STUDY OF SHORT RANGE ORDER IN Au3 Cu BY EXAFS

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STUDY OF SHORT RANGE ORDER IN Au3 Cu BY EXAFS

M. Bessiere, E. Dartyge, S. Lefebvre

To cite this version:

M. Bessiere, E. Dartyge, S. Lefebvre. STUDY OF SHORT RANGE ORDER IN Au3 Cu BY EXAFS.

Journal de Physique Colloques, 1986, 47 (C8), pp.C8-1033-C8-1036. �10.1051/jphyscol:19868199�.

�jpa-00226106�

C o l l o q u e C8, s u p p l 6 m e n t a u n o

STUDY OF SHORT RANGE ORDER IN Au,Cu BY E X A F S

M. B E S S I E R E * , * * , E. DARTYGE* a n d S. LEFEBVRE* * *

" L U R E , Bdtiment 209 D , F-91405 Orsay Cedex, France

" " C E C M , 15, rue G. Urbain, F-94407 Vitry Cedex, France

RESUME

Le b u t d e c e t t e e t u d e est de determiner s i I'EXAFS est une bonne methode pour - rechercher l ' o r d r e l o c a l dans l e s s o l u t i o n s s o l i d e s concentrees. Deux e c h a n t i l l o n s d e AUaCU ordonnes a c o u r t e d i s t a n c e (T/T,=1.02 e t 1 . 2 ) o n t ete e t u d i e s a u v o i s i n a g e d e s s e u i l s K du Cu e t L I I I d e 1'Au. Le nombre d e premiers v o i s i n s est obtenu p a r r e c o n s t r u c t i o n d e s s p e c t r e s EXAFS. Nous concluons que l e degre d ' o r d r e c o u r t e d i s t a n c e ne peut p a s etre e x t r a i t d e s donnees EXAFS pour un a l l i a g e p a r t i e l l e m e n t ordonne : seulement de grandes v a r i a t i o n s d ' o r d r e peuvent etre vues.

ABSTRACT

The purpose o f t h i s s t u d y is t o determine i f EXAFS is a S u i t a b l e method t o i n v e s t i g a t e t h e s h o r t range o r d e r (SRO) i n concentrated s o l i d s o l u t i o n s . Two s.r.0.

A u ~ C U samples annealed a t T/Tc1.02 and 1 . 2 have been s t u d i e d i n t h e v i c i n i t y o f CuK and AllLIII edges. The first neighbour numbers were obtained from fits o f EXAFS s p e c t r a . We conclude t h a t the degree o f SRO cannot b e elrtracted from EXAFS d a t a with a p a r t i a l l y ordered a l l o y : o n l y l a r g e v a r i a t i o n s o f o r d e r can be seen.

I ~ W C T I O N

C r i s t a l l i n e s o l i d s a r e generaly s t u d i e d p r e f e r e n t i a l l y by d i f f r a c t i o n technique. MAFS is choosen when one needs a l o c a l and s p e c t r o s c o p i c prow-, i. e.

when one needs t o r e v e a l t h e i d e n t i t y and t h e p o s i t i o n o f the n e a r neighbour atom o f o n l y the s p e c i f i c element t o which one t u n e s the i n c i d e n t X-Ray energy.

The purpose o f t h i s s t u d y is t o determine i f EXAFS is a s u i t a b l e method t o i n v e s t i g a t e the l o c a l o r d e r (SRO) i n concentrated s o l i d s o l u t i o n s . The Au-Cu system h a s been chosen f o r many reasons :

- example o f o r d e r - d i s o r d e r transformation : t h e phase diagram p r e s e n t s three ordered phases around s t o e c h i o m e t r i c compositions Cu3Au, AuCu, Au3cu at 661 K, 693 K and 470 K r e s p e c t i v e l y . Above t h e s e temperatures, i n the f . c . c . disordered domain, f o r each composition t h e l o c a l o r d e r is s t r o n g and h a s been widely s t u d i e d b y BARDHAN e t a l . (1 ) f o r Cu3Au and by BESSIERE e t a l . ( 2 , 3 ) f o r Au3Cu.

- ^Good c o n t r a s t both f o r d i f f r a c t i o n s t u d i e s and f o r EXAFS'ones : t h e

i n t e r f e r e n c e phenomena which t a k e p l a c e i n both t e c h n i q u e s depend on v a l u e s of t h e e l e c t r o n i c s t r u c t u r e .

The Au-Cu system has been i n v e s t i g a t e d by EXAFS by CLEASON e t a l . ( 4 ) , t h e s e a u t h o r s r e p o r t experiments on CugAu, CuAu and AugCu b u t o n l y t h e experiments on Cu3Au could be analyzed.

SAMPLE PREPARATION

The Au3cu a l l o y was prepared from AU 99.991% (Johnson Mathey) and ASARCO Cu (99.9991 % ) m e l t i n a g r a p h i t e c r u c i b l e . The i n g o t was homogenized by rf melting p r i o r t o r o l l i n g . The s h e e t s were annealed a f t e r a f i r s t r o l l i n g t o g i v e t h e p o s s i b i l i t y t o r e r o l l t h e m t o a t h i c k n e s s o f 1 0 pm g i v i n g a n X-Ray t r a n s m i s s i o n f a c t o r o f about 1 0 % a t t h e a b s o r p t i o n edge.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868199

C8-1034 JOURNAL DE PHYSIQUE

The samples were sheets of 2 cm length and 1 rm width. They were annealed at 973 K for one hour to relieve any strain.

Contrary to Cu3Au, for which the ordering kinetics is so fast that it is difficult to quench the state of high temperature, the kinetics of long range ordering in AujCu is very sluggish. The state of L12 structure was obtained after annealing a sample for 15 days in vacuum at 423 K. The choice of annealing

temperature is dicted by the phase diagram : aSMve 433 K, the structure is a mixture of L12 phase and periodic antiphases one (Au3Cu I and AU3Cu 11). The long range order parameter S is closed to 1 as it appears from calorimetric measurements (5).

For reason of lack of time, it is not possible to obtain a thermodynamical equilibrium state by annealing an ordered sample in a furnace during the EXAFS experiment. Resistivity measurements (6) have shown that the relaxation tune is very short (less than 1') at 573 K but increases faster than exponentially near the order-disorder temperature ; the relaxation time is of 10' second at 485 K.

n*o samples were annealed in vacuum, one at 485 K (T/Tc = 1.02) and the other at 573 5 (T/T,= 1.2) for four hours and ~ater~quenched. The lattice parameters are 3.9818 A for the ordered L12 phase and 3.9843 A for the completly disordered phase

; this parameter decreases with increasing SRO.

EXAFS : EXPERIMENTAL PROCEDURE AND ANALYSIS

The EXAFS experiments are performed on

XIE) ' _{EXRFS I11} spectrometer at LURE (Orsay)

using the synchrotron radiation of DCI storage ring. The CUK and AULIII absorption edge are scanned with two Si 311 crystals monochromator. The energies are calibrated

E(W) with a copper and a gold foil. The spectra

are recordered at 30 K in order to reduce the Debye-Waller factor due to the thermal disorder.

The EXRFS x (k) spectra ( fig. I ) are

-0.4 obtained from the absorption following a

standard procedure :

Figure 1 : Experimental EXAFS spectra on Cu edge for Au3Cu annealed at 485 K.

2 . the sum is taken over shells with Nj atoms at distance r, fromthe absorber, aj ^IS the temperature dependent mean square fluctuation in rj, W A C is a decay constant and corresponds to the inverse mean free path for scattered electron Fj(k) and 6,(k) are the backscattering amplitude and the total phase shift.

Figure: 2 I Cu edge : FT forAu3Cu in L m Figure 3 i Au a g e (t t *far AuaCU in LRa atate (aolid line), 985 X state and rrwO state^.

<dashed line) and SRO 573 X State (dots),

window a r e 80-110 eV and 500-900 eV a t Cu K ,and 80-110 e V and 600-900 eV a t Au L I I I . The amplitude o f the FT o f E X P S s i g n a l o f t h e three samples is given i n f i g u r e s 2 , 3 a t Cu and Au edges.

~t Cu edge the amplitude o f t h e t h r e e states ^{(LRO, SRO} 485 K and 573 K) are v e r y similar b u t n o t i d e n t i c a l . T h e - f i r s t peak p r e s e n t s a bump towards s m a l l R due t o t h e s t r u c t u r e o f b a c k s c a t t e r i n g o f t h e Au atoms. This s h e l l corresponds t o 1 2 atoms which should be a t 2.81 A from t h e c e n t r a l Cu atom, as given by the l a t t i c e parameter.

A t Au edge the amplitude o f ordered and d i s o r d e r e d s t a t e ( f i g . 3 ) around a c e n t r a l Au atom are i n d i s t i n g u i s h a b l e . There are 8 Au and 4 Cu i n t h e ordered s t a t e , 9 Au and 3 Cu i n t h e completely disordered s t a t e : t h i s v a r i a t i o n o f one atom is not even d e t e c t a b l e i n an EXAFS experiment.

DISCUSSION

Our a n a l y s i s c o n c e n t r a t e s on the first neiqhbour s h e l l o f AulCu

-

F i g u r e 4 : EXAFS spectra on Cu edge f o r AU3Cu i n SRO 485 K state :

E(.v) d o t s = i n v e r s e FT o f t h e f i r s t peak

of f i g . 2 ;

s o l i d l i n e = c a l c u l a t e d x(E) w i t h r e s u l t s of column B o f t a b l e 1.

0 0

Cu (12 Cu a t 2.54 A from t h e c e n t r a l atom) and o r d e r e d Au3CU (12 Au a t 2.81 A from t h e c e n t r a l atom Cu )

.

TWO t y p e s o f fits were done : one without any c o n s t r a i n t on the total number o f atoms i n the f i r s t s h e l l (column A table I ) , an other one where t h i s number is f i x e d t o twelve (column B t a b l e 1 ) . I n both cases t h e unknowns which are left f r e e t o vary a r e : t h e number o f NAU and NCU atoms of AU and Cu i n t h e f i r s t shell around t h e Cu atom, t h e d i f f e r e n c e AuCucu and AUAuAu between sample and s t a n d a r d , t h e d i s t a n c e Qucu and RcuAu and t h e o r i g i n of the energy Eo. T h i s o r i g i n is i d e n t i c a l f o r two t y p e o f bounding and c l o s e t o t h e one o f p u r e element (4eV). Au i s very weak (0,001).

The number o f Au atoms found i n c a s e A is c l o s e t o t h e v a l u e found b y d i f f u s e s c a t t e r i n g measurement b u t t h e t o t a l number o f atoms i n t h e first s h e l l is n o t c o n s t a n t and f o r both temperatures t a k e s t h e v a l u e s 11.41 and 11.36. I n c a s e B, the number o f Au atom is v e r y high and independent o f t h e s t a t e o f SRO.

The p r o b a b i l i t y of f i n d i n g an u n l i k e atom i n t h e first s h e l l around Cu c a n be compared w i t h t h e SRO parameter a, d e f i n e d by Warren and Cowley and used t o i n t e r p r e t a t e p h y s i c a l p r o p e r t i e s as r e s i s t i v i t y , mechanical p r o p e r t i e s and s o on :

a = 1

-

I n t h i s experiment t h e values o f al a r e i n c a s e A, f o r T/Tc = 1.02,

-0.1586 and, f o r T/Tc = 1.2, -0.1735 and i n c a s e B, f o r T/TC = 1.02, -0.119 and, f o r T/TC = 1.2. -0.2466. AS i n t h e c a s e o f Cu3Au ( 4 ) t h e parameters i n c r e a s e w i t h temperature i.e. when SRO d e c r e a s e s : t h i s unphysical v a r i a t i o n is explained b y t h e accuracy of t h e experiment where t h e e r r o r can be as h i g h a s 0 . 5 atom. I n fact, t h e

C8-1036 JOURNAL DE PHYSIQUE

variation of local order between T/Tc = 1.02 and T/Tc = 1.2 is very important as it is illustrated (fig.5) by the variation of diffuse intensity for Au3Cu (2,3) along one line 100

-

experiments are -0.0932 for T/Tc = 1.02 and -0.0713 for T/Tc = 1.2.

CONCLUSION

The evolution of SRO with temperature, which involves the variation of less than one atom in the first shell, is not seen by the analysis of EXAFS spectra.

Similar results have already been obtained by CUlESON et al. (1) in a study of the Order-disorder transformation of Cu3Au by EXAFS. But in disagreement with these authors, we conclude that the degree of SRO cannot be extracted from EXAFS data with a partially ordered alloy : only large variations of order can be followed. In addition, to define a SRO state al is not sufficient. The correlation, specially near Tc, extends very far and other a parameters become important.

Table 1 : Pit parameters for Au3Cu Figure 5 : Diffuse intensity (2,3) along one line 100-110 for AU3CU in SRO 485 K (solid line ) and 573 K (dashed line) states.

Column A : the total number of atoms in the first shell is free,

Column B : this total number in fixed to twelve,

NAu and NCu are the number of Au and Cu atoms in the first shell around a Cu atom, RClrAu and kuCu are the distances,

R% is the agreement ratio,

N'Au : number of gold atoms obtained by diffuse intensity measurement.

REFERENCES

(1) BARDHAN P. and COHEN J.B., Acta Cryst. A32, 597-613, 1976

(2) BESSIERE M., LEFEBVRE S. and CALVAYRAC Y., Acts Cryst. B39, 145-153, 1983 ( 3 ) BESSIERE M., LEFEBVRE S., CALVAYFfAC Y., GRATIAS D. and CENEDESE P., Journal de

physique, to be published, 1986.

(4) CLAESON T. and BOYCE J.B., Phys. Rev. B, 29, N4, 1551-1557, 1984.

(5) BESSIERE M., Thesis Paris, 1984

(6) BESSENAY G., these de Docteur-Ingenieur, Paris 1986.