Comparative EXAFS study of (Ag2X)y(As2X3)1−y glasses (X = Se or S)

lOIIRtiAb OF

ELSEVIER

Journal of Non-Crystalline Solids 185 (1995) 274-282

Comparative EXAFS study of (Ag2X) y ( A S 2 X 3) 1-y glasses ( X = Se or S) V. Mastelaro

a,*,

S. B6nazeth a,b, H. Dexpert

a

a LURE, Universit~ Paris-Sud, 91405 Orsay cddex, France b Laboratoire de Chimie Physique et Bioinorganique, Facult~ de Pharmacie, Universit~ Paris-Sud, 92296 Chatenay-Malabry cddex, France

Received 18 January 1994; revised manuscript received 18 October 1994

Abstract The local order of ionic (Ag2X)y(AS2X3) 1_y glasses (X = Se or S) has been studied in a wide composition range using X-ray absorption fine structure spectroscopy; The silver atoms remain in a well defined chalcogenide atom surroundings made of two bonded atoms (RAs_Se = 2.58 A, Ras_ s = 2.46 A). No significant differences between sulphide and selenide glasses were observed.

1. Introduction A g - A s - X (X = S, Se) glasses have been extensively studied relative to their potential applications as solid electrolytes [1,2]. The nature of the conduction - ionic or electronic - depends mainly on the silver content [3]. The glasses studied here belong to the (Ag2X)y(AS2X3)l_y pseudo-binary lines, where sulphide or selenide glasses present approximately the same ionic conductivity [4]. Atomic arrangements of the sulphide glasses with 0.10 < y < 0.75 have been examined and results are

* Corresponding author. Present address: Departamento de Engenharia de Materials, Universidade Federal de S~o Carlos, Caixa Postal 676, S.~o Carlos, S.P., CEP 13565-905, Brazil. Tel: + 55162 748 250. Telefax: +55-162 727 404. E-mall: dvrm@power. ufscar.br.

summarized below. The first structural propositions were made by Kawamoto and co-workers [1,2] in order to interpret the ionic conductivity measurements of glasses with 0.33 < y < 0.56. The coexistence of Ag2S crystalline phases and AsS 3 pyramidal units is supposed. Besides this structural hypothesis supported by the electrical conductivity behaviour, neutron diffraction [5], Raman spectroscopy [6] and extended X-ray absorption fine structure (EXAFS) experiments [7] have been employed. All these authors agree on the existence of AsS 3 pyramidal units. However, some discrepancies in silver surroundings are noted and short-range order models based on Ag2S, AgAsS 2, or AgaAsS 3 crystalline structures are proposed depending on the silver content. The structure of selenide glasses belonging to this same pseudo-binary line have not been described apart from one recent paper [8]. These authors stud-

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V. Mastelaro et al. /Journal of Non-Crystalline Solids 185 (1995) 274-282

ied only glasses with low silver content (y = 0.096 and y = 0.25) by isotopic substitution with neutron diffraction. They concluded that the silver atoms are bonded to three selenium atoms at a distance of 2.68 A. In our previous work [9] on the AsSe-Ag2Se line, we found different results. Each silver atom is coordinated by two selenium atoms at a distance of

energy range extending from 200 eV below the edge up to approximately 1000 eV above it. In the single-scattering and plane-wave approximations, the following equation for EXAFS can be derived [12,13]:

s,

x( k) = - ~ --~l Fi( k, 7r) e x p ( -

2o-i2k z)

2.55 L The aim of this work is to provide a better knowledge of the local structure of (AgzX)y(ms2X3)l_y glasses, and to study the influence of the chalcogenide anion and of the glass composition (when X = S, 0.10 < y < 0.80 and when X = Se, 0.10 < y < 0.75). We performed room-temperature EXAFS experiments at the K edges of the four constitutive elements of these materials to obtain a detailed description of the local order structural parameters.

2. Experimental details and data reduction Bulk glasses were prepared by mixing binary mg2Xand As2X 3 in appropriate proportions. For all compositions of selenide and sulphide glasses with y < 0.67, we used conventional melting at 900°C followed by cold water quenching. For sulphide glasses with y > 0.67, we used a twin-roller apparatus [10]. We obtained glasses with higher silver content than previously described [11]. Metallographic and X-ray diffraction experiments proved that the samples were homogeneous and did not contain crystalline phases. Extended X-ray absorption fine structure experiments were carried out at LURE-DCI (Orsay, France) using different spectrometers (EXAFS I station Si(331) channel-cut monochromator, EXAFS IV station - Si(311) double-crystal monochromator), with ionization chambers as detectors. Glassy and crystalline samples were finely grounded (20 ~m) and uniformily spread between two kapton tapes, used as support for absorption measurements. Their thicknesses were determined by successive attempts to optimize the signal-to-noise ratio. For each K edge (2472 eV for sulfur, 11868 eV for the arsenic, 12658 eV for selenium, and 25514 eV for silver) the spectra were collected in the transmission mode, over an

× exp

A(k)

where k is the photoelectron wave vector, N~ is the number of atoms in the ith shell, R i is the distance from the central absorbing atom to atoms in the ith shell, Fi(k) is the scattering amplitude, tri is the Debye-Waller factor, A(k) is the mean free path of the photoelectron and ~i(k) is the phase shift. In order to determine the structural parameters R, N and o-, the Fourier filtering method was used [14]. In all cases, the parameter A was considered equal to that of the reference compounds, and it was kept constant during the fitting procedure. The energy threshold was adjusted in all cases, but it did not vary significantly. The pre- and post-edge background substraction and Fourier transform (uncorrected from phase shift) were performed using the available program on VAX computers at LURE [15]. The EXAFS spectra were k 3 weighted, a Hanning apodization function being applied for unknown and reference spectra, over similar ranges of k (from 3 to 13 A -1) in order to allow a direct comparison between the various data sets collected. Crystalline samples were taken as structural references to obtain the backscattering phases and amplitudes used in the EXAFS fitting procedure: Ag3AsX 3 [16,17] for the As-X and Ag-X pairs; As2Se 3 [18] for the Se-As pairs. For the Se-Ag pairs, no crystalline compound is known to present a regular coordination around selenium atoms. Considering that cadmium is next to silver in the Periodic Table, and that their backscattering amplitude functions are quite similar, we used the CdSe crystalline compound as reference for the Se-Ag pairs [19].

3. EXAFS results For each K edge spectra analysis for selenide and sulphide glasses and related references, we present

V. Mastelaro et al. /Journal of Non-Crystalline Solids 185 (1995) 274-282

276

0.1

0.1

i

~

L

b

Ik

k 0

1

~ 3

0

R (i)

0

Fig. 1. Fourier transform moduli at arsenic K edge for (Ag2Se)y(As2Se3)l_y samples: (a) Ag3AsSe 3 crystal; (b) y = 0.10; (c) y = 0.60 and (d) y = 0.75 glasses.

the Fourier transform moduli (FF figures) which corresponds to the radial pseudo-function distributions around each surrounding atom studied. For the arsenic K edge, the FT figures (Figs. 1 and 2) show only one shell for the glasses as well as for the Ag3AsX 3 reference compounds. However, in both cases the intensity of this peak is a function of the parameter, y, and presents a minimum for the compositions y = 0.60 (selenide glasses) and y = 0.50 (sulphide glasses). For the selenium and sulfur K edges, the decrease of the X - A s subshell area noted on the FT figures ,2

i

i

i

i

1

2

3

4

R(A)

Fig. 3. Fourier transform moduli of different (Ag2Se)i (As2Se3)l_y compositionsat the selenium K edge: (a) y = 0.10; (b) y = 0.40; (c) y = 0.75; (d) Ag3AsSe3 crystaland (e) AgAsSe2 crystal.

(Fig. 3 for X = Se and Fig. 4 for X = S) is related to structural changes in the X - A s bonds induced by addition of silver. In the glasses with highest silver content, a shoulder appears on the right-hand side of the mean peak which is attributed to X - A g bonds. For the sulphide glasses, the peculiarly low energy of its K edge presented low signal-to-noise ratio, small energy range acquisition and strong absorption for samples with high silver content. For these reasons, we cannot present fitted structural parameters. However, as is shown in the FT figures (Fig. 3 and 4), the variations observed around sulfur

¢0.06

•~

d

II I1 I

v

u~

0

'~'8'q 0

1

r 2

~3

R (A) Fig. 2. Fourier transform moduli at arsenic K edge for (Ag2S)y(As2S3) l_y samples: (a) Ag3AsS 3 crystal; (b) y = 0.10; (c) y = 0.50 and (d) y = 0.80 glasses.

o 0

2

4

R(.~) Fig. 4. Fourier transform moduli of different (Ag2S)y(AS2S3) 1_y compositions at the sulfur K edge: (a) y = 0.10; (b) y = 0.50.

277

V. Mastelaro et al. /Journal of Non-Crystalline Solids 185 (1995) 274-282

4. Discussion

0.03

b

C--

0

1

2

R(A)

3

4

Fig. 5. Silver K edge Fourier transform moduli for the (Ag2le)y(ms2Se3) l_y compositions: (a) Ag3AsSe 3 crystal; (b) y = 0.50; (c) y = 0.75.

and s e l e n i u m atoms are quite similar and the results we present c o n c e r n i n g the surroundings of the selen i u m atoms will be transferred to sulfur atoms. For the silver K edge only one peak is observed on the F'I" figures (Fig. 5 for X = Se and Fig. 6 for X = S) and its intensity is lower for high silver contents.

0.035

I

I

I

A

b--II (-'-a V m

It |

Table 1 Data analysis results of the arsenic K edge for (Ag2Se)y (As2Se3)l _y glasses Composition, y

NAs_Sc (_+0.1)

RAs_Se (~k) (___0.01)

ACrAs_S e (,~k) (+0.003)

0.10 0.40 0.50 0.60 0.75 mg3AsSe3 a

3.0 3.0 3.0 3.0 3.0 3.0

2.41 2.41 2.41 2.41 2.41 2.41

0.026 0.025 0.030 0.045 0.020 -

a Values found in Ref. [17]. N, nearest neighbours; R, mean bond length; Ao-, Debye-WaUer factor.

c- 1 --d

g

The results of fitting (Tables 1 and 2) confirm that the arsenic atom is coordinated by three selenium (or sulfur) atoms whatever the silver composition. W e exclude the eventuality of homopolar A s - A s b o n d ing in these glasses because they are based on stoichiometric A s 2 X 3. Figs. 7 and 8 illustrate for the compositions y = 0.60 (X = Se) and y = 0.80 (X = S) typical adjustment of the experiments to the structural model. The quality of fit is quite the same for all samples. The simulation explains the changes in intensity observed on the F T figures by the evolution of the D e b y e - W a l l e r factor, A o-, its m a x i m u m values b e i n g reached for y = 0.60 (X = Se) and y = 0.50 (X = S) glasses (Fig. 9). A n explanation for this evolution m a y be advanced considering the local order in the two crystalline phases that b e l o n g to these phase diagram lines and that are equal or close

Table 2 Data analysis results at the arsenic K edge for (Ag2S)y(As2S3) 1_y glasses

0

1

2

R(A)

3

4

Fig. 6. Silver K edge Fourier transform moduli for the (Ag2S)y(As2S3) 1_y compositions: (a) Ag3AsS 3 crystal; (b) y = 0.33; (c) y = 0.67 and (d) y = 0.80.

Composition, y

NA~_s (+0.1)

RAs_s ( , ~ ) (+0.01)

ArrAs_s (,~) (_+0.003)

0.10 0.33 0.50 0.67 0.80 mg 3AsS 3 a

3.0 3.0 3.0 3.0 3.1 3.0

2.27 2.27 2.26 2.26 2.25 2.27

0.018 0.025 0.035 0.022 0.000 -

a Values found in Ref. [19].

V. Mastelaro et al. / Journal of Non-Crystalline Solids 185 (1995) 274-282

278 0.2

i

i

i

0.|

I

& -0.2

71 1

.... 1

2

3

r

(ea)

-0.1

0

i

i

!

2

R (A)

4

R IA) 0.03

0.03

F

I

0

i

i

i

OL!

ml

~_,

i

-

-0.03

8 0

(Tb

-0.03

t i 175 350 525 Energy (eV)

0

L 350 ]Energy (eV)

k 525

7011

700

Fig. 7. Typical fitting quality at the arsenic K edge for the y = 0.60 selenide glass: (a) magnitude and imaginary part of the Fourier transform and (b) x(E) filtered EXAFS spectra ( - - , experimental data; O, fitting).

to AgAsX 2 for y 0.50 and Ag3AsX 3 for y -- 0.75. Previous X-ray diffraction studies of these crystalline compounds [17,20] have shown that the surrounding of the arsenic atom is more disordered in A g A s X 2 than in Ag3AsX 3. Thus the Debye-Waller maxima observed in the glasses with y close to 0.50 are related to this higher disorder which implies a similar local order around the arsenic atoms between the crystalline and glassy states. The fitted results for selenium are presented in Table 3 and the quality of this adjustment for y = 0.10 and y = 0.75 is illustrated by Figs. 10 and 11, respectively. The interpretation of these simulations is summarized below as a function of increasing silver content. (i) The Se-As bond number decreases while the mean bond length, R s e _ g s , remains unchanged, (ii) The Se-Ag bond number increases from 0.4 =

,

175

Fig. 8. Typical fitting quality at the arsenic K edge for the y = 0.80 sulphide glass: (a) magnitude and imaginary part of the Fourier transform and (b) x(E) filtered EXAFS spectra ( - - , experimental data; O, fitting).

to 1.8. The mean bond length, RSe_Ag , remains quite constant, (iii) The total number of first neighbours around the selenium atom varies from two (as in the As2Se 3 crystalline or amorphous state) to three (as in the Ag3AsSe 3 crystalline phase), 0.06

...t. Selenide glasses !i Sulphide glasses

0.04 ,<

0.02 \\ \

0

0.2

0.4

0.6

0.8

A g z X (%)

Fig. 9. Variation of the parameter, Ao', with the Ag2X content at the arsenic K edge.

V. Mastelaro et al. /Journal of Non-Crystalline Solids 185 (1995) 274-282 0.08

i

--

(lOall 0

r

0.07

2

=,2

1

I l

3

(11a) -0.07

4

-0.015

(lOb) 4.0

i

+

I 230

3

4

0.015

al A

tu o

I 420 Energy (eV)

2

R (A)

V,-i

1

0

R (A)

0.015

279

610

01b)

-O.OlS 800

40

L 230

I

I

420 Energy

610

800

(eV)

Fig. 10. Typical fitting quality at the selenium K edge for the y = 0.10 selenide glass: (a) magnitude and imaginary part of the Fourier transform and (b) x(E) filtered EXAFS spectra ( - - , experimental data; O, fitting).

Fig. 11. Typical fitting quality at the selenium K edge for the y = 0.75 selenide glass: (a) magnitude and imaginary part of the Fourier transform and (b) x(E) filtered EXAFS spectra ( - - , experimental data; O, fitting).

(iv) The Debye-Waller factor, A ~r, related to the Se-Ag pairs increases due to a more important local disorder. The results of the analytical procedure concerning the silver K edge (Tables 4 and 5) show that in both cases silver atoms are coordinated by two selenium

(or sulfur) atoms whatever the silver composition, as in the crystalline samples. Confirming the analysis of the selenium K edge data, we note that the addition of silver increases the local disorder. The quality of the simulations (Figs. 12 and 13) is quite the same

Table 3 T w o shell fitting results at the selenium K edge for (Ag2Se)y(ms2Se3) 1_y glasses Composition, y

N~_As (_+0.1)

RSe_As (,~) (_+0.01)

AO'se_As(,~) (_+0.003)

N"so A~

AS2 Se3 a 0.10 0.40 0.50 0.60 0.75 mg3 AsSe3 b

2.0 1.7 1.5 1.4 1.2 1.3 1.0

2.41 2.39 2.37 2.38 2.38 2.40 2.41

. 0.000 0.005 0.010 0.017 0.024 -

. 0.4 0.8 1.0 1.8 1.8 2.0

a Values found in Ref. [15]. h Values found in Ref. [17]. N' + N" are the total neighbour numbers.

.

(_+0.2)

RSo-A, (-~) (_+0.02)

a~rso-A, (,~) (_+0.005)

2.60 2.59 2.58 2.58 2.58 2.58

0.015 0.023 0.030 0.035 0.042 -

.

N' +N" 2.0 2.1 2.3 2.4 3.0 3.1 3.0

V. Mastelaro et al. /Journal of Non-Crystalline Solids 185 (1995) 274-282

280

Table 4 Data analysis results at the silver K edge for (Ag 2 Se)),(As 2Se 3)1 - y glasses Composition, y

NAg_Se (-+0.2)

RAg-Se (A) (+0.02)

AO'Ag Se ('~) (-+0.005)

0.40 0.50 0.60 0.75 Ag3AsSe3 a

2.1 2.3 1.9 2.2 2.0

2.58 2.55 2.56 2.55 2.54

0.010 0.020 0.023 0.034 --

a Values found in Ref. [17]. Table 5 Fitting results at the silver K edge for (Ag2S)y(As2S3) 1_y glasses Composition, y

NAg_s (+0.2)

RAg S ('~) (+0.02)

AtYAg_S(.~k) (--+0.05)

0.33 0.67 0.80 mg3msS 3 a

2.0 2.2 2.3 2.0

2.46 2.48 2.48 2.46

0.030 0.050 0.070 -

for all glassy compositions. The small discrepancy ( ~ 0.03 A) between RSe_Ag (Table 3) and RAg_Se (Table 4) may be due to the silver-cadmium approximation used in analyzing the Se-Ag pairs at the selenium K edge data. Thus, related to the silver coordination sphere, we observed well defined bonds with two chalcogenide a t o m s (Rgg_Se-'~ 2.58 ,~, R A g - S = 2.46 ,~). These values are smaller than those found by Benmore and Salmon [5] (RAg_Se = 2.68 ,~ and NAg_Se = 3), and quite equal to that by Okuno et al. [7] for the glass with y -- 0.75. Taking into account these structural results, we present here a tentative structural model for these two glassy systems, based on the role of silver cation modifier. A possible evolution of the ASEX 3 structure [20-22] when silver is introduced is illustrated by Fig. 14. The addition of Ag2X 3 in the binary matrix (Fig. 14(a)) is possible by cutting the X - A s - X chains or opening the A s - X - A s rings. According to

a Values found in Ref. [19]. 0.03

0.02

i

r (12a)

-0.03

(13a) I

d

I

!

2

3

-0.02

I

I

1

2

3

R (,~)

0.005

,

,

,

0.009

,

( 1 3 b )

-o.oos 40

172

I 3114

436

Energy

(eV)

I

568

-o.o09

700

Fig. 12. Typical fitting quality at the silver K edge for the y = 0.75 selenide glass: (a) magnitude and imaginary part of the Fourier transform and (b) x(E) filtered EXAFS spectra ( experimental data; O, fitting).

40

172

304 Energy

436

568

700

(eV)

Fig. 13. Typical fitting quality at the silver K edge for the y = 0.80 sulphide glass: (a) magnitude and imaginary part of the Fourier transform and (b) x(E) filtered EXAFS spectra (. experimental data; O, fitting).

V. Mastelaro et al. /Journal of Non-Crystalline So~Ms 185 (1995) 274-282

X .......

X.--

I

/

14b

I

X

,

14a

×

\, ......

/\

References 14c

\

,.___/

talline AgAsX 2 and Ag3AsX 3. When silver is introduced, sulfur and selenium atoms are a sensitive probe to observe the changes occuring in these glasses due to the strong interaction with silver. We exclude the existence of Ag2X previously proposed. We found no evidence for an influence of sulfur or selenium for glasses which belong to the pseudobinary line based on stoichiometric A s 2 X 3. The authors thank the team who operated the 'DCI' storage ring. Thanks also go to Mrs. Ollitrault-Fichet at the Universit6 de Paris V for her guidance in the sample preparations. One author (V.M.) was supported by a PhD Fellowship from CAPES (Brazil).

X

I

281

/

Fig. 14. Local structure schema based on our EXAFS results: (a) As2X 3 binary composition; (b) low-silver-content glass and (c) high-silver-content glass (X = Se or S).

our EXAFS study, we note that the selenium or sulfur atoms still remain two-coordinated with the addition of a small silver content (Fig. 14(b)) although they become three-coordinated for enriched silver glasses (Fig. 14(c)). In this case, X - A g - X A g - X chains similar to those of the Ag3AsX 3 crystalline compound are formed [16,17]. Thus, the characteristics of network former and modifier disappear to give place to a more compact and covalent structure.

5. Conclusions

In both sulfur and selenium systems, the arsenic atom belongs to the A s X 3 pyramidal units. The variation of the A s - X Debye-Waller factor observed over the wide range of glass compositions is interpreted by similarity with the local order in crys-

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