HAL Id: jpa-00226095
https://hal.archives-ouvertes.fr/jpa-00226095
Submitted on 1 Jan 1986
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
SUBSTITUTION EFFECTS ON THE MIXED VALENCE BEHAVIOUR OF Eu in EuCu2Si2
Sujata Patil, B. Padalia, R. Nagarajan, L. Gupta, R. Vijayaraghavan
To cite this version:
Sujata Patil, B. Padalia, R. Nagarajan, L. Gupta, R. Vijayaraghavan. SUBSTITUTION EFFECTS
ON THE MIXED VALENCE BEHAVIOUR OF Eu in EuCu2Si2. Journal de Physique Colloques,
1986, 47 (C8), pp.C8-983-C8-986. �10.1051/jphyscol:19868189�. �jpa-00226095�
JOURNAL DE PHYSIQUE
Colloque CS, supplement au n o 12, Tome 47, decembre 1986
SUBSTITUTION EFFECTS ON THE MIXED VALENCE BEHAVIOUR OF Eu in EuCu,Si,
SUJATA PATIL, B.D. PADALIA, R. NAGARAJAN*, L.C. GUPTA* and R. VIJAYARAGHAVAN*
D e p a r t m e n t of P h y s i c s , I n d i a n I n s t i t u t e of T e c h n o l o g y , Bombay 4 0 0 0 7 6 , I n d i a
' ~ a t a I n s t i t u t e of Fundamental R e s e a r c h , Bombay 4 0 0 0 0 5 , I n d i a
A b s t r a c t
-
R e s u l t s o f t h e L 3 XANES and 1 5 ' ~ u Mossbauer measurements onE U C U ~ ( S ~ ~ - ~ S ~ ~ ) ~ , E U ( C U ~ - ~ A U ~ ) 2 S i 2 w i t h Nx(0.2 and E U ( C U ~ . ~ M ~ . ~ ) S ~ ~ where M=Au, Ag, Ru and N i , a r e r e p o r t e d here.
I n t r o d u c t i o n :
The mixed v a l e n t (MV) systems, EuCu S i and EuPd S i have been e x t e n s i v e l y s t u d i e d b y s e v e r a l i n v e s t i g a t o r s 11-83.
' ~ t f l i t ~
o f ~ o < s b % u e r s p e c t r o s c o p y (MS) and L3-X r a y a b s o r p t i o n near edge s t r u c t u r e (L3-XANES) i n s t u d y i n g t h e Eu v a l e n c e s t a t e i n MV systems has been e s t a b l i s h e d [10]. S u b s t i t u t i o n o f Sn and Au i n EuPd S i 2 has been r e p o r t e d t o a f f e c t t h e v a l e n c e s t a t e o f Eu [10,13]. These c o n s i d e r a t i 6 n s have m o t i v a t e d us t o c a r r y o u t s y s t e m a t i c L3-XANES and MS on t h e s u b s t i t u t e d EuCu2Si2 samples. The r e s u l t s o f o u r L3-XANES and MS measurements on EuCu ( S i Sn ) Eu(Cu Au ) Si2where O(xt.2 and E C ( C ~ ~ . ~ M ~ . ~ ) ~ S ~ ~ where M =Au, Ag, 6u a Q a X ~ i X a R r e p o r k 8 h$&.Experimental :
The samples were p r e p a r e d by a r c m e l t i n g t h e c o n s t i t u e n t s i n argon atmosphere.
S i n g l e phase f o r m a t i o n o f t h e samples was checked b y x - r a y d i f f r a c t i o n method.
L3-XANES was r e c o r d e d w i t h a c o n v e n t i o n a l X - r a y a b s o r p t i o n s p e c t r o s c o p i c setup u s i n g l a b o r a t o r y source o f c o n t i n u o u s X - r a d i a t i o n [9,14]. A t r a n s m i s s i o n geometry Mossbauer spectrometer w o r k i n g i n t h e c o n s t a n t a c c e l e r a t i o n mode was used i n t h e p r e s e n t work.
R e s u l t s and D i s c u s s i o n :
L3-XANES o f a d i v a l e n t o r a t r i v a l e n t Eu i o n i s , g e n e r a l l y dominated b y a s t r o n g peak ( c a l l e d R a i e b l a n c h e o r w h i t e l i n e ) c o r r e s p o n d i n g t o an e l e c t r o n t r a n s i t i o n f r o m 2p c o r e l e v e l t o t h e f i r s t unoccupied l e v e l i n t h e c o n d u c t i o n band w i t h h i g h % $ s i t y o f s t a t e s . I n a MV system, t h e a b s o r p t i o n p r o f i l e c o n s i s t s o f two-peaks p e r t a i n i n g t o d i v a l e n t and t r i v a l e n t components, s e p a r a t e d by -7 eV.
I n t h e p r e s e n t work, a two-peak p a t t e r n w i t h 7ev s e p a r a t i o n c h a r a c t e r i s t i c o f Eu based MV systems has been observed i n t h e L3-XANES o f Eu i n EuCu ( S i Sn ) f o r each o f t h e s e c o m p o s i t i o n s ( F i g . 1 ) . - T h e peak p o s i t i o n s f o r t h e ' ~ n lilbsfi?utlg sample& appear a t t h e same energy v a l u e s as have been observed f o r t h e MV Eu (Eu and Eu ) i n EuCu S i [2]. T h i s i m p l i e s t h a t Eu i n EuCu2(Si Sn ) samples r e t a i n s i t s MV c k a r i i c t e r . I t i s apparent f r o m F i g . 1 t h a t t h e r e l ! i f f v e X i K t e n s i t i e s
o f EU'+ and E U ~ + components a r e s e n s i t i v e t o Sn c o n c e n t r a t i o n . The i n t e n s i t y o f t h e d i v a l e n t component i s seen t o i n c r e a s e w i t h i n c r e a s i n g Sn c o n c e n t r a t i o n . T h i s may b e due t o t h e v a r i a t i o n i n t h e average l o c a l v a l e n c e w h i c h c a n n o t be i d e n t l g f e d b y t h e L3-XANES s t u d y alone. I t was, t h e r e f o r e d e s i r a b l e t o r e c o r d t h e Eu Mossbauer s p e c t r a o f the same samples.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868189
C8-984 JOURNAL DE PHYSIQUE
F i g . 2 shows t h e l5' Eu Mossbauer s p e c t r a of EuCu ( S i Sn ) a t 300K and 77K The s p e c t r a o f t h e Sn s u b s t i t u t e d samples a r e s i m ? l a r 1 t 6 t # a ? o f t h e MV EuCu S i compound e x c e p t f 4 ~ t h e presence o f a s a t e l l i t e l i n e which appears a t t h e p o s i Z i o 6 expected f o r Eu
.
The isomer s h i f t ( I S ) o f t h e s a t e l l i t e l i n e appears t o be t e m p e r a t u r e independent b u t i t s r e l a t i v e i n t e n s i t y ( w i t h r e s p e c t t o t h e main l i n e ) seems t o be t e m p e r a t u r e dependent. Also, t h e s a t e l l i t e l i n e grows i n i n t e n s i t y w i t h i n c r e a s e i n Sn c o n c e n t r a t i o n .F i g u r e s 3,4,5 and 6 show L3-XANES and 1 5 1 ~ u Mossbauer s p e c t r a o f Eu(Cu M ) S i w i t h M = Au ,Ag ,Ru and N i . S i m i l a r t o EuCu ( S i Sn ) t h e L ~ - X A ~ E ~ P6? Zu ~ C I J 2 ) 2 S i Z e x h i b i t t h e c h a r a c t e r i s t i c two-pgak
'p8tt6r6'
f o r each o f t h e s e sa~p?!~: I t$3
e v i d e n t from F i g . 3 t h a t s u b s t i t u t i o n o f Au has enhanced t h e i n t e n s i t y o f t h e Eu component compared t o t h a t o f EuCu S i.
TheL3-XANES r e c o r d e d f o r E ~ ~ C U Au S i f o r O t x f ~ 2 s u g g e s t t h a t s u b s t i t u t i 6 n 6f Au i s d r i v i n g t h e MV Eu(Eu a~d"Eu"f tdwards Eu
.
T h i r would i m p l y t h a t t h e s i n g l e Mossbauer l i n e w i t h IS=-7.9 mm/sec observed f o r 20% Au s u b s t i t u t e d sample i s a mixed v a l e n t l i n e ( F i g . 5 ) . F u r t h e r c o n c e n t r a t i o n o f Au may s t a b i l i z e t h e Eu i n E U ~ + s t a t e as a r e s u l t of i n c r e a s e i n t h e volume and r e d u c t i o n i n s c r e e n i n g o f Eu c o r e h o l e due t o t h e 3d band o f Cu. F i g . 4 i n d i c a t e s t h a t f o r 20% Ag s u b s t i t u t e d sample, t h e enhancement o f d i v a l e n t component i s l e s s t h a n t h a t f o r 20% Au s u b s t i t u t e d sample eventhough t h e y have s i m i l a r e l e c t r o n i c s t r u c t u r e . T h i s d i f f e r e n c e i n b e h a v i o u r may be due t o volume e f f e c t s a r i s i n g o u t o f d i f f e r e n c e i n t h e i o n i c r a d i i . I t should be n o t e d t h a t t h e I S o f t h e Mossbauer l i n e f o r t h e 20% Ag s u b s t i t u t e d sample does n o t show a s h i f t towards t r i v a l e n t p o s i t i o n as one would e x p e c t , b u t shows a b r o a d e r l i n e a t 300K and t h e development o f a satellite l i n e towards t r i v a l e n t p o s i t i o n a t 77K. T h i s i n t e r e s t i n g aspect w i l l be d i s c u s s e d elsewhere.The L3-XANES (Fig. 4) of Ru substituted sample is similar to that of However, t h e MS measurements show two l i n e s i n t h e MV regime; t h e m a j o r
:Y::2SI~.
n e a r l y a t t h e same p o s i t i o n a k t h a t o f EuCu2SiZ and t h e m i n o r l i n e has IS= -6-9 mm/sec. which i s more towards Eu.
The s u b s t i t u t i o n o f N i f o r Cu i n EuCu S i may n o t change t h e u n i t c e l l volume as N i and Cu have almost t h e same i o n q c p a d i u s b u t may t r y t o f i l l u p i t s incornglete 3d s h e l l demanding e l e c t r o n s f r o m Eu, t h e r e b y t r y i n g t o s t a b i l i z e t h e Eu i n Eu s t a t e . T h i s has indeed been r e f l e c t e d i n t h e L3-XANES and Mossbauer s p e c t r a (Fig.4,5,6).
The m i x e d v a l e n c e b e h a v i o u r o f Eu i n EuCu S i has been i n t e r p r e t e d i n terms of t h e e x c i t a t i o n energy (E ) and a p o s s i b l e d i s ? r i g u t i o n i n E e i t h e r due t o t h e w i d t h o f t h e 4 f l e v e l
8P
due t o t h e d i f f e r e n t l o c a l e n v i r o 8 4 e n t[a].
The p r e s e n t r e s u l t s i n d i c a t e t h a t t h e v a l u e and d i s t r i b u t i o n o f E i s a f f e c t e d b y t h e s u b s t i t u t i o n o f Sn, Au, Ag, Ru and N i . An a t t e m g l i s in b e i nEUCUifi$
m de t o deduce t h e v a l u e o f E and i t s c o r r e l a t i o n t o t h e s u b s t i t u t i o n a l atoms.~ c k n o $ ? e d ~ e m e n t : We express o u r thanks t o P.L. Paulose f o r u s e f u l d i s c u s s i o n s and h e l p i n d a t a a n a l y s i s .
References:
[l] Sampathkumaran, E.V., Gupta, L.C. and Vijayaraghavan, R., Phys. L e t t . 43 (1979) 1189.
121 Hatwar, T.K., Nayak, R.M., P a d a l i a , B.D., G h a t i k a r , M.N., Sampathkumaran, E.V., Gupta, L.C., and Vijayaraghavan, R., S o l i d S t a t e Commun.
2
(1980) 617 [3] Gupta, L.C., Sampathkumaran, E.V., Vijayaraghavan, R., Prabhawalkar, V.,Prabhawalkar, P.D., and P a d a l i a , B.D., Phys. Rev. B
3
(1981) 4283 [ 4 ] Nagarajan, R., Sampathkumaran, E.V., Gupta, L.C., Vijayaraghavan, R.,Darshan, B., and P a d a l i a , B.D., Phys. L e t t . 81A (1981) 397.
[S] R o h l e r , J., and K a i n d l , G., S o l i d S t a t e Commun 36(1981) 1055.
[ 6 ] R o h l e r , J., Wohlleben, D., K a i n d l , G., and B a l s t e r , H., Phys. Rev. L e t t . 49 (1982) 65.
[7] Sampathkumaran, E.V., Gupta, L.C., Vijayaraghavan, R., Gopalakrishnan, K.V., P i l l a y , R.G., and Devare, H.G., J . Phys. C 14 (1981) 2237.
[8] Nowik, I . , H y p e r f i n e I n t e r a c t i o n s 13 (1983) 89.
[g] M a l i k , S.K., and Vijayaraghavan, R., Phys. Rev. B 30 (1984) 4031.
[ l 0 1 Kemly, E., C r o f t , M., Murgai, V., Gupta, L.C., Godart, C., Parks, R.D., and Segre, C.V., 3. Mag. Mag. Mat.
47&48
(1985) 403.[l11
Segre, C.V., C r o f t , M., Hodges, J.A., Murgai, V., Gupta, L.C., and Parks, R.D., Phys. Rev. L e t t . 49 (1982) 1947.[ l 2 1 Gupta
,
L.C., Murgai, V., Yeshurun, Y., and Parks, R.D., i n Valence I n s t a - b i l i t i e s , e d i t o r s P. Wachter and H. Boppart ( N o r t h H o l l a n d ) (1982) 225 and see o t h e r r e f e r e n c e s .[ l 3 1 Mishra, S.N., and Devare, H.G., H y p e r f i n e I n t e r a c t i o n s 15/16 (1983) 633.
[ l 4 1 P a d a l i a , B.D., Darshan, B., Dhar, S.K., Nagarajan, R., M a l i k , S.K., and Vijayaraghavan, R., Phys. L e t t . 103A (1984) 79.
ENERGY IeV) --r
F i g . 1. L XANES o f Sn s u b s t i - t u t e d ~ u ~ u ~ s i 2.
F i g . 2. Mossbauer s p e c t r a o f Sn s u b s t i t u t e d EuCu2Si2.
VELOCITY l M M / S E C )
CS-986 JOURNAL DE PHYSIQUE
ENERGY leV1
-
F i g . 3. L -XANES o f Au s u b s t i - t u t e d ~ u ~ u $ i
*.
I 1 I ,
*
.I , I I I-13 -8 -4 0 4 8 12 VELOCITY ( MM /SEC)
ENERGY I e V ) --r
F i g . 4. L3-XANES o f
Eu(Cu ) S i where
M = AI;~!~;'R~ azd N i .
[-12 I -8 1 -4 I . . =\. '5 i 2 !0 I 4 I 8 I 12
I I
VELOCITY (MM/SEC) F i g . 5. Mossbauer s p e c t r a o f F i g . 6. Mossbauer s p e c t r a o f
Eu(Cu M ) S i where M = Au, Eu(Cu M ) S i where M = Au,
Ag,
