A neutron diffraction study of glassy GeS2

Journal of Non-Crystalline Solids 293±295 (2001) 169±174

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A neutron diraction study of glassy GeS2 I. Petri, P.S. Salmon * Department of Physics, University of Bath, Bath BA2 7AY, UK

Abstract The total structure factor for glassy GeS2 was measured by using neutron diraction. The possibility of broken chemical ordering was then explored by interpreting the results in terms of recently measured partial pair distribution functions for the corresponding diselenide glass GeSe2 which show a signi®cant number of homopolar bonds. It was found that the latter do not give an accurate account of the nearest-neighbour correlations in the disulphide glass and no compelling evidence could be found for a substantial deviation from heteropolar Ge±S bonding. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 61.12.Ex; 61.72.Dd

1. Introduction The object of this paper is to examine whether there is evidence from neutron diraction for broken chemical ordering in the network chalcogenide glass GeS2 . Motivation for the work is provided by the recent measurement of the full set of partial structure factors for the corresponding diselenide glass GeSe2 by using the method of isotopic substitution in neutron diraction [1]. It was found that the basic building block of the network is the GeSe1=2 4 tetrahedron in which 34(5)% of the Ge reside in edge-sharing con®gurations but that the intrinsic chemical order of the glass is broken with a maximum of 25(5)% Ge and 20(5)% Se being involved in homopolar bonds. It is therefore of interest to examine the extent to which these results can explain the structure of

* Corresponding author. Tel.: +44-1225 323 698; fax: +441225 826 110. E-mail address: [email protected] (P.S. Salmon).

glassy GeS2 since information on the intrinsic chemical ordering in chalcogenide materials is important for understanding their optoelectronic properties [2,3] and there are strong analogies between germanium sulphide and selenide systems. For example, the high temperature crystal structures of GeS2 and GeSe2 are the same [4,5], similarities between the structures of glassy GeS2 and GeSe2 are found from X-ray diraction work [6], and the composition dependence of the frequency of the A1 symmetric stretch mode of corner-sharing tetrahedra is comparable for Gex S1 x and Gex Se1 x glasses 0 6 x 6 1 [7].

2. Theory In a neutron diraction study of glassy GeX2 (X S or Se), the coherent scattered intensity can be represented by the total structure factor F Q ASGeX Q CSXX Q

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 6 7 - 6

1 BSGeGe Q 1;

1 1

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I. Petri, P.S. Salmon / Journal of Non-Crystalline Solids 293±295 (2001) 169±174

where Q is the magnitude of the scattering vector and Sab Q denotes a Faber±Ziman partial structure factor. The coecients A±C take values of 103.6(3), 74.4(4) and 36.02(3) mb for GeS2 and 289.9(8), 74.4(4) and 282.3(6) mb for GeSe2 using the coherent scattering lengths of bGe 8:185(20) fm, bS 2:8471 fm and bSe 7:9709 fm given by Sears [8]. The corresponding total pair distribution function in real-space is given by Gr AgGeX r CgXX r

1 BgGeGe r 1;

1 2

where gab r denotes a partial pair distribution function. The mean number of particles of type b contained in a volume de®ned by two concentric spheres of radii ri and rj , centred on a particle of type a, is given by Z rj r2 gab r dr; 3 nba 4pn0 cb ri

where cb is the atomic fraction of species b and 3 for GeS2 [6]) is the number n0 0:03591 A density of the glass.

3. Experimental procedure The samples were prepared following a procedure designed to avoid contamination [1] by sealing elemental Ge (99.9999%, Aldrich) and S (99.998%, Aldrich) in a silica ampoule of 5 mm inner diameter and 1 mm wall thickness. The ampoule containing 3.4 g of reactants was heated in a rocking furnace at 0.9 °C/min to 950 °C with 4 h equilibrium periods at 110 °C and 500 °C. It was maintained at 950 °C for 22 h, cooled at 1 °C/min to 900 °C, kept at this temperature for 5 h, and subsequently quenched in an ice/salt±water mixture at )5 °C. The melting point of GeS2 is 840 °C [9] giving a ratio of quench to melting point temperatures (in Kelvin) of 1.054. The onset and midpoint glass transition temperatures of the asquenched glass were measured to be 487(5) and 491(5) °C using a TA Instruments Thermal Analyst 2000 dierential scanning calorimeter

operating at a scan rate of 10 °C/min. These compare with literature values of 473 °C [10], 490 °C [11], 495 °C [12,13], and 520 °C [7] for the glass transition temperature of GeS2 . The glass was coarsely powdered and held in a vanadium can (6.8 mm inner diameter, 0.1 mm wall thickness) for the neutron diraction experiment, which was made using the instrument D4B (Institut Laue-Langevin, Grenoble) with an inci The complete exdent wavelength of 0.7047 A. periment comprised measurement of the diraction patterns for the sample in its container at room temperature (26(1)°C), the empty container, and a vanadium rod of diameter comparable to the sample for normalisation purposes. The measured intensity for a cadmium neutronabsorbing rod of similar diameter to the sample was also collected to account for the eect of sample self-shielding on the background count rate at small Q-values [14]. The cross-sections used in the data analysis procedure [15] were taken from [8] and full experimental details are given in [16]. It was checked that the resultant F Q tends to the correct high-Q limit, obeys the usual sum±rule relation, and that there is good overall agreement between F Q and the back-Fourier transform of the corresponding Gr after the unphysical low-r oscillations are set to their calculated limit G0 A B C. 4. Results The measured total structure factor for glassy GeS2 is compared in Fig. 1 with that for glassy GeSe2 of natural isotopic abundance [1,16]. The same instrument operating at the same incident wavelength was used for the GeSe2 neutron diraction experiment such that the two F Q cover the same Q-range and correspond to comparable Q-space resolution functions. The F Q for glassy GeS2 has a strong ®rst sharp diraction 1peak (FSDP) at a position QFSDP 1:022 A in agreement with the neutron diraction results of Lin et al. [17]. By comparison with the structures of glassy [1] and liquid [18] GeSe2 , this FSDP will have a dominant contribution from the Ge±Ge correlations. Its full width at


Fig. 1. The measured total structure factors F Q for glassy GeS2 and GeSe2 at 26(1)°C. The bars represent the statistical errors on the data points and the solid curves are the Fourier back-transforms of the corresponding Gr given by the solid curves in Fig. 2.

half maximum, obtained by re¯ecting the low-1 Q region about QFSDP , is DQFSDP 0:351 A which corresponds to a correlation length [19]. The FSDP in of 2p=DQFSDP 18:05 A F Q for 1 glassy GeSe2 occurs at QFSDP and DQFSDP 0:301 A 1. 1:002 A The total pair distribution function, Gr, for glassy GeS2 is illustrated with that for glassy GeSe2 in Fig. 2. By comparison with the partial pair distribution functions for glassy GeSe2 , the will have a dominant ®rst peak at r1 2:212 A contribution from Ge±S correlations and its inte 6 2:52 gives a gration over the range 1:96 6 r A S co-ordination number of nGe 4:11 if there are a negligible number of homopolar bonds. The second peak will have a contribution from S±S correlations and the ratio of the second to ®rst peak positions r2 =r1 1:579 which compares with a

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Fig. 2. The total pair distribution functions Gr for glassy GeS2 and GeSe2 obtained by Fourier transforming the F Q given by the error bars in Fig. 1. The unphysical low-r oscillations about the G0 limits are shown by the broken curves.

p value of 8=3 1:633 for perfect tetrahedra. A similar analysis of Gr for glassy GeSe2 yields nSe Ge 4:22, by integrating the ®rst peak at over the range 2:09 6 r A 6 2:61, and 2.36(2) A the ratio r2 =r1 1:631. These parameters are typical of those previously measured for glassy GeSe2 using neutron diraction [20,21]. The results for both glasses therefore support a model in which GeX1=2 4 tetrahedra are the dominant structural motifs.

5. Discussion As shown in Table 1, the results obtained from the present neutron diraction work assuming no Ge±Ge or S±S homopolar bonds are in accord with those previously measured for glassy GeS2 using X-ray diraction and extended X-ray ab-

Table 1 The local co-ordination environment in glassy GeS2 as measured using neutron diraction (ND), X-ray diraction (XRD) or extended X-ray absorption ®ne structure (EXAFS) spectroscopy r1 (A) nS r2 =r1 Probe Ref. Ge

2.21(2) 2.2 2.23 2.22(1) 2.23(1)

4.1(1) 4 4 3.8(4) 3.8(2)

1.579 1.614 1.592 ± 1.571

The nSGe co-ordination numbers were obtained assuming no homopolar bonds.

ND XRD XRD EXAFS XRD

Present work [22] [6] [23] [24]

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sorption ®ne structure (EXAFS) spectroscopy methods. However, there is evidence for broken chemical ordering in glassy GeS2 from M ossbauer [10] and Raman spectroscopy experiments [25]. Furthermore, homopolar bonds are found in the measured partial pair distribution functions for the corresponding diselenide glass GeSe2 [1]. An attempt was therefore made to analyse the nearestneighbour correlations in glassy GeS2 by using the gab r for as-quenched glassy GeSe2 as measured using the same neutron diractometer under the same conditions. The GeSe2 glass was made using a comparable method to the disulphide glass [1] with 4.3 g of reactants and a ratio of quench to melting point temperatures (in Kelvin) of 1.106. The large contrast between the coherent scattering lengths of S and Se of natural isotopic abundance [8] means that the Ge±Ge correlations are given a much larger relative weighting for the GeS2 glass which should enhance the visibility of any homopolar Ge±Ge correlations. In glassy GeSe2 the nearest-neighbour co-ordiGe nation numbers are nSe Ge 3:71, n Ge 0:255 Se and nSe 0:205 with corresponding bond dis rGeGe 2:422 A and tances of rGeSe 2:362 A, rSeSe 2:322 A, respectively which places all these correlations under the ®rst peak in Gr. In glassy GeS2 the Ge±S distance is rGeS 2:212 A and if homopolar bonds are a feature then is expected from studies of glassy rGeGe 2:42 A GeSe2 and Gex S1 x x > 1=3 [1,23] and rSS 2:06 is expected from studies of liquid and glassy A sulphur and Gex S1 x [26,27]. The ratio of the nearest-neighbour bond distances in glassy GeS2 is therefore anticipated to be rGeGe =rGeS 1:095 and rSS =rGeS 0:932. To ensure this distance ratio for the nearest-neighbours, the measured partial pair distribution functions for glassy GeSe2 were plotted as the functions gGeSe r=rGeSe ; gGeGe 1:095r= rGeGe and gSeSe 0:932r=rSeSe . They were then weighted by the relevant coecients A±C for the disulphide glass (see Eq. (2)) and summed to compare with the total pair distribution function for GeS2 plotted in the form Gr=rGeS G0. The results presented in Fig. 3 show that Ge±Ge homopolar bonds contribute to the high-r side of the ®rst peak in Gr and the total area of this ®rst peak is larger than can be reproduced using the

Fig. 3. The measured total pair distribution function Gr=rGeS G0 for glassy GeS2 (thick solid curve) plotted as A a function of the reduced distance r=rGeS where rGeS 2:21 A. comparison is made with the scaled partial pair distribution functions for glassy GeSe2 ; AgGeSe r=rGeSe (thin solid curve), BgGeGe 1:095r=rGeGe (dashed curve) and CgSeSe 0:932r=rSeSe (dotted curve), together with their sum (chained curve) ± see text.

gab r for the corresponding diselenide glass. One conjecture to explain the shortfall in area would be the appearance in glassy GeS2 of a larger number of homopolar bonds than in glassy GeSe2 , in accordance with M ossbauer spectroscopy experiments where Sn was used as a Ge probe [10]. In these experiments the fraction of Ge in homopolar bonds was estimated to be 16(1)% in glassy GeSe2 and 29(2)% in glassy GeS2 . However, the homopolar bond peaks in Fig. 3 already involve a substantial number of atoms i.e. about 25% of the Ge and 20% of the X. Furthermore, the missing peak intensity is evenly distributed about the central peak position i.e. it could be readily accounted for by increasing the height of the Ge±X partial pair distribution function. This would increase the Ge± X co-ordination number from 3.7(1) found in glassy GeSe2 to a value nearer the limit of 4.1(1) deduced by assuming a negligible number of homopolar bonds in glassy GeS2 . For example, if the ®rst peak in Gr for glassy GeS2 is analysed in terms of the GeSe2 homopolar bond co-ordination Ge numbers [1] i.e. nSS nSe Se 0:2 and n Ge 0:25=2 (to account for the reduced overlap between the ®rst peaks in Gr and gGeGe r see Fig. 3) then it is found that nSGe 3:91. In this case the total coordination numbers of germanium and sulphur


would be nGe 4:21 and nS 2:157, respectively which, within the experimental error, contradict the `8-N' rule. This rule may, however, be satis®ed if the number of homopolar bonds is reduced relative to glassy GeSe2 and in the limit of perfect chemical ordering total co-ordination numbers of nGe 4:11 and nS 2:055 are obtained from the experimental data. Hence, although the details of the homopolar bonding in glassy GeSe2 may well depend on the details of the sample preparation [28], there is no compelling evidence from the present work for a substantial number of homopolar bonds in glassy GeS2 . It is interesting to note that the ratio of the number of Ge±Ge (or X±X) bonds to the total number of bonds in the glass is estimated by using the law of mass action [29] to be 4.2% for GeSe2 and 0.6% for GeS2 . The former value is in agreement with the ratio obtained from the diffraction results for glassy GeSe2 [1] and compares with a value of 6% estimated from an X-ray emission spectroscopy study [30]. A reduction in the number of homopolar bonds on changing from GeSe2 to GeS2 is consistent with the data shown in Fig. 3 and is also found from a recent analysis of Raman spectroscopy data [25]. Furthermore, there is little evidence for Ge±Ge bonds in the lowtemperature Ge K-edge EXAFS spectra for Gex S1 x glasses unless x > 1=3 [23].

6. Conclusions The ®rst peak in the measured total pair distribution function, Gr, for glassy GeS2 cannot be accurately accounted for by using the scaled partial pair distribution functions for glassy GeSe2 . The detailed local structure in glassy GeS2 is therefore signi®cantly dierent to the glassy diselenide, at least for the present glasses that were prepared using typical bulk-quenching methods. The area of the ®rst peak in Gr is greater than that accounted for by the scaled gab r and this shortfall can be understood in terms of an increase in the number of heteropolar Ge±X bonds relative to glassy GeSe2 to give a Ge±S co-ordination number close to four.

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Acknowledgements We would like to thank Dr Henry Fischer and Pierre Palleau for their help with the experiment. IP thanks the University of East Anglia for ®nancial support.

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