The structure of AgI-Ag2O-V2O5 glasses

SOLID SrAtE lofws

Solid State Ionics 90 (1996) 125-128

EISEVIER

The structure of AgI-Ag,O-V,O, H. Takahashi”‘*,

glasses

Y. Hikib, T. Sakuma”, Y. Moriid

“Applied Physics Group, Faculty of Engineering, Ibaraki University, Hitachi 316, Japan ‘Tokyo Institute of Technology, Emeritus 39-3-303 Motoyoyogi, Shibuya-ku, Tokyo 151, Japan ‘Department of Physics, Faculty of Science, Ibaraki University, Mito 310, Japan *Japan Atomic Energy Research Institute, Tokai 319-11, Japan

Received 5 December 1995; accepted 14 May 1996

Abstract Structures of lOAgI-3Ag,O-2V,O,, 3AgI-3Ag,O-2V,O, and 2AgI-2Ag,O-V,O, glasses have been investigated by neutron diffraction experiments. The characteristic features of observed structure factors S(Q) in lOAgI-3Ag,O-2V,O, glass is similar to those of other superionic conducting glasses and molten AgI. From the standpoint of the pair distribution functions, it is clarified that the Ag-I and I-I correlation strength and Ag-Ag correlation length increase with increasing AgI concentration. Observed results suggest that the local AgI structure accompanied by the re-arrangement of silver ions is formed with highly doped iodide ions. Keywords: A@-Ag,O-V,O,

glasses; Neutron diffraction; Glass structure

1. Introduction In recent decades, much attention has been focused on superionic conducting glasses because of their high ionic conduction and application to batteries and other electrochemical devices. Among these, AgI doped glasses are frequently investigated, since for these systems it is easy to form the glassy state and they possess high ionic conductivity near room temperature. Silver oxysalts such as AgIAgPO, or AgI-Ag,O-B,O, systems are typical superionic conducting glasses [l]. It can be clarified that Ag and I ions occupy nearest neighbour sites in the glass, and that the structure of the glass forming component is not affected by the addition of AgI, as *Corresponding author.

confirmed by neutron [2] and X-ray [3] diffraction. However, the local structure of the AgI component and the conduction mechanism of the Ag ions are not known. One of the difficulties in analysing the structure of superionic conducting glasses is that the X-ray or neutron scattering by the glass forming component is dominant compared with the scattering by the AgI component. The structures of AgI-Ag,O-V,O, glasses have been investigated by X-ray diffraction and EXAFS experiments [4,5]. The glass system AgI-Ag,OV,O, is suitable for studying the local structure of AgI in the glass, because (i) wide glass forming region of this system and (ii) the reduction of the number of observed pair correlations by the very low coherent scattering cross-section of vanadium for neutron.

0167-2738/96/$15.00 Copyright 01996 Elsevier Science B.V. All rights reserved PII SOl67-2738(96)00413-4

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H. Takahashi et al. I Solid State tonics 90 (1996) 125-128

The purpose of the present investigation is to measure the structure factors of the system AgIAg,O-V,O, glasses by the neutron diffraction and to obtain information about characteristic features of the AgI local structure.

2. Experimental The glass forming region for the system AgIAg,O-V,O, has been determined by Minami et al. [6]. Glasses of the compositions, lOAgI-3Ag,O2V,O,, 3AgI-3Ag,O-2V,O, and 2AgI-2Ag,OV,O,, were prepared by rapidly quenching the melt. Hereafter, the glass with composition xAgI-yAg,OzV,O, is expressed as (x, y, z) glass for the simplicity of notation. Appropriate amounts of AgI, Ag,O and V,O, were mixed and melted in a Pyrex crucible at 500-600°C depending on the sample composition. The sample was poured on a stainless-steel block and pressed by another block. By this method, rapid quench and glass formation of a thin sample can be realized. The densities of (10, 3, 2), (3, 3, 2) and (2, 2, 1) glasses were found to be 6.040 g/cm3, 5.677 g/cm3 and 6.071 g/cm3, respectively. Obtained glass plates with dark red colour were ground and sealed into a cylindrical vanadium container for the neutron diffraction experiment. The inner diameter and length of the container were 12 and 40 mm, respectively. The neutron diffraction measurements were carried out at room temperature with the double-axis diffractometer installed at JRR-2 at the Japan Atomic Energy Research Institute (JAERI). The reflection from the (002) plane of a pyrolytic graphite crystal monochrometer was used to produce a beam with a wavelength A = 1.0 A. In a typical experimental run, the neutron intensity was recorded in the range 5” < 28 < 105” at 0.1” intervals.

multiple scattering and Placzek corrections were made as described in previous work [7]. The observed structure factors S(Q) for the glasses are shown in Fig. 1. Relatively low AgI doped glasses (3, 3, 2) and (2, 2, 1) have a shoulder around 3 A-‘. The intensity of this shoulder seems to increase in (10, 3, 1) glass. These characteristic features in S(Q) correspond with those of (AgI),(AgPO,), _* superionic conducting glasses [3]. Moreover it is worth noting that the structure factor S(Q) of (10, 3, 1) glass is very similar to that of other heavily AgI doped superionic conducting glasses or molten AgI [8]. These facts seem to indicate that the AgI substructure of several superionic conducting glasses and the structure of molten AgI are alike. It is known that much of the superionic conducting glasses have a prepeak at about 0.7-1.0 A-‘. Rousselot et al. [9] have investigated the prepeak of these glasses and discussed in detail the conduction mechanism in relation to the prepeak. The marked prepeak, however, is not observed even in the AgI rich composition of the present glasses. Fig. 2 shows the pair distribution functions g(r) obtained from the experimental structure factor S(Q) in Fig. 1 using the Fourier transformation of Eq. (1).

n

n

3. Results and discussion 0 Experimental scattering profiles obtained by the neutron diffraction were fitted to the smoothed curves by the least squares spline fitting method, and several experimental corrections such as absorption,

2

4

Q Fig. 1. Structure system.

factors

S(Q)

6

8

10

(A-‘> of the AgI-Ag,O-V,O,

glass

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H. Takahashi et al. i Solid State Ionics 90 (1996) 125-128

+ Ag-I

-1

1

2

1 Ag-Ag

3

4

5

6

r (A) 0

2

1

3

4

5

Fig. 3. (a) Difference between pair distribution functions (10, 3, 2) and (3, 3, 2). (b) Difference between pair distribution functions (3, 3, 2) and (2, 2, 1). Upward directed arrows in (a) and (b) indicate that the corresponding ion pairs of (IO, 3, 2) and (3, 3, 2) glasses am dominant, respectively. Downward directed arrows in (a) and (b) indicate that the corresponding ion pairs of (3, 3, 2) and (2, 2, 1) glasses are dominant, respectively.

6

r (A) Fig. 2. Pair distribution glass system.

functions

1 g(r) = 1 + ~ 277’rfo

g(r) of the AgI-Ag,O-V20,

[S(Q) - 1lQ WQd dQ,

(1)

where p0 is the number density of the glass. AgIAg,O-V,O, glass system has 4 elements. So 10 pairs of partial distribution functions are included in the observed g(r). As mentioned in the introduction, however, the coherent scattering cross-section of vanadium for neutron diffraction is very small. The number of partial pair distribution functions effectively reduces to six. Representative interionic distances of ion pairs estimated by crystal data, such as AgI and Ag,O are also indicated in Fig. 2. Depending on the concentration of Ag and 0 ions, the height of the shoulder at 2.3 A increases with decreasing AgI content. The peak at around 2.8 A consists of Ag-I and O-O pairs. The scattering intensity from O-O pair is comparable to that of the Ag-I pair. Therefore quantitative discussion including coordination number is eliminated in this article. To investi-. gate the contribution to pair distribution function of each ion pair, differences between pair distribution functions A&)

= &X10,

3731

A&)

= &X(3,3,2)1

- &-)[(3,3,2)1, - de[G

are defined. Difference functions shown in Fig. 3. The upward

2, 111, described above are and the downward

directed arrows indicated in Fig. 3 correspond to the positive and negative signs of the difference between the scattering power for each of the ion pairs, which depends on the atomic concentration and atomic scattering cross-section for neu$ons. In (a) of Fig. 3, the negative region around 2.3 A, positive region at 3 A, 3.8 A and 4.3 A in the difference between pair distribution functions g(r) (10, 3, 2) and (3, 3, 2) represent that Ag-0, Ag-I, Ag-Ag and I-I pair correlations are dominant, respectively. The positive region at 2.7 A and negative region at 3.7 A in the difference between g(r) (3, 3, 2) and (2, 2, 1) in (b) mainly arises from O-O and Ag-Ag pairs, respectively. According to these results, following information is obtained. Firstly, Ag-I correlation is not notable in (3, 3, 2) and (2, 2, 1) glasses in comparison with (10, 3, 2) glass. Secondly, on the contrary, Ag-0 correlation intensity is appreciable in (3, 3, 2) and (2, 2, 1) glasses. Thirdly, the second peak around 3.7 A observed in Fig. 2 mainly originates from Ag-Ag correlation. And fourthly, I-I correlation appears in (10, 3, 2) glass. Therefore Ag-0, Ag-I, O-O and Ag-Ag correlations are predominant in these system. On the basis of above results, the nearest neighbour distances of ion pairs determined by the Gaussian fitting to the radical distribution function are shown in Table 1. As a whole, interionic distances of Ag-0 and Ag-I pairs scarcely vary with com-

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H. Takahashi et al. I Solid State Ionics 90 (1996) 125-128

Table 1 Interionic distances (A) of predominant ion pairs in AgI-Ag,OV,O, glasses by the Gaussian curve fitting method Ion pair

(2, 2, 1)

(3,3, 2)

(10,3,2)

Ag-0 Ag-I o-o

2.35 2.87 3.00 3.66

2.30 2.82 2.93 3.63

2.33 2.88 2.98 3.80

Ag-Ag

position. On the other hand, interionic distance of the Ag-Ag pair clearly increases with increasing AgI concentration, although the average number density of Ag ions is almost the same with composition. The increase of Ag-Ag interionic distance is also clear from the position of the second peak in Fig. 2. In conclusion, there is strong Ag-I correlation at 2.85 A, I-I correlation at about 4.3 A and rearrangement of Ag ions appears in the heavily AgI doped glass. In (10, 3, 2) glass, Ag/I ratio reaches 1.6. It is considered that the conduction path for silver ions is formed by an array of adjacent iodide ions. It has been proposed that the AgI cluster exists in the superionic conducting glasses. The possibility of density fluctuation which is caused by the large clusters was not found in the present experiment. Rather it seems that a dispersed conduction network is formed in the present glass system. It is necessary for the understanding of conduction mechanism in AgI doped superionic conducting glasses to clear the iodide network structure. It is expected that the detailed information on the AgI substructure would also be obtained by X-ray diffraction in the heavily AgI doped glass, such as (10,

3, 2) glass. An X-ray diffraction experiment on (10, 3, 2) glass is now in progress. Quantitative discussion of AgI substructure in (10, 3, 2) glass in terms of both X-ray and neutron diffraction measurements will be published elsewhere.

Acknowledgments The authors thank Dr. N. Minakawa and Mr. Y. Shimojo (JAERI) for their help with the neutron scattering measurements.

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