MÖSSBAUER STUDY ON INTERCALATION COMPOUNDS OF GRAPHITE WITH FeCl3-x

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JOURNAL DE PHYSIQUE Colloque Cl, supplément au n" 3, Tome 40, mars 1979, page C2-269

M8SSBAUER STUDY ON INTERCALATION COMPOUNDS OF GRAPHITE WITH F e C l , 3-x K. Ohhashi, T. Shinjo , T. Takada and I. Tsujikawa

Kyoto College of Pharmacy, Xamashina, Kyoto, Japan

* Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu, Japan

**Department of Chemistry, Faculty of Science, Kyoto University, Kyoto, Japan

Résumé.- On a étudié par spectrométrie Mossbauer des composés interfolliaires de graphite G-FeCl (0 ^ x-v ! ) • L'emploi de feuille de Grafoil entraîne un effet de surface beaucoup plus important3que celui attendu d'un graphite massif; il apparaît en plus des états F e2 +, dont la concentration augmen- te la température de Néel de G-FeCl .

Abstract.- A Mossbauer study on intercalation compounds of graphite with iron chloride, G-FeCl3_x

(0 ^ x •$ 1) was reported. The use of Grafoil sheets as host graphite led to a surface effect to be more important than the use of bulk graphite, i.e. the appearence of an additional Fe + form. It was found that the Neel temperature regarded as for G-FeCl3 became higher with increase of F e2 + concentration.

1. Introduction.- Properties of intercalation compounds of graphite with FeCl3 or FeCl2 have been extensively investigated /1,2/. Nevertheless, their essentials have not been made clear, namely, the oxidation state of iron, the arrangement of inter- calated species FeCl3 and FeCl2 and the nature of bonding between carbon and iron chloride layers. In the case of G-FeCl3, Mossbauer experiments suggested an Fe2 +/Fe3 + ratio of less than 0.05 /3,4/, on the other hand, previous Hall data indicated Fe2 +/Fe3 +

to be 0.33 /5/. In the case of G-FeCl2, there were at least two F e^{2 +} forms; the number of the forms will depend on the methods of reduction of original G-FeCl3, on the kinds of host graphite, and on other experimental conditions. When the compounds with FeCl2 were obtained by heating G-FeCl , prepared from bulk graphite, in flowing N2 gas, they contai- ned two forms of Fe ; the form Fe2+-1 (IS relative to Fe metal; IS=1.lmm/s, QS=0.8mra/s at RT), identi- cal with the form F e2 + of bulk FeCl , and the form

2

Fe2+-2 (IS-1. lmm/s, QS=1.6mm/s) M / . On the other hand, Pritzlaff-Stahl recently found an additional form Fe2+-3 (IS.= 1.2mm/s, QS=2.3mm/s) for G-FeCl with high iron content of C^^FeClj /6/. Shechter et al. reported a Mossbauer study of adsorbed FeCl2

monolayers.on Grafoil and suggested the existence of at least two forms of FeCl2 and the magnitude of quadrupole splittings of the forms dependent on adsorbed quantity of FeCl2 / 7 / .

With regard to the above, we report here the observation of hyperfine components split by inter- nal fields, H , for F e2 + forms in G-FeCl,_x at He

n •*

temperatures.

2. Experimental.- Several samples of the compounds with varying composition C FeCl._„ were prepared as

n 3 x

follows, where the values of n were determined by measuring the increase of the weight of dried prepared samples from that of the host Grafoil (grade GTA, 1.5 x 1.5cm2 x 0.13mm thick), and the values of x were determined by estimating F e2 + pattern area of RT spectra. C1 2 8FeCl3 (x -f 0.05, sample-a) was synthesized by direct reaction of FeCl3 with Grafoil at 300°C during 5 days in an evacuated glass tube.

The tube was then opened and excess anhydride was washed off with dilute HC1 solution. C „FeCl „„

9.6 2-32 (x = 0.68, sample-b) was prepared by the same method as for the sample-a except the reacting temperature of 310°C. C50FeCl2 (x 5.0.95, sample-c) was made by heating sample-b further at 600°C in flowing N, gas, but this process led to some amount of loss of iron chloride from the graphite compound.

A Mossbauer effect analyzer in combination with a multichannel analyzer and a source 57Co in Cu were used. As a reference, iron foil at RT was used. Throughout all measurements the direction of the y r a y propagation was perpendicular to the basal plane of the graphite compounds.

3. Results and discussion.- Typical RT Mossbauer spectra for some of the compounds are shown in fi- gure 1. The spectrum of the compound (x « 1.0) shows six absorption lines, which can be classified to three doublets which correspond to three different kinds of F e2 + forms (-1, -2 and - 3 ) . The inner two doublets are in good agreement with those of G-FeCl prepared from bulk graphite /h/. We find the additional form Fe2+-3 whose parameters (IS and QS) are

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979295

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C2-270 JOURNAL DE PHYSIQUE

t h e same a s t h o s e n o t o n l y f o r C,.,FeC12 o b t a i n e d by P r i t z l a f f - S t a h l / 6 / b u t a l s o f o r a d s o r b e d FeC12 monolayer on G r a f o i l w i t h coverage 0.2-0.5 / 7 / . I n a d d i t i o n , t h e i n t e n s i t y r a t i o I+/I- f o r t h e l a t t e r i s a l m o s t i d e n t i c a l t o t h a t of t h e form Fe2+-3.

Fez'- 3

LL-2-I

^{-8 - 6} ^-4 ^{- 2} ⁰² ⁴ ⁶ ⁸

Velocity ( m m / s

1

F i g . 1 : Mijssbauer s p e c t r a a t room t e m p e r a t u r e of CnFeCl,-, ( a ) n=12.8, xzO, sample-a; (b) n=9.6, x=0.68, sample-b and ( c ) n=50, x= I , sample-c.

The s p e c t r a of t h e s e compounds a t 4.2 K and 1.5 K a r e shown i n f i g u r e s 2 and 3 , r e s p e c t i v e l y . The Ndel t e m p e r a t u r e Tn of G-FeC1 was below 4.2 K

(3.6-3.9 K / 4 / ) which was e a s i l y s e e n i n comparison of f i g u r e s 2 a and 3a.

l ~ l l l t ~ ~ ~ t

- 8 - 6

- 4 - 2 0 _Velocity

2

4 _(mm6 8 _1s)

'

Fig. 2 : Mijssbauer s p e c t r a a t 4.2 K of t h e same samples a s i n f i g u r e 1.

F i g . 3 : Mzssbauer s p e c t r a a t 1.5 K o f t h e same sam- p l e s a s i n f i g u r e s 1 and 2 and t h e s t i c k diagrams

c a l c u l a t e d u s i n g t h e p a r a m e t e r s g i v e n i n t a b l e I.

It i s noted t h a t t h e s p e c t r u m of t h e compound ( x = 0.68) c o n s i s t e d of Fe3+ h y p e r f i n e components s p l i t by an i n t e r n a l f i e l d (Hn = 490 kOe), Fe3+ cen- t r a l l i n e and Fe2+ components a s shown i n f i g u r e 2b.

The a p p e a r e n c e of Fe3+ h y p e r f i n e p a t t e r n a t 4.2 K s u g g e s t s t h a t Tn r e g a r d e d a s f o r G-FeC13 became h i g h e r w i t h i n c r e a s e of Fez+ c o n c e n t r a t i o n . It can b e e x p l a i n e d by t h a t most o f Fe3+ and F e z + c o n t e n t s

c o e x i s t e d i n t h e same i r o n c h l o r i d e l a y e r and t h e a n t i f e r r o m a g n e t i c i n t e r a c t i o n between Fe3+ and Fe2*

i s l a r g e r t h a n t h a t between ~ e and Fe ~ +3* a s found i n b u l k FeC1,:< 1 % FeC1, /8/. The d e c r e a s e of t h e i n t e n s i t y of F e 3 + c e n t r a l l i n e a t 1.5 K can be i n - t e r p r e t e d by t h e d i m i n u t i o n of superparamagnetism.

I n t h e c a s e of G-FeCIZ ( x z 1 . 0 ) , t h e d i s t i n c t d i f f e - r e n c e s among t h e s p e c t r a a t RT, 4.2 K and 1.5 Kwere found. We c a l c u l a t e d t h e i n t e n s i t y and p o s i t i o n of s p l i t components o f t h e s p e c t r a a t 1.5 K by t a k i n g t h e f o l l o w i n g assumptions; t h e r e l a t i v e c o n c e n t r a - t i o n r a t i o of t h e forms ~ e ' + - l , 2 and 3 i s k e p t t o be n e a r l y 2 : 1 : 1 which i s t h e same a s a t RT, and t h e p a t t e r n s of forms Fez+-1 and 2 a r e t h e same a s t h o s e f o r G-FeC12 p r e p a r e d from b u l k g r a p h i t e / 4 / . The v a l u e s of I S , Hn,

.

1 / 2 e Z q Z q , asymmetry parameter, q, t h e a n g l e s p e c i f y i n g t h e d i r e c t i o n of H n w i t h r e s p e c t t o t h e EFG, BE, and t h e d i r e c t i o n of EFG,Z, a r e l i s t e d i n t a b l e I. The s t i c k diagrams c a l c u l a - t e d by u s e of t h e s e p a r a m e t e r s a r e i n good a g r e e m n t w i t h t h e o b s e r v a t i o n a s shown i n f i g u r e 3c.

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T a b l e I : ~ E s s b a u e r p a r a m e t e r s of CnFeC13-x a t 1.5 K.

IS

i

^H I i / 2 e Z q z ~ i _g

eH I

z

i

^(mm/s)

i

^(kOe)

i

(mm/s)

i i

("1

i

j F e 3 + - h

i

^{0 . 6 3}

j

490 : + 0 . 2 5

i

⁰

i

90

i

¹¹ c - a x i s

i F e 2 + - 1 1.20

i

10 i + 1 . 3 5

i

⁰

j

0 11

i

~ e ' + - 2

i

1.20

i

50 i - 2 . 3 0

i

⁰

i

0

i

I (

~ e ' + - 3

i

^1.25 ²⁶⁰

1

⁺^2.44

i

^{0 . 2} ⁹⁰ ¹c - a x i s

i

R e f e r e n c e s

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5

(1976) 181.

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,

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5

(1968) 681.

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Japan

36

(1974) 980 and

37

(1974) 63.

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79 (1957) 1051.

-

/ 6 / P r i t z l a f f , B. and S t a h l , H . , C a r b o n s (1977) 399 / 7 / S h e c h t e r , H. e t a l . , Phys. Rev.

B14

(1976) 1876.

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15

(1965) 854.