EXAFS MEASUREMENTS IN Ni33Y67 AND Cu33Y67 AMORPHOUS RIBBONS AND COMPARISON WITH DIFFRACTION RESULTS

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EXAFS MEASUREMENTS IN Ni33Y67 AND

Cu33Y67 AMORPHOUS RIBBONS AND

COMPARISON WITH DIFFRACTION RESULTS

M. Maret, M. Belakhovsky, F. Merdrignac, A. Flank

To cite this version:

J O U R N A L

DE

PHYSIQUE

Colloque C8, supplkment a u n012, Tome 46, dkcembre 1985 page c 8 - 2 1 1

EXAFS MEASUREMENTS IN

N i 3 3 Y 6 7

AND

C U ~ AMORPHOUS RIBBONS ~ Y ~ ~

A N D

COMPARISON WITH DIFFRACTION RESULTS

M. Maret, M. ~ e l a k h o v s k ~ * , F. ~ e r d r i ~ n a c * and A.M.

lank**

I n s t i t u t Laue Langegin, B. P. 256 X , 38042 GrenobZe Cedex, France *Centre drEtudes NucZe'aires de GrenobZe, Departement de Recherche

Fondamentale, B.P. 85 X , 38041 Grenoble Cedex, France

*%.

U . R. E., B6timent 209c, Universite' Paris-Sud, 91405 Orsay France

Rdsumd - Des e x p d r i e n c e s d7EXAFS o n t Q t d r d a l i s d e s s u r l e s s e u i l s K du N i , du Cu e t de 1 ' Y t t r i u m dans l e s a l l i a g e s amorphes Ni33Y67 e t Cu33Y67, e t dans l e s composds c r i s t a l l i n s d e r d f d r e n c e NiY3 e t CuY. Les v a l e u r s d e s p a r a m s t r e s s t r u c t u r a u x ( N j , R j ) dans a-Ni33Y67 e t a-Cu33Y67 d s d u i t e s d e l ' a n a l y s e EXAFS s o n t compardes

B

c e l l e s obteriues d e s e x p e r i e n c e s de d i f f r a c t i o n .

A b s t r a c t - EXAFS experiments have been performed on N i , Cu and Y K-edges i n t h e m e t a l l i c g l a s s e s Ni33Y67 and Cu33Y67, and i n t h e c r y s t a l l i n e model com- pounds NiY3 and CuY. The s t r u c t u r a l p a r a m e t e r v a l u e s ( N j , R j ) i n a-Ni33Y67 and a-Cu33Y67 deduced from t h e EXAFS a n a l y s i s a r e compared t o t h o s e o b t a i n e d from d i f f r a c t i o n e x p e r i m e n t s .

I - INTRODUCTION

Neutron d i f f r a c t i o n u s i n g i s o t o p i c s u b s t i t u t i o n i s c o n s i d e r e d t h e most a c c u r a t e method t o s t u d y t h e l o c a l s t r u c t u r e of m e t a l l i c g l a s s e s p r o v i d e d s c a t t e r i n g c o n t r a s t i s s u f f i c i e n t . U n f o r t u n a t e l y t h i s method i s u s u a l l y e x p e n s i v e and does n o t a p p l y t o e l e m e n t s w i t h o u t s t a b l e i s o t o p e s . T h e r e f o r e d u r i n g t h e p a s t y e a r s t h e EXAFS t e c h n i q u e has been w i d e l y used t o s t u d y b i n a r y amorphous a l l o y s / 1 / , b e c a u s e EXAFS a l l o w s t o p r o b e t h e n e a r n e i g h b o r environment of e a c h a t o m i c s p e c i e s . However i t s a ~ v l i c a t i o n t o d i s o r d e r e d systems had g i v e n r i s e t o some c o n t r o v e r s y 121. I n f a c t t h e r e a r e v e r y few EXAFS r e s u l t s i n m e t a l l i c g l a s s e s which c a n be compared w i t h a complete s e t of i n t e r a t o m i c d i s t a n c e s and p a r t i a l c o o r d i n a t i o n numbers deduced from s e v e r a l s c a t t e r - i n g e x p e r i m e n t s . I n t h i s p a p e r we p r e s e n t such a comparison i n amorphous r i b b o n s of Ni33Y67 and Cu33Y67 f o r which t h e t h r e e p a r t i a l d i s t r i b u t i o n f u n c t i o n s were d e t e r - mined from t h r e e n e u t r o n d i f f r a c t i o n measurements f o r Ni33Y67, and two n e u t r o n and one X-ray d i f f r a c t i o n f o r Cu33Yb7 131. For t h i s , EXAFS measurements were performed on melt-spun r i b b o n s of Ni33Y67 and Cu33Y67, and a l s o i n o r d e r t o p r o b e t h e a m p l i t u d e s and phase s h i f t s c a l c u l a t e d by Teo and Lee /4/ on t h e N i Y 3 and CuY c r y s t a l l i n e model compounds.

I1

-

EXPERIMENTAL TECHNIQUE AND DATA ANALYSIS

EXAFS s p e c t r a were r e c o r d e d on b o t h K-edges a t LURE u s i n g t h e s y n c h r o t r o n r a d i a t i o n of D C I ; a t 30 K f o r t h e amorphous samples and a t room t e m p e r a t u r e f o r t h e c r y s t a l - l i n e ones s i n c e t h e CuY compound undergoes a m a r t e n s i t i c t r a n s f o r m a t i o n below 150 K / 5 / . The melt-spun r i b b o n s were p r e p a r e d a t C a l t e c h by M. Atzmon, and i n g o t s of CuY and NiY3 a t t h e L a b o r a t o r y o f Magnetism i n Grenoble which were t h e n t r e a t e d a t 500°C d u r i n g 24 h and f i n a l l y reduced t o p m d e r f o r EXAFS specimen.

The s t r u c t u r a l p a r a m e t e r s ( c o o r d i n a t i o n numbers N j

,

i n t e r a t o m i c d i s t a n c e s Rj

,

and mean s q u a r e d i s p l a c e m e n t s crj) were o b t a i n e d by f i t t i n g t h e E-YAFS f i l t e r e d s l g n a l r e l a t i v e t o t h e n e a r e s t n e i g h b o r environment u s i n g t h e EXAFS formula f o r g a u s s i a n p a i r d i s t r i b u t i o n :

2

exp(-ZK o.)exp(- J

3)

A

(K) F j (K) s i n (2KRj

+

O j

(K)) (1)

C8-212 JOURNAL DE PHYSIQUE

where Fj(K) i s t h e b a c k s c a t t e r i n g a m p l i t u d e of atom j , 0j(K) t h e t o t a l phase s h i f t , and A(K) t h e mean f r e e p a t h w i t h A(K) = K / T ( r i s a p a r a m e t e r ) . The v a l u e s o f t h e t h r e s h o l d energy Eo e q u a l t o 8344, 8991 and 17028 eV r e s p e c t i v e l y f o r t h e N i , Cu and Y K-edges were d e t e r m i n e d from p u r e c r y s t a l l i n e e l e m e n t s and checked i n c r y s t a l l i n e compounds, and t h e r e , i n most of c a s e s , k e p t f i x e d f o r amorphous samples. The p a r a - m e t e r s

r

were d e t e r m i n e d from c r y s t a l l i n e compounds. The b a c k s c a t t e r i n g a m p l i t u d e s of Cu and Y were e x t r a c t e d from c r y s t a l l i n e Cu and Y and f o r N i a s g i v e n from t h e o r y / 5 / .

111

-

CRYSTALLINE COMPOUNDS

I n t h e NiY3 compound w i t h an orthorhombic Fe3C t y p e s t r u c t u r e , N i atoms h a v e no n e a r e s t n e i g h b o r N i atom b u t 6 n e a r e s t n e i g h b o r s Y atoms a t a n a v e r a g e d d i s t a n c e of 2.806

;i

( c a l c u l a t e d from 2 Y a t 2.76:

a,

1 Y a t 2.782

1,

2 Y a t 2.21 A and 1 Y a t 2.90 1 ) and f u r t h e r 2 Y atoms a t 3.395 A . Y atoms have 2 N i atoms a t RN; = 2.806

A,

0.67 N i atom a t 3.395

1

and 9.33 Y atoms between 3.527

1

and 3.619

1

( i . e .

-

Ry = 3.564

A).

I n t h e CuY compound w i t h a c u b i c CsCl t y p e

structure,

Cu atoms have 8 n e a r e s t n e i g h - b o r s Y atoms a$ 3.01 A , and 6 Cu atoms a! 3.477 A and i n v e r s e l y Y atoms have 8 Cu atoms a t 3.01 A , and 6 Y atoms a t 3.477 A . I f we assume t h e n e a r e s t n e i g h b o r e n v i r o n - ment of N i o r Y atoms i n NiY3 c a n b e d e s c r i b e d by one a v e r a g e d i s t a n c e , phase s h i f t s f o r t h e f o u r h e t e r o a t o m i c p a i r s ( e x p l i c i t l y N i Y , E Y , Y N i and YCu, t h e u n d e r l i n e d element d e f i n i n g t h e a b s o r b i n g atom) can b e e x t r a c t e d from EXAFS measurements s i n c e around t h e a b s o r b i n g atom t h e r e i s a s i m p l e n e a r e s t n e i g h b o r environment w i t h atomsof the samechemical s p e c i e s . Indeed we o b s e r v e d t h a t on N i o r Cu K-edge t h e o r e t i c a l p h a s e s h i f t s l e a d t o good d i s t a n c e s N i Y o r CuY b u t e x t r a c t e d p h a s e s h i f t s a l l o w e d t o improve t h e f i t t i n g o n t h e whole K range. And, on Y-edge, t h e t h e o r e t i c a l phase s h i f t s y i e l d a s i m i l a r good f i t a s e x t r a c t e d phase s h i f t s b u t d i s t a n c e s Y N i and YCu 0.1 s h o r t e r than t h e v a l u e s mentioned above. Thus, i n t h e EXAFS a n a l y s i s , t h e e x t r a c t e d p h a s e s h i f t s a r e used f o r h e t e r o a t o m i c p a i r s and t h e Teo Lee t h e o r e t i c a l p h a s e s h i f t s f o r monoatomic p a i r s .

C-NiY3 : On Ni-edge t h e s i g n a l c o r r e s p o n d i n g t o t h e back t r a n s f o r m a t i o n o f t h e main

peak

t h e F o u r i e r t r a n s f o r m (FT) o f 5 3 X ( k ) i s s i m u l a t e d by 6 Y atoms ( w i t h

r

e q u a l t o 0.6 a t 2.81

1

and oy = 0.125 A ( F i g . l a ) . On Y-edge t h e EXAFS s i g n a l c o r r e z p o n d i n g t o t h e n e a r e s t n e i g h b o r s i s w e l l r e p r e s e n t e d by 0 . 5 oxygen atom a t 2.25 A (coming from Y2O3), 2 , 7 ? 0 . 3 N i atoms a t 2.81

1

w i t h ON; = 0.11

&

and 10.5

+

0.4 Y atoms a t 3.52 f 0.01

A,

w i t h uy = 0.12

1,

and a common v a l u e o f

r

e q u a l t o 0 . 8

x-2

( F i g . I b ) . The e r r o r s i n d i s t a n c e s a r e e s t i m a t e d from t h e f i t t i n g p r o c e d u r e . The h i g h v a l u e s o f O i a r e confirmed by t h e b r o a d Bragg peaks on t h e X-ray p a t t e r n s o f NiY3 and CuY due t o t h e p r e p a r a t i o n of powder.

F i g . 1 - F i l t e r e d s i g n a l of c -NiY3 (dashed l i n e ) and b e s t f i t ( s o l i d l i n e ) :

c-CuY : On Cu e d g e , t h e c o n t r i b u t i o n o f Cu atoms a t 3.477 i n t h e TF of k 3 ~ ( k ) =ry weak r e l a t i v e t o t h e one of Y atoms a t t h e same d i s t a n c e o n Y-edge. The f i t t i n g o f t h e f i l t e r e d s i g n a l when f i x i z g t h e number of Y an$ Cu atoms a t 8 and 6 l e a d t o 8 Y atoms a t 3.00

1

w i t h c ~ y = 0.13 A , 6 Cu atoms a t 3.6 A w i t h ocu = 0.15

1

and

r

= 1.3 The d i s t a n c e CuCu i s t o o l a r g e by 0 . 1 2

1

; t h e e x a c t d i s t a n c e

(3.477 8 ) i s r e g a i n e d through a s h i f t of Eo (- 20 e v ) ( F i g . 2 a ) . I f

r

i s f i x e d a t 1.1, t h e v a l u e s of Ny and

NcU

a r e e q u a l t o 6 and 4 ; t h i s emphasizes t h e c o r r e l a t i o n between

r

and N j . On Y-edge a good f i t of t h e s i g n a l w i t h NCU and Ny f i x e d a t 8 and 6 i s o b t a i n e d w i t h 8 Cu atoms a t 3.02

1

w i t h ucu = 0.14

8

and 6 Y atoms a t 2.45

k

w i t h 0 = 0.11

1,

t h e a d j u s t e d v a l u e of

r

i s 1.1 ( F i g . 2b).

F i g . 2 - F i l t e r e d s i g n a l of C-CuY (dashed l i n e ) and b e s t f i t ( s o l i d l i n e ) :

a ) Cu-edge, b ) Y-edge.

I V - AMORPHOUS SAMPLES

a-Ni33Y : On Ni-edge t h e modulus of t h e TF o f k 3 X ( k ) o e x h i b i t s a main peak a t 2.25+ w l t h a s h o u l d e r a t low r and a s m a l l e r peak a t 3.09 A ( F i g . 3 a ) ( t h e p o s i - t i o n s of maxima i n Ix(R)

(

a r e u n c o r r e c t e d f o r p h a s e - s h i f t )

.

On ~ w h

I

TF e

I

of k ~ ( k ) p r e s e n t s two main maxima a t 2.25 and 3.1 1

8

( F i g . 3 b ) . On Ni-edge t h e back

t r a n s f o r m a t i o n o f x(R) between 0.9 and 3.4

1

i s f i t t e d w i t h one i n n e r N i s h e l l and two o u t e r Y s h e l l s ; t h e v a l u e s o f t h e a d j u s t a b l e p a r a m e t e r s c o r r e s p o n d i n g t o t h e f i t i n F i g . 4a a r e l i s t e d i n Table 1 w i t h t h o s e o b t a i n e d from d i f f r a c t i o n . On Y-edge

E E E

Table 1

-

S t r u c t u r a l p a r a m e t e r s o b t a i n e d from EXAFS (N R U ) and from d i f f r a c t i o n ( N ~ R ~ ) f o r a-Ni33Y67.

JOURNAL

DE

PHYSIQUE

3

Fig. 3

-

F o u r i e r transforms of k ~ ( k ) on Ni-edge (a) and of kx(k) on Y-edge (b) f o r a-Ni

33'67'

Fig. 4 - F i l t e r e d s i g n a l of a-Ni33Y67 (dashed l i n e ) and b e s t f i t w i t h parameters of Table 2 ( s o l i d l i n e ) .

a ) Ni-edge, b) Y-edge.

0

v a l u e s given i n Table 1. The a d d i t i o n of a second s h e l l of N i atoms around 3.5 A o r a n o t h e r of Y atoms around 3.80

1

do not improve t h e f i t a t low K.

a-Cu73Y67 : On Cu-edge t h e TF of kx(k) e x h i b i t s a main peak a t 2.36 d i s t o r t - ed oneboth s i d e s (Fig. 5a) and on Y-edge t n e TF of kx(k) two maxima a t 2.2

1

and 3.24 A (Fig. 5b). On Cu-edge a f i t t i n g of t h e s i g n a l o b t a i n e d by back-transformation of t h e main peak i s r e a l i z e d w i t h two s h e l l s of Y and Cu atoms ; t h e f i t i s p e r f e c t when Eo i s taken a s a n a d j u s t a b l e parameter f o r t h e Cu s h e l l , a s i n t h e CuY compound

T a b l e 2

-

S t r u c t u r a l p a r a m e t e r s o b t a i n e d from EXAFS and from d i f f r a c t i o n f o r a-Cu33Y67. a ) Value o b t a i n e d w i t h a n a d j u s t m e n t o f - 11 eV f o r Eo.

F i g . 5

-

F o u r i e r t r a n s f o r m s o f k ~ ( k ) f o r a-Cu33Y67.

a ) Cu-edge, b) Y-edge.

F i g . 6

-

F i l t e r e d s i g n a l o f a-Cu 33Y67 (dashed l i n e ) and b e s t f i t w i t h p a r a m e t e r s o f T a b l e 2 ( s o l i d l i n e ) .

JOURNAL

DE

PHYSIQUE

V

-

CONCLUSION

T h i s s t u d y shows t h e importance of EXAFS measurements on c r y s t a l l i n e model compounds ;

t h e y a l l o w t o e x t r a c t more r e l i a b l e p h a s e - s h i f t s and g i v e e s t i m a t e s of t h e

r

para- meter of t h e mean f r e e p a t h of t h e p h o t o e l e c t r o n . The s h o r t e s t d i s t a n c e s i n amorphous

samples a r e i n ood agreement w i t h t h o s e o b t a i n e d from d i f f r a c t i o n , w h i l e t h e d i s - t a n c e s above 3

1

a r e s l i g h t l y t o o s h o r t . The c o o r d i n a t i o n numbers from EXAFS a r e , i n g e n e r a l s m a l l e r t h a n t h e v a l u e s from d i f f r a c t i o n ; t h e s e d i s c r e p a n c i e s become l a r g e r when t h e s h e l l only c o n t a i n s a few atoms. Our d i f f r a c t i o n d a t a have helped t o exclude o t h e r models of t h e l o c a l environment which could a l s o l e a d t o a good f i t of t h e s i g n a l . The problem of nonuniqueness of EXAFS a n a l y s i s i n amorphous m a t e r i a l s can be g e n e r a l l y solved by completing t h e EXAFS measurements w i t h two d i f f r a c t i o n e x p e r i - ments (one X-ray and one n e u t r o n ) .

ACKNOWLEDGEMENTS

We g r a t e f u l l y thank A. Sadoc f o r h e l p during t h e experiments and LURE f o r a l l o c a t i o n of beam time. The l o c a l programs w e r e i n i t i a l l y i s s u e d from A. Michalowicz (LURE) and J . Goulon (Nancy U n i v e r s i t y ) .

REFERENCES

/ l / Sadoc, A . , Raoux, D . , Lagarde, P., Fontaine, A . , J. Non C r y s t . S o l i d s

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(1982)

33 1.

/ 2 / E i s e n b e r g e r , P . , Brown, G.S., S o l i d S t a t e Commun.

29

(1979) 481.

/ 3 / Maret, M . , Chieux, P . , H i c t e r , P . , Atzmon, M . , Johnson, W.L., Rapidly Quenched Metals V, ed. S. S t e e b , H.Warlimont ( E l s e v i e r Science P u b l i s h e r s , 1985) 521. / 4 / Teo, B.K., Lee,P.A., J. Am. Chem. Soc. 101 (1979) 2815.

/ 5 / B a l s t e r , H . , I h r i n g , H., Kockel, A . , ~ e m e s s e l , S . , 2. Physik B

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(1975) 241.