LATTICE DYNAMICS OF GRAPHITE-IRON CHLORIDE INTERCALATION COMPOUNDS FROM MÖSSBAUER SPECTROSCOPY

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LATTICE DYNAMICS OF GRAPHITE-IRON

CHLORIDE INTERCALATION COMPOUNDS FROM MÖSSBAUER SPECTROSCOPY

M. Katada, R. Herber

To cite this version:

M. Katada, R. Herber. LATTICE DYNAMICS OF GRAPHITE-IRON CHLORIDE INTERCA-

LATION COMPOUNDS FROM MÖSSBAUER SPECTROSCOPY. Journal de Physique Colloques,

1979, 40 (C2), pp.C2-663-C2-665. �10.1051/jphyscol:19792230�. �jpa-00218612�

JOURNAL DE PHYSIQUE Colloque C2, suppl6ment au n o 3, Tome 40, mars 1979, page C2-663

L A T T I C E DYNAMICS O F G R A P H I T E - I RON C H L O R I D E I N T E R C A L A T I O N COMPOUNDS FROM MOSSBAUER SPECTROSCOPY

M. ~atada'and R.H. Herber

Department of Chemistry, Rutgers U n i v e r s i t y , New B m s w i c k , New J e r s e y 08903, U . S . A .

R6surnb.- Les spectres Mijssbauer des composds lamellaires, graphite-FeC13 et graphite-FeC12, sont me- sur6s entre 4,2 et 320 K. La temp6rature de rdseau Mijssbauer,

eM

obtenue par la variation de la frac- tion d'effet sans recul en fonction de la tempbrature, est plus faible que celle des chlorures de fer correspondants. Les rgsultats suggPrent que les atomes de fer dans les compos6s lamellaires sont plus labiles que dans les chlorures.

Abstract.- Mijssbauer spectra of graphite-FeC13 and graphite-FeC12 intercalation compounds have been measured over the temperature range 4.2 5 T 1 3 2 0 K. The MEssbauer lattice temperature, 8, obtained from the temperature dependence of recoil-free fraction for the intercalates were smaller than those of the corresponding neat iron chlorides. This result suggests that the iron atoms in the intercala- tion compounds are more labile than that in the corresponding iron chloride.

A n m b e r of graphite intercalation compounds are known and have been studied by a variety of physicochemical techniques /1,2/. For the cases where intercalation compounds contain one or more appropriate atoms, the Gssbauer effect can be used to elucidate chemical bonding, structure and lattice dynamical properties in these compounds 13-71.

In the present investigation, Gssbauer effect data have been obtained for graphite-FeClg and gra- phite-FeC12 intercalation compounds over the tempe- rature range 4.2 T <_ 320 K.

The starting matrix material was highly orien- ted pyrolytic graphite obtained from Union Carbide Corp. and was used without further purification. The

1st stage FeC13-graphite compound was prepared by a procedure similar to that described in the literatu- re 181. Chemical analysis of two samples gave the composition of 57.3% and 59.0% FeC13, respectively, corresponding to the formula C~oFeCla (57.5%). The 1st stage graphite-FeC12 compound (49.4% FeC12) was obtained by reduction of the 1st stage graphite- FeC13 compound at 375'~ in a hydrogen flow for 5 hours. Spectrometer calibration was effected using NBS standard reference material iron foil and the hyperfine interaction parameters reported in the li- terature 191.

The MEssbauer data for graphite-FeC13 and gra- phite-FeC12 intercalation compounds are summarized in table I. Typical MEssbauer spectra of the gra- phite-FeC13 compound are shown in figure 1 , while

.L

Present Address : Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Setagaya- Ku, Tokyo 158, Japan.

the corresponding spectra of the graphite-FeC12 compound are shown in figure 2. In figure I, in the spectrum at 78 K can be observed a very small con- tribution of a high spin ~ e doublet with one com- ~ + ponent of the doublet overlapping the main peak.

l . l . l . I , 1 . 1 . 1 , 1 . 1 . 1 . 1 . 1 . 1

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 Velocity , m m / s

Fig. 1 : FeC13-graphite Mijssbauer spectra at

(a) 300 and (b) 78 K. The difference in the ordinate scales reflects the temperature dependence of the recoil-free fraction of the Fe(II1) species. The small resonance peak at an isomer shift of 1.75 w s-' is due to an Fe(I1) impurity.

Graphite-FeC13 intercalate : The isomer shift of the graphite-FeC13 intercalation compound at 300 K is observed to be slightly more positive than that of anhydrous FeC13. This result may be accounted for partial transfer of the sr electrons of the graphite to the unoccupied iron d orbitals. The absence of a quadrupole splitting in the spectra of the graphi- te-FeC13 compound indicates that the electric charge distribution around iron is not changed by the for-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792230

JOURNAL DE PHYSIQUE

mation of the intercalation compound.

Table I : Summary of MEssbauer data for anhydrous FeC13 and FeC12 and FeCl3 graphite intercalation compounds.

j

Isomer shift*

j

dIS(dT j ~ u a d r u ~ o l e splittini dlr.~:~~)/d~ OM

Compound mm s-I

:

mm s- deg-' ¹ I ¹ I I

1 I 1 1 I

:

^{78 K 300 K}

:

^{78 K 300 K}

:

^{deg-' deg}

I I 1 1 I

I I 1 I I

Graphite-FeC1, 0.53 0.49

:

^{-5.18 x}

^{lob4 ;} :

-5.08 x 10-

:

¹⁶⁴

+

⁵

I 1 I 1 f

Graphi te-FeC12

:

I ^{1 7} I ¹

^,

Fe(I)

_*

1.23 1.09

;

-6.56 x

' o l :

^{1.11 0.79}

i

^{-5.17 x}

1

¹⁶²

+

⁵

1 I I 1

Fe(O) 1.23 1.09

;

^{-6.68 x}

:

^{1.96 1.60}

:

^{-4.67 x}

:

¹⁷¹

+

⁵

1 I I I 1

FeC13

:

^{0.55 0.43}

:

^{-7.28 x}

lo-' : :

^{-2.68 x}

:

²²⁵

+

⁵

(anhydride) I I I 1 I

I 1 I I I

I

,

^{I 1 1}

%ith respect to a-Fe.

The temperature dependence of the recoil-free The decrease in the

OM

for the graphite-FeC13 fraction of the absorber is given from the Debye mo- compound compared with that of anhydrous FeC13 can del of solids /lo/ by an expression of the form be explained by postulating that the iron ion pac- dlnf

-

6 E ~

-

- OM

( T _ c T ) king in the intercalation compound is looser than dT

k0; that in anhydrous FeC13, and executes a large vibra-

tional motion at a given temperature.

where ER and k are the recoil energy of the free Graphite-FeC12 intercalate : The Gssbauer spectra molecule and ~oltzmann constant, respectively, and of graphite-FeC12 indicate the presence of two dis- OM is the assbauer temperature of the lattice. tinct iron sites, referred to as Fe(1) and Fe(0) in the subsequent discussion. The isomer shifts of both

Fig. 2 : FeC1,-graphite Mossbauer spectra at (a) 300 and (b) 78 K. The difference in the ordinate scales reflects the temperature dependence of the recoil- free fraction of the Fe(I1) species. The presence of two distinct Fe(I1) sites as well as the aniso- tropy of the recoil-free fraction is clearly discer- nable by a comparison of two spectra.

sites are the same and are similar to that of anhy- drous FeC12 /Ill. This result indicates the absence of significant transfer of n electrons of the gra- phite matrix to the iron d orbitals as was observed in the graphite-FeC13 case. Thus there exists at best only a very weak bonding interaction between

the graphite lattice and FeC12 in the stage one com- pound. h e smaller values of OM for Fe(1) and Fe(0) sites compared with that of anhydrous FeC12 1121 also support above result. The quadrupole splittings of the two sites, however, are very different. Its value for site Fe(1) is about the same as that of anhydrous FeC12, while that of the other site, Fe(O), is approximately twice as large as that of ferrous chloride /II/. Therefore, the local structure around Fe(1) is probably almost the same as that of anhy- drous FeC12, while that of Fe(0) site is signifi- cantly different.

Angular dependence of f for the graphite-FeC12 in- tercalate : The Angular dependence of the intensity ratio, R=I(x)/I(a) for the Fe(1) and Fe(0) sites are represented graphically in figure 3. In the case of the Fe(1) site, the values of R obtained experi- mentally are smaller than those calculated for the ideal case. This deviation from the ideal value may be due to the non-identity of the crystallographic symmetry axis and the principal axis of the electric

field gradient at the " ~ e nucleus and/or a polari- zation effect in the Msssbauer absorption by single crystals such as those reported by Housley et al.

/13/. The area ratio of the Fe(0) site is not depen- dent on the observational angle 8, implying that the Fe(0) site is oriented randomly.

-10 0 02030435083

@ In Dogroes

Fig. 3 : Intensity ratio R = I(n)/I(a) for the Fe(1) and Fe(0) sites in a single crystal sample of the FeCln-graphite intercalation compound at 295

+

^{2 K.}

The angle 8 refers to the orientation of the optical (gamma r a ~ - ~ r o ~ a ~ a t i o n ) axis with respect to the crystallographic c axis. The open and closed circles represent the experimental data for the Fe(1) and Fe(0) sites, respectively, and the full line is the theoretical ratio.

Acknowledgements.- This research was supported in part by the National Science Foundation under grant DMR-7600139 and by a grant from the Rutgers Center for Computer and Information Services. This support is herewith gratefully acknowledged. The authors

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are also grateful to Dr. A.W. Moore for providing the pyrolytic graphite.