EXAFS study of amorphous Cu33Zr66 alloy

Journal of Non-Crystalline Solids 61 & 62 (1984) 403-408 North-Holland, Amsterdam

403

EXAFS STUDYOF AMORPHOUSCu33Zr66 ALLOY Anne SADOC Laboratoire de Physique des Solides, BAt.510, et LURE, BAt 209C, Universit~ Paris-Sud, 91405 Orsay, France. ABSTRACT The Exafs spectra of Cu33Zr66 alloys have been recorded on Cu and Zr absorption edges f o r both c r y s t a l l i n e and amorphous phases. The spectra of CuZr2 have been well reconstructed using c r y s t a l l o g r a p h i c data. The spectra of glassy Cu33Zr6fi have been modelled using two subshells f o r the Cu-Cu pairs and f o r t~e Zr-Zr ones. The RDF of the Cu-Cu pairs is very d i f f e r e n t to the amorphous and in the c r y s t a l l i n e a l l o y , but i t is s i m i l a r to that found in CuxZr1_x glasses of copper richer concentrations. INTRODUCTION Because of their potential in technology, metallic glasses are of considerable current interest both experimentally and theoretically. standing their properties is the local structure.

A key to under-

As a matter of fact, the

information contained in a set of partial radial distribution functions is sufficient to express many bulk properties such as the equation of state and even to describe chemical short range order. On the other hand, the a b i l i t y of Exafs to identify the type of scattering atoms and in providing their numbers and the bond distances is of major significance in determining the local architecture of metallic glasses.

We chose

the Cu33Zr66 system for the present Exafs study for, at this composition, i t has a well-defined crystalline phase (CuZr2)(1).

The existence of the Cu33Zr66

composition in both the crystalline and glassy phases makes i t an attractive model system for quantitative Exafs investigation of the local atomic environment in metallic glasses.

I t w i l l be moreover interesting to compare the local

structure with data for other compositions and to see how i t changes as one goes from a Zr rich a l l o y to a Cu rich one (2'3)

MATERIALS AND METHODS Amorphous ribbons of Cu33Zr66 composition were prepared by melt-spinning (J. Bigot, C.E.C.M., V i t r y , France). to v e r i f y the amorphous state.

X-ray d i f f r a c t i o n measurements were made

In order to get the CuZr2 c r y s t a l l i z e d a l l o y ,

an amorphous sample was annealed f o r 30' at 460°C under vacuum (2 10-6 t o r r s ) . Exafs measurements were performed at LURE (Orsay) using the synchrotron r a d i a t i o n of DCI.

The spectra were recorded at 30 K on both Cu and Zr

0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

A. Sadoc / EXAFS study of amorphous Cu33Zr 66 alloy

404

K - absorption edges. conventional manner (2).

The e x t r a c t i o n of the x(k) o s c i l l a t i o n s was done in a Fourier transforms of k3x(k) were achieved w i t h d i f f e -

rent k - windows f o r the d i f f e r e n t spectra depending on t h e i r e x t e n t in k. For example, the Exafs o s c i l l a t i o n s can be observed up to 650 eV above the Cu edge in the amorphous case and up to 1000 eV in the c r y s t a l l i n e one.

F(R

F(R)' 0-02

0.001

-

0 0-01

0.0;

o

FIGURE 2

FIGURE I

Same as f i g . l

Cu edge: Fourier transforms f o r (a) c - CuZr2, (b) a - Cu33Zr66 On Cu edge, the F T ( f i g . l )

R(A)

on Zr edge

consist b a s i c a l l y of a single broad peak cente-

red approximately at 2.2 ~ f o r (uncorrected f o r phase s h i f t ) f o r a-Cu33Zr66 and The p o s i t i o n o f the main peak at higher R f o r

at 2.5 - 2.6 ~ f o r c-CuZr 2.

c-CuZr 2 does not depend on the choice of the k-window in the Fourier filtering.

With a shorter k - window, i d e n t i c a l with the one used in the

amorphous case, the peak remains centered a t 2.5 A in the c - Cu Zr 2 case. Therefore, i t is an i n d i c a t i o n t h a t there is some short bond distance around a Cu atom in the glass, which does not e x i s t in the c r y s t a l .

On Zr edge, the

F T are dominated by two peaks in the region 2 - 3.5 ~ ( f i g . 2 ) .

The main peak

of the F T on Cu edge and the two ones on Zr one have been backtransformed in the k space r e s u l t i n g in inverse F T which have then been f i t t e d Exafs formula f o r gaussian d i s t r i b u t i o n s ( 4 , 5 ) ,

using the

and the amplitudes and phase

s h i f t s calculated by Teo and Lee (8). C - Cu Zr 2 The s t r u c t u r e o f c r y s t a l l i n e CuZr 2 is body centered tetragonal (Si2Mo type(1)).

The local environment around a Cu atom consists of 8 Zr atoms at a

distance of 2.89 X and 4 Cu ones at 3.22 X.

So there are no Cu next nearest

A. Sadoc / EXAltS study of amorphous Cu33Zr66 alloy neighbours and t h e r e f o r e no short Cu-Cu bond distance. relatively,

405

This appears in the,

high R p o s i t i o n of the central peak in the F T on Cu edge.

Around a Zr atom there are 4 Cu at 2.89 ~, 4 Zr at 3.05 ~ and 4 Zr at 3.22 ~.

The bond distances in t h i s double shell of Zr-Zr pairs are separated

from 0.17 ~. The Exafs signals have been simulated using the values given in table 1, which compare well with the X-ray data. of Zr-Zr pairs is shown in f i g . 3 .

The c o n t r i b u t i o n of the second shell

I t s t r o n g l y affects the zeros and the i n -

t e n s i t i e s of the spectrum so t h a t the second subshell is e a s i l y evidenced by Exafs.

0

1o

5 i

!

t

k (A-')

FIGURE 3

13

I~---i

Exafs spectra of c-Cu Zr 2 on Zr edge.

;

The dots are the inverse F T and the

" }. "" '"

s o l i d lines the spectra calculated

,

0.oi.

with (a) 4 Cu at 2.85 ~ and 4 Zr at

0

3.09 ~ ; (b) idem plus 4 Zr at 3.20~.

(b) O.C

-r-.4~_-e~-~.c,:~,.~.,~

0

V I

~

~

200

;

!

~00

!

700 E (eV)

a - Cu33 Zr66 The best f i t

of the spectrum above the Zr edge of amorphous Cu33Zr66 was

obtained with one shell of Cu atoms and two subshells of Zr ones ( f i g . 4b). Using the heterogeneous pairs so determined on Zr edge, we have simulated the spectrum on Cu edge with two subshells of Cu atoms ( f i g .

4a).

The distances

and c o o r d i n a t i o n numbers so obtained are given in t a b l e i . Around a central Zr atom, the coordination numbers remain in the glass very s i m i l a r to those of the c r y s t a l w i t h , however, considerable rearrangement of the distances.

A closer packing of Cu atoms appears while the shortest

406

A. Sadoc / EXAFS study of amorphous Cu33Zr 66 alloy

x

O 5 ~il-

10 I

k (kb i

I

FIGURE 4

o.o,flri, 0

Exafs spectra of a-Cu33Zr66 (a) on Cu edge, (b) on Zr one.

l--..i.i~C1~-:,... -.-,.~,.~..;....,, n

The dots are the inverse F T and the .
o.oq.

s o l i d l i n e s the spectra c a l c u l a t e d

(a)

,:

from the model presented in the t e x t .

I--.~Ac~-L~-Y~-,/CX;X-.~,,,I .. {l.i ! ~ ~, v '

o

o

'

2~o

:

"

:

60o

~

E (¢V)

c - Cu Zr 2 R(~)

Cu

a(~)

~(~)

3.5 Cu

2.54

.10

8.5 Zr

2.87

.065

6

Zr

2.71

.12

4

3.16

.085

1.5 Cu

3.05

.I0

Cu

....................................

Zr

a - Cu33 Zr66 R(~)

N

....................................

4

Cu

2.85

.070

3.5 Cu

4

Zr

3.09

.095

4

Zr

3.19

4

Zr

3.20

.055

3

Zr

3.43

Exafs parameters

2.71

TABLE 1 f o r Cu Zr 2 c r y s t a l l i n e and amorphous a l l o y s .

.12

N is the coor-

d i n a t i o n number, R the distance and ~ the mean square displacement (aN = 0 . 5 , AR = ± .05 X, ao = ± . 0 1 X ) .

Zr-Zr distance vanishes.

The Zr-Zr d i s t r i b u t i o n shows two distances, separated

by 0.24 X, with a d i r e c t contact which occurs at 3.19 X l i k e in pure zirconium. Strong m o d i f i c a t i o n s of the local order appears also around a Cu atom since there now e x i s t Cu next nearest neighbours a t 2.54 X.

The d i s t r i b u t i o n of the

Cu-Cu pairs consist o f two w e l l - r e s o l v e d gaussian ones l i k e in amorphous

CU6oZr40(2) and Cu46Zr54(3).

A. Sadoc / EXAFS stud}' of amorphous Cu33Zr66 alloy

407

CONCLUDING REMARKS

Highlights of our experimental findings in this Cu33Zr66 amorphous a l l o y as modeled by a subshell simulation are (I) around the central Cu atom there e x i s t Cu nearest neighbours, (2) around the Zr atom the structure of the Cu coordination shell remains similar to that of the crystal but closer to the central Zr atom and (3) as in the crystal the Zr neighbours e x i s t in subshells but substantially rearranged and farther from the central atom.

Therefore,

the d i s t r i b u t i o n of the Zr-Zr pairs is an asymmetric one. We have shown by Exafs the existence of asymmetric RDF in several metallic glassest2'3)and'' modellized them with two gaussian subshells.

The r e l i a b i l i t y

of this model with recent theoretical developments, such as the structural model for two-site atoms proposed by Phillips~6)or" " the double-well potentials considered by Ignatiev et al(7)is s t i l l

an open question.

ACKNOWLEDGEMENT I t is a pleasure to thank P. Marin and his collaborators for the runs dedicated by the "Laboratoire de l'Acc~16rateur Lin~aire" to LURE. Discussions with Y. Calvayrac and A. Quivy are g r a t e f u l l y acknowledged. REFERENCES 1) M.V. Nevitt, J.W. Downey, AIME Trans. 224 (1962) 195. 2) A. Sadoc, D. Raoux, P. Lagarde, A. Fontaine, J. Non-cryst. Solids, 50 (1982) 331. 3) A. Sadoc, A.M. Flank, D. Raoux, P. Lagarde, J. de Physique Colloque C9 (1982) 43. 4) D.E. Sayers, F.W. Lytle, E.A. Stern, Adv. X-ray Anal. 13 (1970) 248. 5) E.A. Stern, Phys. Rev. BIO (1974) 3027. 6) J.C. P h i l l i p s , Phys. Rev. B24 (1981) 1744. 7) F.N. Ignatiev, V.G. Karpov, M.I. Klinger, J. Non-Cryst. Sol. 55 (1983) 307. 8) B.K. Teo, P.A. Lee, J. Am. Chem. Soc. I01 (1979) 2815.