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STRUCTURAL INVESTIGATIONS OF
AMORPHOUS METALS BY NMR
P. Panissod
To cite this version:
S T R U C T U R A L INVESTIGATIONS OF AMORPHCUS METALS
BY NMR
P. P a n i s s o dL .
M.
S . E.S.(U.A.
au
C . N.R. S .n o 3 0 6 ) , Universite' Louis Pasteur,
4 ,r u e B l a i s e Pascal, 67070 Strasbourg Ccdez, France
RQsum6
-
Dans c e t a r t i c l e nous p r 6 s e n t o n s une revue de q u e l q u e s 6 t u d e s p a r RMN dans d e s m6taux amorphes q u i m o n t r e n t l ' a p p o r t d e c e t t e t e c h n i q u e h 1 ' 6 - l u c i d a t i o n d e l a s t r u c t u r e d e c e s syst&mes.A b s t r a c t - I n t h i s p a p e r , t h r o u g h t h e r e v i e w o f some r e l e v a n t s t u d i e s of amorphous m e t a l s , we show what i n f o r m a t i o n NMR measurements c a n g i v e on t h e s t r u c t u r e of s u c h systems.
I - INTRODUCTION
NMR h a s now l a r g e l y proved u s e f u l i n i d e n t i f y i n g s t r u c t u r a l s u b u n i t s i n i n s u l a t i n g m a t e r i a l s : m o l e c u l e s o r ,submolecular u n i t s i n o r g a n o c h e m i s t r y a n d , c l o s e r t o o u r t o p i c
,
f o r example b o r a t e u n i t s i n c l a s s i c a l g l a s s e s 111. The aim of t h i s p a p e r i s t o p r e s e n t some of t h e few e x p e r i m e n t s done i n t h a t d i r e c t i o n i n amorphous m e t a l s . I n NMR measurements, n u c l e i probe t h e i r l o c a l e l e c t r o n i c environment t h r o u g h hyper- f i n e i n t e r a c t i o n s ( m a g n e t i c o r e l e c t r i c s h i e l d i n g i n t e r a c t i o n ) . Most of t h e s t r u c t u - r a l i n f o r m a t i o n i s g a i n e d t h r o u g h t h e a n a l y s i s of t h e a n i s o t r o p i c p a r t of t h e s e i n t e r a c t i o n s ( q u a d r u p o l a r i n t e r a c t i o n , a n i s o t r o p i c f r e q u e n c y s h i f t ) which r e f l e c t s t h e symmetry of t h e l o c a l s t r u c t u r e . The i s o t r o p i c p a r t (Knight o r c h e m i c a l s h i f t ) and t h e i r dynamic c o n t r i b u t i o n s ( r e l a x a t i o n t i m e s ) c a n a l s o b e u s e f u l a s a s i g n a - t u r e of t h e c h e m i c a l environment p r o v i d e d t h a t NMR d a t a i n r e f e r e n c e s y s t e m s c a n be used a s "f i n g e r p r i n t s T 1 .The n e x t s e c t i o n ( 1 1 ) w i l l b e d e v o t e d t o t h e i n v e s t i g a t i o n o f t h e l o c a l symmetry and t h e s t r u c t u r a l c o h e r e n c e t h r o u g h t h e measure of t h e e l e c t r i c f i e l d g r a d i e n t
(EFG) and r e l a t e d p r o p e r t i e s . T h i s i s t h e most s p e c i f i c o u t p u t o f h y p e r f i n e i n t e r - a c t i o n s s t u d i e s of t h e amorphous s t r u c t u r e s i n c e c o n v e n t i o n a l s t r u c t u r a l s t u d i e s c a n h a r d l y go beyond p a i r c o r r e l a t i o n s measurements. The l a s t s e c t i o n (111) w i l l b e a b r i e f o u t l i n e o f measurements which a r e o r i e n t e d towards t h e s t u d y of t h e e l e c - t r o n i c s t r u c t u r e (Knight s h i f t and h y p e r f i n e f i e l d s , r e l a x a t i o n t i m e ) b u t which c a n y i e l d some i n f o r m a t i o n about t h e c h e m i c a l s h o r t - r a n g e o r d e r by comparison w i t h c r y s t a l l i n e m a t e r i a l s .
I1 - LOCAL SYMMETRY AND STRUCTURAL COHERENCE
As t h e r e l a t e d n u c l e a r t e c h n i q u e s (Mcssbauer e f f e c t , p e r t u r b e d a n g u l a r c o r r e l a - t i o n s . ..) NMR p r o v i d e s t h e o p p o r t u n i t y t o measure t h e l o c a l EFG on s e l e c t e d s i t e s and hence i t c a n p r o b e t h e s i t e symmetries of t h e s t r u c t u r e .
I n t h e p r e s e n c e of an EFG t h e NMR s p e c t r u m i s s p l i t t e d i n 2 1 l i n e s ( I i s t h e n u c l e a r s p i n ) t h e s e p a r a t i o n of which depends on t h e EFG s t r e n g t h and symmetry and on t h e p o l a r a n g l e s of t h e m a g n e t i c f i e l d i n t h e EFG frame. The EFG i s c h a r a c t e r i z e d by two independent p a r a m e t e r s : i t s s t r o n g e r p r i n c i p a l component V measured i n NMR by t h e q u a d r u p o l a r f r e q u e n c y v = 3e Q V Z Z / 2 1 ( 2 1 - l ) h (v = 0 f o g Z i s o t r o p i c o r c u b i c
9
Q
C8-242 JOURNAL
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PHYSIQUEsymmetry) and t h e assymmetry p a r a m e t e r q =
I
(V -V )/VI
( q = 0 f o r a x i a l symme- xxt r y ) . I n a p o l y c r y s t a l o r an amorphous system t h g Y e x t g ? n a l f i e l d i s randomly o r i - e n t e d i n t h e EFG frame hence t h e NMR l i n e s a r e d i s t r i b u t e d i n p o s i t i o n y i e l d i n g c h a r a c t e r i s t i c "powder p a t t e r n s " . From t h e s i m p l e o b s e r v a t i o n of t h e s p e c t r a i t i s r a t h e r e a s y t o d e c i d e i f t h e symmetry of t h e s t r u c t u r e around t h e probe n u c l e i i s i s o t r o p i c ( a s i n g l e l i n e i s o b s e r v e d ) , a x i a l
( n
= 0) o r s i g n i f i c a n t l y non-axial. To go beyond t h i s q u a l i t a t i v e i n f o r m a t i o n i t i s u s e f u l f o r amorphous m e t a l s t o per- form computer s i m u l a t i o n s o f t h e observed s p e c t r a s o t h a t q u a n t i t a t i v e v a l u e s c a n be o b t a i n e d f o r t h e a v e r a g e v and q and t h e i r d i s t r i b u t i o n s . I t must be n o t e d however t h a t , c o n t r a r y t o ~ ~ s g b a u e r e f f e c t , NMR cannot u s u a l l y p r o v i d e t h e s i g n o f t h e EFG. Comparison w i t h c r y s t a l l i n e systems ( e x p e r i m e n t a l ) o r amorphous s t r u c - t u r e models ( s i m u l a t i o n s ) a r e t h e n done.S u c c e s s f u l s t u d i e s were c a r r i e d o u t i n m e t a l - m e t a l l o i d amorphous systems mainly on t h e m e t a l l o i d e l e m e n t / 2 / which show obvious s i m i l a r i t i e s between t h e s i t e sym- metry i n t h e g l a s s y s t a t e and i n t h e c r y s t a l l i n e s t a t e f o r a-Ni P B and C - N i B
(non a x i a l , q = 0 . 6 ) and f o r a-Mo B and C - M O ~ B ( a x i a l , q =
0j8
l 4 ' ' ~ n a-La $a a s i n g l e l i n e i s observed a s expez?edOfor a n i s o t r o p i c symmetry (c-La Ga i s 75 25 c u b i c ) b u t a computer a n a l y s i s shows t h a t t h e l i n e shape i s due t o a 2 r o a d EFG d i s t r i b u t i o n c o n s i s t e n t w i t h computer s i m u l a t i o n s of random p a c k i n g o f s p h e r e s (RPS) 131.Combined measurements of t h e s i t e EFG and s p i n - s p i n r e l a x a t i o n t i m e s a l l o w a f u r t h e r i n v e s t i g a t i o n of t h e s t r u c t u r a l c o h e r e n c e a s shown by t h e s t u d y d e s c r i b e d below of t h e amorphous n i c k e l b o r i d e s i n a wide c o n c e n t r a t i o n range / 4 / .
The ''B NMR s p e c t r a i n a-Ni B (18
<
x5
40) and c r y s t a l l i n e n i c k e l b o r i d e s ( F i g . 1 ) and t h e i r computer1%.?y:is i n d i c a t e t h a t t h e s i t e symmetry around boron i n t h e g l a s s y s t a t e e v o l v e s g r a d u a l l y from t h a t of C - N i B ( q = 0 . 6 ) t o t h a t of3
C - N i B3
(?
= 0 . 4 ) . The a x i a l symmetry of C - N i B i s n o t observed i n t h e amorphous 2 .modification
( x = 0 . 3 3 ) nor i s t h e broad d i s t r ~ b u t i o n of h i g h v a l u e s of q which i s c h a r a c t e r i s t i c of a RPS. On t h e c o n t r a r y a t b o t h ends of t h e c o n c e n t r a t i o n range t h e EFG d i s t r i b u t i o n i s found q u i t e narrow which shows t h a t t h e boron environmentc-Ni
B - l i k e environment of boron. 4 3111 - CHEMICAL SHORT-RANGE ORDER
A supplementary i n f o r m a t i o n i s given by t h e t r a n s v e r s e r e l a x a t i o n time T2 which, i n n i c k e l b o r i d e s , i s only due t o "B n u c l e a r s p i n s d i p o l a r i n t e r a c t i o n . T mea- sured on t h e c e n t r a l l i n e (- 112 + + 112 t r a n s i t i o n ) i s n e a r l y n o t - a f f e c t e i c b y t h e EFG ; i t i s only r e l a t e d t o t h e average B-B d i s t a n c e which i s shown i n t h e g l a s s e s t o be equal t o t h a t i n t h e c r y s t a l l i n e b o r i d e s of same composition. This i m p l i e s B-B c o n t a c t s f o r t h e h i g h e s t B-concentrations. C o n t r a r i l y t o T2c t h e r e l a x a t i o n time T measured on t h e wings of t h e s p e c t r a (+ 312 -t i- 112 t r a n s i t i o n s ) i s strong-
2w
l y dependent on t h e EFG d i s t r i b u t i o n on neighbour B s i t e s : i f a l l B neighbours experience t h e same EFG ( s t r e n g t h and o r i e n t a t i o n ) t h e n T i s c l o s e t o T
2w 2 c
(T2c/T2w % 1 ) ; o p p o s i t e l y i f t h e r e i s a broad d i s t r i b u t i o n of EFG on B-nelghbours t h e d l p o l a r r e l a x a t i o n i s quenched (T i n c r e a s e s , T / T d e c r e a s e s s t r o n g l y ) . Hence t h e r a t i o T2c/T2w i s a measure
$f
t h e s t r u c t u r ? ? c8Xerence on a few i n t e r - atomic d i s t a n c e s . As shown on Fig. 2, T2c/T2w i s c l o s e t o 1 i n C - N i B i n which2
a l l B s i t e s a r e e q u i v a l e n t and i d e n t i c a l l y o r l e n t e d ( i n t h e same m i c r o c r y s t a l ) , t h i s r a t i o d e c r e a s e s i n C - N i B i n
3
which B s i t e s a r e e q u i v a l e n t but w i t h
Beside t h e d e t e r m i n a t i o n of t h e s i t e symmetries i n amorphous m a t e r i a l s , t h e s e n s i - t i v i t y of NMR t o t h e n a t u r e of t h e neighbours can be used t o probe a t l e a s t par- t i a l l y t h e chemical s h o r t range o r d e r (CSRO) i n the samples. However f o r such pur- poses the NMR d a t a have t o be compared t o those o b t a i n e d i n r e f e r e n c e samples t h e s t r u c t u r e of which i s known ; hence t h e information obtained i s r a t h e r i n d i r e c t . Two examples of such s t u d i e s a r e given below f o r the non-magnetic compound a-PdCuP (A) and t h e amorphous ferromagnet a - C O B (B).
I
T
2
~
0.5
0
3 1
4) P Knight s h i f t s and r e l a x a t i o n times T were measured i n a-(Pd Cu ) P 11-x x 80 20 and C - ( P ~ ~ - ~ C U ) P 161. As can be seen on F i g .
4
b o t h K and R = (TIT)- varyx 3
much l e s s w l t h i n c r e a s i n g Cu c o n c e n t r a t i o n i n t h e g l a s s t h a n i n t h e c r y s t a l l i n e compound and t h e i r v a l u e s remain c l o s e t o t h o s e measured i n pure c-Pd P . This
3
shows t h a t some kind of CSRO e x i s t s i n t h e amorphous m a t e r i a l such t h a t P atoms a r e e s s e n t i a l l y surrounded by Pd atoms. Such CSRO can be compared t o t h e w e l l - e s t a b l i s h e d
2
0
4Q
X
prisms s h a r i n g more and more N i atoms allowing B-B c o n t a c t s when they F i g . 2 - S t r u c t u r a l coherence a s measured by s h a r e r e c t a n g u l a r f a c e s . This conclu- t h e t r a n s v e r s e r e l a x a t i o n times ( s e e t e x t ) . s i o n i s c o n s i s t e n t w i t h t h e increased b r i t t l e n e s s of t h e samples f o r high B-content and i n agreement w i t h neutron d a t a151.
C8-244
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F i g . 3 - K n i g h t - s h i f t and r e l a x a t i o n F i g . 4
-
5 9 ~ o h y p e r f i n e f i e l d i n c o b a l t - r a t e o n 3 1 ~ i n t h e g l a s s m e t a l l o i d compounds v s m e t a l l o i d concen-a-
(Pdl-xCux)80P20 ( x ) and t h e c r y s t a l t r a t i o n ( x ) and number of v a l e n c e e l e c - t r o n s ( Z = 3 , Z = 4 ) . Open c i r c l e s r e - c-(Pdl-xCux)3P ( 0 ) . p r e s e n t B t h e ex:Qcted h y p e r f i n e f i e l d onCo f o r 1 t o 5 boron atoms i n i t s f i r s t c o o r d i n a t i o n s h e l l .
metalloid-metalloid avoidance i n m e t a l - ~ r t e t a l l o i d g l a s s e s and i t s u g g e s t s t h a t Cu might p l a y a r o l e o f " g l a s s former" i n t h e system.
B)
I n f e r r o m a g n e t s t h e h y p e r f i n e f i e l d (HF) on t h e m a g n e t i c e l e e n t i s v e r y s e n s i t i v e t o i t s environment : f o r example a s shown on F i g . 4 , t h e 5yC0 HF decrea- s e s l i n e a r l y w i t h i n c r e a s i n g B c o n t e n t . With t h e c r y s t a l l i n e b o r i d e s f o r r e f e r e c c e which f o r t h e numbers of B atoms around Co a r e known one c a n r e a s o n a b l y a t t r i b u t ea g i v e n h y p e r f i n e f i e l d t o a g i v e n number (1 t o 5 ) of B i n t h e Co c o o r d i n a t i o n s h e l l Then from t h e NMR s p e c t r u m which i s e s s e n t i a l l y t h e h y p e r f i n e f i e l d d i s t r i b u t i o n one c a n deduce t h e d i s t r i b u t i o n o f c o o r d i n a t i o n numbers a t a g i v e n c o n c e n t r a t i o n 171. On t h e non-magnetic element t h e e f f e c t of l o c a l f l u c t u a t i o n s i n t h e number and n a t u r e of t h e n e i g h b o u r i n g atoms a r e u s u a l l y weaker and on t h e same b a s i s one c a n o n l y deduce a n a v e r a g e c o o r d i n a t i o n number which i s more d i r e c t l y o b t a i n e d by s t r u c t u r a l d i f f r a c t i o n t e c h n i q u e s .
I V - CONCLUSION
/ I / B r a y , P . J . a n d B u c h o l t z , F . , G e i s s b e r g e r A.I. and H a r r i s , I . A . , N u c l . I n s t . Methods
199
(1 982) 1./ 2 / P a n i s s o d , P. e t a l , Phys. Rev. L e t t . ( 1 9 8 0 ) 1965.
/ 3 / C z j z e k , G., Nucl. I n s t . Methods
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(1982) 37 a n d t h i s c o n f e r e n c e . / 4 / P a n i s s o d , P . B a k o n y i , I . and Hasegawa, R . , Phys. Rev. B 28 (1983) 2374.151
G a r d n e r , P . P . , Cowlam, N. and D a v i e s , H.A., J. Phys. F ~ T ( 1 9 8 5 ) 769. 1 6 1 A l i a g a - G u e r r a , D., T h e s i s , S t r a s b o u r g ( 1 9 8 0 ) .1 7 1 P a n i s s o d , P . , Qachaou, A., Durand, J. a n d Hasegawa, R., Nucl. I n s t . Methods 199 (1982) 231.
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/ 8 / A l i a g a - G u e r r a , D., P a n i s s o d , P. and Durand, J . , J. P h y s i q u e ( P a r i s ) C o l l . (1 980) 674.
