EXAFS AND XANES STUDIES ON THE STRUCTURE OF LIQUID Rb-RbBr SYSTEMS

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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

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 1 7 2 ) , 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 l e s 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 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 windows of t h e high-temperature c e l l . S i l i c a windows were chosen f o r t h e ce11 i n order t o a v o i d t h e s t r o n g r e a c t i v i t y o f the m a t e r i a l s and due t o t h e i r transparency i n the X-ray domain. 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 systema- t 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.

JOURNAL DE PHYSIQUE

I I I

-

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 tempe-

rature and in t h e 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 proce-

dures. 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 t h e 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 p a i r , questions of phase and amplitude transferabil i t y a r e un-important.

So we can use the low

k

EXAFS regime o r XANES t o determine the local order i n

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 (10-

110 eV) f o r al1 the samples ( f i g u r e 2). Fourier transformation of the

EXAFS

spectra

yields a pattern t h a t i s q u a l i t a t i v e l y similar t o a radial d i s t r i b u t i o n function

except, however, t h a t i t takes i n t o 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 c l e a r l y 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 s c a t t e r i n g formula. The

r e s u l t s of the curve f i t t i n g procedure a r e l i s t e d in t a b l e 1 and a typical f i t

i s shown in figure

3. a

denotes the absolute mean square r e l a t i v e displacement

obtained i n RbBr a t room temperature with the theoretical amplitudes and phases,

while

A U

a r e 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 a r e 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. 0.01 O

-

i n RbBr compared ( a ) i n t h e s o l i d (a)

.

.

s t a t e a t room temperature and

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 s h e l l (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

F i t t i n g 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 s o l i d sample and the 700°C liquid one, but the number of Rb atoms remains i d e n t i c a l .

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 s i g n i f i c a n t l y smaller. Al1 these r e s u l t s a r e

i n

good agreement with neutron r e s u l t s /3/.

To investigate the s t r u c t u r e of R b 0 . 8 ( R b B r ) ~ . ~ beyond the f i r s t coordination shell i s a challenging problem.

B u t

the second peak could be assigned conclusively a s

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 b e t t e r 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

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.

Table 1 : S t r u c t u r a l parameters o f RbBr and Rbg.g(RbBr)o.2 AN =

+

0.5 ; A R =

+

0.05

A

a t room temperature and

f

0.10

fi

a t h i g h temperature ; A (AU) =

2

0.03

1

REFERENCES Di sorder parameter

(1)

u = 0.14 AU = 0.17 AU = 0.20 AU = 0.11 System T RbBr s o l i d 20 O C 500 O C l i q u i d 700 O C R b 0 . 8 ( R b B r ) ~ . ~ l i q u i d 700 O C

/1/

Lagarde, P., EXAFS and Near Edge S t r u c t u r e , Ed. Bianconi, A., Incoccia, L., S t i p i c h , S., (Springer Verlag, B e r l i n , 1983) p. 294

/2/ Teo, B.K. and Lee, P.A., J. Ame. Chem. Soc.

101

(1979) 2815 N R ~

6 6 6 4

/3/ Chabrier, G., J a l , J.F., Chieux, P. and Oupuy, J., Phys. L e t t . .93J (1982) 47 (')

3.38 3.32 3.32 3.32