EXAFS AND XANES STUDIES ON THE
STRUCTURE OF LIQUID Rb-RbBr SYSTEMS
J. Jal, J. Dupuy, A. Sadoc
To cite this version:
J. Jal, J. Dupuy, A. Sadoc. EXAFS AND XANES STUDIES ON THE STRUCTURE OF
LIQUID Rb-RbBr SYSTEMS. Journal de Physique Colloques, 1985, 46 (C9), pp.C9-107-C9111. <10.1051/jphyscol:1985913>. <jpa-00225279>
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JOURNAL DE PHYSIQUE
page C9-107
Colloque C9, supplément au n012, Tome 46, décembre 1985
EXAFS AND XANES STUDIES ON THE STRUCTURE OF L I Q U I D Rb-RbBr SYSTEMS
J.F. Jal, J. Dupuy and A.
ado oc*
Département de physique des kkztériaux (UA 172), Université CZaude ~ e m a r d
Lyon 1, 69622 ViZZeurbanne Cedex, France
*Laboratoire pour 2 'UtiZisation du Rayonnement Ezectromagnétique (LURE),
Université de Paris Sud, 92405 Orsay Cedex, France
-
Résumé
En vue d ' é t u d i e r l a t r a n s i t i o n métal-non métal que présentent
les
systèmes désordonnés t e l s que l e s s o l u t i o n s métaux-sels, nous avons
e n t r e p r i s des expériences EXAFS dans l e système Rb-RbBr.
A b s t r a c t - To study t h e metal-non metal t r a n s i t i o n observed f o r t h e metals
i n molten s a l t systems, extended X-ray a b s o r p t i o n f i n e s t r u c t u r e (EXAFS)
experiments are c a r r i e d o u t f o r t h e Rb-RbBr systems.
1
-
INTRODUCTION
I n order t o i n v e s t i g a t e t h e metal-non metal t r a n s i t i o n ,
we have performed an EXAFS
study above t h e bromine K a b s o r p t i o n edge i n Rb-RbBr s o l u t i o n s .
We r e p o r t here
Our p r e l i m i n a r y r e s u l t s i n l i q u i d RbBr and Rbo~8(RbBr)0.2. This i s the f i r s t EXAFS
i n v e s t i g a t i o n on l i q u i d m e t a l - s a l t
systems and t h e l i m i t a t i o n s and c a p a b i l i t i e s
o f EXAFS as a s t r u c t u r a l t o o l f o r t h i s k i n d o f systems w i l l be discussed.
II - EXPERIMENT
The RbBr and Rbo.8(RbBr)0.2
windows of
in
order
samples were prepared by s p u t t e r d e p o s i t i o n on t h e
t h e high-temperature
to
avoid
the
transparency i n the X-ray
strong
cell.
S i l i c a windows were chosen f o r t h e ce11
reactivity of
domain.
the m a t e r i a l s
and due t o
their
The ce11 was mounted i n a furnace which has
been described p r e v i o u s l y /1/.
The experiments were c a r r i e d o u t a t LURE w i t h t h e EXAFS-1 spectrometer using a
Si 220 channel c u t monochromator. The B r EXAFS above 13475 eV were measured systemat i c a l l y i n RbBr as a f u n c t i o n o f temperature from room temperature up t o 700°C
through the m e l t i n g p o i n t (692°C).
For Rboe8(RbBr)0.p, t h e EXAFS were taken above
the m e l t i n g p o i n t (631°C), a t 700°C.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985913
JOURNAL DE PHYSIQUE
III
-
DATA ANALYSIS
The normalized . ( k ) o s c i l l a t i o n s f o r RbBr are plotted in figure 1 a t room temperature and in the liquid s t a t e , a t 700°C. The e f f e c t of thermal disorder shows
up immediately since, a t 700°C, the o s c i l l a t i o n s decrease very rapidly versus
the energy so t h a t they completely disappear about 100 eV above the edge. Therefore
a XANES (X-ray Absorption near Edge Structure) study was necessary f o r the liquid
systems.
F l r s t , the RbBr spectrum a t room temperature was analyzed following standard procedures. The f i r s t shell contribution was isolated by Fourier f i l t e r i n g and was
f i t t e d using the amplitudes and phase s h i f t s of Teo and Lee /2/. Six Rb atoms
were found a t a 3.381 distance of a Br atom ; t h i s compares nicely w i t h the 3.431
crystallographic distance. As the Teo and Lee functions are given only f o r
k > 4 1-1 (E > 55 eV), we have used, in the following, experimental amplitudes
and phase s h i f t s generated from the f i r s t shell contribution of the room temperature
spectrum. These functions include any supplementary process which can take place
a t low energies. Since we always deal with the same atomic species, i.e.
the Br-Rb pair, questions of phase and amplitude transferabil i t y are un-important.
So we can use the low k EXAFS regime o r XANES t o determine the local order in
similar systems. Moreover these systems are highly disordered and only a rough
estimation of the interatomic distance and the coordination number can be expected.
This does not require a more refined analysis.
Then Fourier transforms of the kx(k) spectra were done using the same window (10110 eV) f o r al1 the samples (figure 2). Fourier transformation of the EXAFS spectra
yields a pattern t h a t i s qualitatively similar to a radial distribution function
except, however, t h a t i t takes into account the phase s h i f t s of the photoelectron
wave function by absorber and backscatterer atoms. The Fourier transforms of the
high temperature data exhibit mainly one peak centered a t a shorter distance than
f o r room temperature RbBr. For RboS8(RbBr)0.p a second shell peak clearly appears.
The f i r s t peak was isolated and backtransformed i n t o k space. Thereafter these
inverse Fourier transforms were f i t t e d using the single scattering formula. The
r e s u l t s of the curve f i t t i n g procedure are l i s t e d in table 1 and a typical f i t
i s shown in figure 3. a denotes the absolute mean square relative displacement
obtained i n RbBr a t room temperature with the theoretical amplitudes and phases,
while AU are the differences between the values a t high temperature and a t room
temperature. The disorder i s so high in these systems t h a t the A U values are a s
large as the absolute a value a t room temperature. Due t o these great Debye Waller
factors, coordination numbers and distances are subject to large uncertainties.
For example, the Br-Rb distance does not appear greater a t 500°C than a t room
temperature, but even smaller.
0.01
O
-
. ..
. .: ..-..
(a)
*
:
. .,.. ...
I
.
.
r. l.
"
.*,'.
LZ..
..Ç'...-*..
- a
S.
Figure 2. F o u r i e r transforms o f k x ( k ) i n
t h e range 10-110 eV f o r s o l i d RbBr
( a ) a t room temperature and ( b ) a t 500°C,
( c ) f o r l i q u i d RbBr a t 700°C,
( d ) f o r l i q u i d R b o . g ( R b B r ) ~ . ~a t 700°C.
Figure 1. Experimental XANES and
EXAFS spectra v ( k ) above t h e Br edge
i n RbBr compared ( a ) i n t h e s o l i d
s t a t e a t room temperature and
( b ) i n t h e l i q u i d s t a t e a t 700°C
JOURNAL DE PHYSIQUE
Figure 3. Liquid RbBr a t 700°C : Fourier f i l t e r e d f i r s t shell (dots)
and i t s simulation ( l i n e ) with 6 Br atoms a t 3.32 A and A G = 0.20 fi
Fitting the three high-temperature spectra in the same way gives the same 3.32A
distance a t f 0.10 A . For RbBr, the disorder parameter increases a l i t t l e between
the 500°C solid sample and the 700°C liquid one, but the number of Rb atoms remains
identical. On the opposite, f o r RboS8(RbBr)0.p, the number of Rb atoms i s l e s s
than f o r liquid RbBr and the disorder parameter significantly smaller. Al1 these
r e s u l t s are i n good agreement with neutron r e s u l t s /3/.
To investigate the structure of Rb0.8(RbBr)~.~
beyond the f i r s t coordination shell
i s a challenging problem. B u t the second peak could be assigned conclusively as
Br atoms, using experimental amplitudes and phase s h i f t s extracted from the second
shell contribution (Br-Br) of the room temperature RbBr spectrum.
DISCUSSION
To f u l l y determine the local order will require better data ; but even with noisy
spectra, extending out only t o 100 eV above the edge, we can obtain, roughly,
the f i r s t shell of coordination. In f a c t , t h i s simple approach applied t o low
k data i s possible because the systems are highly disordered /4/.
Acknowledgements
We vould l i k e t o thank A.
LEYCURAS f o r h i s advice i n preparing t h e s p u t t e r e d
sampl es.
System
T
RbBr
solid
Di sorder
parameter (1)
(')
NR~
20
OC
6
3.38
u
=
0.14
500
OC
6
3.32
AU
=
0.17
liquid
700
OC
6
3.32
AU
=
0.20
Rb0.8(RbBr)~.~l i q u i d
700
OC
4
3.32
AU
=
0.11
Table 1 : S t r u c t u r a l parameters o f RbBr and Rbg.g(RbBr)o.2
AN =
and
+ 0.5
f
0.10
; AR =
fi
+
0.05
A
a t room temperature
a t h i g h temperature
; A (AU) =
2
0.03
1
REFERENCES
/1/ Lagarde, P., EXAFS and Near Edge S t r u c t u r e , Ed. Bianconi, A.,
S t i p i c h , S.,
/2/ Teo, B.K.
and Lee, P.A.,
/3/ Chabrier, G.,
/4/
(Springer Verlag, B e r l i n ,
J a l , J.F.,
1983)
J. Ame. Chem. Soc.
Incoccia, L.,
p. 294
101
Chieux, P. and Oupuy, J.,
(1979)
2815
Phys. L e t t . 93.J
(1982) 47
p. 2, o r F. Comin p. 238, o r Bouldin C.E. and
See f o r example Schaich W.L.
Stern E.A. p. 273 i n EXAFS and Near Edge S t r u c t u r e III,ed. K.O. Hodgson,
B. Hedman, J.E. Penner-Hahn ( S p r i n g e r Verlag, 1984)
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