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>
HAL Id: jpa-00226046
https://hal.archives-ouvertes.fr/jpa-00226046
Submitted on 1 Jan 1986
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JOURNAL DE PHYSIQUE
Colloque C8, suppl6ment au no 12, Tome 47, d6cembre 1986
EXAFS AND XANES STUDIES ON La,-,Sr,Co03
.
COMPOUNDS (x=0,0.2,0.5)
H KAGEYAMA, N. KAMI JO, M. NARUKAWA* , F
and K. TANIGUCHI*
. MARUYAMA* ,. Y .
NAKAO*
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
3
from 12.0 at 60 K to 8.5 at 300 K. There was little difference etween the 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 conductivity 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
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 Physics8)
both at 60 K and 300 K, and with a laboratory-type X-ray spectrometer
Data analysis was made according to a standard method.'')
at 300 K.')
SrTiOg and Co(a~ac)~were used as standard samples for Sr-0 and Co-0
atom pairs, respectively.
J
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
4J
lower energy by about 2 eV as compared
with that of LaCo03 (Fig. 1). Both the
main absorption peak and the threshold
:.
w
of Sr K-edge of La0.5Sr0.5C003 measured iec
at 300 K are found to be shifted to
o
U
lower energy by 2 eV as compared with
c
that measured at 60 K(Fig. 2). The
-rl
4~
a
XANES regions of Co K-edge of the
&I
conpounds at 60 K and 300 K are all
e
similar except for the main absorption
peaks(Fig. 3).
The Fourier transform
3
of k -weighted EXAFS oscillation about
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 KFig. 1. The XANES spectra of La
edges are summarized in Tab. 1. The L -edge of three compounds. The
3
coordination number of oxygen around
energies labelled " ~ d ~ ecorrespond
"
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.
-5
V)
co
K-edge
Peak
I
Photon Energy (eV)
Photon Energy ( e V )
Fig. 3. The XANES s p e c t r a of Co
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 of 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 peaks.
Fig. 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 of 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 peaks.
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.
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 about
Co K-edge a t 60 K and 300 K.
JOURNAL DE PHYSIQUE
C8-760
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
N
SrTiO (standard)
(12.0)
3
Co (acac) (standard)
LaCo03(60 K)
(300 K)
12.0
La0.8Sr0.2C~03(60 K)
(300 K)
8.5
La0.5Sr0.5C00 (60 K)
11.3
3 ( 3 ~ ~ ~ ) 6.7
R[L]
(2.761)
-
2.37
2.63
2.67
2.64
Co-0
o[KI
N
-
-
0.09
0.10
0.10
0.10
(6.0)
6.4
5.6
4.7
5.4
5.3
5.0
~ [ i l
(1.888)
1.92
1.92
1.91
1.92
1.91
1.91
o[J(l
0.09
0.09
0.10
0.09
0.09
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.
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211(1978).
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(19771, p. 129.
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1679.
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