POLARIZATION-DEPENDENT XANES OF POTASSIUM HYDRIDE AND POTASSIUM INTERCALATED GRAPHITE

HAL Id: jpa-00226078

https://hal.archives-ouvertes.fr/jpa-00226078

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

POLARIZATION-DEPENDENT XANES OF POTASSIUM HYDRIDE AND POTASSIUM

INTERCALATED GRAPHITE

G. Loupias, J. Chomilier, J. Tarbes, I. Ascone, J. Goulon, D. Guerard, N.

Elalem

To cite this version:

G. Loupias, J. Chomilier, J. Tarbes, I. Ascone, J. Goulon, et al.. POLARIZATION-DEPENDENT

XANES OF POTASSIUM HYDRIDE AND POTASSIUM INTERCALATED GRAPHITE. Journal

de Physique Colloques, 1986, 47 (C8), pp.C8-891-C8-895. �10.1051/jphyscol:19868173�. �jpa-00226078�

POLARIZATION-DEPENDENT XANES OF POTASSIUM HYDRIDE AND POTASSIUM INTERCALATED GRAPHITE

G. LOUPIAS*

* * , ^J. CHOMILIER* , J. TARBES* , ^I. ASCONE* * ,

J . ^{GOULON* *}

, D . ^{GUERARDI *} and N. ELALEM'

'~aboratoire de MinBralogie-Cristallographie, U.P.M.C. and UniversitB Paris VII, T 16, 4 , place Jussieu,

F-75252 Paris Cedex 05, France

* * L.U.R.E. (C.N.R.S., C.E.A., M.E.N.), Bdtiment 209 D , F-91405 Orsay Cedex, France

+Laboratoire de Chimie ThBorique, Universite Nancy I. BP 239, F-54506 Vandoeuvre-16s-Nancy Cedex. France

++Laboratoire de Chimie du Solide Mineral, Universitt? Nancy I , BP 239, F-54506 Vandoeuvre-16s-Nancy Cedex, France

ABSTRACT

XANES of p o t a s s i u m h y d r i d e g r a p h i t e i n t e r c a l a t i o n compounds a r e r e c o r d e d and compared t o p o t a s s i u m - g r a p h i t e compounds. T h i s s t u d y i s a t r y t o l i n k t h e d e t a i l s of t h e band s t r u c t u r e above t h e Fermi l e v e l t o t h e superconductivi:y.

INTRODUCTION

Because t h e i n t e r l a y e r i n t e r a c t i o n i n g r a p h i t e i s r e l a t i v e l y weak, i t i s p o s s i b l e t o d i f f u s e f o r e i g n atoms o r m o l e c u l e s i n t o t h e s p a c e between c a r b o n l a y e r s . The b i n a r y compounds a r e c o n s t i t u t e d by one s p e c i e s of i n s e r t e d atoms forming a l a y e r i n between two c a r b o n l a y e r s . The number of c a r b o n l a y e r s between two s u c c e s s i v e l a y e r s of i n t e r c a l a t e d s p e c i e s i s c a l l e d t h e s t a g e n of t h e g r a p h i t e i n t e r c a l a t i o n compound (GIC). I n s e r t e d m e t a l s a r e e l e c t r o n d o n o r s t o c a r b o n l a y e r s , i . e . t h e m e t a l c o n d u c t i o n e l e c t r o n s a r e m a i n l y t r a n s f e r r e d t o t h e g r a p h i t e l a y e r s .

I n t h e s u p e r c o n d u c t i n g GIC's r e p o r t e d t o d a t e , t h e i n t e r c a l a t e s p e c i e s a c t s a s a donor. The p r e s e n t t e n d e n c y t o i n c r e a s e t h e t r a n s i t i o n t e m p e r a t u r e T i s t o i n s e r t two d i f f e r e n t s p e c i e s i n g r a p h i t e ; s u c h compounds a r e c a l l e d C t e r n a r y o n e s . Promising r e s u l t s a r e o b t a i n e d w i t h i n t e r c a l a t e d amalgam ( I ) , and r e c e n t s t u d i e s ( 2 , 4 ) a r e performed on f i r s t and second s t a g e s a l k a l i h y d r i d e G I C (KH0.8C4n w i t h n = l o r 2 ) o f t h e same s t r u c t u r e a s amalgam GIC ( f i g u r e

jn

- - - - -

. . ^. . . ^. . . . ."

- - - - - ^- -

Carbon layer

peak

F i g 1: S t r u c t u r e of a l k a l i h y d r i d e GIC F i g 2: Dependence of t h e f i r s t peak

- KHO 8C4n. (H) means a s u b s t i t u t i o n i n t e n s i t y of KC8 v e r s u s s i n 2 0 of R a t t h e c o r n e r s of t h e 2-D c e l l .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868173

C8-892 JOURNAL DE PHYSIQUE

I n t h e b i n a r y potassium compounds s e r i e s , t h e s u p e r c o n d u c t i v i t y i s observed i n t h e f i r s t s t a g e compound KC8, b u t cannot be d e t e c t e d i n t h e second s t a g e KClzn down t o 60mK(3). I n t h e hydrogen second s t a g e potassium t e r n a r y G I C , t h e s u p e r c o n d u c t i v i t y i s c o n t r o v e r s e d ( 2 , 4 ) , while low c o n c e n t r a t i o n of d i l u t e d hydrogen i n KC i n c r e a s e s t h e temperature T ( 5 ) .

8 ~ v e n i f t h e s u p e r c o n d u c t i S i t y mechanism i s n o t w e l l understood, i t depends s t r o n g l y upon t h e d e t a i l s of t h e band s t r u c t u r e above t h e Fermi l e v e l . Polarization-dependent X-ray a b s o r p t i o n (XANES) i s a powerful t o o l f o r s t u d y i n g such l a y e r e d compounds where t h e two main problems t o s o l v e a r e :

- t h e charge t r a n s f e r from metal t o carbon and hydrogen

- t h e symmetry of f i n a l s t a t e s .

COMPOUNDS PREPARATION

The b i n a r i e s s e r i e (KC , KCz4, KC

KCg4) has been s y n t h e t i z e d w i t h t h e a l r e a d y e s t a b l i s h e d methods ? 6 ) , a s we13 a s t e r n a r y compounds of s t o i c h i o m e t r y

8C4n(7). A l l samples a r e prepared from HOPG (Highly Oriented P y r o l i t i c G r a p h i t e ) . T h e ' l n s e r t i o n of KH i n t o g r a p h i t e t o produce t h e t e r n a r y compound o c c u r s i n two s t e p s :

-1- Almost e x c l u s i v e i n s e r t i o n of potassium i n t o g r a p h i t e -2- Simultaneous i n s e r t i o n of potassium and hydrogen

A c t u a l l y , a t t h e end of s t e p 1 , t h e b i n a r y compound c o n t a i n s some hydrogen d i l u t e d i n t h e potassium l a y e r s . This i n t e r m e d i a t e "binary" compound i s t h e n

KC H , w i t h x l e s s t h e n 0 . 1 ( 8 ) ; t h i s compound i s n o t a t h r e e - l a y e r s s t r u c t u r e . 24 XThe q u a l i t y of samples i s checked w i t h X-rays, and f o r a good c o n s e r v a t i o n , they a r e k e p t i n c e l l s w i t h 12 um-thick kapton windows.

EXPERIMENTAL CONDITIONS

The measurements of t h e potassium a b s o r p t i o n t h r e s h o l d have been performed a t LURE-DCI, by t r a n s m i s s i o n through t h e sample, on t h e EXAFS 2 s t a t i o n equipped w i t h m i r r o r s f o r harmonics r e j e c t i o n .

Depending upon t h e potassium c o n c e n t r a t i o n , t h e sample t h i c k n e s s (homogeneous) v a r i e s from 5 p t o 5 0 u m t o optimize t h e s t a t i s t i c s . I n t h e s e l a y e r e d m a t e r i a l s t h e c r i s t a l l o g r a p h i c a x i s i s always p e r p e n d i c u l a r t o t h e sample.

The 2-D n a t u r e of G I C i s due t o t h e d i r e c t i o n a l i t y of t h e n a t i v e g r a p h i t e o r b i t a l s . The

type o r b i t a l s l i e i n t h e b a s a l p l a n e , s o t h e d i p o l e s e l e c t i o n r u l e l e a d s t o an int+ensity f o r a pure transi+t&on, e x c i t e d by a p o l a r i z e d l i g h t of e l e c t r i c f i e l d E, 1 s - o p r o p o r t i o n a l t o s i n 2 ( ~ , c ) and 1s-*proportional t o + c o s 2 ( d , ~ ) . As a r e s u l t , by v a r y i n g t h e a n g l e

between t h e e l e c t r i c f i e l d and t h e c-axis, t h e symmetry of t h e f i n a l s t a t e s can be s e l e c t e d . Because t h e g r a p h i t e h o s t i s HOPG, no d i r e c t i o n a l i n f o r m a t i o n can be o b t a i n e d i n t h e b a s a l plane.

We have performed measurements f o r a n g l e s

ranging from 9 0 " t o 30' by s t e p s of 15". For each spectrum the energy s t e p i s 0.15 eV.

DISCUSSION

1. Charge t r a n s f e r and e l e c t r o n i c s t r u c t u r e of t h e s t a g e 1 potassium GIC.

I n KC8,the p r e d i c t e d charge t r a n s f e r of potassium v a r i e s from 0.6(9) t o l ( 1 0 ) . A r e c e n t r e v i e w ( l 1 ) of experimental r e s u l t s does n o t c l e a r l y f a v o r one of t h e two c a l c u l a t i o n s . N e v e r t h e l e s s , t h e models(l2) used t o e x p l a i n i t s s u p e r c o n d u c t i v i t y a r e supported by a n o - t o t a l charge t r a n s f e r . The experimental a b s o r p t i o n edges f o r t h e s t a g e s 1 t o 3 potassium b i n a r y compounds a r e compared t o KH and t o t h e s t a g e 7(KC8&)edges. The charge t r a n s f e r i s n o t t o t a l f o r K 4 s e l e c t r o n i n KH (0.7 according t o P a u l i n g ' s e l e c t r o n e g a t i v i t i e s ) , and i t corresponds t o a measured chemical s h i f t of 1.7 eV ( f i g u r e 6 ) compared t o t h e t o t a l l y i o n i z e d potassium i n d i f f e r e n t surroundings (face-centered c u b i c K C 1 a s w e l l a s KC24 o r KC,' " . f o r which t h e t o t a l t r a n s f e r

i s u n i v e r s a l l y admitted). All t h e edges f o r t o t a l l y i o n i z e d K i n d i f f e r e n t surroundings have i d e n t i c a l p o s i t i o n s and a r e superimposed w i t h t h e KC8 one.

Then, a t t h e p r e c i s i o n of t h e experiment, t h e potassium i n i t i a l l y 4s e l e c t r o n

i n KC8 i s t o t a l l y t r a n s f e r r e d . T h i s confirms t h e p r e d i c t i o n s of DiVincenzo and

Normalized 70 m t e r t h e edge.

R a b i i ( i 0 ) who argue f o r a model w i t h an empty K4s band.

The s p e c t r a show ( f i g u r e 3 ) d r a m a t i c v a r i a t i o n s i n peak i n t e n s i t i e s a s t h e a n g l e

i s v a r i e d and l e a d s t o i d e n t i f y t h e symmetry bands above t h e Fermi l e v e l . 0 n l y two s p e c t r a a r e r e p o r t e d b u t f o u r a r e measured. The i n i t i a l peak has been shown t o be a K ( l s ) t o

e x c i t a t i o n by t h e l i n e a r dependence of t h e peak i n t e n s i t y v e r s u s s i n 2 O ( f i g u r e 2).A more e l a b o r a t e d comparison w i t h t h e band s t r u c t u r e c a l c u l a t i o n of Rabii t a k i n g i n t o account t h e h y b r i d i z a t i o n of C and K ( s , p and d ) o r b i t a l s , a s w e l l a s t h e e x p l a n a t i o n of small d i f f e r e n c e s i n energy p o s i t i o n s of t h e near-edge s t r u c t u r e s a r e i n p r o g r e s s ( l 3 ) .

2. P o l a r i z a t i o n dependent XANES of h i g h e r s t a g e s potassium G I C .

The s p e c t r a f o r two v a l u e s of O ( 9 0 " and 45") a r e r e p o r t e d f o r second KCz4 and t h i r d KC s t a g e s on f i g u r e s 4 and 5. I n t h e energy range of 60 eV

above t h e edge, 3fhe p h o t o e l e c t r o n s have a mean f r e e p a t h l a r g e enough t o be m u l t i p l e s c a t t e r e d . The p o l a r i z a t i o n e f f e c t i s s p e c t a c u l a r and i n c r e a s e s w i t h t h e s t a g e . This shape resonance depends upon t h e c l u s t e r geometry+related t o t h e d i l u t i o n of potassium which i n c r e a s e s w i t h t h e s t a g e both a l o n g t h e c-axis and a l s o i n t h e 2-D l a y e r s ( 1 potassium f o r 8 carbon i n t h e f i r s t s t a g e and only 1 f o r 12 i n h i g h e r s t a g e s ) . The s e n s i t i v i t y of t h e 2-D surrounding i s confirmed by t h e s i m i l a r peak p o s i t i o n s and amplitudes f o r both t h e s t a g e s 2 and 3 s p e c t r a measured w i t h 8=90° compared t o t h e f a r d i f f e r e n t spectrum f o r s t a g e 1 ( f i g u r e 3) because of a d i f f e r e n t s t o i c h i o m e t r y i n t h e plane.

Energy (eV)

3640 3660

Absorption

__ _ _

(arb.unit)

KC,,H,

-

6000 GolO Energy (eV) 6020

w: XwES of KC f o r 0-90' and 45' m: XANES of KH, KH 8C4, KC2,+$: KCZ4.

Normalized 38 eV a f t e r t h e edge. Normalized a t tfk f i r s t pea 3. Charge t r a n s f e r i n potassium hydride GIC's.

-Ternary compounds ( f i r s t and second s t a g e s ) :

The second s t a g e KH C edge has t h e same p o s i t i o n a s a l l t h e b i n a r y

compounds: t h e e x p e r i m e n k f 8charge t r a n s f e r of t h e K4s e l e c t r o n i s t o t a l . For t h e

C8-894 JOURNAL DE PHYSIQUE

s t a g e one t h e e x p e r i m e n t a l a b s o r p t i o n edge i s compared t o t h e KH and KC 022'~k' f i g u r e 6 : i t c o r r e s p o n d s t o a c h e m i c a l s h i f t e q u a l t o 0.6 eV comparei4 t o t h e t o t a l l y i o n i z e d potassium.

a r e s u l t , t h e t r a n s f e r of t h e K4s e l e c t r o n i s n o t t o t a l i n t h i s compound.

- Binary compound w i t h d i l u t e d H (second s t a g e KC H w i t h x<0.1)

The edge i s r e p o r t e d on t h e f i g u r e 6 and e z f f i ~ i t s a c h e m i c a l s h i f t from KCz4 e v a l u a t e d t o 0.3 eV. T h i s e x p e r i m e n t a l r e s u l t i s s u r p r i s i n g because t h e c h a r g e t r a n s f e r i s l e s s t h a n i n KC and t h e o n l y known r e s u l t c o n c e r n i n g t h e e l e c t r o n i c s t a t e of hydrogen i n suc?i4a compound i s claimed by Enoki t o be a n a n i o n H by p o s i t r o n a n n i h i l a t i o n measurements(l4).

Table 1:Summary of c h a r g e t r a n s f e r and s u p e r c o n d u c t i v i t y r e s u l i s

*: c h e m i c a l s h i f t between t h i s compound edge and t h e t o t a l l y i o n i z e d K edge.

We c a n o b s e r v e ( t a b l e 1 ) t h a t a p a r t i a l c h a r g e t r a n s f e r i s n o t s u f f i c i e n t t o p r e d i c t s u p e r c o n d u c t i v i t y : KC i s a s u p e r c o n d u c t o r w i t h a q u a s i - t o t a l c h a r g e t r a n s f e r , b u t i f a s m a l l amount of t h e i n i t i a l l y K4s e l e c t r o n remains i n potassium, 8 i t p a r t i c i p a t e s a t a c o n d u c t i o n band. On t h e o p p o s i t e , i n KHO C4 a l a r g e r

amount of t h e K i n i t i a l l y 4 s e l e c t r o n remains on i n s e r t e d K a d moreover i t o c c u p i e s l o c a l i z e d e l e c t r o n i c s t a t e s because o f t h e i o n i c i n s u l a t o r c h a r a c t e r o f p o t a s s i u m h y d r i d e KH.

I n c o n c l u s i o n , a s i t i s p o i n t e d o u t by P r e i l and F i s h e r ( l 5 ) f o r t h e amalgam G I C , t h e symmetry of s t a t e s a t t h e Fermi l e v e l i s p r e p o n d e r a n t t o u n d e r s t a n d t h e s u p e r c o n d u c t i v i t y mechanism. E l a b o r a t e d band c a l c u l a t i o n s a s w e l l a s measurements of e l e c t r o n i c d e n s i t y o f v a l e n c e e l e c t r o n s i n ground s t a t e by Compton

s c a t t e r i n g ( l 6 ) a r e needed t o i n t e r p r e t t h e hydrogenated p o t a s s i u m G I C s u p e r c o n d u c t i v i t y .

s u p e r c o n d u c t i v i t y measured t r a n s f e r

BIBLIOGRAPHY

Second s t a g e compounds b i n a r y d i l u t e d H t e r n a r y

1. IYE I., and TANUMA S. I. Phys Rev B (1982) 4583.

2. ENOKI T., SANO M., INOKUCHI H., Phys. Rev. B (1985) 2497.

3. ALEXANDER M. GOSHORN D. P., GU6RARD D., LAGRANGE P., ELMAKRINI N., ONN D. G., S o l i d S t a t e Commun. 38 (1981) 103.

4. SUZUKI K., T S U J I ~ A I., KOBAYASHI M., KIMISHIMA Y., Synth. M e t a l s 12 ⁽¹⁹⁸⁵⁾

389.

5. KANEIWA S., KOBAYASHI M., TSUJIKAWA I., J. Phys. Soc. J a p a n +1 (1982) 2375.

6 . HERQLD A. B u l l . Soc. Chim. France (1955) 999.

7. GUERARD D . , ELALEM N., TAKOUDJOU C., ROUSSEAUX F., Synth. M e t a l s 12 (1985) 196.

8.

ELALEM N., GUERARD D., ROUSSEAUX F., LOUPIAS G., ESTRADE-SZWARCKOPF Ha, LAUGINIE P., CONARD J., P r o c e e d i n g s of t h e 4 t h I n t e r n a t i o n a l Conference on Carbons, Baden-Baden, FRG, (19861, p 433.

9. OHNO T., NAKAO K., KAMIMURA H., J. Phys. Soc. Japan 47 (1979) 1125.

10. DIVINCENZO D., RABII S., Phys. Rev. B25 (1982) 4 1 1 z

11. DRESSELHAUS M. S., SUGIHRA K., ~ x t e n G A b s t r a c t s G r a p h i t e I n t e r c a l a t i o n Compounds , MRS, E d t s EKLUND P. C., DRESSELHAUS M. S., DRESSELHAUS G. D. (1984) p39.

12. AL-JISHI R., Phys. Rev. B28 (1983) 112.

TAKADA Y., J. Phys. Soc. J a p a n 3 (1982) 63.

F i r s t s t a g e compounds b i n a r y d i l u t e d H t e r n a r y

K H O . 8'8 y e s ( 4 ) n o ( 2 )

t o t a l K C z 4

n o ( 5 ) t o t a l K H O . 8 C 4

n o ( & ) no t o t a l

0.6 eV*

K C 2 4 H x

-

no t o t a l 0.3 eV*

K C 8 y e s ( l )

t o t a l

K C 8 H x y e s ( 3 )

-

Metals 12 (1985) 225.

15. PREIL

FISCHER J . E., Synth. Metals 8 (1983) 149.

16. LOUPIAS

CHOMILIER J . , RABII S . , ~ n n a l e s d e Physique 11 ⁽¹⁹⁸⁶⁾