EXAFS AND XANES STUDY ON RARE-EARTH CHROMITES OF Gd1-xSrxCrO3

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EXAFS AND XANES STUDY ON RARE-EARTH CHROMITES OF Gd1-xSrxCrO3

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

Taniguchi, H. Maeda

To cite this version:

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

STUDY ON RARE-EARTH CHROMITES OF Gd1-xSrxCrO3. Journal de Physique Colloques, 1986,

47 (C8), pp.C8-761-C8-764. �10.1051/jphyscol:19868145�. �jpa-00226047�

JOURNAL DE P H Y S I Q U E

C o l l o q u e C8, suppl6rnent a u n o 12, Tome 47, dbcembre 1986

EXAFS AND XANES STUDY ON RARE-EARTH CHROMITES OF Gdl-,SrxCrO, (x=0,0.2,0.5)

N . KAMIJO, H . KAGEYAMA, M . NARUKAWA*, F. MARUYAMA", Y . NAKAO*,

K . TANIGUCHI* and H . MAEDA*'

Government Industrial Research Institute, Osaka. Ikeda, Osaka 563, Japan

'0saka Electro-Communication University, Neyagawa, Osaka 573, Japan

Department of Chemistry, Okayama University, Okayama 700, Japan

A b s t r a c t

EXAF'S and XANES measurements a r e made f o r Gd S r Cr03 system (x= 0,0.2,0.5) a t 1-x x

60 K and 300 K f o r S r K-edge and Gd L3-edge r e s p e c t i v e l y , and f o r C r K-edge a t 300 K t o e l u c i d a t e t h e i r oxygen d e f e c t s t r u c t u r e and t h e i r e l e c t r i c conduction mechanism.

It i s found t h a t t h e c o o r d i n a t i o n number of oxygen around t h e S r i o n i n Gdo.5Sro.5- CrO i s extremely decreased a t 300 K a s compared w i t h one of t h a t compound a t 60 K.

3

S i m i l a r tendency i s found i n Gd 0.8SrQ.2Cr03: The r e s u l t i s c o n s i s t e n t w i t h t h a t t h e t h r e s h o l d energy of S r K-edge i n Gd0,5Sr0.5Cr0 a t 300 K i s s h i f t e d t o low energy

3

s i d e a s compared with one a t 60 K. S i m i l a r tendency i s a l s o found i n Gdo-8Sro.2- Cr03. These r e s u l t s imply t h a t m o b i l i t y of oxygen i o n around S r i o n i s i n c r e a s e d w i t h e l e v a t i n g temperature even i n such low temperature r e g i o n . Oxygen c o o r d i n a t i o n numbers around Gd i o n s i n Gd 1-x S r x Cr03(x= 0.2 and 0 . 5 ) a r e a l s o found t o be s m a l l a s compared w i t h one of GdCrO

3-

1. I n t r o d u c t i o n

Metal o x i d e m a t e r i a l s with a l a r g e e l e c t r i c c o n d u c t i v i t y a t e l e v a t e d tempera- t u r e s have a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n a s a way of s t u d y i n g e l e c t r i c conduction mechanism and a l s o a s p o t e n t i a l m a t e r i a l s f o r f u e l c e l l e l e c t r o d e . LaCr03 h a s been known t o e x h i b i t a prominent h e a t - r e g i s t a n c e and have a l a r g e e l e c t r i c c o n d u c t i v i t y

2+ 24- ( e l e c t r o n i c conduction mechanism) a t e l e v a t e d temperatures. The use of Ca , S r ,

~ g p a r t i a l l y s u b s t i t u t i n g La3+ h a s l e d u s t o i n c r e a s i n g e l e c t r i c c o n d u c t i v i t y and ~ f improving t h e p r o p e r t i e s a s t h e e l e c t r o d e m a t e r i a l . We, t h e r e f o r e , i n t e n d t o make EXAFS and XANES s t u d y of r a r e - e a r t h chromite w i t h v a r i o u s temperatures and oxygen atmospheres t o e l u c i d a t e t h e oxygen arrangement around each m e t a l i o n . However, We must make experiment f o r t h e X-ray a b s o r p t i o n s t u d y o f Gd S r CrOj i n s t e a d of

1-x x

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868145

C8-762 JOURNAL DE PHYSIQUE

Lal-xSrxCrO system, because t h e a b s o r p t i o n edge p o s i t i o n of La L -, ^L -edge a r e s o

3 2 3

c l o s e t o one of C r K-edge t h a t i t i s n o t s u i t a b l e f o r t h e p r e s e n t s t u d y . 2. Experimental-

The samples were p r e p a r e d according t o t h e u s u a l method. The formation of a s i n g l e p e r o v s k i t e - l i k e phase was confirmed by X-ray d i f f r a c t i o n .

X-ray a b s o r p t i o n measurements were made with X-ray e m i t t e d from an e l e c t r o n s t o r a g e r i n g a t Photon Factory i n N a t i o n a l Laboratory f o r High Energy P h y s i c s , u s i n g a S i ( 3 1 1 ) channel-cut monochromator on BL-1OB EXAFS s t a t i o n 1 ) and with a l a b o r a t o r y - type X-ray s p e c t r o m e t e r 2)

.

Data a n a l y s i s was made a c c o r d i n g t o a s t a n d a r d procedure3! A f t e r t h e s u b t r a c - t i o n of t h e Victoreen-type b a s e - l i n e of pre-edge r e g i o n , EXAFS spectrum k ~ ( k ) 3 v s . k

O - 1

( i n A ) was e x t r a c t e d from t h e background determined by c u b i c - s p l i n e f u n c t i o n o r moving average method o r automatic background removal 4

.

3. R e s u l t and Discussion

I n F i g . 1 a r e shown XANES s p e c t r a on S r K-edge of Gd S r Cr03 (x= 0.2 and 0.5) 1-x x

a t 60 K and 300 K. Both white l i n e peak and t h r e s h o l d energy o f S r K-edge of Gd0.5Sr0.5Cr0 measured a t 60 K a r e found t o be s h i f t e d t o high energy s i d e of *3 eV

3

a s compared with t h a t measured a t 300 K. S i m i l a r tendency i s found f o r ~d

3 0 . 8 ~ ~ 0 . 2 -

Cr03. I n Fig.3 EXAFS s i g n a l s k ~ ( k ) and t h e i r F o u r i e r transforms f o r S r K-edge i n Gdl-xSrxCr03 (x= 0 . 2 and 0.5) a r e compared a t 60 K ( s o l i d l i n e ) and a t 300 K ( d o t t e d l i n e ) . The f f r s t peaks i n the r a d i a l d i s t r i b u t i o n i n d i c a t e t h e oxygen atoms a r o w d S r i o n . Coordination numbers of t h e oxygen around S r i o n o b t a i n e d by c u r v e - f i t t i n g

\

a n a l y s i s a r e l i s t e d i n Table 1. It should be n o t e d t h a t t h e c o o r d i n a t i o n numbers of oxygen around S r i o n i n Gd S r Cr03 (x= 0.2 and 0.5) a r e decreased a t 300 K a s

1-x x

compared w i t h t h o s e a t 60 K. The r e s u l t i s i n good agreement w i t h t h e t h r e s h o l d edge s h i f t o f t h e XANES s p e c t r a . The d i s t a n c e s between S r and oxygen i n Table 1 a r e found t o be s h o r t e r t h a n t h a t i n s t a n d a r d sample SrTi03. I n Fig.2 a r e shown t h e XANES s p e c t r a on G d L -edge of Gd0.5Sr0.5Cr0 a t 60 K and 300 K r e s p e c t i v e l y : a

3 3

s m a l l e r t h r e s h o l d edge s h i f t t h a n t h a t i n S r K-edge i s found between t h e one a t 300 K and t h e one a t 60 K. A s i m i l a r r e s u l t i s a l s o o b t a i n e d i n Gd0.8Sr0.2Cr03. A s shown i n Table 1 t h e c o o r d i n a t i o n numbers o f oxygen around Gd i o n i n Gd S r C r O (x= 0.2 and

1-x x 3

0.5) a r e a l s o s m a l l compared t o t h a t i n GdCr03.

I n conclusion Gdl-xSrxCr03 (x= 0.2 and 0.5) c o n t a i n l a r g e amount of oxygen vacancies a s compared w i t h GdCr03. Most of t h e v a c a n c i e s c o n c e n t r a t e around A-site i o n of p e s o v s k i t e - l i k e s t r u c t u r e . Oxygen atoms around S r i o n a r e extremely mobile a s i n c r e a s e temperatures (60 K 'L 300K). We i n t e n d t o make f u r t h e r EXAFS and XANES s t u d y o f t h e s e chromite i n h i g h e r temperatures and under v a r i o u s oxygen atmospheres.

I I I

7 . 2 3 7.25 7 . 2 8

PHOTON ENERGY (keV)

Fig.2 Gd L -edge XANES o f Gd0.SSr0.5Cr0

3 3

a t 60 K and 300 K .

16.10 16.13 16.16 16.19 16.22 PHOTON ENERGY (keV)

F i g . 1 S r K-edge XANES of Gdl-xSrxCrO 3 (x= 0.2 and 0.5) a t 60 K and 300 K. S o l i d and d o t t e d l i n e s i n the f i g u r e show t s o s e a t 60 K and 300 K r e s p e c t i v e l y .

1.6 3.2 4.8

w \ D I h I , DISTANCE (A)

Fig.3 EXAFS s p e c t r a and t h e i r F o u r i e r t r a n s f o r m s on S r K-edge Of Gdl-xSrxCr03 (x= 0.2 and 0.5) a t 60 K and 300 K.

JOURNAL DE PHYSIQUE

Table 1 The parameters obtained by the least-squares fits in the k-space, bond distance R, coordination number N and Debye-Waller factor a. Those of SrTiO and GdFeO given by X-ray diffraction are used3for referznce sample.

Acknowledgements

The authors wish to express their sincere thanks to Drs. M. Nomura and A. Koyama of Photon Factory and Prof. T. Murata of Kyoto University of Education for their kind suggestions on EXAFS measurements.

Compound

Gd0.5Sr0.5Cr03 (EXRFSI

GdO+8Sro.2Cr03 (EXAFS)

GdCr03 (EXAFS)

ISrTi031 (X-ray

d~ffractron) IGdFe031 (X-ray

d~ffraction)

References

1) H-Oyanagi, T-Matsushita, M.Ito and H.Kuroda: KEK 83-80 March (1984).

2) K.Taniguchi and Y.Nakao, Shimazu Scientific Instrument News

26

3) H-Maeda, H.Terauchi, K.Tanabe, N.Kamijo, M.Hida and H-Kawamura, Jpn. J. Appl.

phys.

1

.

4) ~.W.cook, Jr. and D.E. Sayers: J. Appl. Phys. 52

-

M

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sr

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Sr

-

-

Gd

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cr

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sr

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Sr - O(300 K) 2.60 0.11

Gd

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Gd - O(300 K) 6.7 2.54 0.08 Cr - ^{O(300 K) 5.8 1.84 0.04}

Gd - ^{O(60 K)} 8.3 2.53 0.08

-

Gd

-

Cr - ^{O(300 K)} 4.5 1.82 0.04 [Sr

-

[Gd

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