EXAFS AND XANES STUDIES ON La1-xSrxCoO3 COMPOUNDS (x=0, 0.2, 0.5)

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EXAFS AND XANES STUDIES ON La1-xSrxCoO3 COMPOUNDS (x=0, 0.2, 0.5)

H. Kageyama, N. Kamijo, M. Narukawa, F. Maruyama, Y. Nakao, K.

Taniguchi

To cite this version:

H. Kageyama, N. Kamijo, M. Narukawa, F. Maruyama, Y. Nakao, et al.. EXAFS AND XANES

STUDIES ON La1-xSrxCoO3 COMPOUNDS (x=0, 0.2, 0.5). Journal de Physique Colloques, 1986,

47 (C8), pp.C8-757-C8-760. �10.1051/jphyscol:19868144�. �jpa-00226046�

EXAFS AND XANES STUDIES ON La,-,Sr,Co03 COMPOUNDS (x=0,0.2,0.5)

H

.

KAGEYAMA, N. KAMI JO, M. NARUKAWA*

,

F

.

^MARUYAMA*

,

. Y

.

^NAKAO*

and K. TANIGUCHI*

Government Industrial Research Institute, Osaka, Ikeda, Osaka

563,

Japan

"0saka Electro-communication University, Neyagawa, Osaka

573,

Japan

Abstract

The local structure around the Sr and the Co atoms in the title compounds calcined at 850°C-900°C were investigated by EXAFS and XANES spectroscopy. Both the main absorption peak and the threshold energy of Sr K-edge of La0.5Srg.5CoOp at 300 K was found to be shifted to lower energy by about 2 eV as compare with that measured at 60 K. The coordination number of the oxygen around the Sr atom of La

0 . 5 ~ ~ 0 . 5 ~ 0 ~ reduced from 11.1 at 60 K to 6.7 at 300 K, and that of Lao.8Sro.6Co0 also decrease2 from 12.0 at 60 K to 8.5 at 300 K. There was little difference etween the 3 XANES regions of Co K-edge and the coordination numbers of the oxygen around the Co atoms of three compounds at 60 K and 300 K. The oxide ion vacancy localized around the Sr atom already existed at 300 K before reaching to the higher temperature that was reported to be enough for the creation of the oxide ion vacancy.

1. Introduction

Perovskite compounds Lal-xSr~Co03(x=O-0.6) are well-known ionic conductors that have large diffusion coefficients of

02-

ions as compared with other metal oxides. It is generally considered that the doping of the Sr ion to the A site in LaC00~(La~,~Sr~Co0~) gives rise to the creation of the co4+ species

when x

is less than 0.4, and leads to the formation of the oxygen deficiency when x is greater than 0.4. The electrical conductivity of this solid solution system is

considered to be caused by the

-co~+-o-co~+-

superexchange interaction.

3 , It was reported that the drastical decrease of electrical conduc-

tivity observed at temperatures higher than 650°C-850°C for these

compounds indicates the creation of considerable amount of oxygen deficiency at this temperature.

4 ,

Recently title compounds have been elucidated to have high catalytic activity 05 the oxidation of CO, hydrocarbons and alcohols5) and the elimination of NO. 6, Especially the highly active catalysts prepared at about 850°C are of much interest. According to Misono

et al.

the temperature programed desorption(TPD) study of the catalysts indicates that

the

desorptron

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868144

C8-758 JOURNAL DE PHYSIQUE

of oxygen and the reduction tend to become easier as the amount of the sr2+ increase^.^) In this study the structural difference between Perovskite samples synthesized at 850°C-900°C are investigated by EXAFS and XANES spectroscopy.

2. Experimental

The samples were prepared as follows: a concentrated solution of citric acid was added to a concentrated solution of metallic nitrates.

The solution was dried up and calcined finally at 850°C for LaCoO, and

J

La0.8Sr0.2C003, and at 900°C for La0.5Sr0.5C003. The formation of a single Perovskite phase was confirmed by X-ray diffraction. The measurement of X-ray absorption spectra was carried out at Beam Line 10B of Photon Factory in National Laboratory for High Energy Physics 8) both at 60 K and 300 K, and with a laboratory-type X-ray spectrometer at 300 K . ' ) Data analysis was made according to a standard method.'') SrTiOg and Co(a~ac)~ were used as standard samples for Sr-0 and Co-0 atom pairs, respectively.

3. Results and Discussion

The XANES spectra of La L3-, Sr K- and Co K-edges of three samples are shown in Figs. 1-3, respectively. The

main absorption peak of La L3-edge of La0.8Sr0.2C003 is found to shift to

lower energy by about 2 eV as compared

4J

with that of LaCo03 (Fig. 1). Both the -5

main absorption peak and the threshold .:

of Sr K-edge of La0.5Sr0.5C003 measured

w ^iec

at 300 K are found to be shifted to

_U o

lower energy by 2 eV as compared with

c

that measured at 60 K(Fig. 2). The

-rl 4~

XANES regions of Co K-edge of the

^a &I

conpounds at 60 K and 300

K

are all

e V)

similar except for the main absorption peaks(Fig. 3). The Fourier transform of k -weighted EXAFS oscillation about 3 Sr K- and Co K-edges of three compounds are shown in Figs. 4 and 5, respectively.

5460 5480 5500 5520

The results of curve fitting analysis

Photon Energy(eV) of the compounds about Sr K- and Co K-

Fig. 1. The XANES spectra of La

edges are summarized in Tab. 1. The

L -edge of three compounds. The

coordination number of oxygen around

energies labelled " ~ d ~ e " 3 correspond to the first maxima of the derivative

the

S r

atom of La0. 5Sr0. 5C003 reduces

spectra, and those labelled "Peak" to

from 11.1 at 60 K to 6.7 at, 300 K, and

the top of the main absorption peaks.

P h o t o n Energy ( e V ) F i g . 2. The XANES s p e c t r a o f S r K-edge of t h r e e compounds. The e n e r g i e s l a b e l l e d "Edge" c o r r e s p o n d t o t h e f i r s t maxima o f t h e d e r i v a t i v e s p e c t r a , and t h o s e l a b e l l e d "Peak" t o t h e t o p of t h e main a b s o r p t i o n p e a k s .

Distance

(A)

F3g. 4. The F o u r i e r t r a n s f o r m of k -weighted EXAFS o s c i l l a t i o n a b o u t S r K-edge a t 60 K and 300 K.

I

Photon Energy (eV) F i g . 3. The XANES s p e c t r a o f Co K-edge o f t h r e e compounds. The e n e r g i e s l a b e l l e d "Edge" c o r r e s p o n d t o t h e f i r s t maxima o f t h e d e r i v a t i v e s p e c t r a , and t h o s e l a b e l l e d "Peak" t o t h e t o p o f t h e main a b s o r p t i o n p e a k s .

Distance (A)

F3g. 5. The F o u r i e r t r a n s f o r m of k -weighted EXAFS o s c i l l a t i o n a b o u t Co K-edge a t 60 K and 300 K.

C8-760 JOURNAL DE PHYSIQUE

Tab. 1. Results of curve fitting analysis for the firstoSr-0 and Co-0 peaks.

Coordination numbers N, interatomic distances R [ A ] , and Debye-Waller factors o [ i ] determined by the least-squares refinement.

Sample Sr-0 Co-0

N

R [ L ]

o[KI N ~ [ i l o[J(l

SrTiO (standard) (12.0) (2.761) - - - -

3

Co (acac) (standard) - - - (6.0) (1.888) -

LaCo03(60 K)

- -

- 6.4 1.92 0.09

(300 K)

-

^- - 5.6 1.92 0.09

La 0.8Sr0.2C~03(60 K ) 12.0 2.37 0.09 4.7 1.91 0.10 (300 K) 8.5 2.63 0.10 5.4 1.92 0.09 La0.5Sr0.5C00 (60 K) 11.3 2.67 0.10 5.3 1.91 0.09 3 ( 3 ~ ~ ~ ) 6.7 2.64 0.10 5.0 1.91 0.09

that of La0,8Sr0.2C003 decreases from 12.0 at 60 K to 8.5 at 300 K.

The Sr-0 distances of these two compounds are shortened

by

0.03-0.04 A

as temperature elevates. These two results mean that the removal of the oxygen around the Sr ions occurs, which is in agreement with the shift

of Sr K-edge to lower energy side. While the coordination numbers of the oxygen around the Co atoms and the Co-0 distances of the compounds are not varied by doping of the Sr ion or elevating temperature. The results above imply that the oxide ion vacancy localized around the Sr atom already exists at relatively low temperature like 300 K. The order of the facility of the oxygen desorption as given by the TPD study7) is also explained by these results. As for the Co atom, the -Co-0-Co- chain utilized for the electrical conduction is considered to be maintained even at 300 K. More precise measurements under various pressures or at higher temperatures should be necessary to understand the changes in the electrical conductivity, the mechanism of ionic conduction and the catalysis.

REFERENCES

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