X-RAY ABSORPTION SPECTROSCOPY ON AMORPHOUS MIXED-VALENT ALLOYS

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X-RAY ABSORPTION SPECTROSCOPY ON AMORPHOUS MIXED-VALENT ALLOYS

D. Malterre, A. Siari, J. Durand, G. Krill, G. Marchal

To cite this version:

D. Malterre, A. Siari, J. Durand, G. Krill, G. Marchal. X-RAY ABSORPTION SPECTROSCOPY ON AMORPHOUS MIXED-VALENT ALLOYS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8- 991-C8-995. �10.1051/jphyscol:19868191�. �jpa-00226098�

X-RAY ABSORPTION SPECTROSCOPY ON AMORPHOUS MIXED-VALENT ALLOYS

D. MALTERRE, A. SIARI, J. DURAND, G. KRILL and G. MARCHAL

L a b o r a t o i r e d e P h y s i q u e d u S o l i d e , U n i v e r s i t e d e Nancy I , BP 239, F-54506 Vandoeuvre-1.5s-Nancy Cedex. F r a n c e

and

L.U.R.E., B S t i m e n t 209 C, F-91405 O r s a y Cedex, F r a n c e

R6sum6.

-

Les i n f o m t i o n s s-bructurales obtenues indiquent l a pr6sence de deux environnements locaux associks aux deux 6 t a t s de valence enti& de 1'Eu.

A b s t r a c t . - X-ray absorption msurements were performed on & Pdl, and Eux armrphous alloys. I n both systems, LIII edgesclearly de- m n s t r a t e a concentration-induced valence change. The Eu valence a&- t u r e i s found t o be inhomgeneous in nature. XANES and

=

of those encountered f o r !&2+ and E U ~ + i n t h e Eu/Pd system.

INTRODUCTION.

-

.

EXPERIMENTAL.

-

lov8

a.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868191

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The X-ray absorption experiments w e r e p e r f o m d using the syn- chrotron radiation beam of the D. C. I. storage ring on the EWE3 I1 station a t L. U. R. E. (Orsay).

EXPERIMENTAL RESULTS. 1 ' LIII edge

-

a) Eux Pdl-x alloys

.

LIII edge m e a s ~ t s were p e r f o m d on the Ey, P ~ L - ~ m r p b u s alloys. The data were taken a t 300 K with an energy resolution of about 1.5 eV. The spectra clearly exhibit a double peaked s-tructure with a peak separation of about 7 eV which is characteristic of a mixed-valent s t a t e of Eu ( f i g w e 1)

.

*

electronic configmation whereas the high energy peak i s a signature of the 4 f (5d 6s 1 configuration. The average valence is obtained f r o m the- s e spectra with a standard f i t t i n g procedure as discussed elsewha(2).The Eu ions are t r i v a l e n t i n EuO, 17 PdO. 8 3 alloys and the average Eu valence decreases with increasing Eu concentration. The valence change in m r p h o u s s t a t e i s slightlyslowerthm in crystalline one : m r p h o u s Euo 2 5 Pdo-75 i s mixed valent whereas i t s crystalline counterpart Eu Pd3 i s &valent.

%ssbauer specbmscopy a l s o p e r f o m d on these alloys indicates t h e inhorn- geneous character of the Eu mixed valent s t a t e in m r p h o u s alloys. The ave- rage Eu valences estimated f r o m Nijssbauer spectra are in good agreement with those estirrated from L I I I edges (figure 2). ?his r e s u l t shows that f i n a l sta- te-effects in the present case ca? be neglected. Therefore LIII edge measu- rements allow an accurate determuation of the valence ( 3 ) .

Fig. 1 : Average Eu valence versus Fig. 2 : Average Yb valence versus concenmtion i n m r p h o u s Eux Pdl, concentration in amrphous Y4, P ~ I - ~

alloys and in crystalline alloys

Eu Pd5, Eu Pd3 and Eu Pd2.

30.

-

-

I>

20.

b ) Ybx Pdl-x alloys

.

Same measurements have also been performed on the Y b LIII edge i n m r p h o u s Y& alloys. We report i n figure 2 the concentra- tion dependence of the Yb valence ; it decreases from 2.90

+

x = 0.29 t o 2.12

+

I% e t i c measurements on these alloys clearly demns-ate the presence of Y b g . Bllf disagreement on the valences deduced fmn b t h technics suggests admixture between s t a t i c and dynamic 4 f fluctuations. These r e s u l t s w i l l be discussed i n a forthcoming paper with mw d e t a i l .

c-EuPd, c-EuPd,

4 I

4-t--k

,-,

+

\t

\

.

#\( \t

+'

\

t, -+

c - ~ t ! ~ d ~

0.20 0.25 x ^{0.30 035}

In the (20 eV- 100 eV) above an absorptipn edge, t h e photoelec- Won has a long mean-free path. Then d t i p l e s c a t t e r i n g o c c u r which a r e d i f f i c u l t t o take i n t o account. Particularly, t h e single s c a t t e r i n g theo- ry which w r k s i n t h e EXWS regime (100 - 1500 eV from t h e edge) f a i l s t o describe t h e XANES regime ( 4 ) .

However, as discussed a t t h i s conference (51, it i s possible in a simple way t o use d i r e c t l y t h e XANES structure in order t o deduce t h e interatomic distancesin a mixed-valent mterial ( i . e . the imprtance of atomic relaxation). The basic idea we use is that t h e XANES s-tructures simply scale w i t h energy : (E - = c s t / ~ ~ . This simple forrrolla holds strikingly even i n the case of 4f nnterials (6). I n an inhomgeneous mixed-valent mteria!

,

.

The XANES structures f o r several Eux P ~ I - ~ alloys are repor- t e d in figure 3. They are obtained by a difference between experimental spec- and t h e f i t t e d LIII edges. In order t o t e s t t h e

v a l i d i t y of t h e rescaling procedure in t h e present case, we try t o re- produce t h e XANES s t r u c t u r e s of t h e divalent a l l o v (x = 0.35) using tho- s e of t h e t r i v a l e n t one (x = 0.19). A s shown in figure 3, this procedure works q u a l i t a t i v e l y , at l e a s t on t h e positions. The rescaling fac- t o r we found experimentally is (0.94) in a good agreement with t h e r a t i o between t h e interatomic distances in divalent (ELI Pd2) and -trivalent

(Eu Pd3) c r y s t a l l i n e compounds (0.92). D i f f i c u l t i e s t o reproduce t h e am plitudes can be explained by t h e differences in t h e l o c a l environments around Eu atoms in t h e two amxphous alloys (as shown by t h e EXWS r e s u l t s ) A s shown in figure 3 f o r t h e x 0.25 mixed-valent case, it i s possible to obtain a good f i t t o t h e data using t h e divalent and .trivalent XANES as references. Thus :

Fig. 3 :

XANES spectra f o r severdl mrpbus alloys.

The f u l l l i n e s are t h e results of t h e f i t t i n g procedure (see t e x t ) .

C8-994 JOURNAL DE PHYSIQUE

where i s the avemge Eu valence. (This procedure works on the whole range of concen-tration). This r e s u l t i s i.pOrtant ( i ) because it means that i n I. V. m r p h o u s alloys, the local environments m u n d E U ~ + and E U ~ + ions a r e roughly identical t o those of the reference alloys, (ii) it shows t h a t XANES experiments can be used t o t e s t the importance of atomic relaxations and thus t o decide i f the mixed-vdlent ground s t a t e i s s t a t i c (inhomgenous) o r dynamic (homgeneous)

.

wS e g e r i ~ n t s 'nave only been p e r f o m d on two Eu alloys:

t h e divalezt x=C. 35 and the n-ly trivalerit x=0.235. (k

.

3 2

that t5e f i r s t s h e l l i s constituted by 12f0.5 Pd atom a t 2.95 A whereas in x=0.35 Eu ions are surrounded by 10+2 Pd a t o m a t 3.21 A and 3k1 Eu atoms a t 3.28 A.

These r e s u l t s confirm the interatomic distances deduced from t h e rescaling procedure a t t h e XANES spectra.

Fig. 4 : (k

.

R e f u l l l i n e i s the r e s u l t of our f i t

C O N C L U S I O N .

-

the XANES. In such system, the XANES ~ f i e c d ~ ~ c a n be reproduced by a direct sumntion of two XANES contributions associated t o each integral configu- ration. We can also deduce by a simple way the r a t i o of interatomic distan- ces around w i v a l e n t and divalent Eu. Thus XANES seerns t o be a very powerful technic t o investigate the mixed valent systems.

REFERENCES.

-

(1) - P o l i t t , B., Dlrkop, D., and Weidner, P., J. fig. h g . M3t.

5,

(1985) 585

W a l t e r , U., Weidner, P., and Wohlleben, D., Phys. Rev. L e t t . ,

54,

(1985) 4 8 1

( 2 ) - fiijhler, J . , J. Phg. Mag. %t.

47-48,

( 3 ) - filterre, D., Delcroix, P., S i a r i , A . , M a n d , J., K r i l l , G., and

%l, G., Sol. S t a t . C m . , 58 (1986) 5 9 1

( 4 ) - N a t o l i , C. R., in EXAFS and ~ ~Structure Proceeding ~ d of ~t h e e I n t e r n t i o r a l Conference, ed. B i a m n i ; A., T o m c i a , L., and Stipack, S., ( F r a s c a t i 1982) 43.

(5) - K r i l l . , G., in t h i s conference

( 6 ) - Pkterlik., G., m l e r , J. E . , and Wilkins, J. W., Phys. Rev. L e t t . , 50, (19831 267

( 7 ) - Krill., G . , G p p l e r , J. P., Ravet, M. F., Godart, C., and S e n a t e w , J. P., J. %g. k g . M a t . ,

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