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Raman scattering in GeS2 glass and its crystalline polymorphs compared
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 435 (1997) 193-198
Raman scattering in GeS2 glass and its crystalline polymorphs compared Z. (~erno~ek a, E. (~ernogkov~ b, L.
Beneg b
"Department of General and h~organic Chemistry, Faculty of Chemical Teehnology. UniversiO' of Pardubice. n(un.Legi1565, CZ-532 10 Pardubice, Czech Republic bJoint Laboratoo" of Solid state Chemistr), of Academy of Sciences of Czech Republic and University ofPardubice, Studentskdt 84, CZ-530 09 Pardubice, Czech Republic
Abstract
Raman spectrum of glassy GeS2 and low-resolution ones of polycrystalline c~- and ~-GeS2 were studied. It was shown that the medium range structure of glassy GeS 2 is similar to the three-dimensional structure of 3-GeS 2. Our conclusion of similarity of medium range order of glassy GeS 2 and 3-GeS 2 was also confirmed by detection of 3-GeS 2 microcrystals grown from glassy GeS2 at annealing temperature sufficiently below glass transition temperature. © 1997 Elsevier Science B.V. Keywords: Medium range order; Glass; Raman scattering
1. Introduction
The structure of GexS 100-x glassy system has been the center of interest in glass science for many years. Since long range order in a glass is absent, conventional crystallographic diffraction method become less useful. Thus a consensus on structure of glasses has not been achieved up to now. It is possible to infer the structure of G e f i 100-x glasses over the glassforming region ( - - 1 5 - 4 4 at %Ge) from far IR and Raman scattering [1,2] provided structure of glassy GeS2 is well understood. For Ge-rich glasses (x > 33.3), glass counterpart of GeS compound has also to be taken into account. Up to 1971, results of structural studies [3-8] were interpreted in terms of a three-dimensional (3D) network of glassy GeS> However, authors [9] showed that crystalline GeS2 at ambient pressure exists in two polymorphs; a high temperature modification (o~-GeS2) and a low temperature one (3-GeS2). o~-GeS2 has a two-dimensional
(2D) layered structure while 3-GeS2 has a threedimensional (3D) network and is stable below - 4 9 7 ° C [9]. For more details see [10-13]. Studies dealing with structure of glassy GeS2 after 1970 speak for the benefit of 2D structure [ 14,15] or of existence of ~-GeS2 type clusters (so-called outrigger rafts) [16-18] as well as for coexistence of 2D and 3D structure, e.g. [19]. Similar conclusions about the coexistence of both these structures was suggested by Sugai [2]. Using DTA M~ilek found that 3-GeS2 phase forms during the first stages of the crystallization process of glassy GeS2 and the 3 --' ~ transformation takes place at a higher temperature [20,21]. Our study is based on the idea that the structure of the stoichiometric glass differs from its crystalline counterparts to a first approximation in bonds angles and bond lengths displaying some fluctuations about a mean value characteristic of the crystalline phases. Then we suppose that a transition from crystalline to glassy state leads to a broadening of Raman spectral
0022-2860/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0022-2860(97)00184-1
194
Z. Cernogek et al./Journal of Molecular Structure 435 (1997) 193-198
bands. This broadening can be computer simulated by lowering of resolution limit during FT-Raman spectra computing from interferogram. The aim of this paper is a study of medium range structure of glassy GeS2 by comparing Raman spectrum of glassy GeS: with "computer amorphized" Raman spectra of ~-GeS2 and/3-GeS2.
(z - G e S 2
13- G e S 2 2. Experimental
The bulk glasses of chemical composition GeS2 were prepared by direct synthesis from elements of high purity (5N, total weight 5 g) in evacuated silica ampoules in a rocking furnace (1050°C, 24 h). The ampoules containing the melt were quenched in cold water after synthesis and homogenization. The non-crystalline state of prepared glasses were confirmed by X-ray analysis. The chemical composition and homogeneity were checked by microprobe analysis. c~-GeS2 was obtained by crystallization of glassy GeS2 at 700°C for 3 days in evacuated silica ampoule, /3-GeS 2 by crystallization of the glassy GeS 2 at 460°C for 56 h. Glassy GeS2 was used for a study of first step of crystallization in solid state, as well. A GeS2 bulk glass sample was placed in a silica ampoule under an overpressure of dry argon and heated at a rate of 20°C/min to 420°C, and then at a rate of 3°C/min to 440°C (Tg(GeS2) -4950C [20]). After annealing at 440°C for 30 min the sample was cooled to room temperature in air. XRD analyses of powdered samples were performed using a X-ray vertical diffractometer HZG-4 (Germany) with diameter of goniometer of 25 cm. CuK~ line and proportional counter were used. Diffraction angles were measured from 4 ° to 90 ° (219). Theoretical X-ray patterns for both a-GeS2 and 13-GeS2 polymorphs were calculated from crystallographic data [10,11] using Lazy Pulverix software [22]. Raman spectra of bulk glassy GeS z and polycrystalline samples were measured at room temperature in back scattering geometry using an FT spectrometer IFS 55 with FRA 106 accessory (Bruker) using a diode pumped Nd:YAG laser (1064 nm) and a Ge detector cooled by liquid nitrogen. Incident laser power was 9 W/cm 2 and 255 scans were averaged for all samples. Both asquenched GeS2 glass and
a
b
60
50
40
30
20
2O
10
Fig. l. Theoretical X-ray patterns of ~-GeS2, 13-GeS2 and experimental patterns of crystallization of glassy GeS 2 (a) after annealing at 700°C for 3 days and (b) after annealing at 460°C for 56 h.
glassy GeS2 annealed at 440°C were measured at a resolution of 2 cm -1. Crystalline samples were measured at a resolution of 1 cm -1. Scattering and spectrometer optics influence were corrected for all spectra using standard procedures of Opus 2.0 software (Bruker). Broad Raman spectra of both polycrystalline polymorphs, hereafter labeled LR-spectra (low-resolution spectra), were computed from measured polycrystalline samples' interferograms with lowered resolution limit of Fourier transformation. For the best fit of Raman spectrum of glassy GeS2 a resolution limit of 24 cm -l was found empirically.
3. Results and discussion
Theoretical X-ray patterns for both c~-GeS2 and t3-GeS2 polymorphs and measured experimental
195
Z. Cernogek et al./Journal of Molecular Structure 435 (1997) 193-198
-GeS 2
,.--?.
o~-GeS 2
/ ~
,--?. "-s
>.,
-GeS 2
r~ c"
-GeS 2
(/) c-
/ ' ~
r-
C
E n,"
E ~3 rY
.... 500
[ .... 400
I .... I ' ' ' ' I ' 300 200 100 Raman shift [cm -1]
'
500
'''
I'
450
'''
['''
' I ' ' ' '
400 350 Raman shift [cm -1]
300
Fig. 2. Experimental Raman spectra of both GeS2 crystalline polymorphs together with their computed low-resolution spectra and experimental spectrum of GeS2 bulk glass.
Fig. 3. Computed low-resolution spectra of both crystalline polymorphs and experimental spectrum of GeS2 bulk glass.
patterns of prepared polycrystalline samples are compared in Fig. 1. The diffraction patterns of prepared c~- and 3-GeS 2 agree with theoretical diffraction's patterns. Raman spectra of both polycrystalline modifications and spectrum of glassy GeS2 are presented in Fig. 2 as well as corresponding LR-spectra of polycrystalline polymorphs. The comparison of LRspectra with spectrum of glassy GeS2 is presented in Fig. 3. For the further discussion the spectral region 3 0 0 - 5 0 0 cm -~ will be in the center of our attention. Raman bands of all basic structural units can be found in this spectral region, see e.g. [12,13]. Lower energy parts of spectra are mainly ascribed to the vibrations of larger structural units and for their discussion are not relevant data in the literature. From Fig. 3 it is clear that the most intensive
Raman band of glassy GeS 2 at 345 cm ~ is completely identical with the most intensive one of LR-spectrum of 3-GeS2. On the other hand the most intensive line of LR-spectrum of o~-GeS 2 is centered at 362 cm -1 and thus differs considerably from this one of glassy GeS 2. Owing to the good agreement of the most intensive spectral line of glassy GeS2 and of LR-spectrum of 3GeS2 as well as the same position of so-called A~ line ( - 3 7 5 cm -L) in both spectra (Fig. 3) we suppose that the medium range structure of glassy GeS2 can be better interpreted assuming a three-dimensional network of/3-GeS2 type. Nevertheless, it is evident from Fig. 3 that considerable difference of scattering strength of glassy GeS2 and of LR-spectrum of 3GeS2 exists at a energy higher than - 4 0 0 cm -~ This difference is quantitatively shown in Fig. 4. In
196
Z C'ernogek et al./Journal of Molecular Structure 435 (1997) 193-198
Table 1 Raman bands and modification of their intensities used for computation of low-resolution spectrum [Fig. 5(b)], see text for details Position (cm q) Multiple of crystal band intensity
375.5 1.75
382.8 1.75
our view, if the medium range order of crystal and glass of the same chemical composition differ mainly due to some fluctuations of nearest-neighbor bond lengths and bond angles, then it is appropriate to think that the crystal to glass transition must influence relative intensities of Raman bands but not their energies. To test this idea we numerically changed the relative intensities of some Raman bands of/3-GeS2 polycrystal spectrum (Table 1) and we used this modified spectrum for low-resolution spectrum computation, Fig. 5. In our view the LR-spectrum obtained by this simple way agrees quite well with the Raman spectrum of glassy GeS2. It would be interesting to discuss the difference spectrum [Fig. 5(d)] in detail but unambiguous vibrational analyses of Raman
396.8 2
403.8 2
432.9 0
438.8 8
445.1 0
spectra of both GeS2 polymorphs are not available up to now. On the basis of results described above we assume that medium range order of GeS2 glass consists of a 3D arrangement of Ge[S 1/214tetrahedral units as in/3GeS2. Our assumption of a similarity in the short and medium range structure of glassy GeSe and/3-GeSe is also in agreement with our study of the first step of crystallization of glassy GeS2 at the temperature below glass transition temperature. Experimental Raman spectrum of annealed GeSe glass composed
--t
-'1
t
i-
a
\
~
e-
rt-
E
r,,-
r~
t'-
\
(~
E
C
n,"
,,/"
,~"~.__~/' /
i I1
\',
\/' I
I
d 500
450
400
350
300
Raman shift [cm -1] 500
450
400 350 300 Raman shift [cm q]
Fig. 4. Experimental Raman spectrum of glassy GeS 2 (a), computed low-resolution spectrum of/3-GeS 2 (b) and difference spectrum (c).
Fig. 5. Comparison of (a) experimental Raman spectrum of glassy GeS2 with (b) computed modified low-resolution spectrum of/3GeS2. Experimental Raman spectrum of polycrystalline /3-GeS2 (full line) and its modification (dashed line) (c) and their difference spectrum (d).
Z Cernogek et al./Journal of Molecular Structure 435 (1997) 193 198
a
7 .Q
/f //
O)
!I
g
/
f
b //
197
very similar to/3-GeS2 can be obtained. Raman spectrum of these microcrystals is very similar to glassy GeSe2. Three-dimensional structure of GeSe2 (comparable with 3-GeS2 one) is probably thermodynamically much less stable and it leads to erroneous conclusion that only two-dimensional layered structure of glassy GeSe2 and glassy GeS2 can exist, as well.
r~
4. Conclusion o", ;, : ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I '
500
400
300 200 100 Raman shift [cmq]
Fig. 6. Raman spectra of (a) bulk glassy GeS2 (annealed at 440°C for 30 min) with grown isolated microcrystalsand (b) polycrystalline 3-GeS2.
of the mixed structure of the glassy GeS2 and of the isolated crystallites as well as the spectrum of/3-GeS2 is presented for comparison in Fig. 6. Spectral bands of growing crystalline phase in glassy matrix can be well compared with 3-GeS2. It is known that only 3GeS2 polymorph crystallize when glassy GeS2 is annealed at temperature below --497°C. It is clearly verified by our method of preparation of 3-GeS2. Because the /3 --~ o~ phase transformation is irreversible, one can hardly expect that in the case if glassy matrix would be mainly a-like type [17,18] the crystallization will start by o~ --~ /3 transformation of 2D clusters. Thus observed crystallization of only 3-GeS 2 polymorph in the glassy matrix of GeS2 is in good agreement with our opinion that the medium range order of glassy GeS2 is 3D 3-GeS2-1ike. Because the structures of glassy GeS2 and glassy GeSe2 are sometimes compared one to other, e.g. [13,19], we would like to make a comment. A lot of studies of structure of glassy GeSe2 were made up to now, e.g. [16,23,24], and it was concluded that structure of glassy GeS 2 is layered two-dimensional of o~GeSe2 type. It is necessary to emphasize that only the ~-GeSe2 crystal modification is known to be stable at higher temperature. However using Raman spectroscopy Sugai [2] found that by low power light irradiation of glassy GeSe2 the microcrystals with a structure
On the basis of a comparison of Raman spectra of glassy GeS2 and low-resolution ones of polycrystalline o~- and B-GeS2, we have shown that the medium range structure of glassy GeS2 is similar to that of the 3D structure of 3-GeS 2. Growth of 3-GeS 2 microcrystals from solid phase of glassy GeS2 at temperature sufficiently below glass transition temperature Tg w a s found, as well. It confirms our conclusion of similarity of medium range order of glassy GeS2 and 3-GeS2.
Acknowledgements This work was supported by the grants no. 203/96/ 0876 and no. 203/96/0184 of The Czech Grant Agency.
References [11 G. Lucovsky, F.L. Galeener, R.C. Keezer, R.H. Geils, H.A. Six, Physical Review B 10 (1974) 5134. [2] S. Sugai, Physical Review B 35 (1987) 1345. [3] Y. Kawamoto, Journal of American Ceramics Society 52 (1969) 626. [4] Y. Kawamoto, Journal of American Ceramics Society 54 (1971) 131. [5] S.C. Rowland, S. Narasimhan, A. Bienenstock, Journal of Applied Physics 43 (1972) 2741. [6] L. Cervinka, A. Hrub~, Amorphous and Liquid Semiconductors, Taylor and Francis, London, 1973. [7] G. Lucovsky, J.P. deNeufville, F.L. Galeener, Physical Review 9 (1974) 1591. [8] K. Arai, N. Koshizuka, H. Namikawa, Proceedings of the International Conference of Structure and Excitations in Amorphous Solids, AIP Conference Series No. 31, 1976, p. 217. [9] W. Viaene, G.H. Moh, N. Jb. Miner. Mh. 6 (1970) 283. [10] G. Dittmar, H. Sch~ifer, Acta Cryst. B 31 (1975) 2060.
198 [11] [12] [13] [14] [15] [ 16] [17] [18]
Z. Cernogek et al./Journal of Molecular Structure 435 (1997) 193-198 G. Dittmar, H. Sch~ifer, Acta Cryst. B 32 (1976) 1188. Z.V. Popovic, H.J. Stolz, Phys. Stat. Sol. 106 (1981) 337. Z.V. Popovic, Fizika 15 (1983) 11. Y. Kawamoto, Ch. Kawashima, Mar. Research Bulletin 17 (1982) 1511. B.A. Weinstein, R. Zallen, M.L. Slade, J.C. Mikkelsen Jr., Physical Review B 25 (1982) 781. J.C. Phillips, Journal of Non-Crystal Solids 43 (1981) 37. J. Grothaus, P. Boolchand, Journal of Non-Crystal Solids 72 (1985) 1. P. Boolchand, J. Grothaus, M. Tenhover, M.A. Hazle, R.K. Grasselli, Physical Review B 33 (1986) 5421.
[19] [20] [21] [22]
K. Arai, Journal of Non-Cryst Solids 59 (1983) 1059. J. Mztlek, Journal of Non-Crystal Solids 107 (1989) 323. J. M~ilek, Journal of Thermal Analysis 40 (1993) 159. K. Yvon, W. Jeitschklo, E. Parthe, Journal of Applied Crystal 10 (1977) 73. [23] M. Stevens, P. Boolchand, J.G. Hernandez, Physical Review B 31 (1985) 981. [24] K. Murase, K. Inoue, O. Matsuda, in: Y. Sakurai, Y. Hamakawa, T. Masumoto, K. Shirae, K. Suzuki (Eds.), Currents Topics in Amorphous Materials: Physics and Technology, North-Holland, London, 1993, p. 47.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 435 (1997) 193-198
Raman scattering in GeS2 glass and its crystalline polymorphs compared Z. (~erno~ek a, E. (~ernogkov~ b, L.
Beneg b
"Department of General and h~organic Chemistry, Faculty of Chemical Teehnology. UniversiO' of Pardubice. n(un.Legi1565, CZ-532 10 Pardubice, Czech Republic bJoint Laboratoo" of Solid state Chemistr), of Academy of Sciences of Czech Republic and University ofPardubice, Studentskdt 84, CZ-530 09 Pardubice, Czech Republic
Abstract
Raman spectrum of glassy GeS2 and low-resolution ones of polycrystalline c~- and ~-GeS2 were studied. It was shown that the medium range structure of glassy GeS 2 is similar to the three-dimensional structure of 3-GeS 2. Our conclusion of similarity of medium range order of glassy GeS 2 and 3-GeS 2 was also confirmed by detection of 3-GeS 2 microcrystals grown from glassy GeS2 at annealing temperature sufficiently below glass transition temperature. © 1997 Elsevier Science B.V. Keywords: Medium range order; Glass; Raman scattering
1. Introduction
The structure of GexS 100-x glassy system has been the center of interest in glass science for many years. Since long range order in a glass is absent, conventional crystallographic diffraction method become less useful. Thus a consensus on structure of glasses has not been achieved up to now. It is possible to infer the structure of G e f i 100-x glasses over the glassforming region ( - - 1 5 - 4 4 at %Ge) from far IR and Raman scattering [1,2] provided structure of glassy GeS2 is well understood. For Ge-rich glasses (x > 33.3), glass counterpart of GeS compound has also to be taken into account. Up to 1971, results of structural studies [3-8] were interpreted in terms of a three-dimensional (3D) network of glassy GeS> However, authors [9] showed that crystalline GeS2 at ambient pressure exists in two polymorphs; a high temperature modification (o~-GeS2) and a low temperature one (3-GeS2). o~-GeS2 has a two-dimensional
(2D) layered structure while 3-GeS2 has a threedimensional (3D) network and is stable below - 4 9 7 ° C [9]. For more details see [10-13]. Studies dealing with structure of glassy GeS2 after 1970 speak for the benefit of 2D structure [ 14,15] or of existence of ~-GeS2 type clusters (so-called outrigger rafts) [16-18] as well as for coexistence of 2D and 3D structure, e.g. [19]. Similar conclusions about the coexistence of both these structures was suggested by Sugai [2]. Using DTA M~ilek found that 3-GeS2 phase forms during the first stages of the crystallization process of glassy GeS2 and the 3 --' ~ transformation takes place at a higher temperature [20,21]. Our study is based on the idea that the structure of the stoichiometric glass differs from its crystalline counterparts to a first approximation in bonds angles and bond lengths displaying some fluctuations about a mean value characteristic of the crystalline phases. Then we suppose that a transition from crystalline to glassy state leads to a broadening of Raman spectral
0022-2860/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0022-2860(97)00184-1
194
Z. Cernogek et al./Journal of Molecular Structure 435 (1997) 193-198
bands. This broadening can be computer simulated by lowering of resolution limit during FT-Raman spectra computing from interferogram. The aim of this paper is a study of medium range structure of glassy GeS2 by comparing Raman spectrum of glassy GeS: with "computer amorphized" Raman spectra of ~-GeS2 and/3-GeS2.
(z - G e S 2
13- G e S 2 2. Experimental
The bulk glasses of chemical composition GeS2 were prepared by direct synthesis from elements of high purity (5N, total weight 5 g) in evacuated silica ampoules in a rocking furnace (1050°C, 24 h). The ampoules containing the melt were quenched in cold water after synthesis and homogenization. The non-crystalline state of prepared glasses were confirmed by X-ray analysis. The chemical composition and homogeneity were checked by microprobe analysis. c~-GeS2 was obtained by crystallization of glassy GeS2 at 700°C for 3 days in evacuated silica ampoule, /3-GeS 2 by crystallization of the glassy GeS 2 at 460°C for 56 h. Glassy GeS2 was used for a study of first step of crystallization in solid state, as well. A GeS2 bulk glass sample was placed in a silica ampoule under an overpressure of dry argon and heated at a rate of 20°C/min to 420°C, and then at a rate of 3°C/min to 440°C (Tg(GeS2) -4950C [20]). After annealing at 440°C for 30 min the sample was cooled to room temperature in air. XRD analyses of powdered samples were performed using a X-ray vertical diffractometer HZG-4 (Germany) with diameter of goniometer of 25 cm. CuK~ line and proportional counter were used. Diffraction angles were measured from 4 ° to 90 ° (219). Theoretical X-ray patterns for both a-GeS2 and 13-GeS2 polymorphs were calculated from crystallographic data [10,11] using Lazy Pulverix software [22]. Raman spectra of bulk glassy GeS z and polycrystalline samples were measured at room temperature in back scattering geometry using an FT spectrometer IFS 55 with FRA 106 accessory (Bruker) using a diode pumped Nd:YAG laser (1064 nm) and a Ge detector cooled by liquid nitrogen. Incident laser power was 9 W/cm 2 and 255 scans were averaged for all samples. Both asquenched GeS2 glass and
a
b
60
50
40
30
20
2O
10
Fig. l. Theoretical X-ray patterns of ~-GeS2, 13-GeS2 and experimental patterns of crystallization of glassy GeS 2 (a) after annealing at 700°C for 3 days and (b) after annealing at 460°C for 56 h.
glassy GeS2 annealed at 440°C were measured at a resolution of 2 cm -1. Crystalline samples were measured at a resolution of 1 cm -1. Scattering and spectrometer optics influence were corrected for all spectra using standard procedures of Opus 2.0 software (Bruker). Broad Raman spectra of both polycrystalline polymorphs, hereafter labeled LR-spectra (low-resolution spectra), were computed from measured polycrystalline samples' interferograms with lowered resolution limit of Fourier transformation. For the best fit of Raman spectrum of glassy GeS2 a resolution limit of 24 cm -l was found empirically.
3. Results and discussion
Theoretical X-ray patterns for both c~-GeS2 and t3-GeS2 polymorphs and measured experimental
195
Z. Cernogek et al./Journal of Molecular Structure 435 (1997) 193-198
-GeS 2
,.--?.
o~-GeS 2
/ ~
,--?. "-s
>.,
-GeS 2
r~ c"
-GeS 2
(/) c-
/ ' ~
r-
C
E n,"
E ~3 rY
.... 500
[ .... 400
I .... I ' ' ' ' I ' 300 200 100 Raman shift [cm -1]
'
500
'''
I'
450
'''
['''
' I ' ' ' '
400 350 Raman shift [cm -1]
300
Fig. 2. Experimental Raman spectra of both GeS2 crystalline polymorphs together with their computed low-resolution spectra and experimental spectrum of GeS2 bulk glass.
Fig. 3. Computed low-resolution spectra of both crystalline polymorphs and experimental spectrum of GeS2 bulk glass.
patterns of prepared polycrystalline samples are compared in Fig. 1. The diffraction patterns of prepared c~- and 3-GeS 2 agree with theoretical diffraction's patterns. Raman spectra of both polycrystalline modifications and spectrum of glassy GeS2 are presented in Fig. 2 as well as corresponding LR-spectra of polycrystalline polymorphs. The comparison of LRspectra with spectrum of glassy GeS2 is presented in Fig. 3. For the further discussion the spectral region 3 0 0 - 5 0 0 cm -~ will be in the center of our attention. Raman bands of all basic structural units can be found in this spectral region, see e.g. [12,13]. Lower energy parts of spectra are mainly ascribed to the vibrations of larger structural units and for their discussion are not relevant data in the literature. From Fig. 3 it is clear that the most intensive
Raman band of glassy GeS 2 at 345 cm ~ is completely identical with the most intensive one of LR-spectrum of 3-GeS2. On the other hand the most intensive line of LR-spectrum of o~-GeS 2 is centered at 362 cm -1 and thus differs considerably from this one of glassy GeS 2. Owing to the good agreement of the most intensive spectral line of glassy GeS2 and of LR-spectrum of 3GeS2 as well as the same position of so-called A~ line ( - 3 7 5 cm -L) in both spectra (Fig. 3) we suppose that the medium range structure of glassy GeS2 can be better interpreted assuming a three-dimensional network of/3-GeS2 type. Nevertheless, it is evident from Fig. 3 that considerable difference of scattering strength of glassy GeS2 and of LR-spectrum of 3GeS2 exists at a energy higher than - 4 0 0 cm -~ This difference is quantitatively shown in Fig. 4. In
196
Z C'ernogek et al./Journal of Molecular Structure 435 (1997) 193-198
Table 1 Raman bands and modification of their intensities used for computation of low-resolution spectrum [Fig. 5(b)], see text for details Position (cm q) Multiple of crystal band intensity
375.5 1.75
382.8 1.75
our view, if the medium range order of crystal and glass of the same chemical composition differ mainly due to some fluctuations of nearest-neighbor bond lengths and bond angles, then it is appropriate to think that the crystal to glass transition must influence relative intensities of Raman bands but not their energies. To test this idea we numerically changed the relative intensities of some Raman bands of/3-GeS2 polycrystal spectrum (Table 1) and we used this modified spectrum for low-resolution spectrum computation, Fig. 5. In our view the LR-spectrum obtained by this simple way agrees quite well with the Raman spectrum of glassy GeS2. It would be interesting to discuss the difference spectrum [Fig. 5(d)] in detail but unambiguous vibrational analyses of Raman
396.8 2
403.8 2
432.9 0
438.8 8
445.1 0
spectra of both GeS2 polymorphs are not available up to now. On the basis of results described above we assume that medium range order of GeS2 glass consists of a 3D arrangement of Ge[S 1/214tetrahedral units as in/3GeS2. Our assumption of a similarity in the short and medium range structure of glassy GeSe and/3-GeSe is also in agreement with our study of the first step of crystallization of glassy GeS2 at the temperature below glass transition temperature. Experimental Raman spectrum of annealed GeSe glass composed
--t
-'1
t
i-
a
\
~
e-
rt-
E
r,,-
r~
t'-
\
(~
E
C
n,"
,,/"
,~"~.__~/' /
i I1
\',
\/' I
I
d 500
450
400
350
300
Raman shift [cm -1] 500
450
400 350 300 Raman shift [cm q]
Fig. 4. Experimental Raman spectrum of glassy GeS 2 (a), computed low-resolution spectrum of/3-GeS 2 (b) and difference spectrum (c).
Fig. 5. Comparison of (a) experimental Raman spectrum of glassy GeS2 with (b) computed modified low-resolution spectrum of/3GeS2. Experimental Raman spectrum of polycrystalline /3-GeS2 (full line) and its modification (dashed line) (c) and their difference spectrum (d).
Z Cernogek et al./Journal of Molecular Structure 435 (1997) 193 198
a
7 .Q
/f //
O)
!I
g
/
f
b //
197
very similar to/3-GeS2 can be obtained. Raman spectrum of these microcrystals is very similar to glassy GeSe2. Three-dimensional structure of GeSe2 (comparable with 3-GeS2 one) is probably thermodynamically much less stable and it leads to erroneous conclusion that only two-dimensional layered structure of glassy GeSe2 and glassy GeS2 can exist, as well.
r~
4. Conclusion o", ;, : ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I '
500
400
300 200 100 Raman shift [cmq]
Fig. 6. Raman spectra of (a) bulk glassy GeS2 (annealed at 440°C for 30 min) with grown isolated microcrystalsand (b) polycrystalline 3-GeS2.
of the mixed structure of the glassy GeS2 and of the isolated crystallites as well as the spectrum of/3-GeS2 is presented for comparison in Fig. 6. Spectral bands of growing crystalline phase in glassy matrix can be well compared with 3-GeS2. It is known that only 3GeS2 polymorph crystallize when glassy GeS2 is annealed at temperature below --497°C. It is clearly verified by our method of preparation of 3-GeS2. Because the /3 --~ o~ phase transformation is irreversible, one can hardly expect that in the case if glassy matrix would be mainly a-like type [17,18] the crystallization will start by o~ --~ /3 transformation of 2D clusters. Thus observed crystallization of only 3-GeS 2 polymorph in the glassy matrix of GeS2 is in good agreement with our opinion that the medium range order of glassy GeS2 is 3D 3-GeS2-1ike. Because the structures of glassy GeS2 and glassy GeSe2 are sometimes compared one to other, e.g. [13,19], we would like to make a comment. A lot of studies of structure of glassy GeSe2 were made up to now, e.g. [16,23,24], and it was concluded that structure of glassy GeS 2 is layered two-dimensional of o~GeSe2 type. It is necessary to emphasize that only the ~-GeSe2 crystal modification is known to be stable at higher temperature. However using Raman spectroscopy Sugai [2] found that by low power light irradiation of glassy GeSe2 the microcrystals with a structure
On the basis of a comparison of Raman spectra of glassy GeS2 and low-resolution ones of polycrystalline o~- and B-GeS2, we have shown that the medium range structure of glassy GeS2 is similar to that of the 3D structure of 3-GeS 2. Growth of 3-GeS 2 microcrystals from solid phase of glassy GeS2 at temperature sufficiently below glass transition temperature Tg w a s found, as well. It confirms our conclusion of similarity of medium range order of glassy GeS2 and 3-GeS2.
Acknowledgements This work was supported by the grants no. 203/96/ 0876 and no. 203/96/0184 of The Czech Grant Agency.
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