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Atomic structure and chemical order in Ge–As sulfide glasses: a combined Ge and As K-edge EXAFS study
Journal of Non-Crystalline Solids 293±295 (2001) 204±210
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Atomic structure and chemical order in Ge±As sul®de glasses: a combined Ge and As K-edge EXAFS study S. Sen, C.W. Ponader, B.G. Aitken * Glass Research Division, Corning Incorporated, SP-FR-05 Corning, NY 14831, USA
Abstract The coordination environments of Ge and As atoms in Gex Asx S100 2x glasses with 13:3 6 x 6 32:5 have been studied with Ge and As K-edge EXAFS spectroscopy. Ge and As atoms are fourfold- and threefold-coordinated, respectively, in all glasses. The atomic structures of the stoichiometric and S-excess glasses are found to consist of GeS4 tetrahedra and AsS3 trigonal pyramids implying the preservation of chemical order at least over the length scale of the ®rst coordination shell. As±As homopolar bonds are found to appear at low and intermediate levels of S-de®ciency. Ge±Ge bonds are formed in extremely S-de®cient glasses only after all As atoms participate in homopolar As±As bonds, implying clustering of like metal atoms and violation of chemical order in S-de®cient glasses. Intermediate-range structural order induced by such clustering is shown to play a critical role in controlling the compositional variation of physical properties in these glasses. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Chalcogenide glasses are promising materials for wide ranging technological applications including devices for fast non-linear optical switching, ®ber optics and sub-micron lithography [1]. The related physico-chemical properties of these glasses have therefore been studied intensively in the last 20 years [2]. These glasses have also been used as model systems for understanding the in¯uence of structure and topology on the compositional variation of various physical properties in amorphous covalently bonded materials [3,4]. It has been argued that the compositional dependence of physical properties of these glasses is so-
* Corresponding author. Tel.: +1-607 974 3111; fax: +1-607 974 2410. E-mail address: [email protected] (B.G. Aitken).
lely determined by their average coordination number hri de®ned as X hri n i Xi : In this equation ni is the coordination number of species i (Ge, As, S, Se) according to the (8-n) valence rule and Xi is its mole fraction in the multicomponent solution [3,4]. Non-linear compositional variations of various physical properties in these glasses have been ascribed to either a rigidity percolation type of transition at hri 2:4 or a topological phase transition at hri 2:67 [5,6]. However, these theories are simply based on the average nearest-neighbor coordination numbers of the constituent atoms and do not dierentiate between the characters of homo- and hetero-polar bonds and their relative concentrations in the coordination environment of an atom. These distinctions may become important in controlling the compositional variation of physical properties in
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 6 7 2 - X
S. Sen et al. / Journal of Non-Crystalline Solids 293±295 (2001) 204±210
the case of Ge±As±S glasses with constituent ions that have stronger electronegativity dierences than those in the case of Ge±As±Se glasses. A systematic study of the coordination environments of the constituent atoms in ternary Ge±As±S(Se) glasses over a wide composition range and hri values is therefore needed in order to test these hypotheses. This paper reports the results of a detailed study of the local atomic environments of Ge and As atoms in ternary Ge±As±S glasses with Ge and As K-edge X-ray absorption ®ne structure (EXAFS) spectroscopy. A series of nine glass compositions with a Ge:As atomic ratio of 1:1 and with hri values ranging from 2.400 to 2.975 have been studied. Besides the determination of the nearestneighbor coordination environments for Ge and As atoms, a major objective of this study is to investigate the degree of chemical order in these glasses as a function of composition. 2. Experimental 2.1. Sample preparation GeAsS glasses were synthesized by melting mixtures of the constituent elements ( P99.9995% purity, metals basis) in evacuated (10 6 Torr) and ¯ame-sealed fused silica ampoules at 1200 K for at least 24 h in a rocking furnace. The ampoules were quenched in water and subsequently annealed for 1 h at the respective glass transition temperatures. A more detailed description of the preparation method can be found elsewhere [7]. The nominal compositions of the GeAsS glasses studied here are listed in Table 1 and are also shown in Fig. 1. 2.2. EXAFS spectroscopy 2.2.1. Data collection All Ge and As K-edge EXAFS spectra were collected at beam line X10C at the National Synchrotron Light Source at Brookhaven National Laboratory. A Si(2 2 0) monochromator was used with the focusing mirror tuned to reject higher harmonics. All samples were ®nely ground to
Fig. 1. Ternary plot of the Gex Asx S100 investigated in this study.
205
2x
glass compositions
Table 1 Compositional parameters of GeAsS glasses Chemical composition
%S in excess of stoichiometry
Average coordination number hri
Ge13:3 As13:3 S73:4 Ge17:5 As17:5 S65:0 Ge18:2 As18:2 S63:6 Ge20:0 As20:0 S60:0 Ge22:5 As22:5 S55:0 Ge25:0 As25:0 S50:0 Ge27:5 As27:5 S45:0 Ge30:0 As30:0 S40:0 Ge32:5 As32:5 S35:0
57.1 6.1 0.0 )14.3 )30.2 )42.9 )53.3 )61.9 )69.2
2.400 2.525 2.546 2.600 2.675 2.750 2.825 2.900 2.975
powders and were mounted on mylar tapes on the sample holder of a liquid nitrogen cooled cryostat. Sample temperatures were maintained within a range 80±90 K in order to lower the eect of the thermal Debye±Waller factor on the signal. The EXAFS data were collected in the transmission mode in energy steps of 2 eV. Ionization chambers were used as detectors for measuring the incident and transmitted X-ray beam intensities. Synthetic As2 S3 , AsS, GeS and GeS2 crystals were used as model compounds. 2.2.2. Data analysis The Ge and As K-edge EXAFS data have been analyzed using the standard software packages EXBROOK and EXCURV92 developed by the Daresbury Laboratory [8]. The EXBROOK package has been used to subtract a linear background in the pre-edge region and a quadratic background in the post-edge region from the raw
206
S. Sen et al. / Journal of Non-Crystalline Solids 293±295 (2001) 204±210
absorption spectra. The EXAFS oscillations are subsequently k 3 -weighted and ®tted using the nonlinear least squares ®tting routine in EXCURV92, based on the curved-wave theory of EXAFS [9]. The three structural parameters that are varied in order to obtain the best ®t are: (i) the radial distance R of the neighboring atoms around the central photo-excited Ge or As atom, (ii) the number N of the neighboring atoms around the central atom within a shell of radius R, and (iii) the Debye±Waller factor 2r2 . The calculated electron scattering phase shifts for As±S, Ge±S and Ge±As atom pairs are tested by ®tting the Ge and As EXAFS spectra of the model compounds and proved to be adequate without further re®nement. It is important to note that the Debye± Waller factor and the coordination number are correlated quantities in EXAFS data analysis and the quality of the ®t can be kept unchanged by varying both the quantities simultaneously over a limited range. The related uncertainties in the coordination numbers of Ge and As atoms in
these glasses are found to be of the order of 0:2 or less.
Fig. 2. k 3 -weighted Ge K-edge EXAFS spectra of the Gex Asx S100 2x glasses with various levels of S-excess as indicated alongside each spectrum. Solid lines represent experimental data and dashed lines correspond to least-squares ®ts obtained using EXCURV92.
Fig. 3. Magnitudes of the Fourier transform of the k 3 -weighted Ge K-edge EXAFS spectra of Gex Asx S100 2x glasses with various levels of S-excess. Solid lines represent experimental data and dashed lines correspond to least-squares ®ts obtained using EXCURV92.
3. Results 3.1. Ge EXAFS The k 3 -weighted experimental Ge EXAFS spectra and the ®tted curves for the Gex Asx S100 2x glasses are shown in Fig. 2. The corresponding Fourier transforms are shown in Fig. 3. The structural parameters obtained from ®tting the Ge EXAFS spectra are listed in Table 2. The Ge atoms are found to be tetrahedrally coordinated in all glasses. However, more interestingly, the atomic constitution of the ®rst coordination shell of Ge changes progressively from one consisting of 4 S atoms in S-excess glasses to one consisting of a combination of S and X (X Ge, As) atoms in Sde®cient glasses (Table 2). It must be noted in this regard that the Ge and As backscatterers cannot
S. Sen et al. / Journal of Non-Crystalline Solids 293±295 (2001) 204±210
207
Table 2 Ge K-edge EXAFS structural paramaters for GeAsS glasses Composition Ge13:3 As13:3 S73:4 Ge17:5 As17:5 S65:0 Ge18:2 As18:2 S63:6 Ge20:0 As20:0 S60:0 Ge22:5 As22:5 S55:0 Ge25:0 As25:0 S50:0 Ge27:5 As27:5 S45:0 Ge30:0 As30:0 S40:0 Ge32:5 As32:5 S35:0
First coordination shella
Second coordination shella 2 ) (A
Ns
Rs (A)
2r2s
4.0 4.0 4.0 4.0 4.0 4.0 3.4 3.0 2.5
2.20(6) 2.20(2) 2.20(4) 2.20(5) 2.21(7) 2.22(4) 2.21(9) 2.23(0) 2.22(7)
0.012 0.012 0.014 0.015 0.012 0.009 0.010 0.006 0.012
2 ) (A
Nx
Rx (A)
2r2x
0.0 0.0 0.0 0.0 0.0 0.0 0.6 1.0 1.5
± ± ± ± ± ± 2.46(0) 2.46(9) 2.46(0)
± ± ± ± ± ± 0.005 0.004 0.004
Nx
Rx (A)
2 ) 2r2x (A
1.9 1.9 1.6 2.0 1.5 ± ± ± ±
3.38(7) 3.39(4) 3.45(0) 3.38(7) 3.39(0) ± ± ± ±
0.022 0.020 0.020 0.024 0.024 ± ± ± ±
a The subscripts s and x denote structural parameters corresponding to S and Ge/As neighbors, respectively. For the ®rst coordination shell the subscript x denotes Ge neighbors only (see text for details).
be directly distinguished by EXAFS due to the similarity in their electronic structures. The Ge±S bond lengths in these glasses vary between 2.20 a range characteristic of GeS4 tetraand 2.23 A, hedra in crystalline GeS2 [10]. The corresponding Debye±Waller factors range between 0.006 and 2 . The Ge±X bond lengths in the ®rst co0.015 A ordination shell range between 2.46 and 2.47 A (Table 2). These inter-atomic distances in the ®rst coordination shell of Ge do not show any signi®cant compositional dependence. The corresponding Debye±Waller factors are found to vary 2 (Table 2). between 0.004 and 0.005 A Two peaks are present in the Fourier transforms of the Ge EXAFS spectra for all but the four most S-de®cient glasses (Fig. 3). These EXAFS spectra have been ®tted using a second shell of 1.5±2 Ge/As next-nearest neighbors around the with central Ge at an average distance of 3.40 A Debye±Waller factors ranging between 0.020 and 2 (Table 2). The ®tting of this second shell 0.024 A has no detectable eect on the ®t parameters of the ®rst coordination shell and has been found to have at least 95% probability of being statistically signi®cant according to the criterion of Joyner et al. [10]. These Ge±X second shell distances are similar to those observed between Ge atoms in cornersharing GeS4 tetrahedra in crystalline GeS2 [11]. The S atoms in the GeS4 tetrahedra in stoichiometric and S-de®cient glasses are expected to be `bridging', i.e., bonded to two metal atoms. Hence, for corner-sharing GeS4 tetrahedra, 4 next-nearest neighbor X atoms are expected to be present in the
second coordination shell of the Ge atoms in these glasses. Therefore, the Ge±X second shell coordination numbers derived from ®tting a second shell to the Ge EXAFS spectra seem to be substantially underestimated, at least in the case of stoichiometric and S-de®cient glasses (Table 2). Such a discrepancy may originate from a large distribution of inter-atomic distances in this shell, which would be consistent with the associated relatively large Debye±Waller factors (Table 2). 3.2. As EXAFS The k 3 -weighted experimental As EXAFS spectra and the ®tted curves for the Gex Asx S100 2x glasses are shown in Fig. 4. The corresponding Fourier transforms are shown in Fig. 5. The EXAFS spectra show a rather strong change in the intensity distribution with composition as indicated by the gradual shift in the position of the most intense oscillations from low to high wavenumber regions with increasing S-de®ciency (Fig. 4). This result is a clear indication of a change in the nature of the back-scattering atoms in the coordination shells of the photo-excited As atoms. The Fourier transforms of the EXAFS spectra show, in all but one glass, the presence of only one peak corresponding to the ®rst coordination shell of As. The structural parameters obtained from theoretical ®ts of the As EXAFS spectra are given in Table 3. The As atoms in all glasses are found to be 3-coordinated indicating a likely trigonal pyramidal con®guration of the ®rst coordination
208
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Fig. 4. k 3 -weighted As K-edge EXAFS spectra of the Gex Asx S100 2x glasses with various levels of S-excess as indicated alongside each spectrum. Solid lines represent experimental data and dashed lines correspond to least-squares ®ts obtained using EXCURV92.
shell, typical of the As-sul®de crystalline phases [12±14]. However, like Ge, the atomic constitution of the ®rst coordination shell of As changes progressively from one consisting of 3 S atoms in Sexcess glasses to one consisting of 3 As/Ge atoms in S-de®cient glasses (Table 3). The As±S bond lengths in these glasses vary between 2.22 and 2.27
Fig. 5. Magnitudes of the Fourier transform of the k 3 -weighted As K-edge EXAFS spectra of Gex Asx S100 2x glasses with various levels of S-excess. Solid lines represent experimental data and dashed lines correspond to least-squares ®ts obtained using EXCURV92.
a range characteristic of As-sul®de crystalline A, phases [12±14]. The corresponding Debye±Waller factors range between 0.005 and 0.015. The As±X in the bond lengths range between 2.42 and 2.46 A ®rst coordination shell with the corresponding Debye±Waller factors ranging between 0.005 and
Table 3 As K-edge EXAFS structural paramaters for GeAsS glasses Composition Ge13:3 As13:3 S73:4 Ge17:5 As17:5 S65:0 Ge18:2 As18:2 S63:6 Ge20:0 As20:0 S60:0 Ge22:5 As22:5 S55:0 Ge25:0 As25:0 S50:0 Ge27:5 As27:5 S45:0 Ge30:0 As30:0 S40:0 Ge32:5 As32:5 S35:0
First coordination shella
Second coordination shella
Ns
Rs (A)
2r2s
3.0 3.0 3.0 2.5 1.8 1.2 0.0 0.0 0.0
2.25(2) 2.24(6) 2.27(0) 2.23(1) 2.26(3) 2.22(4) ± ± ±
0.014 0.015 0.009 0.010 0.005 0.005 ± ± ±
2
) (A
Nx
Rx (A)
2r2x
0.0 0.0 0.0 0.5 1.2 1.9 3.0 3.1 3.1
± ± ± 2.46(5) 2.46(5) 2.45(1) 2.42(7) 2.44(1) 2.43(0)
± ± ± 0.005 0.005 0.005 0.011 0.012 0.008
2
) (A
2
Nx
Rx (A)
) 2r2x (A
± ± ± 1.0 ± ± ± ± ±
± ± ± 3.41(4) ± ± ± ± ±
± ± ± 0.012 ± ± ± ± ±
a The subscripts s and x denote structural parameters corresponding to S and Ge/As neighbors, respectively. For the ®rst coordination shell the subscript x denotes As neighbors only (see text for details).
S. Sen et al. / Journal of Non-Crystalline Solids 293±295 (2001) 204±210
209
0.012. These quantities do not show any signi®cant compositional trend in these glasses (Table 3). 4. Discussion The Ge and As K-edge EXAFS results reported here indicate that the ®rst shell coordination numbers of Ge and As are 4 and 3, respectively, in all glasses irrespective of composition. However, the coordination environments of Ge and As atoms show gradual and systematic changes as a function of glass composition. The Ge and As atoms in the stoichiometric and S-excess glasses are found, within experimental resolution, to be bonded only to S atoms forming GeS4 and AsS3 polyhedra which necessitates the presence of S±S homopolar bonds in S-excess glasses. This hypothesis is corroborated by the results of a previous study based on Raman spectroscopy which showed the appearance of a vibrational band at 490 cm 1 corresponding to an S±S stretching mode in S-excess Ge±As±S glasses [15]. On the other hand, the initial structural response to Sde®ciency in Ge±As±S glasses is found to be the formation of As±X homopolar bonds. The As±X bonds in the ®rst coordination shell of As ®rst become observable in the Ge20 As20 S60 glass, i.e., the glass with the lowest S-de®ciency of )14.3%. In contrast, the Ge±X bonds begin to form in the ®rst coordination shell of Ge only when the S-de®ciency becomes )53.3%, in the Ge27:5 As27:5 S45:0 glass. At this level of S-de®ciency the ®rst coordination shell around As atoms already consists only of As±X bonds (Tables 2,3). However, the ®rst shell coordination environment of the Ge atoms does not show any evidence of metal±metal homopolar bonding until all the As±X bonds are exhausted in the structure (Tables 2,3). This result implies that the initial reaction of the glass structure to the lowering of the S content below stoichiometry is the formation of As±As homopolar bonds. The concentration of As±As bonds increases with further lowering of the S content to a S-de®ciency level of )53.3% until all As atoms are coordinated to 3 other As atoms in the glass structure (Fig. 6). Such a structural scenario leaves the Ge atoms with the only possibility of forma-
Fig. 6. Compositional variation in the atomic make-up of the ®rst coordination shell of Ge and As atoms in Gex Asx S1 2x glasses as derived from ®tting the Ge and As K-edge EXAFS data.
tion of Ge±Ge homopolar bonds in the highly Sde®cient ( P 53.3%) glasses (Fig. 6). Therefore, although Ge and As backscatterers cannot be directly distinguished by EXAFS, it can be safely concluded that the As±X and Ge±X bonds in these glasses are exclusively between the same atom types, i.e., they are As±As and Ge±Ge bonds (Fig. 6). This result, to our knowledge, is the ®rst direct evidence in favor of an inhomogeneous distribution or clustering of like metal atoms in chalcogenide glasses. The clustering of constituent atoms, as seen in the avoidance of As±Ge bonds and in the formation of As±As and Ge±Ge bonds in S-de®cient Ge± As±S glasses, clearly indicates a complete violation of chemical order. The nature of the intermediaterange structural order induced by such a clustering of As and Ge atoms may play an important role in controlling the compositional variation of the physical properties in Ge±As±S glasses. For example, the molar volume and thermal expansion coecient of Ge±As±S glasses show a non-linear compositional variation with maxima at low levels of S-de®ciency corresponding to hri 2:6 [15]. A
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recent Raman spectroscopic study has shown that this non-linearity is correlated with the concentration of discrete As4 S3 -based molecular units in the glass structure [7]. Formation of such molecular units where the As atoms are coordinated to a combination of S and other As atoms is a direct consequence of clustering of As atoms in Ge±As±S glasses with low to intermediate S-de®ciency levels (from )14.3% to )42.9%) as shown by the present EXAFS results. It should be noted that the degree of chemical order in chalcogenide glasses is likely to be dependent on the nature of the chalcogen atom. A previous Ge and As EXAFS study of Ge±As±Te glasses has shown the existence of metal±metal homopolar bonds even in Te-excess glasses with hri 2:4, whereas this study shows the presence of only Ge/As±S heteropolar bonds in Ge±As±S glasses with similar hri values [16]. On the other hand, the observed dierences between Ge and As in the propensity toward homopolar bond formation indicate that the Ge:As ratio in a Ge±As±S glass could also be a critical factor in determining the degree of chemical order at similar hri values. 5. Summary The Ge and As EXAFS results provide complementary information regarding the atomic structure of Ge±As±S glasses. The Ge and As atoms are always found to be 4- and 3-coordinated in these glasses. The stoichiometric and S-excess glasses are characterized by heteropolar Ge±S and As±S bonds, which implies the presence of chemical order and S±S homopolar bond formation in glasses with excess S. On the other hand, As±As and Ge±Ge bonds are observed in S-de®cient glasses indicating violation of chemical order and clustering of like metal atoms. As±As bonds are energetically favored over Ge±Ge bonds at low to intermediate S de®ciencies. The latter bonds are
formed in strongly S-de®cient glasses only when all As atoms are used up for homopolar bonding. It is argued that clustering of like atoms may result in intermediate-range structural order in these glasses and consequently in the observed non-linear compositional variation of the relevant physical properties. Acknowledgements The authors thank M.L. Powley, D.H. Crooker and L.K. Cornelius for their technical assistance with the preparation of the glass samples used in this study. S.S. and C.P. also wish to thank the sta at the beam line X10C for their valuable technical assistance with EXAFS data collection. The National Synchrotron Light Source is supported by the US Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. References [1] A.B. Seddon, J. Non-Cryst. Solids 184 (1995) 44. [2] U. Senapati, K. Firstenberg, A.K. Varshneya, J. NonCryst. Solids 222 (1997) 153. [3] J.C. Phillips, J. Non-Cryst. Solids 34 (1979) 153. [4] M.F. Thorpe, J. Non-Cryst. Solids 57 (1983) 355. [5] M. Tatsumisago, B.L. Halfpap, J.L. Green, S.M. Lindsay, C.A. Angell, Phys. Rev. Lett. 64 (1990) 1549. [6] K. Tanaka, Phys. Rev. B 39 (1989) 1270. [7] B.G. Aitken, C.W. Ponader, J. Non-Cryst. Solids 274 (2000) 124. [8] N. Binsted, S.J. Gurman, J.C. Campbell, SERC Daresbury Lab. EXCURV092 Program, 1992. [9] S.J. Gurman, N. Binsted, I. Ross, J. Phys. C 17 (1984) 143. [10] R.W. Joyner, K.J. Martin, P. Meehan, J. Phys. C 20 (1987) 4005. [11] G. Dittmar, N. Schafer, Acta Crystallogr. B 31 (1975) 2060. [12] T. Ito, N. Morimoto, R. Sadanaga, Acta Crystallogr. 5 (1952) 775. [13] N. Morimoto, Mineral. J. 1 (1954) 160.
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Atomic structure and chemical order in Ge±As sul®de glasses: a combined Ge and As K-edge EXAFS study S. Sen, C.W. Ponader, B.G. Aitken * Glass Research Division, Corning Incorporated, SP-FR-05 Corning, NY 14831, USA
Abstract The coordination environments of Ge and As atoms in Gex Asx S100 2x glasses with 13:3 6 x 6 32:5 have been studied with Ge and As K-edge EXAFS spectroscopy. Ge and As atoms are fourfold- and threefold-coordinated, respectively, in all glasses. The atomic structures of the stoichiometric and S-excess glasses are found to consist of GeS4 tetrahedra and AsS3 trigonal pyramids implying the preservation of chemical order at least over the length scale of the ®rst coordination shell. As±As homopolar bonds are found to appear at low and intermediate levels of S-de®ciency. Ge±Ge bonds are formed in extremely S-de®cient glasses only after all As atoms participate in homopolar As±As bonds, implying clustering of like metal atoms and violation of chemical order in S-de®cient glasses. Intermediate-range structural order induced by such clustering is shown to play a critical role in controlling the compositional variation of physical properties in these glasses. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Chalcogenide glasses are promising materials for wide ranging technological applications including devices for fast non-linear optical switching, ®ber optics and sub-micron lithography [1]. The related physico-chemical properties of these glasses have therefore been studied intensively in the last 20 years [2]. These glasses have also been used as model systems for understanding the in¯uence of structure and topology on the compositional variation of various physical properties in amorphous covalently bonded materials [3,4]. It has been argued that the compositional dependence of physical properties of these glasses is so-
* Corresponding author. Tel.: +1-607 974 3111; fax: +1-607 974 2410. E-mail address: [email protected] (B.G. Aitken).
lely determined by their average coordination number hri de®ned as X hri n i Xi : In this equation ni is the coordination number of species i (Ge, As, S, Se) according to the (8-n) valence rule and Xi is its mole fraction in the multicomponent solution [3,4]. Non-linear compositional variations of various physical properties in these glasses have been ascribed to either a rigidity percolation type of transition at hri 2:4 or a topological phase transition at hri 2:67 [5,6]. However, these theories are simply based on the average nearest-neighbor coordination numbers of the constituent atoms and do not dierentiate between the characters of homo- and hetero-polar bonds and their relative concentrations in the coordination environment of an atom. These distinctions may become important in controlling the compositional variation of physical properties in
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 6 7 2 - X
S. Sen et al. / Journal of Non-Crystalline Solids 293±295 (2001) 204±210
the case of Ge±As±S glasses with constituent ions that have stronger electronegativity dierences than those in the case of Ge±As±Se glasses. A systematic study of the coordination environments of the constituent atoms in ternary Ge±As±S(Se) glasses over a wide composition range and hri values is therefore needed in order to test these hypotheses. This paper reports the results of a detailed study of the local atomic environments of Ge and As atoms in ternary Ge±As±S glasses with Ge and As K-edge X-ray absorption ®ne structure (EXAFS) spectroscopy. A series of nine glass compositions with a Ge:As atomic ratio of 1:1 and with hri values ranging from 2.400 to 2.975 have been studied. Besides the determination of the nearestneighbor coordination environments for Ge and As atoms, a major objective of this study is to investigate the degree of chemical order in these glasses as a function of composition. 2. Experimental 2.1. Sample preparation GeAsS glasses were synthesized by melting mixtures of the constituent elements ( P99.9995% purity, metals basis) in evacuated (10 6 Torr) and ¯ame-sealed fused silica ampoules at 1200 K for at least 24 h in a rocking furnace. The ampoules were quenched in water and subsequently annealed for 1 h at the respective glass transition temperatures. A more detailed description of the preparation method can be found elsewhere [7]. The nominal compositions of the GeAsS glasses studied here are listed in Table 1 and are also shown in Fig. 1. 2.2. EXAFS spectroscopy 2.2.1. Data collection All Ge and As K-edge EXAFS spectra were collected at beam line X10C at the National Synchrotron Light Source at Brookhaven National Laboratory. A Si(2 2 0) monochromator was used with the focusing mirror tuned to reject higher harmonics. All samples were ®nely ground to
Fig. 1. Ternary plot of the Gex Asx S100 investigated in this study.
205
2x
glass compositions
Table 1 Compositional parameters of GeAsS glasses Chemical composition
%S in excess of stoichiometry
Average coordination number hri
Ge13:3 As13:3 S73:4 Ge17:5 As17:5 S65:0 Ge18:2 As18:2 S63:6 Ge20:0 As20:0 S60:0 Ge22:5 As22:5 S55:0 Ge25:0 As25:0 S50:0 Ge27:5 As27:5 S45:0 Ge30:0 As30:0 S40:0 Ge32:5 As32:5 S35:0
57.1 6.1 0.0 )14.3 )30.2 )42.9 )53.3 )61.9 )69.2
2.400 2.525 2.546 2.600 2.675 2.750 2.825 2.900 2.975
powders and were mounted on mylar tapes on the sample holder of a liquid nitrogen cooled cryostat. Sample temperatures were maintained within a range 80±90 K in order to lower the eect of the thermal Debye±Waller factor on the signal. The EXAFS data were collected in the transmission mode in energy steps of 2 eV. Ionization chambers were used as detectors for measuring the incident and transmitted X-ray beam intensities. Synthetic As2 S3 , AsS, GeS and GeS2 crystals were used as model compounds. 2.2.2. Data analysis The Ge and As K-edge EXAFS data have been analyzed using the standard software packages EXBROOK and EXCURV92 developed by the Daresbury Laboratory [8]. The EXBROOK package has been used to subtract a linear background in the pre-edge region and a quadratic background in the post-edge region from the raw
206
S. Sen et al. / Journal of Non-Crystalline Solids 293±295 (2001) 204±210
absorption spectra. The EXAFS oscillations are subsequently k 3 -weighted and ®tted using the nonlinear least squares ®tting routine in EXCURV92, based on the curved-wave theory of EXAFS [9]. The three structural parameters that are varied in order to obtain the best ®t are: (i) the radial distance R of the neighboring atoms around the central photo-excited Ge or As atom, (ii) the number N of the neighboring atoms around the central atom within a shell of radius R, and (iii) the Debye±Waller factor 2r2 . The calculated electron scattering phase shifts for As±S, Ge±S and Ge±As atom pairs are tested by ®tting the Ge and As EXAFS spectra of the model compounds and proved to be adequate without further re®nement. It is important to note that the Debye± Waller factor and the coordination number are correlated quantities in EXAFS data analysis and the quality of the ®t can be kept unchanged by varying both the quantities simultaneously over a limited range. The related uncertainties in the coordination numbers of Ge and As atoms in
these glasses are found to be of the order of 0:2 or less.
Fig. 2. k 3 -weighted Ge K-edge EXAFS spectra of the Gex Asx S100 2x glasses with various levels of S-excess as indicated alongside each spectrum. Solid lines represent experimental data and dashed lines correspond to least-squares ®ts obtained using EXCURV92.
Fig. 3. Magnitudes of the Fourier transform of the k 3 -weighted Ge K-edge EXAFS spectra of Gex Asx S100 2x glasses with various levels of S-excess. Solid lines represent experimental data and dashed lines correspond to least-squares ®ts obtained using EXCURV92.
3. Results 3.1. Ge EXAFS The k 3 -weighted experimental Ge EXAFS spectra and the ®tted curves for the Gex Asx S100 2x glasses are shown in Fig. 2. The corresponding Fourier transforms are shown in Fig. 3. The structural parameters obtained from ®tting the Ge EXAFS spectra are listed in Table 2. The Ge atoms are found to be tetrahedrally coordinated in all glasses. However, more interestingly, the atomic constitution of the ®rst coordination shell of Ge changes progressively from one consisting of 4 S atoms in S-excess glasses to one consisting of a combination of S and X (X Ge, As) atoms in Sde®cient glasses (Table 2). It must be noted in this regard that the Ge and As backscatterers cannot
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Table 2 Ge K-edge EXAFS structural paramaters for GeAsS glasses Composition Ge13:3 As13:3 S73:4 Ge17:5 As17:5 S65:0 Ge18:2 As18:2 S63:6 Ge20:0 As20:0 S60:0 Ge22:5 As22:5 S55:0 Ge25:0 As25:0 S50:0 Ge27:5 As27:5 S45:0 Ge30:0 As30:0 S40:0 Ge32:5 As32:5 S35:0
First coordination shella
Second coordination shella 2 ) (A
Ns
Rs (A)
2r2s
4.0 4.0 4.0 4.0 4.0 4.0 3.4 3.0 2.5
2.20(6) 2.20(2) 2.20(4) 2.20(5) 2.21(7) 2.22(4) 2.21(9) 2.23(0) 2.22(7)
0.012 0.012 0.014 0.015 0.012 0.009 0.010 0.006 0.012
2 ) (A
Nx
Rx (A)
2r2x
0.0 0.0 0.0 0.0 0.0 0.0 0.6 1.0 1.5
± ± ± ± ± ± 2.46(0) 2.46(9) 2.46(0)
± ± ± ± ± ± 0.005 0.004 0.004
Nx
Rx (A)
2 ) 2r2x (A
1.9 1.9 1.6 2.0 1.5 ± ± ± ±
3.38(7) 3.39(4) 3.45(0) 3.38(7) 3.39(0) ± ± ± ±
0.022 0.020 0.020 0.024 0.024 ± ± ± ±
a The subscripts s and x denote structural parameters corresponding to S and Ge/As neighbors, respectively. For the ®rst coordination shell the subscript x denotes Ge neighbors only (see text for details).
be directly distinguished by EXAFS due to the similarity in their electronic structures. The Ge±S bond lengths in these glasses vary between 2.20 a range characteristic of GeS4 tetraand 2.23 A, hedra in crystalline GeS2 [10]. The corresponding Debye±Waller factors range between 0.006 and 2 . The Ge±X bond lengths in the ®rst co0.015 A ordination shell range between 2.46 and 2.47 A (Table 2). These inter-atomic distances in the ®rst coordination shell of Ge do not show any signi®cant compositional dependence. The corresponding Debye±Waller factors are found to vary 2 (Table 2). between 0.004 and 0.005 A Two peaks are present in the Fourier transforms of the Ge EXAFS spectra for all but the four most S-de®cient glasses (Fig. 3). These EXAFS spectra have been ®tted using a second shell of 1.5±2 Ge/As next-nearest neighbors around the with central Ge at an average distance of 3.40 A Debye±Waller factors ranging between 0.020 and 2 (Table 2). The ®tting of this second shell 0.024 A has no detectable eect on the ®t parameters of the ®rst coordination shell and has been found to have at least 95% probability of being statistically signi®cant according to the criterion of Joyner et al. [10]. These Ge±X second shell distances are similar to those observed between Ge atoms in cornersharing GeS4 tetrahedra in crystalline GeS2 [11]. The S atoms in the GeS4 tetrahedra in stoichiometric and S-de®cient glasses are expected to be `bridging', i.e., bonded to two metal atoms. Hence, for corner-sharing GeS4 tetrahedra, 4 next-nearest neighbor X atoms are expected to be present in the
second coordination shell of the Ge atoms in these glasses. Therefore, the Ge±X second shell coordination numbers derived from ®tting a second shell to the Ge EXAFS spectra seem to be substantially underestimated, at least in the case of stoichiometric and S-de®cient glasses (Table 2). Such a discrepancy may originate from a large distribution of inter-atomic distances in this shell, which would be consistent with the associated relatively large Debye±Waller factors (Table 2). 3.2. As EXAFS The k 3 -weighted experimental As EXAFS spectra and the ®tted curves for the Gex Asx S100 2x glasses are shown in Fig. 4. The corresponding Fourier transforms are shown in Fig. 5. The EXAFS spectra show a rather strong change in the intensity distribution with composition as indicated by the gradual shift in the position of the most intense oscillations from low to high wavenumber regions with increasing S-de®ciency (Fig. 4). This result is a clear indication of a change in the nature of the back-scattering atoms in the coordination shells of the photo-excited As atoms. The Fourier transforms of the EXAFS spectra show, in all but one glass, the presence of only one peak corresponding to the ®rst coordination shell of As. The structural parameters obtained from theoretical ®ts of the As EXAFS spectra are given in Table 3. The As atoms in all glasses are found to be 3-coordinated indicating a likely trigonal pyramidal con®guration of the ®rst coordination
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Fig. 4. k 3 -weighted As K-edge EXAFS spectra of the Gex Asx S100 2x glasses with various levels of S-excess as indicated alongside each spectrum. Solid lines represent experimental data and dashed lines correspond to least-squares ®ts obtained using EXCURV92.
shell, typical of the As-sul®de crystalline phases [12±14]. However, like Ge, the atomic constitution of the ®rst coordination shell of As changes progressively from one consisting of 3 S atoms in Sexcess glasses to one consisting of 3 As/Ge atoms in S-de®cient glasses (Table 3). The As±S bond lengths in these glasses vary between 2.22 and 2.27
Fig. 5. Magnitudes of the Fourier transform of the k 3 -weighted As K-edge EXAFS spectra of Gex Asx S100 2x glasses with various levels of S-excess. Solid lines represent experimental data and dashed lines correspond to least-squares ®ts obtained using EXCURV92.
a range characteristic of As-sul®de crystalline A, phases [12±14]. The corresponding Debye±Waller factors range between 0.005 and 0.015. The As±X in the bond lengths range between 2.42 and 2.46 A ®rst coordination shell with the corresponding Debye±Waller factors ranging between 0.005 and
Table 3 As K-edge EXAFS structural paramaters for GeAsS glasses Composition Ge13:3 As13:3 S73:4 Ge17:5 As17:5 S65:0 Ge18:2 As18:2 S63:6 Ge20:0 As20:0 S60:0 Ge22:5 As22:5 S55:0 Ge25:0 As25:0 S50:0 Ge27:5 As27:5 S45:0 Ge30:0 As30:0 S40:0 Ge32:5 As32:5 S35:0
First coordination shella
Second coordination shella
Ns
Rs (A)
2r2s
3.0 3.0 3.0 2.5 1.8 1.2 0.0 0.0 0.0
2.25(2) 2.24(6) 2.27(0) 2.23(1) 2.26(3) 2.22(4) ± ± ±
0.014 0.015 0.009 0.010 0.005 0.005 ± ± ±
2
) (A
Nx
Rx (A)
2r2x
0.0 0.0 0.0 0.5 1.2 1.9 3.0 3.1 3.1
± ± ± 2.46(5) 2.46(5) 2.45(1) 2.42(7) 2.44(1) 2.43(0)
± ± ± 0.005 0.005 0.005 0.011 0.012 0.008
2
) (A
2
Nx
Rx (A)
) 2r2x (A
± ± ± 1.0 ± ± ± ± ±
± ± ± 3.41(4) ± ± ± ± ±
± ± ± 0.012 ± ± ± ± ±
a The subscripts s and x denote structural parameters corresponding to S and Ge/As neighbors, respectively. For the ®rst coordination shell the subscript x denotes As neighbors only (see text for details).
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0.012. These quantities do not show any signi®cant compositional trend in these glasses (Table 3). 4. Discussion The Ge and As K-edge EXAFS results reported here indicate that the ®rst shell coordination numbers of Ge and As are 4 and 3, respectively, in all glasses irrespective of composition. However, the coordination environments of Ge and As atoms show gradual and systematic changes as a function of glass composition. The Ge and As atoms in the stoichiometric and S-excess glasses are found, within experimental resolution, to be bonded only to S atoms forming GeS4 and AsS3 polyhedra which necessitates the presence of S±S homopolar bonds in S-excess glasses. This hypothesis is corroborated by the results of a previous study based on Raman spectroscopy which showed the appearance of a vibrational band at 490 cm 1 corresponding to an S±S stretching mode in S-excess Ge±As±S glasses [15]. On the other hand, the initial structural response to Sde®ciency in Ge±As±S glasses is found to be the formation of As±X homopolar bonds. The As±X bonds in the ®rst coordination shell of As ®rst become observable in the Ge20 As20 S60 glass, i.e., the glass with the lowest S-de®ciency of )14.3%. In contrast, the Ge±X bonds begin to form in the ®rst coordination shell of Ge only when the S-de®ciency becomes )53.3%, in the Ge27:5 As27:5 S45:0 glass. At this level of S-de®ciency the ®rst coordination shell around As atoms already consists only of As±X bonds (Tables 2,3). However, the ®rst shell coordination environment of the Ge atoms does not show any evidence of metal±metal homopolar bonding until all the As±X bonds are exhausted in the structure (Tables 2,3). This result implies that the initial reaction of the glass structure to the lowering of the S content below stoichiometry is the formation of As±As homopolar bonds. The concentration of As±As bonds increases with further lowering of the S content to a S-de®ciency level of )53.3% until all As atoms are coordinated to 3 other As atoms in the glass structure (Fig. 6). Such a structural scenario leaves the Ge atoms with the only possibility of forma-
Fig. 6. Compositional variation in the atomic make-up of the ®rst coordination shell of Ge and As atoms in Gex Asx S1 2x glasses as derived from ®tting the Ge and As K-edge EXAFS data.
tion of Ge±Ge homopolar bonds in the highly Sde®cient ( P 53.3%) glasses (Fig. 6). Therefore, although Ge and As backscatterers cannot be directly distinguished by EXAFS, it can be safely concluded that the As±X and Ge±X bonds in these glasses are exclusively between the same atom types, i.e., they are As±As and Ge±Ge bonds (Fig. 6). This result, to our knowledge, is the ®rst direct evidence in favor of an inhomogeneous distribution or clustering of like metal atoms in chalcogenide glasses. The clustering of constituent atoms, as seen in the avoidance of As±Ge bonds and in the formation of As±As and Ge±Ge bonds in S-de®cient Ge± As±S glasses, clearly indicates a complete violation of chemical order. The nature of the intermediaterange structural order induced by such a clustering of As and Ge atoms may play an important role in controlling the compositional variation of the physical properties in Ge±As±S glasses. For example, the molar volume and thermal expansion coecient of Ge±As±S glasses show a non-linear compositional variation with maxima at low levels of S-de®ciency corresponding to hri 2:6 [15]. A
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recent Raman spectroscopic study has shown that this non-linearity is correlated with the concentration of discrete As4 S3 -based molecular units in the glass structure [7]. Formation of such molecular units where the As atoms are coordinated to a combination of S and other As atoms is a direct consequence of clustering of As atoms in Ge±As±S glasses with low to intermediate S-de®ciency levels (from )14.3% to )42.9%) as shown by the present EXAFS results. It should be noted that the degree of chemical order in chalcogenide glasses is likely to be dependent on the nature of the chalcogen atom. A previous Ge and As EXAFS study of Ge±As±Te glasses has shown the existence of metal±metal homopolar bonds even in Te-excess glasses with hri 2:4, whereas this study shows the presence of only Ge/As±S heteropolar bonds in Ge±As±S glasses with similar hri values [16]. On the other hand, the observed dierences between Ge and As in the propensity toward homopolar bond formation indicate that the Ge:As ratio in a Ge±As±S glass could also be a critical factor in determining the degree of chemical order at similar hri values. 5. Summary The Ge and As EXAFS results provide complementary information regarding the atomic structure of Ge±As±S glasses. The Ge and As atoms are always found to be 4- and 3-coordinated in these glasses. The stoichiometric and S-excess glasses are characterized by heteropolar Ge±S and As±S bonds, which implies the presence of chemical order and S±S homopolar bond formation in glasses with excess S. On the other hand, As±As and Ge±Ge bonds are observed in S-de®cient glasses indicating violation of chemical order and clustering of like metal atoms. As±As bonds are energetically favored over Ge±Ge bonds at low to intermediate S de®ciencies. The latter bonds are
formed in strongly S-de®cient glasses only when all As atoms are used up for homopolar bonding. It is argued that clustering of like atoms may result in intermediate-range structural order in these glasses and consequently in the observed non-linear compositional variation of the relevant physical properties. Acknowledgements The authors thank M.L. Powley, D.H. Crooker and L.K. Cornelius for their technical assistance with the preparation of the glass samples used in this study. S.S. and C.P. also wish to thank the sta at the beam line X10C for their valuable technical assistance with EXAFS data collection. The National Synchrotron Light Source is supported by the US Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. References [1] A.B. Seddon, J. Non-Cryst. Solids 184 (1995) 44. [2] U. Senapati, K. Firstenberg, A.K. Varshneya, J. NonCryst. Solids 222 (1997) 153. [3] J.C. Phillips, J. Non-Cryst. Solids 34 (1979) 153. [4] M.F. Thorpe, J. Non-Cryst. Solids 57 (1983) 355. [5] M. Tatsumisago, B.L. Halfpap, J.L. Green, S.M. Lindsay, C.A. Angell, Phys. Rev. Lett. 64 (1990) 1549. [6] K. Tanaka, Phys. Rev. B 39 (1989) 1270. [7] B.G. Aitken, C.W. Ponader, J. Non-Cryst. Solids 274 (2000) 124. [8] N. Binsted, S.J. Gurman, J.C. Campbell, SERC Daresbury Lab. EXCURV092 Program, 1992. [9] S.J. Gurman, N. Binsted, I. Ross, J. Phys. C 17 (1984) 143. [10] R.W. Joyner, K.J. Martin, P. Meehan, J. Phys. C 20 (1987) 4005. [11] G. Dittmar, N. Schafer, Acta Crystallogr. B 31 (1975) 2060. [12] T. Ito, N. Morimoto, R. Sadanaga, Acta Crystallogr. 5 (1952) 775. [13] N. Morimoto, Mineral. J. 1 (1954) 160.