X-RAY ABSORPTION IN LAMELLAR COMPOUNDS

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X-RAY ABSORPTION IN LAMELLAR COMPOUNDS

D. Bonnin

To cite this version:

D. Bonnin. X-RAY ABSORPTION IN LAMELLAR COMPOUNDS. Journal de Physique Colloques,

1986, 47 (C8), pp.C8-831-C8-836. �10.1051/jphyscol:19868159�. �jpa-00226062�

Colloque C8, suppl6ment au n o 12, Tome dkcembre 1986

X-RAY ABSORPTION IN LAMELLAR COMPOUNDS

D. B O N N I N

Laboratoire de Physique Quantique, C . N . R . S . U . A . 421,

E.S.P.C.I., 10, rue Vauquelin, F-75231 paris Cedex 05, France

R6sumi: Etant donnd le caractbre bidimensionnel des materiaux lamellaires, les spectres d'absorption des rayons X sont trbs sensibles aux effets de polarisation qui rdvblent I'anisotropie des propri6tds 6lectroniques et structurales. Plusieurs exemples sont donn6s pour le graphite, les dichalcogdnures, ainsi que leurs compos&s d'insertion. En XANES, on obtient la symetrie des ittats finals et la densitd partielle des Btats vides. L'EXAFS permet de ddterminer les angles de liaisons et la dependance en temperature des facteurs de Debye-Waller.

Abstract: Some classes of lamellar compounds are discussed as well as their corresponding intercalated forms. Because of the high anisotropy of these materials, X-ray spectra are very sensitive to the polarization effects which reveal the anisotropy of the electronic and structural properties. Several examples are given for dichalcogenide and graphite intercalation compounds. From XANES, it is possible to obtain the symmetry of the final states and to describe the partial density of the empty states. EXAFS spectra permit the determination of the bond angles and the thermal dependence of the Debye-Waller factors.

The first definition which can be given fbr a lamellar compound is: Every material with a symmetry less than cubic. In that case there is always an anisotropy of the physical properties. For example Fe203 corresponds to this definition, nevertheless it is not usually considered as a lamellar compound, so the definition must be improved. It is necessary

to

say that the bond strengths are largely anisotropic. Then, a lamellar compound is constituted of a stacking of layers in which the atoms are strongly bonded. The binding energy between the layers is very weak, it corresponds generally to Van der Waals interaction.

A lot

of

layered compounds can be intercalated with atoms or molecules which are weakly bonded through the host layers, they correspond to other lamellar compounds. The swelling of the lamellar compound depends on the intercalate and varies in a large range. So it is possible to vary the physical properties by the intercalation and particularly to modify the anisotropy of the material.

Nevertheless, the intercalation is not always possible. Some charge transfer can occur between the intercalate and the host, in that case there is a change in the electrical and thermal properties of the material.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868159

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M a t e r i a l s c a n be c l a s s i f i e d i n s e v e r a l c l a s s e s , t h e most u s u a l a r e : g r a p h i t e a n d r e l a t e d i n t e r c a l a t i o n c o m p o u n d s / d i c h a l c o g e n i d e s a n d r e l a t e d i n t e r c a l a t i o n c o m p o u n d s / 111-V l a y e r e d c o m p o u n d s / FeOCl, MPS3, Ta2S2C, U 0 2 F 2 / Some o r g a n i c a n d b i o l o g i c a l c o m p o u n d s ( f o r e x a m p l e : f e r r o c e n e a n d h e m e i n m y o g l o b i n G r a ~ h i t e a n d its i n t e r c a l a t i o n c o m e o u n d s

The g r a p h i t e is h i g h l y a n i s o t r o p i c , t h e p l a n e c o n s i s t s of c a r b o n a t o m s c o v a l e n t l y b o n d e d b y s p 2 h y b r i d i z a t i o n i n a h e x a g o n a l a r r a n g e m e n t w i t h a c a r b o n - c a r b o n d i s t a n c e o f 1.42

1.

P e r p e n d i c u l a r l y , t h e 3 . 3 5

1

s p a c i n g b e t w e e n t h e g r a p h i t i c p l a n e s is d e t e r m i n e d b y t h e i n t e r a c t i o n s b e t w e e n p Z o r b i t a l s of c a r b o n a t o m s from a d j a c e n t l a y e r s . T h e l a y e r s a r e a r r a n g e d i n a l t e r n a t i n g s t a c k i n g s e q u e n c e ( A B A B . . ) . For a g i v e n i n t e r c a l a n t , s e v e r a l i n p l a n e s t o i c h i o m e t r i c c o m p o s i t i o n s c a n b e o b t a i n e d . The s t a g e o f a c o m p o u n d is t h e n u m b e r of c a r b o n l a y e r s b e t w e e n n e a r e s t i n t e r c a l a n t l a y e r s . F o r e x a m p l e , M C 8 ( M = K , Rb, Cs) which is a s t a g e 1 h a s t h e s t r u c t u r e g i v e n i n t h e f i g u r e 1 , M C Z 4 is a s t a g e 2.

D i c h a l c o g e n i d e s and i n t e r c a l a t i o n c o m e o u n d s

A l a y e r of a d i c h a l c o g e n i d e is c o n s t i t u t e d by a s t a c k i n g of s h e e t s [ X T X ] , t h e c e n t r a l T s h e e t c o r r e s p o n d s t o a c o m p a c t h e x a g o n a l n e t w o r k o f t r a n s i t i o n i o n s between t h e t w o X s h e e t s o f c h a l c o g e n i d e s a t o m s ( S , S e , T e ) , a l s o w i t h a h e x a g o n a l arrangement

.

Bonding i n t h e l a y e r is i n t e r m e d i a t e b e t w e e n i o n i c a n d c o v a l e n t , a n d t h e s t a c k i n g is d u e t o Van d e r Waals i n t e r a c t i o n s . D i f f e r e n t s t a c k i n g c a n e x i s t ( p o l y t y p i s m ) g i v i n g r i s e t o d i f f e r e n t p h y s i c a l p r o p e r t i e s , t h e y a r e r e l a t e d t o t h e r e l a t i v e p o s i t i o n s o f t h e h e x a g o n a l n e t w o r k s of t h e s h e e t s . T h e l o c a l s t r u c t u r e depends o n t h e p o l y t y p i s m a s it is s h o w n f i g u r e 2. I n c o n t r a s t w i t h t h e g r a p h i t e , i n t e r c a l a t i o n g i v e s a l w a y s s t a g e 1 c o m p o u n d s w i t h a v a r i a b l e f i l l i n g o f t h e i n t e r c a l a n t l a y e r . An e x a m p l e is g i v e n f i g u r e 2 f o r t h e i n t e r c a l a t e d 2 H - p o l y t y p e CuO. Nb S2.

XANgS

S e v e r a l i n v e s t i g a t i o n s h a v e b e e n d o n e by a n g u l a r s t u d i e s o r by m o d i f y i n g t h e i n t e r c a l a t e d s p e c i e s . H e a l d a n d S t e r n 1 3 1 h a v e s h o w n t h a t f o r a l a m e l l a r compound t h e K - e d g e s p e c t r u m r e c o r d e d w i t h t h e p o l a r i z a t i o n i n t h e p l a n e c o r r e s p o n d s t o a px o r p y - l i k e f i n a l s t a t e , a n d t o a p z - l i k e s t a t e i f it is p e r p e n d i c u l a r . T h e i n t e n s i t y o f t h e s p e c t r u m d e p e n d s o n t h e l o c a l p a r t i a l d e n s i t y o f s t a t e s . F o r a K - edge, i t m e a n s f o r s t a t e s w i t h a p - l i k e c h a r a c t e r f o r w h i c h t h e d e n s i t y is c o n s i d e r e d a t t h e p o i n t o f t h e e x c i t e d a t o m .

T h i s f a c t is c l e a r l y d e m o n s t r a t e d i n t h e p o l a r i z a t i o n d e p e n d e n c e of t h e K - edge o f c a r b o n a t o m s i n g r a p h i t e ( R o s e n b e r g e t a l . [ 4 ] ) f o r w h i c h d r a m a t i c c h a n g e s i n t h e i n t e n s i t y o f t h e f i r s t p e a k is o b s e r v e d . T h i s p e a k d i s a p p e a r s f o r p o l a r i z a t i o n i n t h e p l a n e a n d c o r r e s p o n d s t o a t r a n s i t i o n t o w a r d a maximum o f t h e d e n s i t y o f states of t h e f i r s t e m p t y R - b a n d ( p z - l i k e b a n d ) ( f i g u r e 4 ) . S e v e r a l x a n d a b a n d peaks a g r e e well w i t h t h e b a n d s t r u c t u r e of g r a p h i t e .

a 1 . [5]. I n t h e c a s e of p o t a s s i u m i n KC8, t h e f i r s t p e a k h a s a Pxy-like b e h a v i o r i n c o n t r a s t w i t h g r a p h i t e , s h o w i n g t h e e f f e c t of t h e a l k a l i a t o m o r b i t a l s . I n t h a t l a t e r c a s e , t h e n u m b e r o f s t a t e s i n c r e a s e s by c o m p a r i s o n t o t h e g r a p h i t e , b e c a u s e of t h e a d d i t i o n a l a l k a l i a t o m o r b i t a l s a n d t h e f o l d i n g o f t h e B r i l l o u i n z o n e . S p e c t r a f o r h e a v i e r a l k a l i a t o m s a r e v e r y b r o a d e n e d b e c a u s e o f s h o r t e r l i f e t i m e s of t h e i r c o r e h o l e s .

F i g . 1: S t r u c t u r e of t h e s t a g e 1 F i g . 2 : P e r s p e c t i v e s t u c t u r e s of TS2

MC8 (M = K,Rb,Cs)(from E l ] ) . (T = Ti,Nb,Mo) compounds (from [ 2 ] ) .

o: S , e : metal

275 285 295 305 315 325 335 345 PHOTON ENERGY (IVI

F i g . 4: G r a p h i t e c a r b o n K-edge a s a f u n c t i o n o f t h e a n g l e (from 1 4 1 ) . dashed l i n e s : n s t a t e s

s o l i d l i n e s : o s t a t e s

F i g . 3: S t r u c t u r e o f t h e i n t e r c a l a t e d 2H-polytype CuOe5 Nb S2 (from [ I l l ) .

Layered t r a n s i t i o n - m e t a l d i c h a l c o g e n i d e s TX2 a r e c o m p o s e d w i t h m e t a l s b e l o n g i n g t o t h e g r o u p s I V a , Va a n d V I a . O c t a h e d r a l c o o r d i n a t i o n is a d o p t e d by g r o u p IV c o m p o u n d s ( T i , Zr, H f ) a n d s o m e g r o u p V c o m p o u n d s ( V ) , a n d t h e t r i g o n a l p r i s m a t i c c o o r d i n a t i o n by g r o u p VI (Mo) a n d t h e o t h e r g r o u p V ( N b ) compounds. Ohno e t a l . [ 6 - 81 a n d by D a v i e s a n d Brown [ 9 ] s u g g e s t e d t h a t t h e r i g i d b a n d model is v a l i d f o r compounds w i t h t h e s a m e c o o r d i n a t i o n . T h e h o s t m a t e r i a l b a n d s t r u c t u r e s a r e d i s c u s s e d i n [6-71, t h e y c o r r e s p o n d t o t h e b a n d d i a g r a m s o f t h e f i g u r e 5. T h e n , a s

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group V compounds e x h i b i t m e t a l l i c p r o p e r t i e s and most of group IV and V I compounds a r e s e m i c o n d u c t o r s , t h e a u t h o r s e x p e c t t h a t t h e a d d i t i o n of an e x t r a e l e c t r o n from an i n t e r c a l a t e d atom t o g r o u p IV ( V ) compounds l e a d s t o t h e s a m e e l e c t r o n i c s t r u c t u r e a s t h a t of group V ( V I ) compounds, r e s u l t i n g i n t h e s e m i c o n d u c t o r - m e t a l

(metal-semiconductor) t r a n s i t i o n .

S t r u c t u r e group and r e p r e s e n t a t i v e m a t e r i a l :

1 T 2H

I -

BDS bot1d (metal s p o r d bond). Thrs band 1%

derived from the admlxture of metal and S s and P BD4 bond (d o r e, band). Tllls bdnd 1s de- g adm~xture of metal d and S 3p orbrtals. However, some s character is also present.

0 3 band (d, 1 1 , or dz2 bond). T h ~ s bdnd IS from the strong adm~xture of metal d and

ED2 bond (cholcogen p or o bond). Thls band admlxture of S 3p and nletdl r,

Drnd-strucfure dlagrams of layer olcogen s band). Thls band is trans~tion-metdl d~sulfides, Id) for the group-IV TS, denved pnmarlly from S s orbitals, but ~ncludes compounds lb) for the group-V IT-TS, compounds, (c) s o m e p chardctenstics, also.

for the group-V 2II(3RLTS, ccmpounds, and Id) for the group-VI TS, compounds

F i g . 5 (from [ 8 ] )

S p e c t r a a g r e e well with t h e s e e x p e c t a t i o n s . Figure 6a r e p r e s e n t s S K-edges of g r o u p I V h o s t TiS2 and i n t e r c a l a t e d TiS3 and g r o u p V VS2 and some i n t e r c a l a t e d s p e c i e s , a l l in o c t a h e d r a l c o o r d i n a t i o n . I t is c l e a r t h a t t h e e d g e s have t h e s a m e shape. A s i m i l a r i t y is o b t a i n e d f o r L I I I - e d g e of Nb i n g r o u p V NbS2 a n d LIII-edge of Mo i n MoSZ ( f i g u r e 6 b ) . Moreover, Ohno e t a l . [ 8 ] show t h a t t h e i n t e r c a l a t i o n e f f e c t s o n X-ray a b s o r p t i o n s p e c t r a d o n o t depend s o much on t h e h o s t m a t e r i a l s t u d i e d , a s on t h e i n t e r c a l a t e d s p e c i e s . I t means t h a t t h e t r a n s f e r of e l e c t r o n from t h e i n t e r c a l a t e d atom t o t h e c o n d u c t i o n band of t h e h o s t l a y e r s d o e s n o t c h a n g e t h e two-dimensional b e h a v i o r of t h e m a t e r i a l and confirms t h a t t h e rigid band model c a n be used i n t h e f i r s t approximation.

I

F i g . 6: a ) S K-edges of 1 T of pure and i n t e r c a l a t e d d i c h a l c o g e n i d e s .

b ) Nb and Mo LIII.-edges of d i c h a l c o g e n i d e s .

m a t e r i a l s a r e u s e d i n EXAFS t o c h e c k f e a t u r e s o f t h e s p e c t r a [ 3 , 9 ] . By c o n t r a s t , t h e i n t e r e s t is e v i d e n t i n t h e c a s e of i n t e r c a l a t i o n of t h e s e compounds, b e c a u s e EXAFS p e r m i t s t o e x a m i n e t h e l o c a t i o n of t h e i n t e r c a l a t e d a t o m a n d t h e bond l e n g t h s i n t h e h o s t l a y e r . F o r e x a m p l e , B o u r d i l l o n e t a l . [ l o ] s t u d i e d t h e i n t e r c a l a t i o n compound R b0.28NbSe2 a n d T h u l k e e t a l . [ I 1

I

C U ~ . ~ N bS2. S p e c t r a w e r e c o m p a r e d r e s p e c t i v e l y t o NbSe2 a n d NbS2. E s s e n t i a l l y n o c h a n g e i n t h e Nb-Se ( N b - S ) n e a r n e i g h b o r d i s t a n c e was f o u n d , while t h e Nb-Nb d i s t a n c e was o b s e r v e d t o i n c r e a s e by 0 . 0 3 upon i n t e r c a l a t i o n . 'The s t r o n g e s t b o n d i n g i n t h e c r y s t a l is b e t w e e n Nb a n d S e ( S ) a t o m s . T h e c o r r e s p o n d i n g o - b a n d , r e f e r e n c e d BD2 i n t h e f i g u r e 5, l i e s w e l l below t h e F e r m i l e v e l . T h a t b a n d r e m a i n s u n a f f e c t e d by t h e c h a r g e t r a n s f e r which f i l l s t h e ED3 b a n d . T h i s B D 3 b a n d h a s a c o n s i d e r a b l e m e t a l d - c h a r a c t e r , t h i s e x p l a i n s t h e r e p u l s i o n between neighboring Nb atoms.

Heald a n d S t e r n [ 3 ] g a v e a g e n e r a l f o r m u l a t i o n o f t h e o r i e n t a t i o n dependence o f t h e EXAFS b o t h a t K a n d L - e d g e s . T h e y o b s e r v e d t h e a n i s o t r o p y of 2 H - WSeZ b o t h o n S e K-edge a n d on W L I I I - e d g e , t a k i n g i n t o a c c o u n t t w o d i r e c t i o n s o f t h e e l e c t r i c f i e l d w i t h r e s p e c t t o t h e c - a x i s , t h e y m e a s u r e d o n t h e F o u r i e r t r a n s f o r m t h e v a r i a t i o n s of a m p l i t u d e s of t h e p e a k s

.

T h e a m p l i t u d e r a t i o s of t h e s e p e a k s a g r e e w e l l w i t h t h e k n o w n s t r u c t u r e o f t h e 2H- WSe2. M o r e o v e r , t h e s e r e s u l t s c o n f i r m t h a t t h e p-s t r a n s i t i o n s a r e s m a l l a n d t h e p-d t r a n s i t i o n s d o m i n a t e .

S e v e r a l i n v e s t i g a t i o n s of i n t e r c a l a t e d g r a p h i t e h a v e b e e n p e r f o r m e d o n Br2. a l k a l i a t o m s a n d t e r n a r y c o m p o u n d s w i t h a l k a l i a n d t e t r a h y d r o f u r a n . Br K-edge was i n v e s t i g a t e d b o t h i n g r a p h i t e a n d G r a f o i l [ 1 2 - 1 4 1 . I t w a s d e m o n s t r a t e d t h a t Br is p r e s e n t a s Br2 molecule i n a l l t h e c a s e s . N e v e r t h e l e s s , s e v e r a l s p e c i e s w e r e found t o p r e s e n t d i f f e r e n t s t r e t c h i n g o f t h e Br-Br d i s t a n c e w h i c h h a v e d i f f e r e n t a n g u l a r d e p e n d e n c e s p e c t r a . I n t h e i n t e r c a l a t e d f o r m , t h e s t r e t c h i n g is a b o u t . 0 3 ( 2 . 3 1 ; i n s t e a d o f 2.28 f o r B r 2 m o l e c u l e ) a n d t h e o r i e n t a t i o n o f t h e m o l e c u l e is found t o b e a b o u t 20" w i t h r e s p e c t t o t h e p l a n e o w i n g t h r e e a n g l e m e a s u r e m e n t s 0'.

4 8 a n d 90" u s i n g g r a p h i t e f i b e r s . T h e s a m e kind of i n v e s t i g a t i o n w a s d o n e f o r Rb- g r a p h i t e w i t h t e t r a h y d r o f u r a n [ 1 5 ] , f o r which t h r e e s t r u c t u r a l models h a v e b e e n proposed. A s e a c h o n e l e a d s t o d i f f e r e n t a n g u l a r v a r i a t i o n s p e c t r a , t h e a n g u l a r s t u d y p e r m i t s t o c h o o s e o n e , which is c o n s i s t e n t w i t h t h e Rb-C d i s t a n c e measured.

The most s t r i k i n g b e h a v i o r is o b t a i n e d f o r t h e b i n a r y c o m p o u n d s a l k a l i - g r a p h i t e . A p r e v i o u s s t u d y o f C a s w e l l e t a1.1161 h a v e s h o w n t h a t a s t a g e o n e KC8 h a v e n o t t h e same s p e c t r u m t h a n a s t a g e 2 KC2& S p e c t r a of RbCZ4 w e r e r e c o r d e d a b o v e a n d below t h e t e m p e r a t u r e o f t h e c o m m e n s u r a t e - i n c o m m e n s u r a t e t r a n s i t i o n [ 1 7 ] . A t l o w t e m p e r a t u r e

-

corn m e n s u r a t e p h a s e

-

o n l y o n e Rb-Rb i n p l a n e d i s t a n c e w a s o b s e r v e d . Above t h e t r a n s i t i o n t w o d i s t a n c e s a p p e a r , o n e s m a l l e r a n d t h e o t h e r o n e l a r g e r t h a n t h e l o w t e m p e r a t u r e d i s t a n c e .

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PHYSIQUE

S p e c t r a of a n o r i e n t e d s a m p l e o f RbC8 [ 1 8 ] w e r e r e c o r d e d f o r a s e t of a n g l e s f r o m 0" u p t o 40°. F o r e a c h p e a k i n t h e F o u r i e r t r a n s f o r m , t h e a m p l i t u d e r a t i o p e r m i t s t o c a l c u l a t e t h e a n g l e o f t h e c o r r e s p o n d i n g R* v e c t o r w i t h r e s p e c t t o t h e p l a n e . M o r e o v e r , t h a t a n a l y s i s d o e s n o t n e e d t h e k n o w l e d g e of t h e n a t u r e of t h e n e i g h b o r . The o b t a i n e d r e s u l t s a g r e e w e l l w i t h t h e k n o w n s t r u c t u r e of t h e f i r s t s t a g e . T h e t h e r m a l s t u d y from 3 0 0 K d o w n t o 3 5 K s h o w s a high a n i s o t r ~ o y o f t h e Debye-Waller f a c t o r , t h e a m p l i t u d e of t h e v i b r a t i o n b e i n g l a r g e r i n t h e p l a n e . T h e s e c o n d s t a g e RbC24 s h o w s a h i g h e r s t a t i c d i s o r d e r t h a n t h e f i r s t s t a g e RbC8 a n d comparison w i t h s i m u l a t i o n s a r e s h o w n , t h e y p e r m i t t o p r o p o s e a p r o b a b l e model of t h e d i s o r d e r e d i n t e r c a l a n t l a y e r .

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