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SHORT RANGE ORDER DETERMINATION IN
AMORPHOUS FeGe2 WITH ANOMALOUS X-RAY
SCATTERING
R. Lorentz, A. Bienenstock
To cite this version:
SHORT RANGE ORDER DETERMINATION I N AMORPHOUS F e G e 2 WITH ANOMALOUS X-RAY SCATTERING
R.D. Lorentz and A.I. Bienenstock
Department of Applied Physics, Stanford U n i v e r s i t y , S t a n f o r d , CA 94305, U.S.A.
Rksum6 - La technique de diffusion anomale diffkrentielle des rayons X
a kt6 utilis6e pour 6tudier la structure atomique locale dans des films minces amorphes de Fe GeZ. Les fonctions de distribution radiale ont 6t6 dgterminkes. Les atomes Ge sont essentiellement entourks de Fe et les distances Ge - Ge sont trop longues pour correspondre h des liai- sons covalentes ou metalliques. La structure de cet alliage amorphe a 6t6 aussi compar6e h celle de la phase cristalline de msme composition. De curieuses differences dans l'environnement des atomes Ge montrent que la phase amorphe n'est pas une forme dksordonn6e du cristal.
Abstract
-
The Differential Anomalous X-Ray Scattering technique has been applied to an amorphous FeGe metallic thin film in order to study its local atomic structure.2
radial distribution function, as well as both Ge and Fe differential distribution functions, have been determined for this alloy. Ge is coordinated, primarily, by Fe with the bulk of the Ge-Ge distances being too long for covalent or metallic bonding. The structure of th.is amorphous alloy has also been compared to that of the crystalline phase with the same composition. Striking differences in the environments about Ge atoms in the alloys demonstrate that the amorphous phase is not a broadened form of the crystal.Introduction
This structural study of amorphous FeGe2 is part of an analysis of the amorphous Fe-Ge system over a broad composition range (1). This particular alloy has a higher metalloid content than many other transition metal-metalloid glasses studied, and the existence of a crystal compound of the same composition enables a comparison of the atomic scale structures of the crystal and of the glass.
An EXAFS experiment (2) was performed on the amorphous FeGe2 sample at both the Fe and Ge K absorption edges. No suitable model compound for this glass was found, as the shells in crystalline FeGep were too close to be separated for determination of empirical backscattering amplitudes and phase shifts, so calculated ( 3 ) values were used. Unfortunately, the backscattering amplitudes are similar enough to prevent identification of the species in the first coordination shell. First shell distances about both Fe and Ge atoms in the amorphous alloy were indicated to be about
0.15
11
shorter than the corresponding crystal distances. We attribute this to the lack of usable low k information and asymmetry of the atomic distributions (4). Only one neighbor shell was visible in the real space transform. By using the differential anomalous x-ray scattering (DAS) technique (51,we have determined the local environments about both Fe and Ge atoms in the amorphous FeGe2 alloy. This technique utilizes the rapid change of an atom's scattering factor f,
C8-188 JOURNAL DE PHYSIQUE
where f is t h e atomic s c a t t e r i n g f a c t o r a t high e n e r g i e s and E i s t h e i n c i d e n t photon %nergy, n e a r one of its a b s o r p t i o n edges t o e n a b l e d e t e r m i n a t i o n o f a d i f f e r e n t i a l d i s t r i b u t i o n f u n c t i o n (DDF). A DDF r e p r e s e n t s t h e average d i s t r i b u t i o n o f neighbors about one o f t h e c o n s t i t u e n t s p e c i e s of an a l l o y , t y p i c a l l y t o a t l e a s t 10
A.
Experiment
The amorphous FeGe, sample was prepared by s p u t t e r i n g from pure Fe and Ge t a r g e t s o n t o a Kapton s u b s t r a t e , r e s u l t i n g i n a f i l m approximately 1 micron t h i c k . The composition was determined by Auger microprobe. The sample was checked f o r c r y s t a l l i n i t y by x-ray d i f f r a c t i o n , and f o r homogeneity w i t h small a n g l e x-ray s c a t t e r i n g and w i t h x-ray near edge a b s o r p t i o n d a t a used i n a novel way ( I ) , which showed no evidence f o r phase s e p a r a t i o n .
Anomalous x-ray s c a t t e r i n g d a t a were taken a t t h e S t a n f o r d Synchrotron R a d i a t i o n Laboratory. A coupled 8-28 scanning mode i n t r a n s m i s s i o n geometry was employed and a t o t a l sample t h i c k n e s s of approximately 15 microns was achieved by s t a c k i n g t h e f i l m s . S c a t t e r i n g d a t a from t h e amorphous sample were c o l l e c t e d a t f o u r e n e r g i e s , 7102ev, 7082ev, 11093ev, and 11073ev, which a r e lOev and 30ev below t h e Fe and Ge K a b s o r p t i o n edges, r e s p e c t i v e l y . Background s c a t t e r i n g from t h e Kapton s u b s t r a t e alone was measured a t 30ev below each edge. Because of t h e n e c e s s i t y o f using e n y r g i e s c l o s e t o an a b s o r p t i o n edge, ,the maximum k v e c t o r magnitude was 10.40
1-
f o r t h e Ge edge d a t a and was 6.561-
f o r t h e Fe edge. The i n c i d e n t beam i n t e n s i t y was monitored by a NaI s c i n t i l l a t i o n d e t e c t o r . The s c a t t e r e d i n t e n s i t y from t h e sample was measured by an i n t r i n s i c G e d e t e c t o r with a n energy r e s o l u t i o n s u f f i c i e n t t o d i s c r i m i n a t e a g a i n s t t h e K a f l u o r e s c e n c e , but n o t t h e Kp.Data Analysis
The c o n t r i b u t i o n t o t h e s c a t t e r i n g from t h e Kapton s u b s t r a t e was removed, and t h e d a t a were then put on a per atom b a s i s through a n o r m a l i z a t i o n procedure d e s c r i b e d by K o r t r i g h t (6). The use of t h e G e d e t e c t o r reduced t h e f l u o r e s c e n t c o n t r i b u t i o n t o t h e measured s c a t t e r e d i n t e n s i t y , making t h e d a t a e a s i e r t o normalize. A F o u r i e r s i n e t r a n s f o r m then y i e l d e d a r a d i a l d i s t r i b u t i o n f u n c t i o n (RDF).
The DAS technique uses t h e r a p i d change i n f 9 as E i s v a r i e d n e a r a c o n s t i t u e n t atom9s a b s o r p t i o n edge. Accuracy of t h e v a l u e s of f 1 t o b e t t e r t h a n 0.5 e l e c t r o n s is e s t i m a t e d by F o n t a i n e e t a l . (7) t o be necessary f o r t h i s method t o succeed. F a r from an a b s o r p t i o n edge, Cromer-Liberman (C-L) (8) f r e e atom c a l c u l a t i o n s of t h e anomalous s c a t t e r i n g f a c t o r s (ASF1s) a r e regarded a s s u f f i c i e n t l y a c c u r a t e , w h i l e n e a r t h e edge chemical e f f e c t s can cause s i g n i f i c a n t d e v i a t i o n s from t h e s e values. We have determined t h e v a l u e s of f 9 and f w f o r Fe and Ge i n t h e amorphous a l l o y near t h e r e s p e c t i v e K a b s o r p t i o n edges by using a b s o r p t i o n d a t a from t h e EXAFS experiment on t h i s sample and t h e d i s p e r s i o n r e l a t i o n , f o l l o w i n g procedures s i m i l a r t o t h o s e o f Hoyt e t a l .
(9).
The d e t e c t o r r e s p o n s e f u n c t i o n a s w e l l a s t h e a b s o r p t i o n from lower s h e l l s and t h e o t h e r atom were removed from t h e experimental a b s o r p t i o n edge scan by f i t t i n g a polynomial t o t h e pre-edge r e g i o n and then s u b t r a c t i n g t h e r e s u l t i n g f u n c t i o n from t h e e n t i r e d a t a s e t . The K s h e l l a b s o r p t i o n was s c a l e d t o C-L f r e e atom c a l c u l a t i o n v a l u e s f a r above t h e edge where chemical e f f e c t s a r e expected t o be n e g l i g i b l e . The o p t i c a ltheorem 2
fn(k=O,E) = (mcE/Q.rre Ti) a(k=O,E)
was a p p l i e d t o determine t h e c o n t r i b u t i o n t o f " from t h e K s h e l l and t h e c o n t r i b u t i o n from t h e lower s h e l l s was then added u s i n a C-L v a l u e s . The Kramers-Kronig r e l a t i o n w i t h a r e l a t i v i s t i c c o r r e c t i o n (10)
Jo
I n t h e c o u r s e o f determining t h e ASFss i n t h i s manner f o r a series of amorphous Fe-Ge a l l o y s , s i g n i f i c a n t v a r i a t i o n s i n t h e Ge edge white l i n e h e i g h t w i t h composition have been observed, causing changes i n t h e f ' v a l u e s n e a r t h e edge. The Ge f ' and f " v a l u e s determined f o r t h i s sample near t h e Ge edge a r e shown i n F i g u r e l a . The pre-edge f e a t u r e seen i n t h e Fe near-edge a b s o r p t i o n ( F i g u r e l b ) c a u s e s a corresponding s t r u c t u r e i n t h e Fe f 1 values. These r e s u l t s show t h a t it is important t o determine t h e ASF's f o r t h e m a t e r i a l under c o n s i d e r a t i o n both t o i n c l u d e chemical e f f e c t s and i n o r d e r t o determine t h e optimum e n e r g i e s f o r d a t a c o l l e c t i o n . Taking Fe edge s c a t t e r i n g d a t a c l o s e r than lOev from t h e edge would i n c r e a s e a b s o r p t i o n and f l u o r e s c e n c e from t h e r e s o n a n t Raman e f f e c t , w i t h no g a i n i n f ' change. T h i s method a l s o allows a c o n s i s t e n t energy c a l i b r a t i o n t o be maintained between t h e s c a t t e r i n g d a t a and t h e f 1 v a l u e s , which change r a p i d l y with energy near an a b s o r p t i o n edge.
Both Fe and Ge DDF1s were determined i n t h e manner described by Fuoss e t a l . (5) from d a t a c o l l e c t e d near each s p e c i e s ' a b s o r p t i o n edge, and a RDF was found from d a t a taken n e a r t h e G e K a b s o r p t i o n edge. The Ge DDF c o n s i s t s o f a l i n e a r combination o f t h e Ge-Ge and Ge-Fe p a r t i a l d i s t r i b u t i o n f u n c t i o n s (PDF's), while t h e Fe DDF is a combination o f Fe-Fe and Fe-Ge PDF's. The RDF i s a weighted sum of a l l t h r e e PDF's.
Ge DDF
=
0.99 Ge-Ge PDF+
1.02 Ge-Fe PDF Fe DDF = 0.71 Fe-Fe PDF+
1.14 Fe-Ge PDFRDF = 0.67 Ge-Ge PDF
+
1.34 Ge-Fe PDF+
0.33 Fe-Fe PDFS i n c e t h i s sample is i n t h i n f i l m form, it has proven impossible t o measure its d e n s i t y . For t h e DDF's used f o r comparison with t h e c r y s t a l , t h e c r y s t a l l i n e d e n s i t y was assumed f o r t h e g l a s s . E r r o r e s t i m a t e s f o r t h e d i s t a n c e s and a r e a s r e f l e c t t h e u n c e r t a i n t y i n d e n s i t y .
Results and Discussion
C8-190 JOURNAL DE PHYSIQUE
Figure 2. RDF, Ge DDF and Fe DDF f o r amorphous FeGe2 shown on an o f f s e t s c a l e .
occur a t a d i s t a n c e which is t o o long f o r covalent o r m e t a l l i c bonding.
We have a l s o compared t h e s t r u c t u r e of amorphous FeGe2 w i t h t h a t of t h e c r y s t a l l i n e compound, which has a t e t r a g o n a l CuA12 type s t r u c t u r e (11). Between 0 and 4
A ,
t h e r e a r e 2 F e l s a t 2.48A
and 8 G e l s a t 2.56A
about each Fe atom i n t h e c r y s t a l . A Ge atom i n t h e c r y s t a l has 4 F e l s a t 2.568
and 11 Ge neighbors, 1 a t 2.591,
2 a t 2.951,
and 4 each a t 3.081
and 3.161.
Published s t r u c t u r a l d a t a were used t o g e n e r a t e q u a s i - c r y s t a l l i n e DDF1s f o r both Fe and Ge atoms i n t h i s a l l o y , i n which t h e simulated d i s o r d e r i n c r e a s e s with i n t e r a t o m i c d i s t a n c e . T h i s simulated broadening provided a n a t u r a l averaging of t h e v a r i o u s c r y s t a l l i n e f i r s t and second s h e l l d i s t a n c e s . For t h i s comparison, t h e c r y s t a l l i n e d i s t r i b u t i o n f u n c t i o n s were transformed i n t o k-space, weighted a p p r o p r i a t e l y and then back-transformed using t h e same k-space ranges a s were used i n t h e transforms of t h e d a t a from t h e amorphous m a t e r i a l . The r e s u l t i n g d i s t r i b u t i o n f u n c t i o n s are d i r e c t l y comparable w i t h t h e DDF1s and RDF obtained from t h e amorphous m a t e r i a l . These f u n c t i o n s a l s o served a s a test of t h e method we used t o determine t h e a r e a s o f t h e s t r o n g l y overlapping peaks i n t h e RDF and DDFls. The agreement w i t h expected v a l u e s f o r t h e c r y s t a l was always b e t t e r than 2.58.The Ge and Fe DDF1s f o r t h e g l a s s and c r y s t a l a r e shown i n F i g u r e s 3a and 3b, r e s p e c t i v e l y . While t h e chemical o r d e r i n g of t h e g l a s s appears t o be s i m i l a r t o t h a t of t h e c r y s t a l , t h e l o c a l environment about G e atoms i n t h e amorphous a l l o y shows a number of s i g n i f i c a n t d i f f e r e n c e s from t h a t i n t h e c r y s t a l . C l e a r l y , t h e n e a r e s t and next n e a r e s t neighbor s h e l l s about Ge a r e s t r i k i n g l y d i f f e r e n t i n t h e two phases. While t h e f i r s t neighbor s h e l l s occur a t t h e same d i s t a n c e , t h e second neighbor s h e l l i n t h e g l a s s is found a t 3.27
8,
0.171
more than i n t h e c r y s t a l . The number of atoms i n t h e s e s h e l l s is a l s o q u i t e d i f f e r e n t i n t h e two phases a s w e l l . I n t h e Ge DDF, t h e Ge and Fe weighting f a c t o r s a r e almost i d e n t i c a l , s o t h a t t h e measured a r e a s a r e d i r e c t l y p r o p o r t i o n a l t o coordination numbers. The number of f i r s t neighbors about G e , i n t h e g l a s s is approximately one more than i n t h e c r y s t a l . This s h i f t of t h e second s h e l l t o higher d i s t a n c e s is expected when t h e number of n e a r e s t neighbors i n c r e a s e s . The g l a s s ' s second neighbor s h e l l about Ge, i n a d d i t i o n t o being a t a l a r g e r d i s t a n c e r e l a t i v e t o t h e c r y s t a l , c o n t a i n s a t l e a s t t h r e e fewer atoms. The n e a r e s t neighbor s h e l l about Fe i n amorphous FeGe2 has t h e 'same a r e a a s i n t h e c r y s t a l , but is centered a t a s l i g h t l y l o n g e r d i s t a n c e . T h i s information is summarized i n Table 1.G e and Fe DDFvs f o r amorphous and c r y s t a l l i n e FeGe,.
amorphous FeGe2 g r y s t a l l i n e FeGe2 Ge DDF lSt s m i s t a n c e : 2.58+0.01
A
2.581
lSt s h e l l a r e a : 6.250.6 5.1 ( 5 atoms) 2nd s h e l l d i s t a n c e : 3.27+0.01A
znd
s h e l l a r e a : 6.750.2 3.10d
9.9 (10 atoms) Fe DDF lSt s h e l l d i s t a n c e : 2.6450.03A
2.58d
lSt s h e l l a r e a : 10.650.6 10.6T h i s work was done a t SSRL which is supported by t h e Department of Energy, O f f i c e of B a s i c Energy S c i e n c e s ; and t h e National I n s t i t u t e s o f Health, Biotechnology Resource Program, D i v i s i o n o f Research Resources, and was supported by the-NSF-MRL program through t h e Center f o r M a t e r i a l s Research a t S t a n f o r d U n i v e r s i t y .
References
1. R.D. Lorentz, Ph.D. T h e s i s , S t a n f o r d U n i v e r s i t y .
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p. 280 (1984).
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4. J . K o r t r i g h t , W. Warburton and A. Bienenstock, i n : "EXAFS and Near Edge
S t r u c t u r e " , ed. by I. Bianconi, L. I n c o c c i a and S. S t i p c i c h , (Springer Verlag, - .
1983) ~ . 3 6 2 .
5. P. Fuoss, P. E i s e n b e r g e r , W.K. Warburton and A . I . Bienenstock, Phys. Rev. Let. 46, 1537 (1981).
6. J . K o r t r i g h t , Ph.D. T h e s i s , S t a n f o r d U n i v e r s i t y .
7. A. F o n t a i n e , W.K. Warburton and K.F. Ludwig, J r . , Phys. Rev. B31, 3599 (1985). 8. D.T. Cromer and D. Liberman, J . Chem. Phy. 53, 1891 (1970).
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10. M.S. Jensen, Phys. L e t t . 74A, 41 (1979).
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