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MÖSSBAUER INVESTIGATIONS OF AMORPHOUS
METALS
H. Wagner, M. Ghafari, U. Gouser, M. Naka
To cite this version:
H. Wagner, M. Ghafari, U. Gouser, M. Naka.
MÖSSBAUER INVESTIGATIONS OF
JOURNAL DE PHYSIQUE Colloque C8, supple'ment au n08, Tome 4 1 , aoat 1980, page C8-199
M ~ S S B A U E R I N V E S T 1 G A T I O N S O F AMORPHOUS M E T A L S
H.G. Wagner, M. Ghafari, U. Gonser and M. ~aka'
Fachbereich Angewandte Physik, Universitat des Sanrlandes, 6600 Saarbriicken, R.F.A.
+on
leave from t h e Research I n s t i t u t e for Iron, S t e e l and Other Metals, Tnhoku Lhziversity, Sendai 980 Japon.;rliissbauer spectroscopy i s a u s e f u l method f o r t h e a n a l y s i s o f phases, l a t t i c e d e f e c t s and s i t e pop- u l a t i o n s ( ' ) . Phases w i t h resonance atoms as c o n s t i t - uents u s u a l l y e x h i b i t c h a r a c t e r i s t i c spectra. For a q u a n t i t a t i v e a n a l y s i s o f a multi-phase system, reference s p e c t r a are s u b t r a c t e d i n a p p r o p r i a t e steps f r o n t h e measured spectrum. This procedure i s continued u n t i l every p a r t of the measured spectrum i s assigned t o a s p e c i f i c phase. I f a unique assoc- i a t i o n o f resonance atoms and l a t t i c e d e f e c t s (vacancy, i n t e r s t i t i a l , i m p u r i t y e t c . ) e x i s t s , one a l s o expects c h a r a c t e r i s t i c spectra from which q u a n t i t a t i v e i n f o r m a t i o n can be e x t r a c t e d .
U n f o r t u n a t e l y , t h e s p e c t r a l l i n e s o f t h e h y p e r f i n e p a t t e r n o f ferromagnetic amorphous metals of the type T80i<20 (T = t r a n s i t i o n metal ; il = m e t a l l o i d atoms) a r e v e r y broad. When e v a l u a t i n g t h e d i s t r i - b u t i o n s o f t h e h y p e r f i n e f i e l d s , one f i n d s a s t r i k - i n g s i m i l a r i t y almost independent o f t h e k i n d o f t r a n s i t i o n metals o r m e t a l l o i d atoms. This f a c t suggests t h a t t h e a t o m i s t i c s t r u c t u r e s o f such a l - l o y s a r e a l s o very s i m i l a r . A t present t h e r e i s general agreement t h a t t h e broadening i s caused by t h e h y p e r f i n e d i s t r i b u t i o n and t h e s t r u c t u r e s o f the Taoi.iZo a l l o y s a r e v e r y s i m i l a r .
The l a c k o f r e s o l u t i o n o f t h e s p e c t r a l l i n e s i n s p i - r e d s c i e n t i s t s t o apply d i f f e r e n t models f o r t h e i n t e r p r e t a t i o n and e v a l u a t i o n o f t h e spectra and t h e d i s t r i b u t i o n o f t h e f i e l d s . I n general t h e r e are t h r e e approaches(2'3'4) using t h e b a s i c elements:
1. m i c r o c r y s t a l l i t e s
2 . molecules, molecular u n i t s , chemical compound c l u s t e r s
,
embryos3. dense random packing
Also a model was suggested: " q u a s i r c r y s t a l l i n e s t r u c t u r e based on a l o c a l l y d i s t o r t e d o f f - s t o i c h - i o m e t r i c l a t t i c e u ( ' ) . I t seems t h a t t h i s phrase covers e v e r y t h i n g and i s acceptable t o everyone be-
cause i t can be used f o r t h e i n t e r p r e t a t i o n o f any amorphous s t r u c t u r e from m i c r o c r y s t a l s t o t h e ran- dom dense packing. However, t h i s model remains meaningless u n t i l t h e terms "quasi", " l o c a l l y " , " d i s t o r t e d " , " o f f - s t o i c h i o m e t r y " i r e p r e c i s e l y defined.
The presence o f m i c r o c r y s t a l l i t e s i n these a l l o y s seems u n l i k e l y because i t would r e q u i r e t h a t a l l t h e m i c r o c r y s t a l l i t e arrangements of borides, phos- phides, carbides and n i t r i d e s e x h i b i t e s s e n t i a l l y t h e same spectra independent o f t h e consituents, s i z e s e t c . Iloreover, one would expect
-
i n c o n t r a s t t o t h e experimental f i n d i n g s-
t h a t i n t h i s hetero- geneous s t r u c t u r e t h e s p e c t r a can be decomposed i n - t o two p a r t s : a broad c o n t r i b u t i o n from t h e reson- ance atoms being l o c a t e d i n r a t h e r d i s t o r t e d inbe- tween regions ( " g r a i n boundaries") and a r a t h e r sharp c o n t r i b u t i o n of theatoms w i t h i n t h e " g r a i n s " . Also w i t h t h e molecular models one encounters d i f - f i c u l t i e s : one has t o assume t h a t t h e molecular u n i t s , embryos e t c . o f a l a r g e v a r i e t y o f composi- t i o n s e x h i b i t t h e same f a m i l i a r h y p e r f i n e d i s t r i - b u t i o n . I n t h e i n t e r p r e t a t i o n o f t h e spectra n o t o n l y t h e i n d i v i d u a l molecules o r t h e molecular u n i t s b u t r a t h e r t h e t o t a l s t r u c t u r e has t o be taken i n t o account and i t i s n o t so easy t o v i s u a l i z e and e v a l - uate t h e assembly o f molecules as a whole i n t h e i r m a n i f o l d o f arrangements and o r i e n t a t i o n s . I n ad- d i t i o n , one again faces the problem o f t h e inbetween regions. How a r e t h e molecules o r u n i t s l i n k e d t o each o t h e r ? Using Ikissbauer spectroscopy f o r phase a n a l y s i s s u c c e s s f u l l y every p a r t o f t h e s p e c t r a has t o be assigned t o t h e occurence o f s p e c i f i c phases o r environments o r , i n o t h e r words, every resonance atom i n t h e amorphous s t r u c t u r e c o n t r i b u t e s t o t h e i n t e n s i t y o f a s p e c i a l h y p e r f i n e p a t t e r n .The dense random packing model (Bernal model) seems
C8-200 JOURNAL DE PHYSIQUE s u i t a b l e f o r t h e f i t t i n g o f t h e spectra. I n t h i s model t h e t o t a l s t r u c t u r e i s g i v e n by t h e coordina- t i o n o f t h e c o n s t i t u e n t s and t h e v a r i o u s holes
5
1.00-.
-
present. I n o u r f i t t i n g s we have found t h a t t h e VI VI
-
occurrences o f Bernal ' s c o o r d i n a t i o n s f i t we1 1 t h e
E
a p p r o p r i a t e i n t e n s i t i e s o f t h e corresponding hyper- C 0f i n e f i e l d components. O f course, these coordina-
+
-
09L-
t i o n s have t o be v i s u a l i z e d as average numbers. a OC
Thi; model a l s o holds when one v a r i e s t h e t r a n s i t i o n -6 -L -2 0 2 L 6
metals o r t h e m e t a l l o i d atoms, temperatures, e x t e r - n a l magnetic f i e l d s , s t r e s s e t c .
The a p p l i c a t i o n o f t h i s model i m p l i e s t h a t t h e mag- n i t u d e o f t h e f i e l d i s mainly determined by t h e t r a n s i t i o n metal near-neighbours. This seems reason- a b l e c o n s i d e r i n g t h e s e n s i t i v i t y i n h y p e r f i n e f i e l d s t o v a r i a t i o n s i n t h e s t r u c t u r e , f o r instance, i n
d - F e and a n t i f e r r o m a g n e t i c and ferromagnetic & - ~ e ( ~ ) . I t m i g h t l o o k i n our model as i f t h e m e t a l l o i d atoms which a r e s t a b i l i z i n g t h e Bernal s t r u c t u r e by t h e i r occupation o f t h e b i g g e r holes a r e n o t considered a t a l l , b u t t h e m e t a l l o i d atoms have t h e i r i n f l u e n c e t o o by t h e i r e l e c t r o n c o n t r i b u - t i o n t o t h e conduction e l e c t r o n band causing isomer s h i f t s o r p o s s i b l y small quadrupole e f f e c t s . And because h y p e r f i n e f i e l d s a r e q u i t e s e n s i t i v e t o i n t e r a t o m i c distances, by t h e i r s i z e t h e m e t a l l o i d atoms m i g h t have some s c a l i n g e f f e c t s . However, we b e l i e v e t h a t t h e h y p e r f i n e d i s t r i b u t i o n i s mainly governed by t h e d i f f e r e n t Bernal near-neighbour c o o r d i n a t i o n s and v a r y i n g distances o f t r a n s i t i o n metal atoms.
Results and Discussion
Mossbauer measurements o f t h e f o l l o w i n g a l l o y system a r e presented:
Fe80Bxi'i20-x
(M
= Ge, S i ,C,
P) Fe80Pxi'20-x (14 =s i ,
C, B)A l l s p e c t r a were recorded a t room temperature i n conventional transmission geometry u s i n g a 50 mCi 5 7 ~ o i n Rh source. By way o f an example, a spectrum o b t a i n e d from FegoBIBSi2 i s shown i n f i g . 1. I n a l l cases ( w e l l over 150 d i f f e r e n t s p e c t r a ) good f i t s were o b t a i n e d by a decomposition o f t h e s p e c t r a
i n t o f i v e s u b i p e c t r a corresponding t o t h e Bernal near-contact c o o r d i n a t i o n as i n d i c a t e d by broken l i n e s i n f i g . 1. The b i g g e s t d e v i a t i o n s i n f i t t i n g s occurred f o r a1 l o y s r i c h i n phosphorus. While we do
Fig. 1: Spectrum o f FegoBI8Si f i t t e d ( s o l i d l i n e ) by decomposition i n t o f i v e subspectra (broken l i n e s )
n o t c l a i m t h i s t o be d i r e c t p r o o f f o r t h e v a l i d i t y o f B e r n a l ' s dense random packing o f hard spheres f o r amorphous metals, i t shows t h a t a l l t h e a l l o y s examined e x h i b i t v e r y s i m i l a r d i s t r i b u t i o n s o f h y p e r f i n e f i e l d s d e s p i t e considerable d i f f e r e n c e s i n t h e chemical p r o p e r t i e s o f t h e i r c o n s t i t u e n t s and d i f f e r e n t s t r u c t u r e s o f t h e corresponding c r y s t a l 1 in e compounds. T h i s lends s t m n g supqort t o a geometrical random model o f t h e s t r u c t u r e o f amorphous metals.
Figs.
2
and 3 show how t h e average h y p e r f i n e f i e l din FeBxli20-x and in Fe P
H
i s affected by sub- a0x
20-xs t i t u t i n g 6 (P) by Ge, S i , C, P ( 6 ) . I t i s surpris- ing t h a t the e f f e c t of carbon subsitution i s
negl igable f o r Fe80BxC20-1 ;I though the outer electron configuration ( s p ) i s d i f f e r e n t from t h a t
2 1
of Boron ( s p ) . I t i s a l s o very d i f f i c u l t t o see how the d i f f e r e n t behaviour of FeaoBxC20-x, FegoBxSi20-x and FeaoBxGe20-x can be understood on the basis of charge t r a n s f e r models a s t h e i r outer electron configuration i s i d e n t i c a l .
m l s Q t l i o b 'l b
P x ( a t Y.1
Fig. 3: Average hyperfine f i e l d s vs. metalloid type and concentration (probable e r r o r f o r
H i :
+
2.5 kOe)-
Furthermore, i n s p i t e of the f a c t t h a t the electro- negativity of Si and Ge a r e the same, t h e i r influ- ence on the average hyperfine f i e l d s i s considerably d i f f e r e n t , which excludes chemical bonding as an explanation. Fol 1 owing Kazama and coworkers(7) these observations can be explained by taking the s i z e of the metalloid atoms i n t o consideration and dividing
t h e m
i n t o groups according t o t h e i r s i z e s : 1) smal- l e r than t h a t of the t r a n s i t i o n metal( B ,
C , N ? ) , 2) equal t o the t r a n s i t i o n metal ( S i , P ) and 3 ) metalloids l a r g e r than the t r a n s i t i o n metal (Ge, As). Fig. 4 shows a plot of t h e average hyperfine f i e l d vs. the mean metalloid radius. Although the choice of absolute values f o r the radius i s somewhat arbi- t r a r y , i t is s t i l l evident t h a t t h i s gives the key to understanding the e f f e c t s on s u b s t i t u t i o n of d i f - f e r e n t metalloids. These arguments a r e f u r t h e r sub- s t a n t i a t e d by the dependence of t h e isomer s h i f t s on metalloid s u b s t i t u t i o n , as displayed i n f i g . 5. Following t h e average metalloid radius we observed an increase in the isomer s h i f t (broken l i n e ) which points t oa
decrease i n electron densityFig. 4: Average hyperfine f i e l d s vs. metalloid radius
Fig. 5: Isomer s h i f t s vs. metalloid radius (proba- ble e r r o r of IS:
+
0.03 mm/sec)compatible with scaling e f f e c t s due t o t h e increas- ing metalloid volume. Within each s i z e group, however, the addition of higher valence meta1,loids increases the isomer s h i f t , 4.e. decreases the electron density a t the nucleus with decreasing metalloid volume. I t i s possible t o plot both average hyperfine f i e l d s and isomer s h i f t s as a function of metal l o i d electronegativity
JOURNAL DE PHYSIQUE
Fig. 6: Average h y p e r f i n e f i e l d s vs. e l e c t r o n e g a t i v - i t y o f m e t a l l o i d
$
i.7 20 22 2.4 i6 28 30 *Elwctronegotlvlty (Accord~ng to Poullng)
Fig. 7: Isomer s h i f t s vs. e l e c t r o n e g a t i v i t y o f m e t a l l o i d
gemeinschaft. We a r e g r a t e f u l f o r t h e t e c h n i c a l assistance o f Mr. H. Staats.
References
1. F.E. F u j i t a i n F:ossbauer Spectroscopy, Topics i n A p p l i e d Physics,
2
(U. Gonser ed.) Springer(1975) 201
2. F.E. F u j i t a , Supplement o f Sci. Rep. RITU,
3
(1980) 13. D.S. Boudreaux, Phys. Rev.
818
(1978) 4039 4. U. Gonser, M. Ghafari and H.G. Wagner, J. Magn.Magn. Mat.
-
8 (1978) 1755. T. Kmeny,
I.
Vincze, 6 . Fogarassy and S. A r a j s , Phys. Rev.-
B20 (1979) 4766. U. Gonser, J. de Physique, Q (1980) 51
7. N.S. Kazama, T. Masumoto and M. Mitera, J. Magn. Flagn. Mat. 15-18 (1980) 1331
Acknowledgements
