LOCAL STRUCTURE IN A Cu2 Ti AMORPHOUS ALLOY BY EXAFS AND X-RAY SCATTERING

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LOCAL STRUCTURE IN A Cu2 Ti AMORPHOUS

ALLOY BY EXAFS AND X-RAY SCATTERING

D. Raoux, J. Sadoc, F. Lagarde, A. Sadoc, A. Fontaine

To cite this version:

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JOURNAL DE PHYSIQUE _{Colloque C8, s u p p z h e n t}_au_{n o}_{8, Tome}_42, _{aoCt 1980, page}

_C8-207

LOCAL STRUCTURE I N A C U ~ T i AMORPHOUS ALLOY BY EXAFS AND X-RAY S C A T T E R I N G

D. Raoux, J.F. Sadoc, P. Lagarde, A. Sadoc and A. Fontaine

Laboratoire de Physique des SoZides, B2t. 510 and L. U. R.E., B2t. 209 C

Universi tS Paris-Sud, 91 405 Orsay Cedex, France.

The local structure of metal glasses gives still rise to some controversy. One of the contro- versial points is the existence or not of a chemical ordering similar to that in the crystalline state. A determination of the local structure in a binary alloy needs the knowledge of the partial radial distribution functions. EXAFS is particu- larly well suited for that because of its unique ability of investigating local order around each of the components. Another experiment is however necessary to get the three partial R D F. We thus have undertaken a joint X-ray scattering and EXAFS

study on several metal-metal glasses. We report here our results on the structure of a Cu2 Ti amorphous alloy. Recently Sakata et al. (1) have perfop med a neutron scattering experiment on the same

O- 1

glass. They found an important prepeak at 1.9 A

in the interference function, which seems to indicate a short range chemical ordering between copper and titanium atoms. They however did not make a modelling of the local structure.

1. Experimental procedure and data handling The amorphous samples have been prepared by

DC sputtering. A X-ray absorption measurement at the argon K edge has not shown any argon contami- nation of the sample. The X-ray scattering data have been obtained at room temperature using Mo Ka and KB radiations. A solid state detector (Si Li)

has been used as a counter in order to avoid fluo- rescence. The experiments have been performed by transmission and reflection techniques ; the maxi-

O - 1

.mum k value was 15 A

.

The diffracted intensities ;were corrected for background and Laue scattering.

EXAFS experiments have been done at Lure using the X-ray beam delivered by the DCI storage ring, which was operated at 1.72 GeV with a mean current of about 150 mA. The experimental set-up has been

described elsewhere (2). We have used a Ge (1 11)

channel cut monochromator for the Titanium K-edge in order to reduce the higher orders, and a ~i(220) one for the copper edge measurements. Experiments have been performed at various temperatures between 10 K and room temperature both for amorphous and crystallized samples. EXAFS data handling has been described previously (2,3) and will just be briefly summarized here. We have used the now clas- sical formulation of the EXAFS ( 4 ) .

and the theoretical values of the backscattering amplitudes A(k) and of the ~hase-shifts +(k) which have been calculated by Teo and Lee (5). The elec

2m

tron momentum k =(-(E-E~))'/~ depends on the h2

E value of the zero of kinetic energy, which has t

: be adjusted to best fit the phase of the signal, as it has been discussed by Lee and Beni(6). We have checked this procedure on elemental copper and titanium. We used a simple form E(k) = k/y for the electron mean free path. Best fits of the amplitudes for elemental Cu and Ti have been obtain- ned with the same value y = .45 a.u. ; this value mythus be used with confidence for a Cu2Ti alloy.

2. Results

The interference function which we obtain from the scattering experiment is very similar to

b

those obtained for other metal-metal glasses (7,8).

It may be however pointed out that contrary to the neutron scattering result

(I),

there is no prepeak at 1.9 This prepeak, which indicates corre-

0

lations at % 4 A , is observed in the neutron scat-

tering data because of the negative scattering length of titanium which allows

a

chemical short range order to be observed easily (1). The radial-

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C8-208 JOURNAL DE PHYSIQUE

Fig. 1

Reduced radial distribution function obtained from the scattering experiment. distribution function which is shown in Fig. 1

exibits several peaks which indicate some medium range ordering. The first main peak which is broad

0

and centered around 2.75 A , is due to the Cu-Cu and Cu-Ti first neighbour distances and, the shoulder

0

at 3.15 A is logically attributed to the Ti-Ti ones. However the intensity of this shoulder is surprising : it is similar to that found for a Cu2 Zr glass by Sadoc and Lienard (7) while the titanium scattering factor is but 0.3 times that of zirconium.

The copper and titanium edges are very similar in the amorphous and crystalline forms, however the titanium edge is shifted by 2.1 eV towards lower energies when going from the crystalline state to the amorphous one. This shift indicates a charge transfert corresponding to a less ionized state of the titanium atoms in the glass. It has also been observed by Stern et al(9) on the LIII edge of dysprosium in Dy Fez. The copper edge spec- tra show EXAFS modulations. The spectrum of the amorphous samples which is shown as a dotted curve in the upper part of Fig.2, is simple and structu- reless, and its Fourier transform exhibits a single peak at 2 (uncorrected for phase-shifts) which is logically attributed to Cu-Cu nearest neighbours pairs. On the titanium edge however, there is no EXAFS modulation, except for a weak wiggle around

150-200 eV, while the crystalline sample does exhi-

L

I I I I I

100 200 300 LOO 5 C ENERGY (eV1

Fig. 2

Exafs spectrum on the copper K edge (room temperature) and its reconstruction of the Ti-Cu and Ti-Ti pairs. This interpretation, which will be quantitatively discussed later, is also supported by an examination of the spectrum of the crystalline form which too is anomalously weak below 150 eV buthas an increasing intensity between 150 and 450 eV.

The copper edge EXAFS has been analyzed using the same E and y values as those used for the ele- mental copper (the edges are very similar

,

except for a shift of 0.8 eV), and assuming that its high energy part is exclusively due to the Cu-Cu pairs. The best fit which is shown by a dashed curve in the lower part Fig.2 is obtained for the values given in table

I,

but it is too weak and even out of phase below 200 eV. It is necessary to add the contribution form a second shell which has to be heavh ly damped with energy. The corresponding partial

bit EXAFS. This is a direct evidence of the large

L

I I

100 200 I 1

disorder which exists in the first coordination

300 LOO ENERGY lev)

shell around .titanium atoms in the amorphous state. However it cannot explain the cancellation of the signal below 150 eV which has to be attributed to a destructive interference between contributions

Fig. 3

Calculated EXAFS contributions of t;e titanium edge from 12 copper atoms-at 2.74 A I - ) and

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EXAFS which is shown by the dotted curve in the lower part of Fig.2 can be attributed to a broadly

0

distributed shell of titanium atoms at about 2.74 A,

0

with a 2.14 A and a coordination number close to 6. The overal fit which is shown by the solid curve in the upper part of Fig.2 compares rather well with experimental EXAFS. Fig.3 is a simulation of partial EXAFS contributions around a titanium atom: the solid curve corresponds to a shell of 12 copper

0 0

atoms at 2.74 A, with

a

= .I4 A, the dotted and dashed ones to hypothetical shells of - 8 titanium atoms at respectively 3.12 and 3.A, with

a

= -12 A. This shows that the copper contribution can be can- celled out over the whole energy range by a distri- bution of titanium atoms at a reasonable distance from the central titanium one. These distances com- pare well with the positions of the main peak at 2.75 and of its shoulder at 3.15

%

on the radial distribution function obtained from the scattering experiment. Of course, we do not claim to give an accurate description of the Ti-Ti pair distri- bution : for this reason, the corresponding para- meters are given whiminparenthesis in Table I.

The copper-copper coordination is however accura- tely determined by EXAFS. We then come back to the scattering data. Fig.4 shows the pair distribution function p(r) corresponding to the first peak of the R.D.F.. Keeping in mind our EXAFS results on the copper-copper coordination, we can fit its low distance side by a distribution of 3 copper atoms

0

at 2.53 A from a given copper. Subtraction of the corresponding peak from the experimental data then

0

yields a peak centered at 2.77 A which can be attri- buted to the Cu-Ti pairs, with coordination numbers

of 5 titanium surrounding a copper atom and 10 copper around a titanium one. Finally the shoulder at 3.15

1

can be attributed to a large coordi- nation by at least 1 1 titanium-titanium pairs. 3. Discussion

We first point out that the two techniques, EXAFS and X-ray scattering, yield a consistent description of the local order in the Cu Ti glass.

2

In table I we have compared the structure of the glass to that of the crystal. The crystalline structure is that of Au2 V (lo), so that coordi- nationnumbers are known. However interatomic dis- tanceshave not been measured by x-;ay diffraction. \*thus have measured then by EXAFS : the titanium

K edge is dominated by the contributions of Cu-Ti pairs, allowing an evaluation of the Cu-Ti distance The Cu-Cu distance could be extracted from the cop- per edge EXAFS, and is quite similar to the corres- pondingdistance in the Cu Ti phase (11). Inspection

3

of table I shows that both Cu-Cu and Cu-Ti distances

0

are shorter in the amorphous form by : .05 to .1 A. The Cu-Ti distance in the amorphous state is shor- ter than the average of Cu-Cu and Ti-Ti distances,

0

which would be around 2.84 A as it is in the crys- bal.Such a shortening has also been found by Givord et al. for Ni2 Y glass (8). It seems to in- dicateappreciable electron interactions between

the two metallic components. It may however be no-

ticed that the Cu-Ti distance is close to the sum

0

of the atomic radii, which would be 2.75 A and not that of the covalent radii, as it is the case in Ni2 Y (8) or Dy Fez and Tb Fez (9). The distance between large atoms is greater than the

0

Goldschmidt diameter (2.94 A for Ti) in the Cu,Ti

L

glass, while it is smaller by about .2

%

for Ni2Y.

A l l that suggests some electron transfert towards titanium atoms, in agreement with the titanium edge shift between amorphous and crystalline forms. tkalso notice that the disorder in the Cu-Cu shell, which is measured by the Debye-Waller like para- meter

a

in the EXAFS analysis, is about the same in both the amorphous and crystalline forms

0

(a

2 0.9 A at 50 K). The distribution of unlike

2.5

3.0

atoms pairs is however much bro?der. Such a broa-

dening has also been observed by Stern et a1.(9)in Fig. 4

First peak in the p(r) function their EXAFS study of the Dy Fez glass, and we also

Points show the experimental results, and curves _{get it from preliminary work on Cu2 Zr,amorphous} the partial and overall distributions calculated

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JOURNAL DE PHYSlQiJE

Table I

Distances, coordination number and disorder in first neighbours shells in amorphous and crystalline Cu Ti

(the center atom is on the left) 2

a) Reference 10

-

b) in the Au ₂V structure there are 4 nearest neighbours,and one further away (10) c) These distances correspond to a best fit. The accuracy is around

+

.05 A.

alloy. This could be a general feature of the local Acknowledgments

structure of metal-metal glasses. We point out here We thank J. Sanchez for having prepared the the sensitivity of EXAFS to disorder which is much amorphous samples and Y. Calva yrac the crystalline

one. Cu-CU

Cu-Ti Ti-Cu Ti-Ti

larger than that of the scattering experiments.

References The coordination numbers around copper are very

-.

Coordination numbers Disorder

(i)

EXAFS Scattering x-raya) EXAFS

. -

similar to those in the crystalline state, but the 1. Sakata M, Cowlam N,Davies H.A., J. Phys. F : Metal Physics,

9,

L 235 (1979).

number of Ti-Ti pairs is much larger. Its measure-

2. Raour D.. Petiau J.. Bondot P..Calas G.

0 Distances (A)

EXAFS Scattering C, EXAFS

ments from the scattering data is also larger than FontaineA., Lagarde P., Levitz P., Loupias G,

the value suggested by the EXAFS interpretation. We Sadoc A, 3 paraItre dans Rev. Phys. App1.(1980). amorphous 2.5 1-2.53 2.749.04 2.749.04 (3-3.12) amorphous 3 - 4 5 - 6 (10-12) (7-8)

suggest that, as it is the case in the crystalline 3. Fontaine A., Lagarde P., Naudon A., Raoux D., Spanjaard D., Phil. Mag.

=,

17, (1979). structure (lo), there is one Cu-Cu pair at a larger

4.

Ashley C.A. Doniach S., Phys. Rev. a 1 , 1279J1975) distance than 2.53

1.

I f this distance lies within

o 5. Teo B.K., Lee P.A., J.Am.~hem.~oc.l01,2815,(1979)

-

the shoulder at 3.15 A , the existence of such apair

6. Lee P.A., Beni G., Phys. Rev.,e,2862,(1977) would reduce the titanium-titanium coordination

7. Sadoc J.F.. Lienard A., Proc. 3rd Int. Conf. amorphous 3 5 10 1 1 crystalline + 5 10 4

number to about 7 yielding the same result as the Rapidly quenched ~ e t a l s ,Brighton 2,405, (1978) 0

EXAFS analysis. Such a Cu-Cu pair at 3.15A could 8. Giword D., Lienard A., Rebouillat J.P.,Sadoc J.F. notbe easily seen on the EXAFS suectrum. The analysis J. Phys.

40,

C5-237 (1979)

amorphous 2.53 2.77 2.77 3.15 amorphous 1 1

-

.12(300 K) '109-.lo( 50K) .14 .14 (.12)

of the neutron scattering data could provide a 9 Stern E.A., Rinaldi S., Callen E., Heald S., Bunker B. J.Mag.Mag. Mater

1

,188, (1978). check of this interpretation because of the very

10 Schubert K., Raman A., Rossteucher W.,

crystalline

2.57-2.59 2.82-2.86 2.82-2.86

weak neutron scattering length of titanium. Anyhow Natuwissenchaften

51,

507, (1964).

at least around the copper atoms, we have clear evi- 1 1 Pearson W.B., A handbook of lattice spacings dence of a chemical ordering in the amorphous state: of metals and alloys (Oxford :Pergamon)

3, 616, (1958).

-

a random distribution of atoms as it has been sug-

12 Mizoguchi T., Kudo T., Irisawa T., Watanabe N.,

gested for Cu2Zr by Mizoguchi et a1.(12) would im- Misawa

M.,

Suzuki K., Proc. 3 rd Int. Conf. ply a copper-copper coordination number two times Rapidly Quenched Metals Brighton,

2,

384,

(1

978)

.

larger than the Cu-Ti one, which our results exclu-

0- 1