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STRUCTURE OF (Pt.8 Ni.2 )73 P27 DETERMINED
BY EDXD TECHNIQUE
S. Aur, T. Egami
To cite this version:
JOURNAL DE PHYSIQUE CoZ Zoque C8, suppZ6ment au n"8, Tome 4 1 , aoct 1980, page C8-234
S T R U C T U R E OF
( ~ t Ni )P
D E T E R M I N E D BY E D X D T E C H N I Q U E
- 8 .2 7 3 27S. Aur and T. Egami
Laboratory for Research on t h e Structure of Matter, University o f Pennsylvania, Philadelphia Pa. 19104, U.S.A.
Abstract.- The structure factor of amorphous (Pt Ni ) P obtained by liq~id~quenching was
determined by the energy dispersive X-ray d i f f r a 6 @ i o n ' f E d h f 7 t e c h n i q u e up to 24 A-l, resolving nine peaks. The radial distribution function (RDF) is free of termination errors, and shows the high degree o f disorder in this alloy and a clear separation of the Pt-P peak from the main Pt-Pt first peak.
1 . Introduction
One of the main advantages of the energy disper- sive X-ray diffraction (EDXD) technique(1,2) is that it can cover wider wave vector (q) space than the conventional angular dispersive method, in de- termining the structure factor of liquids and so- lids (3-5). The angular dispersive method using an X-ray tube with a Mo target can determine the struc- ture factor, or the interference function, i(q),
only up to about 17
8-l.
Since i(q) is often notsmall enough at this value of q , the termination in-
troduces errors in obtaining the RDF by the Fourier transformation, which can only be improved by the use of a damping factor which at the same time wipes out fine details of the RDF, or by the iterational method of which accuracy is sometimes doubtful (6,7). The EDXD, on the other hand, does not require any of these termination cormtions, since it can
0- l
determine the values of i(q) up to 25 A or above. In order to demonstrate the power of the EDXD me- thod, we studied the structure of liquid quenched
(P!8N!2)73P27 by this method. The choice of this
alloy is favored also by the fact that the total Compton scattering intensity from this alloy is rather low, since the atomic number of Pt is high.
major reason why the method has not been used for the analysis of a glassy structure until recently.
Although the theoretical subtraction (5,8) has been
successful in bringing up the absolute accuracy of the EDXD method to the equivalent level with the
conventional method ( 9 ) . the accuracy of the theo-
retical Compton scattering intensity (10) has not been carefully tested. Therefore one expects a better result from the study of the alloy with the lower Compton scattering intensity.
2. Data analysis of the EDXD method
In the conventional method, the structural in- formations are contained in the intensity modula- tion of the characteristic X-ray by the sample, with the variation of the scattering angle. In the EDXD method, they are conveyed by the spectral mo- dulation of the incident white X-ray by the sample. The procedure to extract the structural informa- tions from the spectroscopic data is far from simple for the case of amorphous solids (4,8,11). The spectral intensity measured by a photon detec- tor in the EDXD set-up is, at energy E ,
The elimination of the Compton scattering in the EDXD method has not been a simple problem, and is a
where
C(E) : combined detection efficiency
Iinel(E): inelastic scattering intensity
Iel(E) : elastic scattering intensity
Im(E) : multiple scattering intensity
Ia(E) : air scattering intensity
Is(E) : spurious photon counts, mainly the
escape peaks
In the earlier work (4,8), the last three terms and C(E) have been neglected. Their effects can largely be incorporated, even in such a case, by the self- consistent determination of the primary beam in-
tensity
,
I (E). In other words, the self-consis-P
tent I (E) would be different from the true I ( E ) ,
P P
but includes these effects, and the resultant i(q) is not grossly incorrect. Nevertheless, it is more favorable to make corrections for these effects in order to obtain a high accuracy of the results. since the total intensity of these terms can amount to 10% at high energies. The procedure, however, is a little complex,although straightforward, and will not be described here. The structural informations are contained in the elastic scattering intensity. The calculation of the primary beam spectrum and the determination of the structure factor i(q) is described in Ref.8. In order to eliminate the effect of the polarization of the primary beam (8,12), the
X-ray tube was mounted at 45' away from the dif-
fraction plane.
3. Results and discussion
The interference function, qSi(q), thus deter- mined is shown in Fig.1. The earlier study o n a
similar composition (Pt Nf2)75P25 by the conventio-
r S
nal methdd determined the structure factor up to
17 AO-', covering 7 peaks (13). The value of qoi(q)
was measured here up to 25
8-I
beyond which thescattering factor becomes unreliable. The determi-
nation of q0i(q) beyond 18 x-I is difficult even
with the E D W method, since the amplitude of oscil- lation becomes really small compared to the total scattering intensity. Even after some smoothing to remove statistical errors, noise remains. However, it is clear that we have resolved two or perhaps
three additional peaks beyond 17
X-'.
The v a l ~ e s o fq'i(q) beyond 24
2-I
are sufficiently close to zero,thus not requiring any termination corrections.
The RDF obtained by the direct Fourier trans- formation is shown in Fig.2. The salient features of the result are in agreement with the earlier study (13), particularly with respect to the shape of the second peak and the strong damping of the
oscillation at Large values of r , but the peaks are sharper, since the damping of q'i(q) in the Fourier transformation has not been used here. The oscillation in the RDF is almost completely absent
above 9
2.
This is an indication of the largeamount of disorder in this very stable ternary al- loy, but technically also indicates the accuracy of the EDXD result. Any significant inaccuracy in the value o f qWi(q) tends to introduce spurious oscilla- tions in the RDF. Therefore, the absence of oscilla-
tion above 9
8
also indicates the absence of spuri-ous oscillation. The test by the Rahman's criteria (14) also shows excellent results.
Another important feature of the result is the small peak at the inner side of the first peak. This peak is not a termination error of some kind, but is most likely to be due to a Pt-P peak. The
peak positions and the suggested radii of Pt and p
are shown in Table I. They are very similar to the
JOURNAL DE PHYSIQUE
Fig.1 Structure factor q 0 i ( q ) of ( P t .8 Ni . 2 ) 73 P 27
a1 loy.
D i x m i e r a n d Duwez s u g g e s t e d t h a t t h e s m a l l hump i n - 2 . s i d e t h e f i r s t peak o f RDF t o be t h e Pd-P p e a k .
The s u g g e s t i o n met some c r i t i c i s m ( 6 ) , b u t i n t h e p r e s e n t r e s u l t t h e sub-peak i s c l e a r l y s e p a r a t e d 3 . f r o m t h e main p e a k , making i t v e r y d i f f i c u l t t o d o u b t t h e c o n c l u s i o n . T h u s , t h e EXD t e c h n i q u e , 4 . t h r o u g h i t s h i g h p r e c i s i o n , o f f e r s t h e p o s s i b i l i t y 5 . t o d e t e r m i n e , n o t o n l y t h e t o p o l o g i c a l s h o r t r a n g e 6 . o r d e r b u t t h e a t o m i c r a d i i and some c o m p o s i t i o n a l s h o r t r a n g e o r d e r , a s l o n g a s t h e r e a r e s u f f i c i e n t 7 . B. B u r a s , J . Chwaszczewska, S . S z a r r a s and 2.
Szmid, Rep. 894-11-Ps, I n s t . N u c l . R e s . Warsaw
( 1 9 6 8 ) . M. M a n t l e r a n d W. P a r r i s h , Advance i n X-ray A n a l y s i s ,
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s a m p l e o f (Pt8N$2)73P27
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1892 ( 1 9 6 7 ) . T a b l e I 1 s t p e a k p o s i t i o n sub-peak p o s i t i o n P t r a d i u s P r a d i u s R e f e r e n c e s + A l s o a t D e p a r t m e n t o f P h y s i c s . U n i v e r s i t y o f P e n n s y l v a n i a , P h i l a d e l p h i a , P a . 19104, USA.*
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