Correlation between the structure of (1 - y)GeS2 · yAg2S glasses and their ionic conductivity

JOURIqAL OF

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Journal of Non-Crystalline Solids 192 & 193 (1995) 330-333

Correlation between the structure of( 1 - y)GeS2 and their ionic conductivity P. Armand

a

• yAg2S

glasses

A. Ibanez b,, J.M. Tonnerre b D. Raoux b B. Bouchet-Fabre c E. Philippot a

a LPMS, URA D0407 CNRS, UMII, CNRS, CC003, Place E. Bataillon, F-34095 Montpellier codex 5, France b Laboratoire de Cristallographie, UPR 5031, CNRS, BP166 F-38042 Grenoble codex 9, France c LURE, Bgtt. 209d, UPS, F-91405 Orsay codex, France

Abstract The structural analysis of (1 - y)GeS 2 •yAg2S ternary glasses has been carried out using X-ray absorption spectroscopy, anomalous wide angle X-ray scattering and small angle X-ray scattering. All the results are consistent among themselves, and a combined analysis has provided straightforward structural conclusions about the homogeneous distribution of the Ag ÷ cations in the glassy matrix. This improved understanding of the local and medium range order has allowed a discussion of the validity of the various structural models suggested to explain the ionic transport process at the atomic level. This process cannot be explained by a phase-separation model but by correlated Ag + jumps promoted by their large variety of sites.

I. Introduction The glasses of the A g - G e - X (X = S,Se) systems have a high ionic conductivity at room temperature for silver enriched compositions [1]. A structural study of germanium chalcogenide glasses has been undertaken in order to establish correlations between the Ag ÷ ionic conduction and their microscopic structure. The GeX x (X---S,Se) binary glasses have first been characterized by X-ray absorption spectroscopy (XAS) [2-6], anomalous wide angle X-ray scattering

* Corresponding author. Tel: + 33 76 88 78 05. Telefax: + 33 76 88 10 38. E-mail:[email protected].

(AWAXS) [7] and small angle X-ray scattering (SAXS) [8] experiments to follow in a second step the structural evolution due to the addition of silver sulfide as a glassy network modifier in the ( 1 y)GeS2 .yAg2S ternary glasses. The characterization of (1 - y)GeS 2 • yAg2S glasses has been undertaken following the same structural approach. Ge K-edge EXAFS studies [2,3] have shown that the Ag2S addition induces a simultaneous depolymerization of edge and corner sharing tetrahedra, which compose the g-GeX 2 structure, leading thus to an increase of the structural distortion. For Ag2S-enriched glassy compositions (y > 0.3), the silver environment as shown by EXAFS [9] is close to that in the et-AggGeS 6 cristaUine phase in agreement with other previous characterizations [10,11]. In this case, each Ag atom is bonded to three

0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 3 7 1 - 1

P. Armandet al. /Journal of Non-CrystallineSolids 192&193 (1995)330-333 sulfur atoms (Ag-S = 2.51 A) in very distorted sites. On the other hand, for low Ag2S concentrations ( y < 0.3), the Ag sites are rather similar to those found in [3-Ag2S which have lower coordination numbers (N = 2.5) and Debye-Waller factors. The S K-edge XAS characterization [4] shows that the glasses with high silver concentrations have sulfur environments more similar to those encountered in the c~-AgsGeS 6 crystal structure. In this paper, we present our main SAXS and AWAXS results. This is followed by a discussion of the validity of the various models suggested to explain the fundamental mechanisms involved in the ionic transport process on the basis of the glassy structure. 2. Experiments and data processing The ternary bulk glasses were synthesized from GeS 2 and AgeS starting materials, melted at 900°C and water-quenched [1]. All the scattering experiments were carried out on the DCI storage ring at LURE (Orsay). The SAXS spectra were corrected for sample to detector distance and solid angle, counting time, dimensions of the slits and detectors dead-time and then normalized to a constant incident beam intensity. The spectra were also corrected for background scattering, sample absorption and thickness. All spectra are in relative units and can thus be compared directly. The AWAXS spectra for the 0.5GeSz.0.5AgzS glass were recorded on a powdered sample, using a Si:Li multidetector, at different wavelengths near to and far from the Ge (A = 1.1479 A) and Ag (A =0.4859 A.) K-edges. The monochromatization was carried out by a Si(220) and Si(311) double crystal monochromator. The values of the anomalous scattering factors f ' and f" were obtained from Sasaki's tables [12] for the energies far from the Ge and Ag K-edges. Near the K-edge energies, the f" values were extracted from the X-ray absorption spectra and those for f ' were calculated using the Kramers-Kr6nig relationship [13]. The differential anomalous scattering (DAS) method was used to extract the differential intensities for the two elements analyzed and then by Fourier transformation to get the differential distribution function (DDFs), which reflect the local atomic arrangements around the Ge and Ag atoms.

331

3. Results 3.1. SAXS study of (1 - y)GeS 2 . yAgeS glasses The SAXS spectra show an important evolution as a function of composition [6] (Fig. 1). For glasses with low AgeS concentration (0.05 _ 0.3). Further, the scattering tail deviates from Porod's law, which can be attributed to a broader interface ( l ( q ) = f ( q - 3 ) for y = 0.2 and I(q)___f(q-e.5) for y = 0.3). For y > 0.3, the scattering intensity, I(q), is weaker and decreases. In this case, the l(q) curves can only be simulated using a long-range DebyeBfieche random distribution ( I ( q ) = K 1 3 / ( 1 + leq2) e, where l is a correlation length) [15]. We obtained correlations lengths of about 200-300 A. 3.2. AWAXS analysis of 0.5GeS 2 • 0.5 AgeS glass An AgeS enriched glass whose structure is rather homogeneous as shown by the SAXS study and which presents a high ionic conductivity at room temperature has been characterized by AWAXS. Fig. 2 and 3 show the DDF curves extracted for this material around the Ge and Ag K-edges. I(a'u')l

i

i

i

y=0,1 y=0,2 y=0,3 . . . . . y = 0,4 +

i+

% _

\

o y--o,,

..................

.......

0.02 0.04 0.06 0.08 0. Fig. 1. SAXSspectrafor (1 - y)GeS2.yAg2S glasses.

332

P. Armand et al. / Journal of Non-Crystalline Solids 192& 193 (1995) 330-333

25

I

O_ %.

I

I

I

I

3

++ T H E O . 20

-

EXP.

/

~.

,

15 10

/

S

\+=S/:

/.

Ag

-5 0

1

2

3

4

5

b

Fig. 2. Comparisonof the germaniumenvironmentin the 0.5GeS2 • 0.5Ag2S glass, representedby the DDFo~curve (solid line, experiment; dotted line, simulation), with the interatomic distances in the c~-AgsGeS6 crystallinephase (histogram). The DDF~, curve has been fitted up to 5 A using Gaussian distribution functions, taking the q dependance of the weighting factors into account (Fig. 2). The uncertainties on the structural parameters obtained from these simulations are around 15-20% for the coordination numbers, N, and about + 0.02 ,~ for the interaction distances, R. Moreover, Fig. 2 compares the germanium environment in the glass, given by the DDF~ (solid line) with that of aAg8GeS6, displayed as an histogram. This comparison illustrates the important similarity existing between the Ge environment in the glass and in the a-AgsGeS 6 crystalline network. Accordingly, the DDFa~ curve has been modeled using ~-AgsGeS 6 as 20

D

F

15 10 5-

structural basis. Indeed, the first peak (Fig. 2) characterizes the tetrahedral coordination of germanium by sulfur ( N = 3.8, R = 2.23 ,~). and the second is main!y due to G e - . . Ag interactions ( N = 8, R = 3.8 A). However, in agreement with the chemical composition of the glass (Ag2GeS3), short (2.92 ,~) and long (3.4 A) Ge . . - Ge distances partly remain, corresponding to the edge and corner sharing tetrahedra of the GeS 2 crystalline and glassy network [3]. Finally, the distribution between 4.6 and 5.1 A was simulated by long Ge - -. S and Ge - • • Ag interactions as encountered in ~-AgsGeS 6 (Fig. 2). The DDFAg curve shows only a very broad and weak bump (Fig. 3), in agreement with a wide distribution of the interaction distances involving silver atoms, This result is consistent with the high values for the Debye-Waller factor obtained in the Ag K-edge EXAFS analysis [9] and with the aAgsGeS 6 crystal structure where the silver atoms are localized at eight independent sites [16]. Low temperature ( T = 90 K) AWAXS experiments (in progress) seems to show a more structured silver environment.

O. f

q(A, b 0

f

2

3

,i

g

6

Fig. 3. DDFAgcurve extractedfor the 0.5GeS2.0.5Ag2Sglass.

4. Discussion

The structure of the (1 - y ) G e S 2 - y A g 2 S glasses is phase-separated for low Ag2S content (0
P. Armand et al. /Journal of Non-Crystalline Solids 192& 193 (1995) 330-333

0.1) and comprises two types of cluster: clusters based on the GeX 2 crystal structure and Ag2S aggregates (Rp = 50 A) based on the 13-Ag2S network. This phase-separation has been confirmed by a TEM characterization [17]. When the silver content increases (y -- 0.2), the typical action of the Ag2S as a glassy network modifier gives rise to a noticeable breaking of the double and single G e - S - G e bridges and to an increase in the structural distortion at the short and medium range order. This allows a better 'solubility' of the silver cations in the glass, leading to smaller Ag2S clusters (Rp ~ 20 A). Finally, the Ag2S enriched compositions ( y > 0.3), which are good solid electrolytes, exhibit a more homogeneous glassy structure. However, in these Ag-enriched glasses weak fluctuations of the electron density certainly remain as suggested by the Debye-Bueche simulations of the SAXS spectra. The EXAFS and AWAXS experiments, carried out at room and low temperatures, show that the short and medium range order are in this case rather close to those for aAgsGeS 6. Further, these results confirm the homogeneous distribution of Ag + cations in the glassy matrix ( G e . - - A g around 3.8 ,~) which seem to occupy various distorted sites. Thus, the ionic transport process in these glasses cannot be explained through silver enriched clusters as suggested by Greaves in the 'modified random network model' [18] or by Ingram in the 'Cluster by-pass model' [19]. Our structural study seems to support another hypothesis developed by Elliott ('the diffusion controlled relaxation model') [20] which suggest that ionic conductivity is induced by correlated jumps of the cations. This ionic transport process is probably promoted by a wide variety of sites for the Ag + charge carriers in the glassy structure.

5. Conclusion

All the XAS, SAXS and AWAXS results are very consistent with each other. Thus, their combined

333

analysis leads to straightforward structural conclusions, which allow to be checked the validity of the different models suggested to explain the ionic transport. This process cannot be explained by a phaseseparation picture but more by correlated cation jumps promoted by a large variety of sites. The authors would like to thank D. Bittencourt and C. Williams for the SAXS experiments and for helful discussions on the corresponding analysis.

References [1] E. Robinel, B. Carrette and M. Ribes, J. Non-Cryst. Solids 51 (1983) 49. [2] A. Ibanez, E. Philippot, S. Benazeth and H. Dexpert, J. Non-Cryst. Solids 121 (1991) 25. [3] P. Armand, A. Ibanez, H. Dexpert and E. Philippot, J. Non-Cryst. Solids 131 (1992) 137. [4] P Armand, A. Ibanez and E. Philippot, J. Solid State Chem. 101 (1993) 308. [5] C. Peyroutou, S. Peytavin, M. Ribes and H. Dexpert, J. Solid State Chem. 81 (1989) 70. [6] W. Zhou, M. Paesler and D.E. Sayers, Phys. Rev B43 (1991) 2315. [7] P. Armand, A. Ibanez, Q. Ma, D. Raoux and E. Philippot, J. Non-Cryst. Solids 161 (1993) 37. [8] P. Armand, A. lbanez, E. Philippot, C. Williams and D. Bittencourt, J. Phys. 1V C8 1 (1993) 389. [9] A. lbanez, P. Armand and E. Philippot, Solid State Ionics 51 (1993) 157. [10] L.C. Bourne, S.C. Rowland and A. Bienenstock, J. Phys. C4 10-41 (1981) 951. [li] R.J. Dejus, S. Susman, K.J. Volin, D. Montague and D.L. Price, J. Non-Cryst. Solids 141 (1992) 162. [12] S. Sasaki, KEK report 83-22, Nat. Lab. High Energy Physics, Tsukuba, Japan (1983). [13] R.L. Kr6nig, J. Opt. Soc. Am. 11 (1926) 547. [14] G. Porod, in: Small Angle X-ray Scattering, ed. O. Glater and O. Kratky (1982) p. 18. [15] P. Debye and A.M. Bueche, J. Appl. Phys. 20 (1949) 518. [16] G. Eulenberger, Monat. Chem. 101 (1977) 901. [17] P. Armand, A. lbanez and E. Philippot, Nucl. Instrum. Meth. B97 (1995) 176. [18] G.N. Greaves, J. Non-Cryst. Solids 71 (1985)203 [19] M.D. Ingrain, Philos. Mag. B60 (1989) 729. [20] S.R. Elliott, Phys. Rev. B41 (1991)47.