Zr-EXAFS study of fluoride ion-conducting ZrF4-BaF2-CsF glasses

Solid State lonics 44 ( 1991 ) 181-186 North-Holland

Zr-EXAFS study of fluoride ion-conducting ZrF4-BaF2-CsF glasses Yoji K a w a m o t o , Ryoji K a n n o , Yukihiro U m e t a n i Faculty of Science, Kobe University, Rokkodai, Nada, Kobe 657, Japan

K a z u y u k i Tohji Institutefor Molecular Science, Azasaigonaka, Myoudaiji, Okazaki 444, Japan

and Hideki M o r i k a w a Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Nagatsuta, Midori, Yokohama 227, Japan Received 3 February 1990; accepted for publication 3 October 1990

Fluorine coordination environment around zirconium in fluoride ion-conducting 5 5 Z r F 4 - ( 4 5 - x ) BaF2.xCsF glasses was investigated by means of Zr-EXAFS spectroscopy using SOR. EXAFS experiments were carried out on glasses of x = 0 , 7, 14, 20, 27, 34 and 40, and crystalline Li2ZrF6. The Z r - F peaks in the Fourier transform magnitude curves of glasses shifted to shorter distances with the substitution of CsF for BaF2. In the Z r - F peaks of the x = 27, 34 and 40 glasses, at the same time, small but discernible shoulders emerged at the right sides of the peaks, growing more prominently in this order. The Fourier backfiltering and curve fitting treatments of these Z r - F peaks revealed that, although the F coordination numbers of Zr hardly vary with the substitution of CsF for BaF2, the Z r - F interatomic distances split into two largely different ones with the substitution of CsF more than about 20 mol%. For instance, the Z r - F interatomic distances and the F numbers at the distances were 2.07 A/6.5 in the 55ZrF4.45BaF2 glass, 2.06 A/6.4 and 2.27 A/0.6 in the 55ZrF4.18BaFE.27CsF glass, and 2.05 A/5.1 and 2.23 A/1.9 in the 55ZrF4.5BaF2.40CsF glass. The present Zr-EXAFS result may give an interpretation for the anomalous compositional dependence of the activation energy for conduction in electrical conductivity that was previously observed for ZrF4-BaF2-CsF glasses.

1. Introduction

Zirconium tetrafluoride-based glasses have become of interest as a vitreous solid electrolyte of fluoride ions. The ionic conductivity, however, is the order of 10 -6 S c m - l at 200°C. For practical use of these glasses, therefore, enhancement of the conductivity is necessitated. For improving the conductivity, the elucidation of fluoride ion conduction-governing factors would be required. A previous study on the electrical conductivity of ternary ZrF4-BaFz-MFn (M: the group I - V I metals) glasses clarified that the conductivity of ZrF4based glasses is predominantly determined by the activation energy for conduction and that the activa-

tion energy for conduction decreases with an increase in the average polarizability of glassconstituting cations [ 1 ]. On the other hand, a study on the compositional dependence of the conductivity of ZrF4-BaF2-CsF glasses suggested that the activation energy for conduction is largely influenced by the Z r - F bond distance (rzr-F) and/or by the F coordination number of Zr (CNF) [ 2 ]. The validity of this suggestion should be proved by examining the rzr_F and CNF values in these glasses. The most appropriate technique for proving this is considered to be EXAFS (extended X-ray absorption fine structure) spectroscopy. The compositional variation of rzr-v and C N F in glasses 55ZrF4. ( 4 5 - x ) B a F 2 . x C s F , which was examined by Zr-EXAFS experiments us-

0167-2738/91/$ 03.50 © 1991 - Elsevier Science Publishers B.V. ( North-Holland )

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Y. Kawamoto et al. / Z r - E X A F S study

ing SOR (synchrotron orbital radiation), is reported here.

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Glasses chosen for Zr-EXAFS experiments are 55ZrF4. ( 4 5 - x ) B a F 2 . x C s F glasses of x = 0 , 7, 14, 20, 27, 34 and 40 in tool%. The detailed procedure for the glass preparation is described elsewhere [ 2 ]. No inclusion of crystalline precipitates in the prepared glasses was ascertained by observing the glasses under a polarized optical microscope. Crystalline Li2ZrF6 was prepared as a reference specimen in EXAFS analysis. This was synthesized according to a procedure described in the literature [3 ] by using guaranteed reagent grade chemicals of LiF and ZrF4 as the raw materials. The synthesized Li2ZrF6 was identified to be the single-phase by Xray powder diffraction measurement. For EXAFS measurements these prepared specimens were finely powdered under an Ar atmosphere in a glove box and pressed into thin discs with polyethylene powder.

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2.2. EXAFS measurement The Zr-EXAFS measurements of glasses 55ZrF4. ( 4 5 - x ) B a F 2 . x C s F and crystalline Li2ZrF6 were carried out using the EXAFS facilities at BL10B of Photon Factory in National Laboratory for High Energy Physics. The data were collected in the energy range from about 1000 eV on the lower-energy side of the Z r - K absorption edge (about 18 keV) to about 1000 eV on the higher-energy side. The incident and the transmitted X-ray intensities were measured by the (75%N2+25%Ar) and the 100%Kr ion chambers, respectively. X-ray absorption spectra observed for the x = 0 , 20 and 40 compositions of 55ZrF4" ( 4 5 - x ) B a F z . x C s F glasses and the Li2ZrF6 crystal are shown by solid lines in fig. 1.

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Fig. 1. The Z r - K absorption spectra and the EXAFS oscillation curves of crystalline Li2ZrF6 (L ) and glasses 55ZrF4. ( 4 5 - x ) B a F z . x C s F ( x = 0 (A), 20 (B) and 40 (C)).

2.3. EXAFS analysis For EXAFS analysis the measured X-ray absorption spectra were treated according to the Teo procedure [4]. In the extraction of EXAFS oscillation from X-ray absorption spectra, the background absorption was calculated by employing the Victreen formula extrapolation, while the smooth K-shell absorption due to an isolated Zr atom was estimated by using the iterative smoothing method. The energy of absorption edge for Zr was taken as 18000 eV and the EXAFS oscillation was converted into the k space. The z ( k ) curves obtained for crys-

Y. Kawamoto et al. / Zr-EXAFS study talline Li2ZrF6 and glasses 5 5 Z r F a . ( 4 5 - x ) × BaF2.xCsF ( x = 0 , 20 and 40) are shown by d o t t e d lines in fig. 1. The z ( k ) was weighted by k 3 a n d F o u r i e r - t r a n s f o r m e d in the k region from 3.5 to 13.5 A -1 to obtain the F o u r i e r transform magnitude ( I F ( r ) l). A smooth window function used to select out a range o f the z ( k ) d a t a is the H a n n i n g function

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2.4. Fourier backfiltering and curve fitting In order to obtain the Z r - F i n t e r a t o m i c distance and the F c o o r d i n a t i o n n u m b e r o f Zr in glasses 5 5 Z r F 4 . ( 4 5 - x ) B a F 2 ' x C s F , the F o u r i e r backfilterTable 1 The peak maxima (Rmax) and half-widths (HW) of Zr-F peaks in the Fourier transform magnitude curves of 55ZrF4- (45 - x ) BaF2.xCsF glasses. Glass composition x 0 7 14 20 27 34 40

ZrF4 (mol%)

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Fig. 2. The Fourier transform magnitude curves of glasses 55ZrF4.(45-x)BaFz.xCsF (x=0, 7, 14, 20, 27, 34 and 40). The r regions sandwiched between two broken lines are those filtered for Fourier backfiltering. ing and curve fitting treatments o f the Z r - F peaks in I F ( r ) l were conducted. The curve fitting was perf o r m e d on k 3 z ( k ) in the k range from 4.0 to 10.5 A - l by the least squares method. The phase shift and backscattering a m p l i t u d e values which are necessary in performing the curve fitting treatment were obtained by employing the Li2ZrF6 crystal as a reference specimen. As the Z r - F interatomic distance and the F c o o r d i n a t i o n n u m b e r o f Zr for the Li2ZrF6 crystal, the X-ray structure analysis values o f 2.016 A and 6 were a d o p t e d [3]. In the F o u r i e r backfiltering treatment the Z r - F peaks in the r regions indicated by broken lines in fig. 2 were filtered and the Hanning window function

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Y. Kawamoto et al. / Z r - E X A F S study

184

was employed to make cutoffs smooth. The r 2 and r 3 values taken for the respective glasses are indicated by dotted lines in fig. 2. It can be seen from fig. 2 that small but discernible shoulders emerged around 1.8 A at the right-hand sides of Z r - F peaks of glasses x = 2 7 , 34 and 40. So the curve fitting treatment for these glasses was performed by using a "two shell" model, instead of a "one shell" model used for glasses x = 0 , 7, 14 and 20. The k 3 z ( k ) curves obtained for glasses x = 0 , 20 and 40 by the Fourier backfiltering and by the leastsquares curve fitting are shown by the dotted and the solid lines in fig. 3, respectively. The structure parameters obtained for the 55ZrF4. ( 4 5 - x ) B a F 2 - x C s F glasses are given in table 2.

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Before entering into discussion on the present EXAFS result, the compositional dependence of the activation energy for conduction (AE) observed for the electrical conductivity of ZrF4-BaF2-CsF glasses is described [2 ]. Closed circles in fig. 4 show the compositions of ZrF4-BaF2-CsF glasses for which the ionic conductivities were measured. Fig. 5 shows the AE values of these glasses, which are plotted against the average polarizability of glass-constituting cations ( a t ) . As

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Table 2 Structural parameters ( Z r - F interatomic distances (rz,_v), F coordination numbers of Zr (CNF) and Debye-Waller factors (tr)) obtained for 55ZrF4. ( 4 5 - x ) B a F 2 . x C s F glasses by Fourier backfiltering and curve fitting. Values for glasses x = 0, 7, 14 and 20 were obtained by applying a "one shell" model and those of glasses x = 27, 34 and 40, by applying a "two shell" model. Glass composition x

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Fig. 5. Dependence of the activation energy for conduction (AE) on the average polarizability of glass-constituting cations (ct¢) in ZrF4-BaF2-MF. (M: the group I-VI metals) glasses ( O ) and ZrF4-BaF2-CsF glasses ( • ).

described in introduction, the electrical conductivity study of ternary ZrFa-BaF2-MFn (M: the group 1VI metals) glasses clarified that the conductivity of ZrF4-based glasses is predominantly determined by AE and that the AE value decreases with an increase in the average polarizability of glass-constituting cations, as shown by open circles in fig. 5 [ 1 ]. In the X, Y and Z series of ZrF4-BaF2-CsF glasses

185

in fig. 4, the ZrF4 content is constant and BaF2 is replaced by CsF. In each series of glasses, therefore, the ac value becomes large as BaF2 is replaced by CsF since the polarizability values of Ba 2+ and Cs ÷ are 1.69 and 2.40 A 3, respectively. Accordingly the AE values in each series of glasses should become small with the BaF2--. CsF replacement. As can be seen from fig. 5, however, the AE values of the X, Y and Z series of glasses did not decrease linearly with increasing ac. On the contrary the AE values became large when the replaced CsF contents exceeded about 15 tool%. This anomalous ac dependence of AE was interpreted as follows. On the F coordination environment around Zr in crystalline fluorozirconates, three kinds of the F coordination numbers and seven kinds of the F configurations have been reported so far [ 5 ]. Those are octahedron for 6-coordination; monocapped octahedron, pentagonal bipyramid and monocapped trigonat prism for 7-coordination; and dodecahedron, bicapped trigonal prism and antiprism for 8-coordination. Therefore, a wide variety of F coordination environments may be anticipated for Zr in ZrF4based glasses, depending upon the glass compositions. In the 13-BaZrF6, ct-BaZrF6 and Cs2ZrF6 crystals which are present in the ZrFa-BaF2-CsF system, for instance, the average rzr-v and the CNF values are 2.12 A/8, 2.08 ,~/7 and 2.04 ~,/6, respectively [68]. If ZrF4-BaF2-CsF glasses exhibit similar compositional variations in the F coordination environment around Zr, then the substitution of CsF for BaF2 would decrease rzr-F and/or CNF. As already described, on the other hand, the BaF2~CsF substitution increases ac. As the result, the decreased rzr-v and/or CNF, and the increased ac act oppositely on the change of AE. The combined effect of these two competing factors would be responsible for the anomalous ac dependence of AE. Actually, however, there is no evidence as to whether the substitution of CsF for BaF2 in the ( 1 0 0 - a ) Z r F 4 . (a-b)BaF2.bCsF glasses causes rzrF and/or CNF to change or not. The objective of the present EXAFS study is to examine this. The 55ZrF4-(45-x)BaF2.xCsF glasses chosen for the EXAFS experiment are the X series of ZrF4-BaF2CsF glasses shown by open circles in fig. 4. Then the present EXAFS result is discussed. First

186

Y. Kawamoto et aL / Z r - E X A F S study

let us consider the I F ( r ) l curves of the 5 5 Z r F 4 - ( 4 5 - x ) B a F 2 . x C s F glasses (see fig. 2 and table 1 ). The peaks at around 1.5-1.6 A in the I F ( r ) I curves are due to the Z r - F interatomic distances. The position and shape of these Z r - F peaks qualitatively indicate that the average Z r - F interatomic distance becomes shorter with the progressive substitution of CsF for BaF2 and that the Z r - F interatomic distances of glasses of x = 27, 34 and 40 appear to split into two kinds, i.e. a shorter one and a longer one. Although no distinct shoulder could be noticed in the Z r - F peak of the x = 20 glass, a little wide half-width of the peak, compared with the half-widths of the Z r F peaks of the x = 0 , 7 and 14 glasses, suggests that a splitting o f the Z r - F interatomic distance already takes place even in this glass. Next, let us consider the structure parameters obtained from the Fourier backfiltering and curve fitting of the Z r - F peaks (see table 2). The F coordination number o f Zr appears to increase slightly with the substitution of CsF for BaF2. Taking into account an accuracy of the coordination number evaluation in an EXAFS analysis, however, it will be reasonable to consider that the F coordination of Zr hardly changes with composition. On the other hand, the variation of the Z r - F interatomic distances with the B a F 2 ~ C s F substitution revealed the following: There are little changes in the Z r - F interatomic distances of glasses x = 0, 7 and 14, but those of glasses x = 2 7 , 34 and 40 split into two different distances, one of which being shorter and the other o f which being longer than the Z r - F interatomic distances in the x = 0 , 7 and 14 glasses. In the x = 2 7 , 34 and 40 glasses the two Z r - F interatomic distances and the F numbers participating in the respective distances are largely different with the substituted BaF2 contents. No "two shell" model could be applied to the Z r - F peak of the x = 20 glass because of no distinct shoulder, but there is no doubt as to the occurrence of a splitting of the Z r - F interatomic distance, as already mentioned above. The present EXAFS study clarified that, in the 5 5 Z r F a - ( 4 5 - x ) B a F 2 . x C s F glasses, the F coordination environment of Zr begins to change greatly when the substituted BaF2 content exceeds around 14-20 mol%. In almost the same composition region the glasses reversed the ac dependence o f AE, as can be seen from fig. 5. Accordingly it may be deduced that there is a close correlation between a change in

F coordination environment with the BaF2-~CsF substitution and that in ( A E - a c ) dependence with the BaF~ ~ CsF substitution. Consequently it will be permitted to draw a conclusion that the activation energy for conduction is affected by the F coordination environment around Zr, especially by the Z r F bond length. Thus the present Z r - E X A F S study proved that the assumption proposed in the previous paper [2] is valid and that, besides the average polarizability of glass-constituting cations, the F coordination environment around Zr also is one o f the fluoride ion conduction-governing factors in ZrF4based glasses. In closing, the following should also be added: a more recent ~9F C W - N M R and electrical conductivity studies of ZrF4-BaF2 glasses revealed that F participating in the Z r - F - B a bridging (non-bridging fluorine atoms) is more mobile than F participating in the Z r - F - Z r bridging (bridging fluorine atoms), implying that the proportion of non-bridging fluorine atoms to bridging fluorine atoms is also one o f the fluoride ion conduction-governing factors in ZrF4-based glasses [9 ].

Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research (Priority Areas, New Functionality Materials-Design, Preparation and Control) from the Ministry of Education, Science and Culture (No. 63604570).

References [ 1] Y. Kawamoto and I. Nohara, Nippon Kagaku Kaishi ( 1985 ) 1783. [2] Y. Kawamoto and I. Nohara, Solid State Ionics 22 (1987) 207. [3] G. Brunton, Acta Cryst. B29 (1973) 14. [4] B.K. Teo, in: EXAFS: Basic principles and data analysis (Springer, Berlin, 1986). [5] Y. Kawamoto, Materials science forum (Trans Tech Publ., Switzerland) 6 (1985) 417. [ 6 ] J.P. Laval, D. Mercurio-Lavaudand B. Gaudreau, Rev. Chim. Miner. ll (1974) 742. [ 7 ] J.P. Laval, R. Papiernik and B. Frit, Acta Cryst. B34 (1978 ) 1070. [8] H. Bode and G. Teufer, Z. Anorg. Allg. Chem. 283 (1956) 18. [9] Y. Kawamoto and J. Fujiwara, Phys. Chem. Glasses 31 (1990) ll7.