EXAFS STUDY ON RADIATION INDUCED DEFECTS

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EXAFS STUDY ON RADIATION INDUCED DEFECTS

T. Bolze, J. Peisl

To cite this version:

T. Bolze, J. Peisl. EXAFS STUDY ON RADIATION INDUCED DEFECTS. Journal de Physique

Colloques, 1986, 47 (C8), pp.C8-1045-C8-1048. �10.1051/jphyscol:19868202�. �jpa-00226110�

JOURNAL DE PHYSIQUE

Colloque C8, suppl6ment au n o 12, Tome 47, decembre 1986 C8-1045

EXAFS STUDY ON RADIATION INDUCED DEFECTS

T. B O L Z E ( ~ ) and J. PEISL

Sektion Physik, Universitat Miinchen, Geschwister-Scholl-Platz 1, 0-8000 Miinchen 2 2 , F . R . G .

Nous rapportons sur I'applicatlon de la mbthode EXAFS B 1'6tude des defauts dirradiation dans les alll&pAl(Zn), B faible concentration de Zn. Par irradiation avecdas Blectronsde 2 MeV B 80 K on c r b des defauts interstitiels 6'AI pi@% par Iss atomas de solut6. A p r h irradiation l'amplitude du signal EXAFS du Zn est rtduit du fait de la superposition de &ux distances interatomiquas (distance plus-proche-voisin normals + distance ssmib au defaut). Cette dernibre est t r o u v k Bgale B R&f = 2.45

A.

La amparaison avec les m&les montre que aprbs trradiatton I'atome de Zn sst sltu8 sur un site interstitiel octahedrique.

We report

on

the application of EXAFS to the study of radiation lnducsd defects i n dilute A1( Zn) alloys. By irradiation with 2 MeV electrons at 80 K A1 interstitials are created, which are trapped by the Zn solutes. After irradiation the amplitude of the Zn EXAFS i s reduced due to the superposition of the regular nearest neighbour distance and a new defect distance. The defect distanca was determined to be Rdef = 2.45

A.

A comparison with model calculations shows that the Zn atom Is laceted on the octahedral interstitial site after irradiation.

Introduction

The defect structure after particle irradiation i s influenced considerably by the interaction of point defects with impurity atoms. Void swelling m y be enhanced

or

diminished by alloying a metal with foreign atoms (1,2(. Radiation induced segregation w i l l influence mechanical strength, corrosion resistance and radiation embrittlement 131. An understanding of the role played by solute atoms i n alterlng irradiation behaviour requires a detailed knowledge of the configuration and annealing behaviour of the defect

-

solute complexes that are created during irradiation. EXAFS-spectroscopy offers the opportunity to use the Impurity atoms as measuring probes and thus to obtain direct information on the solute

-

defect Interaction after irradiation.

Experimental details

The interpretation of EXAFS spectra i s pcrssible only for very simple defect patterns. Fast electron irradlatjo generates almost exclusively single interstitials and vmncies, so called Frenkel defects (FD ) 141. TheAl(Zn) m p l e s with 500to 1OOOppm Zn were jrrdiated with 2 MeV electrons at 80 K at the Hahn-Meitner lnstitut in Berlin. At this temperature the created interstitials are mobile and can be trapped by the solute. In order to have mainly single interstitials trapped, the irradiation dose was such that only 20% of the impurities have trapped an interstitial. The irradiated slmples were transported i n a liquid nitrogen dewar to Hamburg. The experiments were performed at the spectrometer EXAFS II i n HASYLAB. Without warming them up the samples were transferred into the measuring cryostate. The EXAFS was measured i n the fluorerscence made with a Na I detector. The background was supprassed by a Z- 1 filter and a sollerslit (5,61. The samples were annealed for a 10 min.period at each annealing stage. After each annealing procedure at last three EXAFS

-

spectra were taken. The mmurements were performed at 80

K.

( ' ' ~ e w address : Siemens AG, ZT MTZ 21, Otto-Hahn-Ring 6, D-8000 ~ G n c h e n 83, F.R.G.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868202

JOURNAL DE PHYSIQUE

Results and dlscusslon

I n Fig. 3 we compare the EXAFS-amplitude wlth the A magnitude of the resistivity change during the

anneal. EXAFS and resbtlvlty have been measured simultaneously at the same sample. At 130 K the resistivity changas, which is attrlbuted to a defect anneal at this temperature. Thfs defect has little influence on the solutes' EXAFS.

In Fig. 1 we compare the absorption spectrum of an Irradlated (broken line) and an annealed sample. The annealed and therefore defect-free sample served as reference system. Obviously the EXAFS

-

amplitude i s reduced after irradiation. The spectra remain unchangxl up to annealing temperatures of 150 K. Between I 8 0 and 200 K the vacancies become moblle and annihilate with the trapped tnterstltals. The spectra of samples annealed above 200K are the same as before irradiation. In the Fourier-transform (Flg. 2) the first neighbour peak has a reduced magnitude after Irradiatlon. Uslng p b s e ~ h i f t ~ from Teo and Lee 11 21 we obtaln for both the irradiated and the annealed crystal the nearest neighbour distance R = (2.88 i 0.04)

A.

In order to prevent lnterstltlal clustering the 50 100 150 200 250 300 irradiatlon

dose

was low. From the IOOO ppm Zn

solutes only 150 to 200 ppm have trapped an

T

( K )

F i g . 3 W S amplitude A ( A , o ) i n corn-,

interstitial after electron Irradiation. Thus most of p a r i s o n with t h e resistivity

the Zn atoms have an unperturbed environment. The a n n e a l . s i s t i v i t y O ( X , + ) anneal f r o m literature s o l i d linc

-

^re- 171

absorotlon structure after irradlation conslsts of

1001

; ^?SO.

2.0

, _-

100-

I S O .

1.8

two cktrlbutions, the structure hr of the Zn atoms whlch have trapped an interstitial and the structure b o f the Zn atoms in the defect-free lattim.The contribution of the Zn atoms which have trapped an interstitial can be obtained by

- 1

1

(E,

300) i s the absorption structure of the annealed and completely defect-free sample, b1 (E,T) 1s the structure of the irradiated sample annealed at the temperature T. The weighting factors i n ( 1 ) were varied untll the structure of the regular lattice vanished I n thedifference spectra The so obtained value was i n good agreement wlth the defect concentration determined from the resistivity change.

1.6

9.6 9.7 9.8 9.9 10.0 10.1 10.2

E(keV)

_{1%. 2} F o u r i e r t r a n s f o r m of t h e W Si n t e r - Fig. 1 h b s o r p t i o n s p e c t n n n of an Al(2n) sample a f t e r f e r e n c e f u n c t i o n x[k) obtained from t h e

i r r a d i a t i o n (broken l i n e ) and a f t e r a n n e a l i n g s p e c t r a given i n F i g . 1 a t 303 K ( s o l i d l i n e )

In Fig. 4 the FT of @E,T) i s plotted for a number of annealing steps. Due to the trapping of the Al lnterstltlal after irradiation a new defect-dlstance appears, whlch amounts to R l = (2.46 i

0.05)

A.

This peak remains unchanged up to 150 K. Between I 8 0 and 200 K the defect anneals and at 230 K we get R = (2.85 i 0.05)

A.

The superpclsitlon of thesesimilar distances gives rise toa beatlng In the EXAFSof the irradiated sample, which can bedescribed by 181:

For the modulated amplitude one obtains wlth AR = R1

-

R :

I . . 1 . 0 I . .

, ( : I

F i g . 5 Amplitude f u n c t i o n s o f t h e l n t r r f e r e n c e f u n c r l o n o f t h e f l r ' s t r h r l l o b t a l n r d b y b a c k - t r r n s f o r m l n r t h c n e r r r ~ t n e l t h b o u r p e r k t I n F I E . 1 . B r o k e n l l n r : . m p l l t u d e A ' o f l r r a d l a t r d s a m p l e : s o l l d l l n r : amplitude A o f r n n r a l r d s a m p l e

F J g . 4 F o u r l r r t r a n s f o r m d a r l v e d [ r o n t h e d l f f e r c n c r o f a b ~ o r p r l o n s p e c t r a according t o E q : ( 1 )

In Fig 5 we have determlned the EXAFS amplltude of the f i r s t shell by back-transforming the nearest nelghbour peak of F l g 2. The amplitude A'of the Irradiated sample I s clearly modulated compared to the amplitude A of the annealed sample. In the ratio A' /A the other k-dependent factors are eliminated:

I n order to determine the structure of the interstitial-solute complex we compared our experimental data of (A'/A12 wlth calculations for dlfferent possible defect configurations. Two posslble configurations the mlxed <100> dumbbell and the Zn atom on the interstitial octahedral slte are shown i n Fig. 6. The defect distances were f l r s t determined by calculatlons at the lattlce relaxations wlth emplric morse potentials I10,11

I.

i n a second step these dlstances were varied until we obtained the best agreement between the experlmental data and the calculated values of A'/A for a given Oefect structure.

The best fits that we c w l d obtain are shown i n Fig. 7. For mlxed dumbbells the agreement was satisfying only for very special cases for which R1 = R2 and R3 = R4 = 2.86

A.

For more general cases i n which the distanm differ only by a few hundredth part of an

A

the calculatlons

C8-1048 JOURNAL DE PHYSIQUE

deviate considerably from our experiment results. Excellent agreement was only achieved for theoctahedral conf!guration i n which the Zn atom sits on the interstitlal octahedral site. From our f i t we obtaln R l = (2.44

+

0.05)h and R2 = (3.59

+

^0.09)A.

Fit. b I and b. ' l v o p o s % i b l e inrrrstlti.1-solute c o n -

p l e r c r ; a n l x t d < 1 0 0 , d u n b b r l l : b I n a c 3 r o n T i p I A - 0 C o m p z r l r o n o f e ~ p e r l n r n t * l r r % u l l r ( A ' / * ) ' i n t e r s l i ~ l ~ l o c t a h e d r a l site 2 n d b e s t I l l s lor t r v c r a l i n l c r r t l l l r l s o l u l c

c o m p l r r c % ;

.

1 % t h o - r a n e r r o r : A n l x r d '100, d u m b b e l l 0 - 1 . 6 ; I m i r e d 'll0, d u n b b r l l e - 1 . 5 ; .

C a i x r d ' 1 0 0 ~ d u n b b c l l . g t n r r r 1 c r r i 0 . 2 . 6 : ' D l n a t o n ' o n o c t a h e d r a l l n t e r s t l t l ~ l l l t r

d . 0 . S

Many thanks are due to R. Poeschke and W. Tietze (Hahn-Meitner Institut Berlln) for thelr asststance wfth electron lrrsdlatlon as well as to P. Rabe (Gesamth~hschule Emden), W.

Malzfeld and W. Nieman ( HASYLAB , Hamburg) for their assistance during the experiments at DESY. This work was supported by the Bundesministerium fur Forschung und Technolcgie.

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