Short-range order in Ge–As–S–I glasses

Journal of Non-Crystalline Solids 232±234 (1998) 688±693

Short-range order in Ge±As±S±I glasses Takeshi Usuki a

a,* ,

Osamu Uemura a, Shigenori Iwabuchi a, Yasuo Kameda a, Masaki Sakurai b

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 di€raction and EXAFS measurements have been carried out on …As2 S3 †1ÿx …GeI4 †x glasses, with 0:0 6 x 6 0:4. The I atoms incorporated into the As2 S3 glass are preferentially bonded to As atoms and form AsI3 molecular units. Three structural units, AsS3=2 , AsI3 and partially I-substituted GeS4=2 units, are present in the system. The As±S and Ge±S bond distances are almost identical to their covalent distances, while the As±I distance is nearer to the ionic distance. The detection of Ge±I bonds from extended X-ray absorption ®ne structure (EXAFS) measurements infers the existence of mixed-anion type units, such as Ge(S3=2 I) tetrahedra. The coordination numbers of the constituent atoms in the system, obtained by least squares analysis, are approximately 4 for Ge, 3 for As, 2 for S and 1 for I. Ó 1998 Published by Elsevier Science B.V. All rights reserved.

1. Introduction Interesting results have been reported from recent structural investigations [1±4], that there is a signi®cant di€erence in bonding properties between Ge- and As-chalcohalide glasses, into which halogen atoms are incorporated. For example, Ge±S±I glasses form mixed-anion structural units involving a central Ge atom, such as Ge(S…4ÿn†=2 In ) tetrahedra with n being 1, 2, and 3 [1,2]. On the other hand, AsS3=2 pyramidal units and AsI3 molecular units are separately present in As±S±I glasses [3,4]. However, at present it is not known why such a di€erence occurs. For this reason, it is very interesting to investigate the structural units formed in chalcohalide glasses in which Ge and As atoms coexist as the metallic el-

* Corresponding author. Tel.: 81 236 284582; fax: 81 236 284591; e-mail: [email protected].

ements. The purpose of this paper is to describe the results of Raman spectroscopic, X-ray di€raction and extended X-ray absorption ®ne structure (EXAFS) measurements for …As2 S3 †1ÿx …GeI4 †x glasses, with x ˆ 0.0±0.4, and to investigate the details of the structural units present in these glasses. 2. Experimental procedures …As2 S3 †1ÿx …GeI4 †x glasses, with x ˆ 0.0±0.4, were prepared by heating a mixture of the required amounts of the elemental materials (all 99.99% purity) at 700°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 50±550 cmÿ1 , using a Raman spectrometer (JASCO NR1100) and excitation with an Ar ion laser (514.5 nm) operated at 100 mW. The calibration of the

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 8 - 4

T. Usuki et al. / Journal of Non-Crystalline Solids 232±234 (1998) 688±693

monochromator was carried out using 89 neon emission lines. The scattering intensities were recorded at 1 cmÿ1 intervals, with a scan speed of 30 cmÿ1 /min. Each run was repeated 20 times to accumulate the required data. The X-ray di€raction measurements were made using a h±h re¯ection-type goniometer (Rigaku) with MoKa radiation, the X-ray tube being operated at 50 kV and 35 mA. The scattered intensities were recorded for 40 s, at intervals of 0.2° over the scattering angle, 2h, range 3° 6 2h 6 150°, corresponding to scattering vector magnitudes, Q, beÿ1 . Each scan was repeated tween 0.46 and 17.1 A twice, to minimize long-term instrumental drift. The procedures for the correction and normalization of the intensity data were similar to those described previously [5]. The EXAFS measurements at the Ge and As K-edges were performed at the BL-10B station of KEK-PF (Tsukuba, Japan), with a silicon (3 1 1) channel-cut monochromator, calibrated with the K-absorption edge of a standard Cu metal-foil. 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

Fig. 1. Raman spectra for the …As2 S3 †1ÿx …GeI4 †x glasses.

689

chambers ®lled with a N2 (85)±Ar(15) mixture for I0 and with a N2 (50)±Ar(50) mixture for I. All of the spectra were taken at room temperature, covering the energy range between 10 610 and ÿ1 ) for Ge K-edge, and 11 800 eV (kmax ˆ 13.5 A ÿ1 ) between 11 370 and 13 300 eV (kmax ˆ 19.4 A for As K-edge. 3. Results The Raman spectra for the …As2 S3 †1ÿx …GeI4 †x samples are presented in Fig. 1. A dominant band around 340 cmÿ1 for As2 S3 , which is assigned to the symmetrical stretching mode of the AsS3=2 structural unit [3,6,7], is observed in the spectral of the compositions investigated, while the intensity of this band decreases with increasing x. The central position of the band shifts to higher

Fig. 2. Structure factors, S(Q). The smoothed curves were used for the Fourier transformation.

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T. Usuki et al. / Journal of Non-Crystalline Solids 232±234 (1998) 688±693

frequency, with increasing x, due to the appearance of the symmetrical stretching mode of the GeS4=2 structural unit (m1 ˆ 345 cmÿ1 [5,8]). Another Raman band appears around 210 cmÿ1 , which is assigned to the symmetrical stretching mode of the AsI3 molecular unit [3,4] and increases in intensity with increasing x. However, the presence of S8 ring clusters (m1 ˆ 475 cmÿ1 [7,9]) and GeI4 molecular units (m1 ˆ 159 cmÿ1 [10]) were not observed in the spectra. Fig. 2 shows the structure factors, S(Q), for the same samples. The pre-peak in S(Q) at about ÿ1 for As2 S3 , which is related to the exisQ ˆ 1.2 A tence of medium-range order in the system, disappears with increasing x. Fig. 3 gives the pair distribution functions, g(r), obtained from the Fourier transformation of S(Q). A well-resolved ®rst peak is obtained at 0.23 nm for As2 S3 . For the samples with x ¹ 0, this peak splits into two peaks and the height of the new peak, located at

Fig. 3. Pair distribution functions, g(r).

0.26 nm increases with increasing x, due to the formation of As±I bonds in AsI3 molecular units. As may be seen from Fig. 4(a), these features are consistent with the concentration dependence of the radial distribution function, jF …r†j, obtained by Fourier transformation of the EXAFS oscillation function, k 3 v…k†, for the As K-edge. In addition, the main peak in jF …r†j for the Ge K-edge has a shoulder on the larger r side for x P 0.2 (Fig. 4(b)). This shoulder may be due to Ge±I

Fig. 4. Radial distribution functions, jF …r†j, obtained by the Fourier transformation of the EXAFS oscillation function, k 3 v…k†, around (a) the As and (b) the Ge K-edge.

T. Usuki et al. / Journal of Non-Crystalline Solids 232±234 (1998) 688±693 Table 1 Bond distances obtained by the curve ®tting from the EXAFS oscillation functions, k 3 v…k†; around the As and Ge K-edges x

0

0.2

0.3

0.4

 rAs±S (A)  rAs±I (A)  rGe±S (A)  rGe±I (A)

2.29(2) ) ) )

2.29(2) 2.61(2) 2.23(2) 2.53(2)

2.28(2) 2.60(2) 2.23(2) 2.54(2)

2.29(2) 2.61(2) 2.23(2) 2.54(2)

bonds in the system, although the vibrational mode of Ge±I bonds could not be distinctly assigned in the Raman spectra. A curve ®tting analysis was performed for the ®ltered EXAFS spectra, using the standard equation [11], X NSf …k† exp…ÿ2r2 k 2 † k 3 v…k† ˆ 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) denote 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 [12] were employed in the present curve ®tting. The best-®t curves were obtained assuming that two atomic pair correlations, namely As±S and As±I for the As K-edge, and Ge±S and Ge±I for the Ge K-edge, are contained in the ®rst coordination shell for the present samples. The results for the bond distances are listed in Table 1. These bond distances are in good agreement with those reported previously for the ternary As- or Ge-chalcohalide glasses [2,4,5]. Unfortunately, the determination of the coordination numbers for the ®rst coordination shell was impossible in the present EXAFS experiment, because no reference material was available.

691

and form AsI3 molecular units in a similar manner to the case of the As±S (or Se)±I chalcohalide glasses. Moreover, the existence of Ge±S bonds and small numbers of Ge±I bonds has been con®rmed by the EXAFS experiment around the Ge K-edge. Therefore, it is reasonable to assume that the ®rst peak in g(r), as obtained from the X-ray di€raction measurements, is composed of As±S, As±I, Ge±S and Ge±I correlations. Hence we attempt to determine the structural parameters for these correlations from a least-squares ®t to the observed S(Q). The contribution of the pair i±j to the total structure factor can be expressed as ci niÿj fi fj Siÿj …Q† ˆ 1 ‡ …2 ÿ dij † ÿ P
2 i ci fi   1 sin…riÿj Q† ; …2† exp ÿ l2iÿj Q2 2 riÿj Q where riÿj ; liÿj and niÿj denote respectively, the i±j interatomic distance, the root mean square displacement for the i±j pair and the number of j atoms around a given i atom. dij ˆ 1 for i ˆ j and 0 for i 6ˆ j. The functional form of S(Q) at larger Q is mainly determined by the atomic pair correlations contained in the ®rst coordination shell and its Fourier transform gives the ®rst peak in g(r). Therefore, the structural parameters for the four correlations can be determined by ®tting the sum of the theoretical functions to the observed S(Q) ÿ1 . The results are summarised in at Q P 8.0 A Table 2 and a typical example of the ®t obtained is shown in Fig. 5. The bond distances for all of the correlations are in good agreement with those obtained from the EXAFS study, within the accuracy of the ®t. These bond distances remain almost constant as a function of the composition. The total coordination numbers for As, S, Ge and I, nAs , nS , nGe and nI , respectively, can be obtained from the equations nAs ˆ nAs±S ‡ nAs±I ;

…3†

4. Discussion

nS ˆ …cAs nAs±S ‡ cGe nGe±S †=cS ;

…4†

The Raman spectra and di€raction data for the present …As2 S3 †1ÿx …GeI4 †x samples show that the I atoms are preferentially bonded to the As atoms

nGe ˆ nGe±S ‡ nGe±I ;

…5†

nI ˆ …cAs nAs±I ‡ cGe nGe±I †=cI :

…6†

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Table 2 Structural parameters, riÿj ; liÿj and niÿj obtained from the X-ray data x

As±S rAs±S

0 0.1 0.2 0.3 0.4

As±I  (A)

2.29(1) 2.29(1) 2.28(2) 2.29(1) 2.29(1)

lAs±S

 (A)

0.122(3) 0.135(3) 0.135(2) 0.128(3) 0.130(2)

Ge±S  (A)

nAs±S

rAs±I

2.92(3) 2.60(4) 2.42(4) 2.20(4) 1.77(4)

) 2.62(2) 2.62(2) 2.62(1) 2.63(2)

lAs±I

 (A)

) 0.118(2) 0.118(2) 0.120(3) 0.122(3)

nAs±I

rGe±S

) 0.25(3) 0.52(2) 0.85(4) 1.20(3)

) 2.24(1) 2.23(2) 2.23(1) 2.23(1)

Table 3 summarises these total coordination numbers for the present system, in which we ®nd nAs  3; nS  2; nGe  4 and nI  1 at any composition. We suggest that this fact shows the valence state of the constituent atoms remain unchanged over our composition range, in spite of the variation of the local atomic arrangement with x. On the basis of the above results, theoretical values of the coordination numbers can be calculated assuming that (a) nGe ˆ 4; nAs ˆ 3; nS ˆ 2 and nI ˆ 1 at any composition, (b) most of the I atoms incorporated into the glass preferentially participate in the formation of AsI3 molecular units and (c) the displaced S atoms and the remaining small number of I atoms contribute to GeS4=2 units which are partially substituted by I atoms. In this case, if the average coordination number of I atoms around Ge can be represented by y,

Fig. 5. A typical example of a ®t for the As2 S3 glass. The dots and lines denote the experimental and theoretical values for both S(Q) and g(r), respectively.

Ge±I

 lGe±S (A)  (A) ) 0.124(3) 0.124(2) 0.126(3) 0.125(2)

nGe±S

 rGe±I (A)

 lGe±I (A)

nGe±I

) 3.50(4) 3.60(4) 3.72(4) 3.70(4)

) 2.53(2) 2.53(2) 2.53(2) 2.53(2)

) 0.128(2) 0.133(2) 0.128(3) 0.128(3)

) 0.40(9) 0.30(9) 0.30(9) 0.30(9)

the coordination numbers for the four correlations can be expressed, respectively, as nAs±S ˆ 3 ÿ nAs±I ˆ

…4 ÿ y†x ; 2…1 ÿ x†

…4 ÿ y†x ; 2…1 ÿ x†

nGe±S ˆ 4 ÿ y; nGe±I ˆ y:

…7† …8† …9† …10†

The experimental nAs±S , nAs±I , nGe±S and nGe±I are plotted against x in Fig. 6, together with the corresponding theoretical values from Eqs. (7)±(10) with y ˆ 0.3. The experimental numbers are in good agreement with the theoretical ones, suggesting that the above assumptions are reasonable, i.e. that there exist AsS3=2 network units, AsI3 molecular units and partially mixed-anion GeS4=2 units in these …As2 S3 †1ÿx …GeI4 †x glasses. According to the present results, the structural features of As- and Ge-chalcohalide glasses are also preserved in the glasses containing both As and Ge atoms as the metallic elements, i.e. the As atoms principally form discrete molecular units with halogen anions, whereas the Ge atoms are coordinated by both the chalcogen and halogen anions. These structural tendencies are a€ected by the bonding between the metallic atoms and the chalcogen or halogen atoms. It has been shown by the present study that rAs±S , rGe±S and rGe±I are almost identical to the corresponding sum of covalent radii, 0.222, 0.226, and 0.250 nm, respectively [13], while rAs±I is somewhat longer than the covalent distance (0.246 nm) and is nearer to the ionic r, 0.263 nm [13]. Therefore, this di€erence in the bonding between the As±S and As±I pairs may

T. Usuki et al. / Journal of Non-Crystalline Solids 232±234 (1998) 688±693

693

Table 3 Total coordination numbers for the As, S, Ge and I atoms x

0

0.1

0.2

0.3

0.4

nAs nS nGe nI

2.92(3) 1.95(2) ) )

2.85(4) 1.86(3) 3.90(9) 1.22(9)

2.94(4) 1.92(3) 3.90(9) 1.11(9)

3.05(4) 2.00(3) 4.02(9) 1.06(9)

2.97(4) 2.00(3) 4.00(9) 0.98(9)

prevent the formation of mixed-anion structural units containing both pairs. In contrast, the similarity in the bonding between the Ge±S and Ge±I pairs makes it possible to form mixed-anion units in the glass. The phase stability of the AsI3 molecular units can also be ascertained, although in the crystalline state, by their relatively large heat of formation compared to GeI4 molecular units [14]. This may be the reason that the I atoms are preferentially bonded to As rather than Ge. 5. Conclusions The present Raman spectroscopic, X-ray di€raction and EXAFS results for the …As2 S3 †1ÿx …GeI4 †x samples may be summarised as follows. 1. The I atoms incorporated into our samples are preferentially bonded to As and form AsI3 mo-

Fig. 6. The coordination numbers, nAs±S (triangles), nAs±I (crosses), nGe±S (circles) and nGe±I (squares), together with the corresponding theoretical curves (dashed lines). Except for nGe±I the experimental errors are within the point symbols.

lecular units. Three structural units, AsS3=2 , AsI3 and partially I-substituted GeS4=2 , are present in the system. 2. The As±S and Ge±S bond lengths are almost identical to the covalent lengths, while the As±I distance is nearer to the ionic radii. 3. The contribution from Ge±I bonds, obtained in the EXAFS measurements, implies the existence of mixed-anion units such as Ge(S3=2 I) tetrahedra, because no indication of GeI4 tetrahedra is observed in the Raman spectra even in the iodine rich range. 4. The coordination numbers of the constituent atoms in these glasses are approximately 4 for Ge, 3 for As, 2 for S and 1 for I.

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