- Email: [email protected]
The local atomic arrangement in the amorphous As–Te–I system
Journal of Non-Crystalline Solids 232±234 (1998) 682±687
The local atomic arrangement in the amorphous As±Te±I system Keigo Abe a, Osamu Uemura a,*, Takeshi Usuki a, Yasuo Kameda a, Masaki Sakurai b a
Department of Material and Biological Chemistry, Faculty of Science, Yamagata University,Yamagata 990, Japan b Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980, Japan
Abstract Raman spectroscopic, X-ray diraction and extended X-ray absorption ®ne structure measurements have been carried out on the amorphous (As0:5 Te0:5 )1ÿx Ix system with 0 6 x 6 0:3. AsI3 molecular units and mixed-anion structural units are not formed in the system. The I atoms incorporated into amorphous As0:5 Te0:5 are preferentially bonded to Te atoms and terminate the As±Te network structure. The bond distances for all atomic pairs contained in the ®rst coordination shell remain almost constant for all compositions studied, being rAs±As 0.252 0.001, rAs±Te 0.265 0.002 and rTe±I 0.274 0.002 nm. These numbers agree well with those determined by curve ®tting the EXAFS oscillation functions. The total coordination numbers of As, Te and I atoms in the system are determined to be three, two and unity, respectively. Ó 1998 Published by Elsevier Science B.V All rights reserved.
1. Introduction The structural and electrical properties of amorphous chalcohalide systems, containing S, Se and I atoms, have frequently been investigated from the point of view of the exploitation of materials transmitting in the mid-infrared region [1]. Following previous structural studies of these systems, amorphous chalcohalides can be classi®ed into two groups in which (1) mixed-anion structural units, such as Ge(S 4ÿn=2 In ) tetrahedra in amorphous Ge±X±I systems (X: S, Se) [2,3], are formed, because of a similarity in bonding between the metallic atoms and the chalcogen or halogen atoms, and (2) both chalcogenide and halide units are sepa-
rately present, e.g., I atoms incorporated into amorphous As±X systems form discrete AsI3 molecular units, with a partial ionicity, in the system As±X±I [4,5]. However, in contrast to the sul®de and selenide systems, structural studies of the short-range order in chalcohalide glasses containing Te atoms, which have many interesting properties such as threshold switching and memory eects [6], are at present limited. We describe the results of Raman spectroscopic, X-ray diraction and EXAFS measurements in the amorphous (As0:5 Te0:5 )1ÿx Ix system, with 0:0 6 x 6 0:30, and the use of this data in determining the structural units formed. 2. Experimental procedures
* Corresponding author. Tel.: 81 236 284580; fax: 81 236 284591; e-mail: [email protected].
Amorphous (As0:5 Te0:5 )1ÿx Ix samples, with x 0, 0.05, 0.10, 0.15, 0.20 and 0.30, were prepared
0022-3093/98/$19.00 Ó 1998 Published by Elsevier Science B.V All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 4 3 7 - 2
K. Abe et al. / Journal of Non-Crystalline Solids 232±234 (1998) 682±687
by heating the required amounts of the elemental materials (all 99.99% purity) at 800°C in an evacuated quartz ampoule and subsequently quenching the ampoule containing the liquid into an ice±water bath. The Raman spectra for the amorphous samples were measured in the frequency range 90±500 cmÿ1 , using a Raman spectrometer (Jasco NR1100) and excitation with Ar ion laser radiation (514.5 nm) at a power of 100 mW. The calibration of the monochromator was carried out using 89 neon emission lines. The scattered intensities were recorded at 1 cmÿ1 intervals with a scan speed of 12 cmÿ1 /min. Each run was repeated 20 times to accumulate the required data. X-ray diraction measurements were made using a h±h re¯ection-type goniometer (Rigaku) with Mo-Ka radiation, operated at 50 kV and 35 mA. The scattered intensities were accumulated with a counting time of 100 s at scattering angle, 2h, intervals of 0.2° over the range 3 6 2h 6 150 , corresponding to scattering vector magnitudes, Q, (Q 4p sin h/k, where k is the X-ray wavelength) ÿ1 . Each scan was repeatbetween 0.46 and 17.1 A ed twice, to minimize long-term instrumental drift. The procedures for the experimental corrections and the normalization of the intensity data are similar to those described previously [3]. EXAFS measurements around the As K-edge were performed using the BL-10B station at KEKPF (Tsukuba, Japan), with a silicon (3 1 1) channel-cut monochromator calibrated with the K-absorption edge of a standard Cu foil (E 8980.3 eV). The storage ring was operated at 2.5 GeV with a maximum beam current of 350 mA. The intensity of the incident beam, I0 , and that of the transmitted beam, I, were measured using two ionization chambers ®lled with a N2 (85)±Ar(15) mixture for I0 and with a N2 (50)±Ar(50) mixture for I. All spectra were taken around the As K-edge (E 11865.0 eV), covering the energy range between 11370 and ÿ1 at room temperature. 12970 eV kmax 17:0 A The random errors in measuring these energies was 0.5 eV (?). The random errors in calculating ion distances was 0.002 nm (?).
683
3. Results The Raman spectra for the amorphous (As0:5 Te0:5 )1ÿx Ix system are presented in Fig. 1. The Raman bands located at 155 and 182 cmÿ1 , related to the structural units formed by As and Te (As±Te units), are observed at all compositions and decrease in amplitude with increasing atomic fraction of I, x. However, the Raman band at 210 cmÿ1 , attributed to discrete AsI3 molecular units, which has been observed in the amorphous As±S(or Se)±I systems [5], does not appear in the present As±Te±I system. In addition, the Raman intensity around 110 cmÿ1 increases in the I-rich composition range, possibly due to the formation of Te±I bonds. Fig. 2 shows the structure factors, S(Q), for the amorphous (As0:5 Te0:5 )1ÿx Ix system. A pre-peak in ÿ1 in the spectrum of the S(Q) at about Q 1.2 A
Fig. 1. Raman spectra for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.30).
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K. Abe et al. / Journal of Non-Crystalline Solids 232±234 (1998) 682±687
Fig. 2. Structure factors, S(Q), for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.30). The smoothed curves were used for the Fourier transformation.
sample, As0:5 Te0:5 , which is related to the mediumrange order, disappears with increasing x. Fig. 3 gives the pair distribution functions, g(r), obtained from the Fourier transformation of S(Q). The well-resolved ®rst peak is obtained at 0.263 nm for As0:5 Te0:5 . With increasing x, this peak shifts consistently to larger r. This shift implies the absence of AsI3 molecular units in the system, because the As±I distance in a molecular AsI3 crystal has been reported to be 0.259 nm [7], which is shorter than the As±Te distance. The shift of the ®rst peak in g(r) may be due to an increase in the longer Te±I correlation. Fig. 4 shows the radial distribution functions, |F(r)|, obtained by the Fourier transformation of the EXAFS oscillation functions, k3 v(k), around As K-edge. The functional form of the main peak in |F(r)| is apparently asymmetric and does not change with regard to its position or shape with x, implying that the shorter As±As bond is includ-
Fig. 3. Pair distribution functions, g(r), for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.30).
ed in the ®rst coordination shell at all of the compositions investigated. The curve ®tting analysis was applied to the ®ltered spectra using the following standard equation [8], X NSf k exp ÿ2r2 k 2 k 3 v k k 3 sin 2kr u k=kr2 ;
1
which describes the EXAFS oscillations for a Gaussian atomic distribution centered at a given atom. N, S, f(k), r, u(k) and r in Eq. (1) represent the coordination number, scaling factor, backscattering amplitude, thermal oscillation factor, phaseshift function and bond distance for a given coordination shell, respectively. The total atomic phase shifts and the backscattering amplitudes derived theoretically by Teo and Lee [9] were employed in the present curve ®tting. The best-®t curve is obtained assuming that two atomic pair correlations, namely As±As and As±Te, contribute to the ®rst coordination shell. The results for the bond
K. Abe et al. / Journal of Non-Crystalline Solids 232±234 (1998) 682±687
685
EXAFS experiment because of the lack of a suitable reference material. 4. Discussion
Fig. 4. Radial distribution functions, |F(r)|, obtained by Fourier transformation of the EXAFS oscillation functions, k3 v(k), around As K-edge for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.20).
distances are listed in Table 1. The As±As bond distance, rAs±As 0.251 0.002 nm, is nearly equal to that in amorphous As [10] and liquid As±Te alloys [11] and is somewhat shorter than that in the cradle-type As4 S4 units in crystalline AsS [12]. The As±Te bond distance, rAs±Te 0.266 0.002, is also in good agreement with that in liquid As±Te alloys. Unfortunately, a determination of the coordination number was impossible in the present
Table 1 As±As and As±Te bond distances obtained by curve ®tting analysis of the EXAFS oscillation functions, k3 v(k), around As K-edge for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0± 0.20) x rAs±As rAs±Te
(A) (A)
0
0.05
0.10
0.15
0.20
2.50(2) 2.65(2)
2.50(2) 2.66(2)
2.50(2) 2.66(2)
2.52(2) 2.67(2)
2.52(2) 2.66(2)
Raman spectra in the amorphous (As0:5 Te0:5 )1ÿx Ix system indicate that AsI3 molecular units are not present, unlike the case of the amorphous As±S (or Se)±I systems. Moreover, no indication of the formation of mixed-anion structural units could be found from the present Raman scattering, X-ray diraction and EXAFS data. In contrast, the incorporation of I atoms into amorphous As0:5 Te0:5 results in an increase in the number of Te±I bonds. Therefore, it is reasonable to assume that the ®rst peak of g(r), obtained from the X-ray diraction measurements, is composed of As±As, As±Te and Te±I correlations and so we have attempted to determine the structural parameters for these correlations with the aid of a least-squares ®t to the observed S(Q). The contribution of the correlations for the i±j pair to the total structure factor can be expressed as ci ni±j fi fj Si±j Q 1 2 ÿ dij ÿ P 2 i ci fi 1 2 2 sin ri±j Q ; exp ÿ li±j Q 2 ri±j Q
2
where ri±j , li±j and ni±j denote, respectively, the interatomic distance, the root mean square displacement of the i±j pair and the number of j atoms around a given i atom and dij 1 for i j and 0 for i 6 j. The functional form of S(Q) in the larger Q region is mainly controlled by the atomic pair correlations forming the ®rst coordination shell and its Fourier transformation gives the ®rst peak in g(r). Therefore, the structural parameters for the three correlations can be determined by ®tting the sum of theoretical values to the observed S(Q) ÿ1 : The results are summarized in at Q P 8:0 A Table 2 and a typical ®t is given in Fig. 5. The rAs±As and rAs±Te are in good agreement with those obtained from the EXAFS study, within the ®tting accuracy. The bond distances for all of the atomic pairs contained in the ®rst coordination shell re-
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K. Abe et al. / Journal of Non-Crystalline Solids 232±234 (1998) 682±687
Table 2 Structural parameters, riÿj , liÿj and niÿj for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.30) x
0 0.05 0.10 0.15 0.20 0.30
As±As
As±Te
Te±I
rAs±As (A)
lAs±As (A)
nAs±As
rAs±Te (A)
lAs±Te (A)
nAs±Te
rTe±I (A)
lTe±I (A)
nTe±I
2.51(1) 2.52(1) 2.52(1) 2.52(1) 2.53(1) 2.55(2)
0.105(3) 0.105(3) 0.101(2) 0.104(3) 0.101(2) 0.101(2)
1.03(3) 1.10(4) 1.18(4) 1.34(4) 1.45(4) 1.85(4)
2.66(2) 2.65(2) 2.65(2) 2.64(1) 2.64(2) 2.64(2)
0.124(3) 0.118(2) 0.118(2) 0.122(3) 0.116(3) 0.118(2)
2.03(3) 1.89(3) 1.81(2) 1.70(4) 1.60(3) 1.23(3)
) 2.75(2) 2.74(1) 2.76(2) 2.74(3) 2.73(2)
) 0.124(2) 0.124(2) 0.126(3) 0.118(3) 0.120(2)
) 0.10(3) 0.25(2) 0.42(2) 0.59(3) 0.87(4)
Fig. 5. A typical example of the ®tting analysis for amorphous As0:5 Te0:5 . The dots and lines indicate, respectively, the experimental and theoretical values for both S(Q) and g(r).
main almost constant at any composition and are somewhat longer than the sum of the covalent radii of atoms i and j. However, rAs±As seems to increase with increasing x for x > 0.15, perhaps due to a change in the electron con®guration of the Te atoms caused by an increased number of Te±I bonds. The nAs±As , nAs±Te , and nTe±I are plotted against x in Fig. 6, together with the corresponding theoretical values, which are calculated under the following assumptions: (1) all of the I atoms added participate in the formation of Te±I bonds, (2) the remaining Te atoms coordinate to As atoms, (3) the remaining As atoms contribute to As±As
Fig. 6. The coordination numbers, nAs±As (triangles), nAs±Te (crosses) and nTe±I (squares), together with the corresponding theoretical values (lines). The error bars are within the size of the point symbols.
bonds, and (4) As, Te and I atoms are trivalent, divalent and monovalent, respectively, at all compositions. The coordination numbers of three correlations can then be expressed as 1x 2 ÿ 4x ; nAs±Te ; 1ÿx 1ÿx 2x : 3 nTe±I 1ÿx The theoretical values are in good agreement with experiment, within the errors, implying again that the I atoms incorporated into amorphous As0:5 Te0:5 are preferentially bonded to Te atoms. These structural features are consistent with the composition dependence of the electrical conductivities of the amorphous (As0:5 Te0:5 )1ÿx Ix system nAs±As
K. Abe et al. / Journal of Non-Crystalline Solids 232±234 (1998) 682±687 Table 3 Total coordination numbers of As, Te and I atoms in the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.30) x
0
0.05
0.10
0.15
0.20
0.30
nAs nTe nI
3.06 2.03 )
2.99 1.99 0.95
2.99 2.06 1.12
3.09 2.12 1.19
3.11 2.19 1.18
3.08 2.10 1.01
[6], for which the electrical conductivity decreases monotonically with increasing I content. The total coordination numbers of the As, Te and I atoms, nAs , nTe and nI , can be obtained from the equations nAs nAs±As nAs±Te ; nTe nTe±I cAs nAs±Te =cTe nI cTe nTe±I =cI ;
and 4
respectively. Table 3 summarizes these total coordination numbers for the present system, for which we ®nd nAs 3, nTe 2 and n1 1 at any composition. These coordinations suggest that the I atoms terminate the As±Te network structure by combining monovalently with Te atoms. As mentioned in the Introduction, whether the system forms mixed-anion or As-halide type molecular structural units has been considered to be determined by the bonding between metallic atoms and the chalcogen or halogen atoms. Both the amorphous As±S±I and As±Se±I systems possess the As-halide type structure. On the other hand, several dierent results have been obtained during the present study which indicate that the amorphous As±Te±I system forms neither mixed-anion nor As-halide molecular structural units, but that the I atoms are preferentially bonded to Te atoms. Te is the most metallic element among the chalcogen group and its electronegativity is almost the same as that of As. Therefore, it may reasonably be assumed that the Te atoms behave as electropositive constituents in the amorphous As±Te±I system. This property makes it possible to form the bonds between Te and I atoms. However, why the I atoms are not coordinated by As atoms in the system is not known. To investigate the local atomic arrangement, characteristic of the Te and I atoms, in the amorphous As±Te±I system more fully, it is desirable to
687
ossbauer spectrosperform, for example, 125 Te M copy and to carry out structural studies of chalcohalide systems containing Te and Br (or Cl) atoms.
5. Conclusions The present results from Raman spectroscopic, X-ray diraction and EXAFS measurements for the amorphous (As0:5 Te0:5 )1ÿx Ix system, with x 0±0.3, may be summarized as follows: (1) AsI3 molecular units and mixed-anion structural units are not formed in the system. Iodine atoms incorporated into amorphous As0:5 Te0:5 are preferentially bonded to the Te atoms and act to terminate the As±Te network structure. (2) The bond distances for all of the atomic pairs contained in the ®rst coordination shell remain almost constant at all of the compositions studied, the values being rAs±As 0.252 0.001, rAs±Te 0.265 0.002 and rTe±I 0.274 0.002 nm. These numbers agree well with those determined by curve ®tting to the EXAFS oscillation functions. (3) The coordination numbers of As, Te and I atoms in the system are determined to be roughly three, two and unity, respectively.
References [1] C.M. Baldwin, R.M. Almeida, J.D. Mackenzie, J. NonCryst. Solids 43 (1981) 309. [2] J.S. Sanghera, J. Heo, J.D. Mackenzie, J. Non-Cryst. Solids 103 (1988) 155. [3] Y. Nagata, S. Kokai, O. Uemura, Y. Kameda, J. NonCryst. Solids 169 (1993) 104. [4] Y. Kameda, Y. Sugawara, O. Uemura, J. Non-Cryst. Solids 156±158 (1993) 725. [5] K. Saitoh, O. Uemura, T. Usuki, Y. Kameda, J. NonCryst. Solids 192&193 (1995) 286. [6] V.V. Khiminets, V.I. Bazhan, Izv. Akad. Nauk. SSSR 20 (1984) 2035. [7] R. Enjalbert, J. Galy, Acta Crystallogr. B 36 (1980) 914. [8] P.A. Lee, G. Beni, Phys. Rev. B 15 (1977) 2862. [9] B.K. Teo, P.A. Lee, J. Amer. Chem. Soc. 101 (1979) 2815. [10] R. Bellissent, G. Tourand, J. Phys. (Paris) 37 (1976) 1423. [11] S. Hosokawa, K. Tamura, M. Inui, H. Endo, J. Non-Cryst. Solids 156±158 (1993) 712. [12] D.J.E. Mullen, W. Nowcki, Z. Kristallogr. 136 (1972) 48.
The local atomic arrangement in the amorphous As±Te±I system Keigo Abe a, Osamu Uemura a,*, Takeshi Usuki a, Yasuo Kameda a, Masaki Sakurai b a
Department of Material and Biological Chemistry, Faculty of Science, Yamagata University,Yamagata 990, Japan b Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980, Japan
Abstract Raman spectroscopic, X-ray diraction and extended X-ray absorption ®ne structure measurements have been carried out on the amorphous (As0:5 Te0:5 )1ÿx Ix system with 0 6 x 6 0:3. AsI3 molecular units and mixed-anion structural units are not formed in the system. The I atoms incorporated into amorphous As0:5 Te0:5 are preferentially bonded to Te atoms and terminate the As±Te network structure. The bond distances for all atomic pairs contained in the ®rst coordination shell remain almost constant for all compositions studied, being rAs±As 0.252 0.001, rAs±Te 0.265 0.002 and rTe±I 0.274 0.002 nm. These numbers agree well with those determined by curve ®tting the EXAFS oscillation functions. The total coordination numbers of As, Te and I atoms in the system are determined to be three, two and unity, respectively. Ó 1998 Published by Elsevier Science B.V All rights reserved.
1. Introduction The structural and electrical properties of amorphous chalcohalide systems, containing S, Se and I atoms, have frequently been investigated from the point of view of the exploitation of materials transmitting in the mid-infrared region [1]. Following previous structural studies of these systems, amorphous chalcohalides can be classi®ed into two groups in which (1) mixed-anion structural units, such as Ge(S 4ÿn=2 In ) tetrahedra in amorphous Ge±X±I systems (X: S, Se) [2,3], are formed, because of a similarity in bonding between the metallic atoms and the chalcogen or halogen atoms, and (2) both chalcogenide and halide units are sepa-
rately present, e.g., I atoms incorporated into amorphous As±X systems form discrete AsI3 molecular units, with a partial ionicity, in the system As±X±I [4,5]. However, in contrast to the sul®de and selenide systems, structural studies of the short-range order in chalcohalide glasses containing Te atoms, which have many interesting properties such as threshold switching and memory eects [6], are at present limited. We describe the results of Raman spectroscopic, X-ray diraction and EXAFS measurements in the amorphous (As0:5 Te0:5 )1ÿx Ix system, with 0:0 6 x 6 0:30, and the use of this data in determining the structural units formed. 2. Experimental procedures
* Corresponding author. Tel.: 81 236 284580; fax: 81 236 284591; e-mail: [email protected].
Amorphous (As0:5 Te0:5 )1ÿx Ix samples, with x 0, 0.05, 0.10, 0.15, 0.20 and 0.30, were prepared
0022-3093/98/$19.00 Ó 1998 Published by Elsevier Science B.V All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 4 3 7 - 2
K. Abe et al. / Journal of Non-Crystalline Solids 232±234 (1998) 682±687
by heating the required amounts of the elemental materials (all 99.99% purity) at 800°C in an evacuated quartz ampoule and subsequently quenching the ampoule containing the liquid into an ice±water bath. The Raman spectra for the amorphous samples were measured in the frequency range 90±500 cmÿ1 , using a Raman spectrometer (Jasco NR1100) and excitation with Ar ion laser radiation (514.5 nm) at a power of 100 mW. The calibration of the monochromator was carried out using 89 neon emission lines. The scattered intensities were recorded at 1 cmÿ1 intervals with a scan speed of 12 cmÿ1 /min. Each run was repeated 20 times to accumulate the required data. X-ray diraction measurements were made using a h±h re¯ection-type goniometer (Rigaku) with Mo-Ka radiation, operated at 50 kV and 35 mA. The scattered intensities were accumulated with a counting time of 100 s at scattering angle, 2h, intervals of 0.2° over the range 3 6 2h 6 150 , corresponding to scattering vector magnitudes, Q, (Q 4p sin h/k, where k is the X-ray wavelength) ÿ1 . Each scan was repeatbetween 0.46 and 17.1 A ed twice, to minimize long-term instrumental drift. The procedures for the experimental corrections and the normalization of the intensity data are similar to those described previously [3]. EXAFS measurements around the As K-edge were performed using the BL-10B station at KEKPF (Tsukuba, Japan), with a silicon (3 1 1) channel-cut monochromator calibrated with the K-absorption edge of a standard Cu foil (E 8980.3 eV). The storage ring was operated at 2.5 GeV with a maximum beam current of 350 mA. The intensity of the incident beam, I0 , and that of the transmitted beam, I, were measured using two ionization chambers ®lled with a N2 (85)±Ar(15) mixture for I0 and with a N2 (50)±Ar(50) mixture for I. All spectra were taken around the As K-edge (E 11865.0 eV), covering the energy range between 11370 and ÿ1 at room temperature. 12970 eV kmax 17:0 A The random errors in measuring these energies was 0.5 eV (?). The random errors in calculating ion distances was 0.002 nm (?).
683
3. Results The Raman spectra for the amorphous (As0:5 Te0:5 )1ÿx Ix system are presented in Fig. 1. The Raman bands located at 155 and 182 cmÿ1 , related to the structural units formed by As and Te (As±Te units), are observed at all compositions and decrease in amplitude with increasing atomic fraction of I, x. However, the Raman band at 210 cmÿ1 , attributed to discrete AsI3 molecular units, which has been observed in the amorphous As±S(or Se)±I systems [5], does not appear in the present As±Te±I system. In addition, the Raman intensity around 110 cmÿ1 increases in the I-rich composition range, possibly due to the formation of Te±I bonds. Fig. 2 shows the structure factors, S(Q), for the amorphous (As0:5 Te0:5 )1ÿx Ix system. A pre-peak in ÿ1 in the spectrum of the S(Q) at about Q 1.2 A
Fig. 1. Raman spectra for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.30).
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K. Abe et al. / Journal of Non-Crystalline Solids 232±234 (1998) 682±687
Fig. 2. Structure factors, S(Q), for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.30). The smoothed curves were used for the Fourier transformation.
sample, As0:5 Te0:5 , which is related to the mediumrange order, disappears with increasing x. Fig. 3 gives the pair distribution functions, g(r), obtained from the Fourier transformation of S(Q). The well-resolved ®rst peak is obtained at 0.263 nm for As0:5 Te0:5 . With increasing x, this peak shifts consistently to larger r. This shift implies the absence of AsI3 molecular units in the system, because the As±I distance in a molecular AsI3 crystal has been reported to be 0.259 nm [7], which is shorter than the As±Te distance. The shift of the ®rst peak in g(r) may be due to an increase in the longer Te±I correlation. Fig. 4 shows the radial distribution functions, |F(r)|, obtained by the Fourier transformation of the EXAFS oscillation functions, k3 v(k), around As K-edge. The functional form of the main peak in |F(r)| is apparently asymmetric and does not change with regard to its position or shape with x, implying that the shorter As±As bond is includ-
Fig. 3. Pair distribution functions, g(r), for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.30).
ed in the ®rst coordination shell at all of the compositions investigated. The curve ®tting analysis was applied to the ®ltered spectra using the following standard equation [8], X NSf k exp ÿ2r2 k 2 k 3 v k k 3 sin 2kr u k=kr2 ;
1
which describes the EXAFS oscillations for a Gaussian atomic distribution centered at a given atom. N, S, f(k), r, u(k) and r in Eq. (1) represent the coordination number, scaling factor, backscattering amplitude, thermal oscillation factor, phaseshift function and bond distance for a given coordination shell, respectively. The total atomic phase shifts and the backscattering amplitudes derived theoretically by Teo and Lee [9] were employed in the present curve ®tting. The best-®t curve is obtained assuming that two atomic pair correlations, namely As±As and As±Te, contribute to the ®rst coordination shell. The results for the bond
K. Abe et al. / Journal of Non-Crystalline Solids 232±234 (1998) 682±687
685
EXAFS experiment because of the lack of a suitable reference material. 4. Discussion
Fig. 4. Radial distribution functions, |F(r)|, obtained by Fourier transformation of the EXAFS oscillation functions, k3 v(k), around As K-edge for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.20).
distances are listed in Table 1. The As±As bond distance, rAs±As 0.251 0.002 nm, is nearly equal to that in amorphous As [10] and liquid As±Te alloys [11] and is somewhat shorter than that in the cradle-type As4 S4 units in crystalline AsS [12]. The As±Te bond distance, rAs±Te 0.266 0.002, is also in good agreement with that in liquid As±Te alloys. Unfortunately, a determination of the coordination number was impossible in the present
Table 1 As±As and As±Te bond distances obtained by curve ®tting analysis of the EXAFS oscillation functions, k3 v(k), around As K-edge for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0± 0.20) x rAs±As rAs±Te
(A) (A)
0
0.05
0.10
0.15
0.20
2.50(2) 2.65(2)
2.50(2) 2.66(2)
2.50(2) 2.66(2)
2.52(2) 2.67(2)
2.52(2) 2.66(2)
Raman spectra in the amorphous (As0:5 Te0:5 )1ÿx Ix system indicate that AsI3 molecular units are not present, unlike the case of the amorphous As±S (or Se)±I systems. Moreover, no indication of the formation of mixed-anion structural units could be found from the present Raman scattering, X-ray diraction and EXAFS data. In contrast, the incorporation of I atoms into amorphous As0:5 Te0:5 results in an increase in the number of Te±I bonds. Therefore, it is reasonable to assume that the ®rst peak of g(r), obtained from the X-ray diraction measurements, is composed of As±As, As±Te and Te±I correlations and so we have attempted to determine the structural parameters for these correlations with the aid of a least-squares ®t to the observed S(Q). The contribution of the correlations for the i±j pair to the total structure factor can be expressed as ci ni±j fi fj Si±j Q 1 2 ÿ dij ÿ P 2 i ci fi 1 2 2 sin ri±j Q ; exp ÿ li±j Q 2 ri±j Q
2
where ri±j , li±j and ni±j denote, respectively, the interatomic distance, the root mean square displacement of the i±j pair and the number of j atoms around a given i atom and dij 1 for i j and 0 for i 6 j. The functional form of S(Q) in the larger Q region is mainly controlled by the atomic pair correlations forming the ®rst coordination shell and its Fourier transformation gives the ®rst peak in g(r). Therefore, the structural parameters for the three correlations can be determined by ®tting the sum of theoretical values to the observed S(Q) ÿ1 : The results are summarized in at Q P 8:0 A Table 2 and a typical ®t is given in Fig. 5. The rAs±As and rAs±Te are in good agreement with those obtained from the EXAFS study, within the ®tting accuracy. The bond distances for all of the atomic pairs contained in the ®rst coordination shell re-
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Table 2 Structural parameters, riÿj , liÿj and niÿj for the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.30) x
0 0.05 0.10 0.15 0.20 0.30
As±As
As±Te
Te±I
rAs±As (A)
lAs±As (A)
nAs±As
rAs±Te (A)
lAs±Te (A)
nAs±Te
rTe±I (A)
lTe±I (A)
nTe±I
2.51(1) 2.52(1) 2.52(1) 2.52(1) 2.53(1) 2.55(2)
0.105(3) 0.105(3) 0.101(2) 0.104(3) 0.101(2) 0.101(2)
1.03(3) 1.10(4) 1.18(4) 1.34(4) 1.45(4) 1.85(4)
2.66(2) 2.65(2) 2.65(2) 2.64(1) 2.64(2) 2.64(2)
0.124(3) 0.118(2) 0.118(2) 0.122(3) 0.116(3) 0.118(2)
2.03(3) 1.89(3) 1.81(2) 1.70(4) 1.60(3) 1.23(3)
) 2.75(2) 2.74(1) 2.76(2) 2.74(3) 2.73(2)
) 0.124(2) 0.124(2) 0.126(3) 0.118(3) 0.120(2)
) 0.10(3) 0.25(2) 0.42(2) 0.59(3) 0.87(4)
Fig. 5. A typical example of the ®tting analysis for amorphous As0:5 Te0:5 . The dots and lines indicate, respectively, the experimental and theoretical values for both S(Q) and g(r).
main almost constant at any composition and are somewhat longer than the sum of the covalent radii of atoms i and j. However, rAs±As seems to increase with increasing x for x > 0.15, perhaps due to a change in the electron con®guration of the Te atoms caused by an increased number of Te±I bonds. The nAs±As , nAs±Te , and nTe±I are plotted against x in Fig. 6, together with the corresponding theoretical values, which are calculated under the following assumptions: (1) all of the I atoms added participate in the formation of Te±I bonds, (2) the remaining Te atoms coordinate to As atoms, (3) the remaining As atoms contribute to As±As
Fig. 6. The coordination numbers, nAs±As (triangles), nAs±Te (crosses) and nTe±I (squares), together with the corresponding theoretical values (lines). The error bars are within the size of the point symbols.
bonds, and (4) As, Te and I atoms are trivalent, divalent and monovalent, respectively, at all compositions. The coordination numbers of three correlations can then be expressed as 1x 2 ÿ 4x ; nAs±Te ; 1ÿx 1ÿx 2x : 3 nTe±I 1ÿx The theoretical values are in good agreement with experiment, within the errors, implying again that the I atoms incorporated into amorphous As0:5 Te0:5 are preferentially bonded to Te atoms. These structural features are consistent with the composition dependence of the electrical conductivities of the amorphous (As0:5 Te0:5 )1ÿx Ix system nAs±As
K. Abe et al. / Journal of Non-Crystalline Solids 232±234 (1998) 682±687 Table 3 Total coordination numbers of As, Te and I atoms in the amorphous (As0:5 Te0:5 )1ÿx Ix system (x 0±0.30) x
0
0.05
0.10
0.15
0.20
0.30
nAs nTe nI
3.06 2.03 )
2.99 1.99 0.95
2.99 2.06 1.12
3.09 2.12 1.19
3.11 2.19 1.18
3.08 2.10 1.01
[6], for which the electrical conductivity decreases monotonically with increasing I content. The total coordination numbers of the As, Te and I atoms, nAs , nTe and nI , can be obtained from the equations nAs nAs±As nAs±Te ; nTe nTe±I cAs nAs±Te =cTe nI cTe nTe±I =cI ;
and 4
respectively. Table 3 summarizes these total coordination numbers for the present system, for which we ®nd nAs 3, nTe 2 and n1 1 at any composition. These coordinations suggest that the I atoms terminate the As±Te network structure by combining monovalently with Te atoms. As mentioned in the Introduction, whether the system forms mixed-anion or As-halide type molecular structural units has been considered to be determined by the bonding between metallic atoms and the chalcogen or halogen atoms. Both the amorphous As±S±I and As±Se±I systems possess the As-halide type structure. On the other hand, several dierent results have been obtained during the present study which indicate that the amorphous As±Te±I system forms neither mixed-anion nor As-halide molecular structural units, but that the I atoms are preferentially bonded to Te atoms. Te is the most metallic element among the chalcogen group and its electronegativity is almost the same as that of As. Therefore, it may reasonably be assumed that the Te atoms behave as electropositive constituents in the amorphous As±Te±I system. This property makes it possible to form the bonds between Te and I atoms. However, why the I atoms are not coordinated by As atoms in the system is not known. To investigate the local atomic arrangement, characteristic of the Te and I atoms, in the amorphous As±Te±I system more fully, it is desirable to
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ossbauer spectrosperform, for example, 125 Te M copy and to carry out structural studies of chalcohalide systems containing Te and Br (or Cl) atoms.
5. Conclusions The present results from Raman spectroscopic, X-ray diraction and EXAFS measurements for the amorphous (As0:5 Te0:5 )1ÿx Ix system, with x 0±0.3, may be summarized as follows: (1) AsI3 molecular units and mixed-anion structural units are not formed in the system. Iodine atoms incorporated into amorphous As0:5 Te0:5 are preferentially bonded to the Te atoms and act to terminate the As±Te network structure. (2) The bond distances for all of the atomic pairs contained in the ®rst coordination shell remain almost constant at all of the compositions studied, the values being rAs±As 0.252 0.001, rAs±Te 0.265 0.002 and rTe±I 0.274 0.002 nm. These numbers agree well with those determined by curve ®tting to the EXAFS oscillation functions. (3) The coordination numbers of As, Te and I atoms in the system are determined to be roughly three, two and unity, respectively.
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