. It is generally believed that the formation of the intermediate phase is driven by free energy considerations. To compare the change brought by adding Sn, we have compared the XRD of two compositions namely: x = 10 and x = 12, where the second one belongs to the intermediate phase regime. The XRD of the annealed samples are shown in Fig. 3(a) and (b). Fig. 3(a) shows that no Sn phase crystallizes with annealing at peak crystallization temperature, while Fig. 3(b) shows the presence of c-SnSe2 peaks along with others viz., c-Sb2Se3, c-Se and c-Sb. This indicates that phase separation of SnSe2 occurs in the present system. 4. Conclusions The glassy state and the effect of Sn addition on c-Sb20Se80 have been studied for Sn–Sb–Se chalcogenide. The addition of Sn leads to the emergence of [SnSe4] tetrahedra, which inhibits crystallization of the parent chalcogenide alloy, whereas higher concentrations yields (x > 18) yield crystalline samples. The glass transition temperature increases with Sn addition with a sharp increase at x = 10, where rigidity percolation threshold (RPT) occurs in the present chalcogenide alloy. A comparison of the annealed samples gives an indication of phase separation for x P 12 in this chalcogenide glass.
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