Graphite intercalation compounds of PdCl2: Structural investigations

Synthetic Metals, 34 (1989) 199-204

199

GRAPHITE INTERCALATION COMPOLINDS OF PdCI2: STRHCTLIRAL INVESTIGATIONS

P. BEHRENS*, J. EHRICH and W. METZ I n s t i t u t fiir Physikalische Chemie der Universitiit Hamburg, Bundesstrafle 45, D-2000 Hamburg 13 (F.R.G.) W. N1EMANN Haldor Tops~e Research Laboratories, DK-2800 Lyngby (Denmark)

ABSTRACT GICs ÷ o f Palladinm(ll)-chloride were investigated by X - r a y diffraction and by X-ray a b s o r p t i o n s p e c t r o s c o p y at t h e Cl K edge and t h e L e d g e s o f Pd. These t w o m e t h o d s give c o m p l e m e n t a r y i n f o r m a t i o n on t h e s t r u c t u r e o f the intercalate layer. This s t r u c t u r e is built up f r o m ~ [ P d C l 2] chains as t h e y occur in ~-PdCl 2. M o n o c h r o m a t i c Laue p h o t o graphs reveal t h a t t h e s e chains are rotationally d i s o r d e r e d with r e s p e c t t o the c r y s t a l l o graphic axes o f the graphene layers, b u t s h o w a t e n d e n c y t o w a r d s an epitactic arrangement. The XANES regions o f all edges also provide evidence o f a g r e a t

similarity

b e t w e e n i n t e r c a l a t e d PdCl 2 and pristine ~-PdCl 2. A n g l e - d e p e n d e n t XANES m e a s u r e m e n t s at the Cl K and t h e Pd LII I edge s h o w t h a t t h e angle b e t w e e n the g r a p h e n e layers and t h e plane o f t h e chloride chains is about 59 °. INTRODtICTION I n t e r c a l a t e d metal chlorides c o n s t i t u t e t h e m o s t n u m e r o u s group o f graphite intercalation c o m p o u n d s . From a s t r u c t u r a l point o f view, the i n t e r c a l a t e d layers span t h e range f r o m m o l e c u l a r quasi z e r o - d i m e n s i o n a l s t r u c t u r e s t o t w o - d i m e n s i o n a l s h e e t s with s t r o n g i n t e r a c t i o n s b e t w e e n t h e g u e s t a t o m s [1]. Here we p r e s e n t s t r u c t u r a l investigations on PdC! 2 GICs, the f i r s t i n t e r c a l a t e d metal chloride which p o s s e s s e s a layer built up f r o m chains. Pristine PdCI 2 is dimorphic [2]. W h e r e a s ~-PdCl 2 is built up f r o m hexameric Pd6C112 c l u s t e r s [3] t h e s t r u c t u r e o f ~-PdCI 2 c o n s i s t s o f chains [4]. In b o t h s t r u c t u r e s the Pd a t o m s p o s s e s s a square coordination o f Cl. P r e s e n t address: Fakult~it fiir Chemie, Hniversit~it Konstanz, Universit~itsstraBe 10. D-7750 K o n s t a n z (F.R.G.) + Abbreviations used: GIC: graphite i n t e r c a l a t i o n compound; XANES: X-ray a b s o r p t i o n near edge s t r u c t u r e ; EXAFS: E x t e n d e d X - r a y a b s o r p t i o n fine s t r u c t u r e . 0379-6779/89/$3.50

© Elsevier Sequoia/Printed in The Netherlands

200

Several a u t h o r s have r e p o r t e d u p o n t h e p r e p a r a t i o n o f s t a g e 3 PdCI2-GICs b a s e d on HOPG as s t a r t i n g m a t e r i a l IS, 6, 7]. Repeat d i s t a n c e s a l o n g t h e ~c

axis

(as i n f e r r e d

f r o m OOl reflections} were specified as c (3)= 1670 p m 15, 6] or 1660 p m [7] for t h e s e c o m p o u n d s . This c o r r e s p o n d s t o

an e x c e p t i o n a l l y large t h i c k n e s s o f t h e

stage

one

p a c k a g e (one g r a p h i t e layer p l u s i n t e r c a l a t e layer} o f c tl) ~ 1000 p m (for m o s t m e t a l c h l o r i d e GICs, c (1) ~ 935...955 pm). N e v e r t h e l e s s , t h i s value is still t o o s m a l l f o r t h e Pd6Cll2 m o l e c u l e s o f [5-PdCl 2 t o be p r e s e n t in t h e i n t e r c a l a t e layer. In t h i s p a p e r we p r e s e n t a f i r s t overview o f o u r r e s u l t s c o n c e r n i n g t h e s t r u c t u r e o f PdCI2-GICs. F u r t h e r

details

concerning the

preparation

and t h e

c h a r a c t e r i z a t i o n by

X - r a y d i f f r a c t i o n as well as f u r t h e r r e s u l t s o f e x t e n s i v e x - r a y a b s o r p t i o n m e a s u r e m e n t s will be given in f o r t h c o m i n g p a p e r s . EXPERIMENTAL Preparation H e a t i n g a m i x t u r e o f PdCI 2 and g r a p h i t e at 410 ° C in a s e a l e d a m p o u l e a t a p r e s s u r e o f c h l o r i n e o f a b o u t 1 bar f o r f o u r w e e k s gives p u r e s e c o n d and t h i r d s t a g e c o m p o u n d s in t h e c a s e o f g r a p h i t e f l a k e s and HOPG as s t a r t i n g m a t e r i a l s , resp. W i t h p o w d e r e d g r a p h i t e a s t a g e d i s o r d e r e d p r o d u c t o f s t a g e n u m b e r 3 to 4 is obtained. Characterization 001 r e f l e c t i o n s were m e a s u r e d in t h e u s u a l way on a Philips p o w d e r d i f f r a c t o m e t e r . M o n o c h r o m a t i c Laue p h o t o g r a p h s were t a k e n as d e s c r i b e d earlier [see, e.g., Ref. 8]. X ray a b s o r p t i o n s p e c t r o s c o p i c m e a s u r e m e n t s were p e r f o r m e d at t h e EXAFS II b e a m l i n e [9] at HASYLAB in H a m b u r g (F.R.G.). The e x p e r i m e n t is e q u i p p e d w i t h a Si 111 d o u b l e c r y s t a l m o n o c h r o m a t o r and a f o c u s i n g m i r r o r

coated

with

a Ni alloy.

The

second

s a m p l e c h a m b e r [9] w a s filled w i t h 10 m b a r o f A r g o n g a s a l l o w i n g a highly r e p r o d u c i b l e e n e r g y c a l i b r a t i o n (EAr K: 3.2029 keV) w i t h an e r r o r o f < 0.1 eV. Llsually t w o s c a n s were taken: f r o m 2.700 t o 3.300 keV t o m e a s u r e t h e a b s o r p t i o n s p e c t r u m o f t h e C1 K (ca. 2.780 keV) a n d t h e Pd LII I e d g e {ca. 3.173 keV); and f r o m 3.100 to 3.600 k e V t o o b t a i n t h e s p e c t r a at t h e LIII, Lii (3.330 keV) and LI (3607 keV) e d g e o f Pd. The e d g e j u m p s of the

four edges showed ratios

of

about

8:4:2:1.

Together with

the

increasing

b a c k g r o u n d a b s o r p t i o n f r o m t h e Cl K a n d t h e Pd LII I e d g e t h i s led t o n o n - o p t i m a l r e c o r d i n g c o n d i t i o n s at t h e Ltl a n d t h e

LI edge o f Pd. R e g a r d i n g t h e

small energy

d i f f e r e n c e b e t w e e n t h e LII I a n d t h e LII edge, an EXAFS e v a l u a t i o n giving reliable d a t a o f a b o u t t h e u s u a l s t a n d a r d w a s p o s s i b l e only at t h e C1 K edge. The u s u a l e v a l u a t i o n p r o c e d u r e (see, e.g., Ref. 10} w a s followed. S a m p l e s o f K2PdCla. a n d o f ~-PdC12 (contain i n g t r a c e s o f t h e [5 phase} were m e a s u r e d f o r c o m p a r i s o n o f XANES s p e c t r a a n d in o r d e r t o o b t a i n t h e s c a t t e r i n g p h a s e f u n c t i o n q~c1-Pd a n d t h e (modified} b a c k s c a t t e r i n g a m p l i t u d e f u n c t i o n F~,d . RESULTS AND DISCLISSION X-ray diffraction 001 r e f l e c t i o n s . The 001 r e f l e c t i o n s o f b o t h t h e s e c o n d a n d t h e t h i r d s t a g e c o m p o u n d proved

high

s t a g i n g fidelity. The v a l u e s o f c (2) a n d c (3) were d e t e r m i n e d as 1322 ± 3 p m

201

a n d 1657 :~ 3 p m , r e s p . T h i s c o r r e s p o n d s

t o a v a l u e o f c (1) o f 987 p m , w h i c h is s m a l l e r

t h a n r e p o r t e d in e a r l i e r p u b l i c a t i o n s [5, 6, 7] b u t s t i l l s i g n i f i c a n t l y l a r g e r t h a n in m o s t o t h e r m e t a l c h l o r i d e GICs. hko

diffraction

PdCI2-G1Cs radiation lattice

as

pattern.

observed

is s h o w n

in Fig.

of graphite.

show small but

A

schematic

drawing

on a monochromatic 1. T h e

discrete

of

the

Laue

hk0

diffraction

photograph

reflection

spots

stem

from

T h e d i f f u s e r i n g s w h i c h h a v e t o be a t t r i b u t e d

significant variations

of their diameter.

pattern

prepared

to

with the

the

of

MoKu

in-plane

intercalate

Their mean d values corres-

p o n d t o d i s t a n c e s as t h e y o c c u r in a n d b e t w e e n t h e c h a i n s o f ~ - P d C l 2. W h e r e a s

a full

d i s c u s s i o n o f t h i s p e c u l i a r h k 0 d i f f r a c t i o n p a t t e r n is p o s t p o n e d , we s t a t e t h a t it i n d i c a t e s that the intercalate ~-PdCl2;

the

layers of PdCI2-GICs

observation

of diffuse

a r e b u i l t u p f r o m c h a i n s as t h e y o c c u r in

rings

shows

that

these

d i s o r d e r e d w h i l e t h e v a r i a t i o n s in t h e d i a m e t e r s of the rings indicate

a tendency towards

an :,

representation

hk0

:

~:

diffraction pattern

of PdCI2-GICs as observed

~:

~i~ ~ . . ' ~ l ~ ~

';;:

spectroscopy

'i !::~,

and

~';i::::~

'::

EXAFS. T h e F o u r i e r t r a n s f o r m s 20 K) o f p r i s t i n e

;~

~

~~

~<

Laue p h o t o g r a p h p r e p a r e d

with MoK~ radiation.

X-ray absorption

rotationally

~¢~. ::::! : / . . ~

Fig. 1. S c h e m a t i c on a monochromatic

the

are

: ~ : ' , .....~: :~;: ::, .,~-..,.:. . . . . ~.. ~i~-. ,,~=~:.

epitactical arrangement.

of

chains

.:.~,i ? . . . . .

intercalated

~::.::

!

,;:,-r:,:"-

o f t h e E X A F S a t t h e Cl K e d g e {data m e a s u r e d PdCl 2 a r e

shown

in Fig. 2. D u e t o

at

a non-linear

s c a t t e r i n g p h a s e f u n c t i o n ¢PcI-Pd t h e f i r s t " c o o r d i n a t i o n s h e l l " o f Cl {i.e. t w o Pd a t o m s a t a d i s t a n c e o f 231 pro} g i v e s

two

distribution

evaluation

function.

Standard

p e a k s (at 160 a n d 230 pro) in t h e m o d i f i e d radial procedures

230 • 1 p m f o r t h e i n t e r c a l a t e w h i c h c o m p a r e d n o s i g n i f i c a n t c h a n g e in t h e m e t a l - c h l o r i n e

Fig. 2. F o u r i e r t r a n s f o r m s

[10] y i e l d a Pd-C1

distance

of

t o p r i s t i n e PdCl 2 {231 p m } s h o w s t h a t

bondlength

occurs upon intercalation.

(FTs)

o f t h e E X A F S a t t h e CI K e d g e from measurements scattering

a t 20 K. T h e

processes

leading

to

t h e p e a k s in t h i s m o d i f i e d radial distribution

function

are

also

shown. Note the "focusing effect" leading to an enhancement

of the

i n t e n s i t y o f t h e p e a k a t a b o u t 4 ~. 0

100 200 300 400 500 600 700 BOO

(pm)

202

XANES. XANES s p e c t r a o f the Pd LII I edge are p r e s e n t e d in Fig. 3. The similarity o f t h e edge f e a t u r e s b e t w e e n pristine and i n t e r c a l a t e d PdCl 2 gives f u r t h e r s u p p o r t t o the a s s u m p t i o n t h a t t h e i n t e r c a l a t e layer is built up f r o m 1 [PdCl2 ] chains. The p r o m i n e n t white line A at a b o u t 3174 eV is ascribed t o a d i p o l e - a l l o w e d t r a n s i t i o n f r o m an initial 2p s t a t e t o the dx2_y2 orbital, t h e l o w e s t unoccupied orbital in the case o f a s q u a r e - c o o r dinated d 8 metal ion. Note t h a t this orbital lies along t h e directions o f t h e Pd-Cl bond. As could be e x p e c t e d , XANES s p e c t r a at the Pd LII edge look similar t o t h o s e p r e s e n t e d in Fig. 3. The CI K edge s p e c t r a reveal more d i f f e r e n c e s b e t w e e n pristine and i n t e r c a l a t e d PdCl 2 (Fig. 4). This is o f c o u r s e due t o the fact t h a t the chlorine a t o m s experience s t r o n g e r c h a n g e s o f t h e i r s u r r o u n d i n g upon i n t e r c a l a t i o n t h a n t h e palladium atoms. The white line B at about 2820 eV can be a s s i g n e d t o a t r a n s i t i o n f r o m t h e ls orbital t o an antibonding a * orbital with (mainly) p c h a r a c t e r which lies along the direction o f t h e Pd-CI bond. Note t h a t t h e edge o f t h e intercalation c o m p o u n d is s h i f t e d t o l o w e r energies by 0.6 * 0.1 eV. A s h i f t o f about the same magnitude is o b s e r v e d for K2PdCI 4, a c o m p o u n d which c o n t a i n s planar [PdCI4] 2- anions. These edge s h i f t s can be i n t e r p r e t e d as chemical s h i f t s . In the case o f t h e GIC the s h i f t probably has its origin in t h e charge t r a n s f e r r e d f r o m the graphite h o s t t o the a c c e p t e r intercalate. This negative charge allows t h e ejection o f t h e absorbing e l e c t r o n at a l o w e r energy. A similar s h i f t has been o b s e r v e d at t h e Zn K e d g e s o f i n t e r c a l a t e d and pristine ZnCl 2 [ I l l No c o n c l u sion can yet be drawn regarding t h e q u e s t i o n w h e t h e r a similar s h i f t e x i s t s also in t h e case o f the palladium a t o m s in PdC12-GICs. The Pd LII I (and LII) e d g e s are o b s c u r e d by t h e s t r o n g white lines A (Fig. 4); at t h e Pd LI edge (not s h o w n here), t h e s i g n a l - t o noise ratio is very low. Therefore, t h e edge p o s i t i o n s c a n n o t be d e t e r m i n e d precisely. Nevertheless, m e a s u r e m e n t s at t h e Pd K edge, which are planned t o be p e r f o r m e d , will probably give more i n f o r m a t i o n s . A n g l e - d e p e n d e n t XANES m e a s u r e m e n t s . The CI K and Pd Lin XANES o f the sample b a s e d on HOPG was m e a s u r e d as a f u n c t i o n o f t h e angle O b e t w e e n the incoming beam and the -> c axis of the sample. The el K edge spectra are s h o w n in Pig. S. W e

fitted

the spectra to obtain the area of the white line peak B normalized with respect to the edge jump. A similar procedure was applied to the Pd LIII edge spectra which are not s h o w n here. Fig. 6 shows the intensities of the white lines A and B as a function of the tilting angle ®. Both s h o w a decrease as O increases. After having modified [12] a theory which exists for similar measurements

on surface-adsorbed species [see, e.g.,

Ref. 13] w e obtained values of the angles c~ (see Fig. 7) between the c-> axis and the direction of the Pd-Cl bond (as has already been mentioned the final states of both white lines are oriented along the direction of the Pd-Cl bond):

0t(from e l K e d g e ) : $3°; tX(from Pd LIII edge): S2°" By simple g e o m e t r i c relations {see Fig. 7) we can f u r t h e r m o r e s t a t e t h a t t h e plane o f t h e ~t[PdClz] chain p o s s e s s e s an angle ~ o f about 59 ° with regard t o t h e plane o f t h e graphene layer. This r a t h e r large value is in g o o d a g r e e m e n t with t h e e x c e p t i o n a l l y large t h i c k n e s s o f t h e s t a g e one package and also with t h e c o r r e s p o n d i n g angle in

203 '

'

A'

'

: lid

'

'

-I

e4e14

o

l--

c

n 3.16

SI

I

317

l

318 E (keV)

I

3.19

2.83 2.8/* E (keY)

2.82

Z85

Fig. 3. X A N E S s p e c t r a a t the

Fig. 4. X A N E S s p e c t r a a t t h e

Pd Lxn e d g e .

C1 K e d g e .

.'t= r"

•

.

L.

~

[~

6.0

e-/

oOO

5.5

-S:."

IlJ rI

Ill r"

.N

~_ 1.10 o >rn .-~_ vl cQ/ 4-r-1,02

E

2.82

283

284

2.85

I

]

I

1

I

•w

1o 20 3'0 ,:o so go

70

~o 90

8 (degrees)

2.86

E (keV)

Fig.

S.

Angle

dependent

XANES

Fig. 6. I n t e n s i t y o f t h e w h i t e l i n e s A in Pd LII I

s p e c t r a a t t h e CI K e d g e o f t h e H O P G

(upper

sample. The inset shows the defini-

X A N E S s p e c t r a a s a f u n c t i o n o f O.

t i o n o f t h e t i l t i n g a n g l e O.

half)

and

B

in

Ci

K

(lower

half)

204

Fig. 7. Geometrical r e l a t i o n s h i p s b e t w e e n t h e angle b e t w e e n t h e -~ axis o f t h e s a m p l e and t h e final s t a t e o r b i t a l s and t h e angle ~ b e t w e e n t h e plane of t h e i n t e r c a l a t e chain and t h e g r a p h i t e layers. p r i s t i n e PdCI 2 [1]. It s h o u l d be m e n t i o n e d t h a t t h e r a t h e r large s c a t t e r i n g of t h e data p r e s e n t e d in Fig. 6 is c o n n e c t e d t o t h e s m a l l n e s s o f t h e e f f e c t in t h e region of ~ values

of about

54.7 °

it f /

/

;

(the

s o - c a l l e d "magic angle"). On t h e o t h e r h a n d t h e r e s u l t s o b t a i n e d are r a t h e r insensitive [12] t o this s c a t t e r i n g a n d also t o t h e degree of p o l a r i z a t i o n o f t h e primary b e a m (the p o l a r i z a t i o n f a c t o r P [13] has been a s s u m e d t o be 0.9 in t h i s work). ACKNOWLEDGEMENTS W e are i n d e b t e d t o g o l f Pilfer and Karin Lochte for t h e i r a s s i s t a n c e during t h e e x p e r i m e n t s . T h a n k s are also due t o t h e HASYLAB staff, especially Ronald Frahm. One o f us (P.B.) a c k n o w l e d g e s s u p p o r t f r o m t h e S t u d i e n s t i f t u n g des D e u t s c h e n Volkes. REFERENCES 1

P. Behrens and W. Metz, Synth. Met. 34, (1989) 223.

2

J.R. Soulen and W.H. Chappell Jr., J. Phys. Chem., 69 (1985) 3669.

3

H. Sch~ifer, U. Wiese, K. Rinke and K. Brendel, Angew. Chem., 79 (1967) 244. Angew. Chem. Int. Ed., _6 (1967) 92.

4 5

A.F. Wells, Z__~.Krist., 10_Q0(1939) 189. Y.N. Novikov, V.A. Postnikov, Y.V. Salyn, V.L. Nefedov, M.E. Vol'pin, Izv. Akad. Nauk SSSR, Set. Khim. (1973) 1689.

6

M.B. Dowell and D.S. Badorrek, Carbon, 16 (1978) 241.

7

J.A. W o o l l a m , E.J. Haugland, M.B. Dowell, A. Yavronian, A.G. Lozier and G. Matulka, Synth. Met. 23, (1988) 139.

8

P. Behrens, V. W o e b s , K. Jopp and W. Metz, Carbon, 2_-6 (1988) 641.

9

W. Malzfeldt, W. Niemann, R. Haensel and P. Rabe, Nucl. Instr. Meth., 2088 (1985) 359.

10

P.A. Lee, P.H. Citrin, P. E i s e n b e r g e r and B.M. Kincaid, Rev. Mod. Phys., 53 (1981) 769.

11

P. Behrens, K. Lochte, W. Metz and W. Niemann, in D. Gu6rard and P. Lagrange (eds.), I n t e r n a t i o n a l S y m p o s i u m on_ Layered C o m p o u n d s , P o n t - h - M o u s s o n , France, 8-10 Mar. 1988, p. 255.

12 W. Niemann, J. Ehrich, W. Metz and P. Behrens, t o be published. 13

J. S t 6 h r a n d D. Outka, Phys. Rev., B36 (1987) 7891.