EXAFS AND XANES STUDY ON SPINEL STRUCTURE MATERIALS AND THEIR APPLICATION TO HUMIDITY SENSOR

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EXAFS AND XANES STUDY ON SPINEL STRUCTURE MATERIALS AND THEIR APPLICATION TO HUMIDITY SENSOR

T. Okamato, T. Suzuki, N. Matsui

To cite this version:

T. Okamato, T. Suzuki, N. Matsui. EXAFS AND XANES STUDY ON SPINEL STRUCTURE MA- TERIALS AND THEIR APPLICATION TO HUMIDITY SENSOR. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-737-C8-740. �10.1051/jphyscol:19868139�. �jpa-00226039�

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ExAPS AND XANES STUDY ON SPINEL STRUCTURE MATERIALS AND THEIR APPbICATION TO HUMIDITY SENSOR

T. OKAMATO, T. SUZUKI and N. MATSUI

Toyota Central Research and Development Laboratories, Inc.

Nagakute-cho, Aichi-ken, Japan

Abstract

The deterioration mechanism of t h e humidity sensors were investigated by Fe K-EXAFS and XANES. The humidity sensors were composed of spinel s t r u c t u r e materi a1 s , Zn o, d a ^0. 02Fe ,04 and Mg o. d a 0. 02Fe ,0 4. The former i s excel 1 ent i n d u r a b i l i t y , while t h e l a t t e r i s poor. I t was suggested t h a t t h e deterioration of Mgo- 98Nao. 02Fe204 i s a t t r i b u t a b l e t o t h e movement of Fe atoms. I t was shown t h a t a laboratory scale equipment i s extremely useful tool t o study not only t h e atomic radial d i s t r i b u t i o n , but a l s o t h e coordination geometry around excited atoms.

1. INTRODUCTION

The EXAFS system of laboratory s c a l e has received special a t t e n t i o n , because i t i s a manageable and useful equipment t o study local s t r u c t u r e s around excited atoms 1"3). Namely, t h e atomic radial d i s t r i b u t i o n and t h e coordination geometry around excited atoms can be c l a r i f i e d 4 ) by using EXAFS and XANES techniques. In measuring XANES, an SR f a c i l i t y has been generally utilized. Here, t h e coordination geometry around excited atoms can a l s o be obtained by derivative curve of XANES spectra, by using t h i s laboratory s c a l e equipment. The objects of t h i s paper a r e t o examine EXAFS and XANES spectra a t t h e Fe K-edge f o r spinel compounds, and t o describe t h e effectiveness of t h i s equipment i n 1 aboratory.

2. SPECIMENS

Commercial ZnO, MgO, C u O , NaC03, and a-Fe203 i n extra pure grades, were used in t h i s study. By mixing these materials a t optimum compositions, pressing and then s i n t e r i n g , iron spinel compounds of ZnFe204, MgFe204, CuFe204, Zn o. 98Na

,.

^02Fe20,,,

and Mgo. 98Nao. 02Fe204 were prepared. Iron oxides, Fe304 and FeO, in extra pure grades, and pure Fe metal f o i l were a l s o used f o r t h e references. The two compounds, Zno. 98Na o. 0$e2O4 and Mg o. ,3$la 0- o$e204, were used f o r humidity sensors which had been reported 'I. I t has been c l a r i f i e d from endurance t e s t s t h a t t h e durabi 1 i t y of t h e former compound i s excel 1 ent but t h e 1 a t t e r i s not.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868139

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C8-738 JOURNAL DE PHYSIQUE 3. MEASUREMENT

The spectra of EXAFS and XANES a t t h e Fe K-edge f o r spinel s t r u c t u r e materials were measured a t room temperature. A conventional X-ray generator of rota anode type (Rigaku, RU-ZOOB) was operated on t h e conditions of 17 kV

-

^{80 mA.} ^{The con-}

tinuous X-ray from t h e Mo r o t a t a r g e t was monochromatized by a Johansson cut Si(220) curved crystal. The ener& resolution of t h i s system was ca. 10 eV i n t h e v i c i n i t y of t h e Cu edge^). The absorption spectra were obtained by t h e transmission method.

4. RESULTS AND DISCUSSION

Fourier transforms, 1 b3(r) 1, and derivative curves of extracted XANES spectra a r e shown, respectively i n Figs. 1-4. The Fourier windows i n k-space of 3.5 t o 12.9 A ' were used f o r t h e Fourier transforms.

The Fe atoms have d i f f e r e n t local s t r u c t u r e s in ZnFeQ,, Fe30,, MgFe9, and CuFe204. A l l Fe atoms of ZnFeg, occupy octahedra1 s i t e s and t h e nearest neighbour (Fe-0) distance i s ca. 2.1 A. The half of Fe atoms i n CuFe204 e x i s t s i n tetrahedral s i t e s with t h e nearest neighbour (Fe-0) distance of ca. 1.9 Pi a n d t h e remaining Fe and Cu e x i s t i n t h e octahedral s i t e s . On t h e other hand, t h e Fe atoms i n MgFe8, e x i s t s i n both types of t h e s i t e s a t random, and FeH and Fe 3+ i n Fe ,, e x i s t i n tetrahedral and octahedral s i t e s , respectively. Therefore, t h e f i r s t peaks of

1 93(r) 1 lying a t -1.6 8, of Fe $3, and MgFe204 were found t o be located between those of ZnFe20

,

and CuFe20

,.

The 1 93(r) 1 f o r t h e Zno. 9$lao. o$e$4 before and a f t e r t h e endurance t e s t a r e found t o be s i m i l a r t o those of ZnFe20, and t h e i n i t i a l s t a t e of t h e

Mgo. 98Na ^0. 02Fe204, respectively and t h e nearest neighbour distances of these speci -

mens were greater than t h a t of ZnFe20,. I t can be deduced from these findings t h a t t h e sodium atoms remain i n t h e specimens. The l a t t i c e constants obtained by X-ray d i f f r a c t i o n a r e a l s o found t o support t h e r e s u l t s f o r t h e sodium mentioned above.

Besides, t h e 1 ^$3(r)1 f o r t h e Mg 0. 9$a 0. 0$e204 a f t e r t h e endurance t e s t was found t o be s i m i l a r t o t h a t of MgFe20,, and t h e nearest neighbour distance i s shorter than i n t h e i n i t i a l s t a t e .

These f a c t s i n d i c a t e t h a t whereas Zn o. 9$a o#e 20, i s a very s t a b l e material, Mg0. g8Na 0. 02Fe204 i s unstable, and t h e local s t r u c t u r e of Fe atoms i n

Mg 9$a o. 02Fe 20 i s changed from ZnFe $

,

t o MgFe204 type by t h e endurance t e s t . Extracted XANES spectra were found t o be somewhat d i f f e r e n t from each others i n t h e region of 7086 t o 7156 eV. I t i s generally convenient t o derive t h e raw spectra t o enhance t h e r e l a t i v e differences between those specimens. From t h e f i r s t deriva- t i v e curves f o r t h e references and t h e humidity sensor materials shown i.n Figs. 3 and 4, t h e following points a r e v e r i f i e d , (1) MgFe204, Fe30, and CuFe204, keeping Fe atoms i n t h e tetrahedral s i t e s , have a weak peak below t h e edge (ca. 7105 eV), while no peak was found i n ZnFe 204. (2) The curves of t h e Zn 0. 9&la 0. O.$e$

,

before and a f t e r t h e endurance t e s t and of t h e Mg 9$la o. o$e 20, i n t h e i n i t i a l s t a t e a r e

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and a f t e r the endurance t e s t , c and d: Mg

,.

^9$4a

.

^o$e^9,before and a f t e r the enjurance t e s t , respectively)

Energy (eV1

Fig. 3 Derivative curves f o r

the references. Fig. 4

a2 Q2 0

a2 7100 7114 a4

Energy ( e V )

Derivative curves f o r the humidity sensors. (a and b: Zn

,.

^9$la^o,^o$e204

before and a f t e r the endurance t e s t , c and d: Mgo.S$ao.o$e204 before and a f t e r the endurance t e s t , respectively)

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C8-740 JOURNAL DE PHYSIQUE

s i m i l a r t o t h a t of ZnFe204, and t h e peak was not found i n both sensor materials.

The curve of t h e Mgo, 98Na o. o$e204 a f t e r t h e endurance t e s t i s similar t o t h a t of MgFe,O,, and t h e peak could be c l e a r l y distinguished. I t i s well known t h a t t h e peak a t t h e low energy s i d e of t h e edge appears when absorbing elements take a Td symmetry ^{6 s}?). Therefore, i t has been thoroughly recognized from t h e peak below t h e edge t h a t Fe atoms a r e coordinated with t h e four oxygen atoms i n t h e specimen.

A l a r g e peak of t h e near edge of t h e Mgo. 9&lao. ozFe20, a f t e r t h e endurance t e s t i s a l s o s i m i l a r t o t h a t of MgFe20,, which i s s p l i t i n t o twin peaks.

5. CONCLUSION

The spectra of EXAFS and XANES a t t h e Fe K-edge f o r spinel s t r u c t u r e materials were measured by t h e EXAFS system of laboratory scale. A pair of spectra of Fourier transformed EXAFS and derived XANES were examined f o r a l l specimens. The r e s u l t s were applied t o analyse t h e deterioration mechanism of a humidity sensor of spinel type.

(1) The cause of deterioration of t h e sensor was found t o be closely related t o t h e movement of Fe atoms from octahedral t o tetrahedral s i t e s in the spinel

structure.

(2) The EXAFS system of laboratory s c a l e was found t o be extremely useful f o r studying not only t h e atomic radial d i s t r i b u t i o n s but a l s o t h e coordination geometries around excited atoms.

ACKNOWLEDGEMENTS

The authors a r e grateful t o Prof. Dr. S. Ikeda f o r h i s helpful discussions and a l s o t o Dr. H. Morimoto, Dr. A. Isogai and Dr. T. Yoshida f o r t h e i r encouragements.

REFERENCES

1) ^G. S. Knapp and P. Georgopoulos: "Laboratory EXAFS F a c i l i t y

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19801', Ed., E. A.

Stern, AIP, N Y , p.2 (1980)

2 ) K. Tohji, e t a l : Rev. Sci. Instrum., 54 (1983) 1482

3) ^T. Okamoto and Y. Fukushima: Advances i n X-ray Chem. Anal. Jpn., 17 (1986) 47 4) A. Bianconi: "Proc. Int. Conf. on EXAFS and Near Edge Structure II", Eds., A.

Bianconi , e t a1 , Splinger-Verlog, Berlin, p.118 (1982)

5) T. Suzuki and N. Matsui: 'Proc. Int. Meeting on Chem. Sensors", Kodansha, Tokyo, p.381 (1983)

6) H. Kuroda: Surface Jpn., 22 (1984) 673 7) P. Rabe, e t a l : J. Phys., C12 (1979) 1173