Chemical reduction of stage-1 FeCl3 graphite intercalation compound with n-butyl lithium : a structural, Mössbauer and magnetic study

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intercalation compound with n-butyl lithium : a

structural, Mössbauer and magnetic study

C. Meyer, R. Yazami, G. Chouteau

To cite this version:

Chemical reduction of

_{stage-1 FeCl3 graphite}

intercalation

compound

with

_n-butyl

lithium :

a

_structural,

Mössbauer and

magnetic study

C.

_Meyer

_(1),

R. Yazami

₍₂₎

and G. Chouteau

₍₃₎

(1)

Laboratoire de

_{Spectrométrie Physique (*),}

Université

Joseph

Fourier, Grenoble 1, B.P. 87,

38402 St Martin d’Hères Cedex, France

(2)

Laboratoire

d’Ionique

et d’Electrochimie du Solide de Grenoble

(**),

ENSEEG, Institut National

_{Polytechnique}

de _{Grenoble, B.P. 75,} 38402 St Martin d’Hères Cedex, France

(3)

Service National des

_Champs

Intenses

_(***),

CNRS, B.P. 166, 38042 Grenoble _Cedex, France

(Reçu

le 2 octobre 1989, révisé le 30 _janvier 1990,

accepté

le 1 er

_{février 1990)}

Résumé. 2014 La réduction

_progressive

du

_composé

d’intercalation du

_graphite

de

_premier

stade

C14FeCl3,04

par une solution de

n-butyl

lithium dans l’hexane à 60 °

C conduit à la formation des

composés C14FeCl3-x(LiCl)x

avec x = 0, 1, 2, 3. On observe une diminution de la valence du fer de 3 à 0 avec x. Le

_composé

final est

_{ferromagnétique}

à la

température

ordinaire. Il est constitué de

_plans

contenant essentiellement du

Fe0

formant un

_composé

de

_premier

stade, en

présence

de

Fe3+

et de

Fe2+.

Un _comportement de verre de

_{spin ferromagnétique}

est observé entre 4,2 et 300 K pour x = 2 _{et x} = 3.

Abstract. 2014

Progressive

reduction of the

_{stage-1 graphite}

intercalation

_{compound, (GIC),}

C14FeCl3.04

using

a solution of

n-butyllithium

into hexane at 60 °C leads to the formation of zero valent iron

_layers

of _stage 1

_containing

small amounts of

Fe3+

and Fe2+. The reduced

_compounds

have the average formula :

C14FeCl3-x(LiCl)x

with x = 0, 1, 2, 3. For x = 2 and 3 the

compounds

behave as

_{ferro-spin glasses}

with a

_freezing

_temperature around 300 K.

Classification

Physics

Abstracts 75.30C - 76.80

Introduction.

Graphite easily

forms intercalation

_compounds

with most alkali or alkali earth metals and

with some rare earth metals

_by

a direct reaction

_using

the _vapor

_transport

_technique

_[1].

However,

this method cannot be

_applied

to transition metals because of their _very low

volatility. Only

indirect

_syntheses

have been

_{tempted involving}

a chemical

_[2-7]

or an

electrochemical

_{[7, 8]}

reduction : the transition metal cation is

preliminarily

intercalated into

(*)

Laboratoire associé au CNRS.

(*.*)

URA-CNRS D 1213.

(***)

Laboratoire associé à l’Université

_Joseph

_Fourier, Grenoble.

graphite

(as

chloride

_{[2-5, 7]}

or oxide

_[6])

and then reduced to lower

_valency

states

_including

the zero one.

The main difficulties

arising

from the indirect methods are first of all the

_{incompleteness}

of

the reaction due to the

_{inhomogeneous}

reactive diffusion within the

_{graphite layers and/or}

secondly

the

_difficulty

of

_maintaining

the two dimensional

_(2D)

character of the structure.

When one uses _gas as a reductive

_reagent

at

_high

_{temperatures,}

_{(vapor phase}

with alkali

metals

[7]

or

_{hydrogen [2]),}

the

_strong

_tendency

to form three dimensional

_(3D)

clusters rather than

_layered

domains results from the

_increasing

_{metastability}

of the 2D state with the reaction

temperature.

With

_liquids

at low

_temperature

the solvent is

_{generally polar}

and

partially

dissolves the transition cation. The reduction reaction tends to occur at the

_graphite

surface.

Thus,

the

_resulting

_compounds

contain

_high

amounts of 3D clusters.

_They

are

regarded

as inclusion

_compounds

rather than intercalation ones.

We

_{recently suggested}

a new route to achieve the in situ reduction of transition metal chlorides GIC’s

_by

_{using n-butyl}

lithium dissolved in hexane at room

_temperature

_[9].

In the case of

_FeCl3-GIC,

X-ray

diffraction and Môssbauer

_experiments

demonstrated the

_efficiency

of this method in

_producing

zero valent state metal

_forming

2D domains

_regularly

stacked

along

the c-axis.

_However,

the Môssbauer

_experiments

_{evidenced the presence of}

Fe3 +

ions even after

_using

an excess

of n-butyl lithium

solution

[10].

In

addition,

reduction occurs in two

_{steps :}

Fe3 +

-->

Fe2 +

---*

Fe°

by increasing

the amount of

n-butyl

lithium. We

suggested

a

simple

model in which the reduction reaction

begins

at the border of the domains and

propagates

towards the core.

_Thus,

each domain is made of 3+ ions surrounded

by

2+ and zero valent ones.

An

_improvement

of the reduction method can be

expected

if the reaction

temperature

and duration are increased.

This paper deals with the observation of the

progressive

reduction of a

_stage-1

FeCl3-GIC,

(FeCl3/graphite/FeCl3/graphite...

stacking

sequence

along

the

c-axis), by

a

titrated solution of

n-butyl

lithium at 60 ° C

_during

more than two months. The Môssbauer

experiments,

the

_magnetic

_properties

and the structure

_changes

induced

_by

the reduction are

analysed

within the same model.

Results and discussion.

1. PREPARATION AND STRUCTURE. - Details of the

_preparation

conditions have been

_given

elsewhere

_{[9]. They}

can be summarized as follows :

_{100 mg}

of fine

powder (= 10 itm )

of

stage-1

FeCl3-GIC

(average

formula,

Cl4FeC13)

were treated with

n-butyl lithium

in solution

in hexane

_[9].

A well defined amount of intercalated

lithium, x

=

1,

2 _or

3,

is obtained. The ideal formula is

_Cl4LixFeCl3.

The reaction tube is sealed under

_{high purity}

water free argon

atmosphere

and maintained at 60 ± 1 ° C

_during

10 weeks. The

samples

were

regularly

stirred

in order to achieve a

_{good homogeneity.}

The

_{liquid phase}

was then eliminated

_through

centrifugation

and the unreacted lithium was

_{analysed [9].}

The

_remaining

solid

_powder

was

repeatedly

washed with hexane and methanol and

_finally

dried under vacuum at 100 ° C. An

X-ray

diffraction

_{analysis (Debye-Scherrer)}

was then

_performed

on the

_{resulting compound.}

The chemical

_analysis

shows that the unreacted

_part

of

_butyl

lithium does not exceed 5 % the initial

_quantity

introduced into the reaction tube. This indicates that within the

experimental

accuracy the intercalation into the

FeC13-GIC

has a _very

_{high efficiency}

even

though

the

_possibility

of a

_{partial oxydization}

of

_butyl

lithium cannot be excluded. As we have shown in a

_{previous study [10],}

the

parent

structure of

_{Cl4FeCl3 completely disappears only}

for a lithium concentration x

_greater

than 2. At this concentration a new

_phase

_appears with a

Table 1. -

Debye-Scherrer

pattern indexation

of

the most reduced

_compound

with

x = 3. The iron _atoms

arrange in a

hexagonal

lattice incommensurate with that

of graphite

leading

to a

_stage-1

GIC

_(G-Fe).

Pristine

_graphite

appears as a _consequence

of

the decrease

of

the domain size. Three-dimensional LiCl is

_encapsulated

between the

_{graphene layers.}

Graphite-iron

x = 3

(hexagonal,

a = 4.15

Â,

c = 10.88

Â)

S : _{strong ;} m = medium ; w : _{weak ;} vw : very weak

This

_phase

was ascribed to a

_{stage-1 Fe°-GIC}

in which the

Fe°

domains have a

_regular

crystalline

structure

_along

the c-axis and in the

_{(ab) plane.}

In table 1 we

_report

the indexation of the

_X-ray

powder diagram

lines for the most reduced

compound (x

=

3).

Besides the characteristic lines of

graphite

and three dimensional

LiCI,

the

_major

_part

of the lines can be attributed to the

_stage-1

Fe°-GIC.

The deduced structure is

interpreted

within the

following

model : the iron atoms _{arrange in} a

hexagonal

lattice with

a = 0.415 nm and c = 1.088 nm. Within this model the

00f

lines

with f

and odd number

should not be

_present.

The

_possibility

of

_attributing

the distances d = 0.367 nm and

d = 0.1546 nm to 003 and 007 lines results from the

_{inhomogeneous interplane}

Fe

_density

and/or

from the existence of

_stacking

defects of the Fe domains. It should be

_emphasized

that the calculated Fe-Fe distance in this

_hexagonal

iron

_(0.2396

_nm)

with the coordination number n = 3 is smaller than that of a-iron

(0.2482

_{nm, n} =

8)

and that of

y-iron

(0.2545

_nm,

n =

12)

[11],

as

_expected.

The iron lattice is not commensurate with the

_graphite

lattice. This is the indication of _strong correlations between the iron atoms and shows that the iron

_layers

should be stabilized

_through

metallic-like

_bondings.

Our model differs from the one of

Vol’pin et

al.

_[5]

who described the structure with a double

_layer

of

Fe°

coordinated with the

graphene

hexagones,

and also from the model of Touzain et al.

_[7]

who

_proposed

a

quadratic

cell.

Four different

_samples

were

_prepared

with x =

0, 1, 2

and 3.

They

_were

hermetically

sealed

under argon

atmosphere

in

_appropriate

holders and used for the Môssbauer and

_magnetic

2. MÔSSBAUER SPECTROSCOPY. - The

_driving

idea was to check the Fe state at different reduction

steps,

that is for the different amounts of lithium

_(x

=

Li/Fe)

intercalated into the

parent

stage-1

FeCl3-GIC,

with the theoretical values x =

0, 1,

2 and 3. The Môssbauer

spectra

were measured as a function of

_temperature

in the range 4.2-300 K with a

conventional

_apparatus

_using

a

57Co

source in Rh.

Figure

1

_represents

the

_spectra

obtained for the four different values of x, at room

temperature

and 4.2 K. The

_analysis

was done with an usual least _square fit

_procedure,

making

use of lorentzian

_shaped

line. The Môssbauer

_parameters

of the

_subspectra

are

reported

in tables

_{III, IV,}

V and VI.

_They

are :

i)

the Isomer Shift 6

_(information

about the Fe

_valency),

with a

_typical

error of 0.01

_mm/s,

ii)

the

_{Quadrupole Splittingà}

=

e 2 qQ /2

J 1

_{+ 11}

2/3

reflects the

_symmetry

of the Fe site

through

the Electric Field Gradient

_(EFG)

parameters :

eq =

Vzz,

main EFG

_component,

11 =

(Vxx -

Vyy)/Vzz

_{asymmetry parameter ;}

typical

_{error :} 0.02

mm/s,

iii)

the

_magnetic

_hyperfine

field

H,

proportional

to the iron

_magnetic

moment

_(at

least in the case of

Fe3 +

and

Fe°) ;

_typical

error : 0.1

_T,

iv)

the

_angle,

0,

between Vzz

_(local

_symmetry

_axis)

and H

_through

the measured

quadrupole

parameter

in the presence of the

hyperfine

field:

_{2 e = à (3 cos 2 0 -1)/2.}

Experimentally,

we never had access to this

_angle

because we

_always

measured a zero mean

quadrupole

parameter

2 e = 0 in the

magnetic

sextets. There are two

_possible

reasons for

that : a cubic

_symmetry

of the Fe site

(Li

=

0)

or a random

_angular

distribution of H versus

Vzz( 3

cos2 e -

_{1 >}

= 0)

due to a

_{spin glass-like freezing}

for instance.

The line width

_{r /2}

can reflect eventual

_broadenings

due to disorder

_{(distribution}

in

d,

H and

_0).

_{Experimentally, hyperfine}

field distributions have been observed for all

magnetic hyperfine

structures. As an

_{approximation,}

it was taken into account in the fit with three different line widths for the six-line

_pattern.

H denotes the mean

hyperfine

field

(typical

error : 0.5

_T).

Finally,

the relative amount of the different Fe

_species

_present

in the

samples

was

determined with the

_spectral

area ratios

(typical

error : 2

%).

The final values

reported

in

figure

9 are taken from the

_spectra

at 4.2 K because the resolution is much better than at

room

_temperature.

In

_addition,

for an easier

_comparison,

we have mentioned in table II the

hyperfine

parameters

for several reference

_{compounds :}

a-Fe,

FeCl3

[12],

FeCl2 [13], FeCl3-GIC [14]

and

_FeC12-GIC

_[15].

x = 0 : The

spectrum

is

quite

in

agreement

with those obtained in a number of

_previous

studies

_{[14-16]. According}

to Prietsch et al.

[14],

the weak electronic

_charge

transfer from the

graphene layers

to the intercalant

(-

0.2 _{electron per}

_FeCI3)

is characterized

_by

a

_thermally

activated electron

_hopping

between iron sites. This

results,

in the static

_limit,

in the _presence of 20 %

Fe2 +

among the

Fe3 +

sites. For the Môssbauer time window these two

_valency

states

are

_only

observable at low

_temperatures

(slow

relaxation time

limit).

At

_higher

_temperatures

an _average

_spectral

line is observed because of the fast electronic relaxation. Our

_spectra

exhibit

_clearly

this situation : the

_{major paramagnetic}

line at room

_temperature

_splits

at 4.2 K into two well

_{separated subspectra,}

i.e. a broad

Fe3 +

line

_(indicating

the onset of

_magnetic

order at T 5 4.2

_K)

and a

Fe2 +

doublet. The

Fe2 +

amount is about 9 % of

FeCl3,

less than

what is obtained in reference

[14].

However,

the

_compound

of these authors was a

_stage-2

prepared

under a _very low chlorine pressure while ours is a

_{stage-1 prepared}

under a 2

Table II. -

57 Fe

Môssbauer

_hyperfine

parameters

for reference compounds :

a-Fe,

FeCI3, FeC’3-GIC, FeCI2,

and

_FeCI2-GIC.

(5 :

isomer

_shift

_referred

to a-Fe at room

temperature ; A :

_{quadrupole splitting ;}

H:

_{hyperfine field).}

case,

however,

an additional

Fe2 +

_quadrupole

doublet is observed in the whole

_temperature

range. This

parasitic

Fe2+

may lie at the surface of the

_graphite.

Indeed,

the

_hyperfine

parameters

seem to

_correspond

to the

_{hydrated compound FeCl2,}

4

_H20

which is

_probably

due to some surface water contamination. Chemical reduction eliminates this

_{parasitic phase.}

x = 1 : The

spectrum

is

_drastically

altered,

as can be seen in

_figure

1

₍₂₉₅

and 4.2

_K)

and

figure

2 which shows the more detailed

_temperature

_dependence.

The fit

_parameters

are

reported

in table IV. The most

_{representative}

_spectrum

is to be considered at 30 K because the

_{decomposition}

into three

_subspectra

is rather obvious. It is the

_{superposition}

of two

Fe2 + symmetrical

quadrupole

doublets of same isomer shift & = 1.2

mm/s

with 2l = 1.4 and

2.4

_mm/s,

and a

Fe3 +

broad

_{magnetic hyperfine}

_pattern

_(H

= 48

T).

When the

_temperature

is lowered the

Fe3 +

_components

_undergo

a

_magnetic

transition at

12 ± 2 K which can be observed on the

_spectrum

_by

the

_{progressive broadening}

and

asymmetry

of the main

Fe2+

doublet due to the presence of a small

hyperfine

field

(H

= 1

T).

These characteristics of the

Fe2+

_spectrum

correspond

rather well to the two

quadrupole

doublets of

_FeC12-GIC

[15]

and our

_hyperfine

_parameters

are

quite

similar to the

Fig.

l. - Môssbauer

spectra at room _temperature

_(left)

and 4.2 K

_(right)

for

CnLixPeC13

(x = 0, 1, 2,

3).

For

_temperatures

_higher

than 30 K the

_{decomposition}

is not _easy. The line width of the

Fe 3 ,magnetic

component

becomes

_larger

and

_larger

when the

_temperature

is raised. The

magnetic

structure

_{completely disappears}

above 60 K without

showing

the usual decrease of the

_hyperfine

field

_magnetic

transition,

but rather

_gives

rise to a curvature of the

_background

level. Around 80 K the appearance of a

_{corresponding Fe3 + paramagnetic}

_spectrum

can be characterized

_by

the occurence of a

_progressive

_asymmetry

in the

Fe2 +

doublets.

_However,

it is difficult to define a transition

point

for

Fe3 + .

Moreover the nature of this

_paramagnetic

component

is

_{questionable.}

The room

_{temperature spectrum}

for instance shown in the

figures

has been fitted

_by

Fig.

2. -

introduce an

_anisotropy

effect for the

Fe2+

doublets

leading

to a much more

_pronounced

inequality

of the two lines for the doublet 4 = 2

mm/s.

This could arise either from texture effects

(no

random orientation of the

_{graphite particles}

in the

_sample)

or from an

_anisotropic

resonance

_absorption

factor

_(anisotropy

of the nucleus mean

_squared

vibrational

_amplitudes

in the

lattice).

Table III.

_{- Hyperfine}

_parameters

_{for C,LixFeC13}

(x

=

0).

The first effect has to be

_rejected

because the

_asymmetry

should be maintained in the whole

temperature range, while the doublets are

_quite

_symmetrical

below 100 K. The second effect

(Goldanskii-Karyagin effect)

may exist and should decrease at lower

_{temperatures.}

Vi-brational

_anisotropy

has

_already

been observed for intercalated

_species

_[14]

because of the low

_{dimensionality,}

but not evidenced for

_{powdered FeCl2-GIC}

_[15].

_{Consequently,}

we must

admit that the extra

_component

(Fe3 + )

should be a doublet. However its isomer shift at room

temperature

is low for a trivalent iron chloride

_(à

= 0.2

_mm/s)

while its value at 4.2 K

(5

= 0.47

mm/s )

is in

_agreement

with that of

FeC13-GIC.

This feature will _occur for all values

of x.

As a first

_qualitative

_conclusion,

the

parent

FeCl3-GIC

has been

_mostly

reduced to

FeCl2-GIC by n-butyl

lithium. About 25 % of iron remains in the trivalent unreduced state. It is not identical to

_FeCl3

as far as the

_magnetic

behavior is concerned.

Finally,

at this

_step

of the reduction no

Fe°

is detected.

x = 2 : The first

appearance of

Fe°

is observed. The room

_temperature

Môssbauer

spectrum

exhibits a

_{magnetic hyperfine}

_pattern

with an isomer shift very close to zero

Table IV.

- Hyperfine

parameters

for

C,,LixFeC13

(x

=

1).

(*

denotes _parameters which are

_kept

constant in the

_fit

and

(referred

to

_a-Fe)

and a

_hyperfine

field of 32 T _(to be

_compared

with 33 T for

a-Fe).

From the

intensity

ratios at low

_temperatures

one can estimate that 35 % of the iron atoms are reduced

to

Fe°

inside the

_graphene

_layers.

Since we did not observe any

X-ray

lines characteristic of

a -Fe,

we conclude that this contribution is not due to bulk iron

_expelled

from

graphite.

In

addition,

the

_temperature

_{dependence of}

the

hyperfine

field and its saturation value

(H(O )

= 38

T)

are

_quite

different from thuse of bulk iron

(Fig.

3).

We find that 45 % of the iron is in the 2+

_valency

state and 20 % in thé _{3+ state,} characterized

_by

the

_hyperfine

field of 49 T at 4.2 K.

_Concerning

the evolution of the

_spectrum

as a function of

_temperature,

the

same features as in the case x = 1 are

_observed,

i.e. a broad «

_magnetic

transition »

_starting

from about 200 K due to

Fe3 +

and an

_ordering

transition of 10 K which we attribute to

Fe2 + .

Here

_again

the nature of the

Fe3 +

doublet raises some

_problem,

as the isomer shift at

room

_temperature

is

_quite

low as in the case x = 1.

In

_conclusion,

with x = 2 the

quantity

of unreduced

Fe3+

has not been

changed

with

respect

to x = 1 but the

proportion

of

Fe2 +

has been

considerably

decreased

showing

that the

production

of zero valent iron is made at the expense of the

FeCl2-GIC phase.

This is

_again

in

agreement

with the islandic

_picture

of the

_compound,

the surface of the islands

being

first reduced while their core remains unmodified.

x = 3 : The

Fe2 +

_component

is

considerably

reduced in the

_spectrum

(18 %

at room

temperature).

Besides this

feature,

the room

_{temperature spectrum}

exhibits two

_major

components :

i)

The

Fe°

_component,

_already

_present

for x =

2,

_{now accounts} for 34 % of total iron. This

only corresponds

to a

_slight

increase with

_respect

to the

_{preceding compound.}

The thermal evolution is identical. A small decrease of H with

_respect

to the case x = 2 is observed :

H = 31 T instead of 32 T.

Fig.

3. -

Hyperfine

field of the different

species

of iron versus _{temperature present} in the

_compound

Table V.

- Hyperfine

parameters

for CnLixFeCl3

(x

=

2).

ii)

A well defined

_quadrupole

doublet with 5 = 0.17

mm/s

and d = 0.6

mm/s

_represents

almost half of the total

_quantity

of iron

_{(48 %).}

It cannot be attributed to

Fe3 +

as was done for x = 1 and x = 2 because its behavior when the

_temperature

is lowered _to 4.2 K is

complex :

it

_splits

into the usual

Fe3 +

_sextuplet

with a

_hyperfine

field of 48 T

_{(corresponding}

to a concentration of 20 %

Fe3 + )

and a new

_type

of

Feo sextuplet

with H = 34

T,

5 = 0.17

_{mm/s representing}

20 % the total iron. It should be noted that these

_parameters

are

quite

similar to those of bulk iron. However the thermal variation of the

_hyperfine

field is characteristic of

_{superparamagnetic particles.}

The room

_temperature

doublet

_{disappears progressively}

below 60 K and transforms into the

_hyperfine

pattern.

At 4.2 K a small

_part

of the doublet remains

_{unsplit (corresponding}

to

7 % the total

_iron).

This is due to the

_particles

whose moment relaxes more

_rapidly

than the Môssbauer time window

₍₃

x

10- g

_s).

In

_conclusion,

61 % of the iron atoms are in the zero valent state for x = 3 : - 34 %

are included in

_{large monolayers magnetically}

ordered at room

_temperature,

with

a

_hyperfine

field at 4.2

_K,

10 %

_higher

than that of bulk iron.

- 27 %

are included in small

_{superparamagnetic}

_particles

with a

_hyperfine

field at 4.2 K

nearly equal

to that of bulk iron.

3. MAGNETIC PROPERTIES. - We have measured the static

_{susceptibility}

_{X st} =

Mlh

in a

10 mT field and the

_{magnetization M( h )}

up to 11 T at various

_temperatures

in the range

4.2-300 K.

Figures

4 and 5 show the variation of _Xst for the four

_{compounds (x}

=

0, 1,

2, 3

).

Fig.

4. - Static

_{susceptibility}

_M/h

=

Xst measured in a 10 mT field for x = 0 and x = 1. The arrows

Fig.

5. - Static

_{susceptibility}

_M/h

_{= Xst} measured in a 10 mT field for x = 2 and x = 3. The _arrows

indicate the direction of the _temperature sweep.

Note that the left hand scale is ten times

_larger

than for x = 0 and x = 1.

x = 0 : _{X st} decreases

_monotonously

with the

_temperature

_(Fig.

_4).

The

_{magnetization}

curves show no saturation effect even in the

_highest

fields

_confirming

the absence of

_ordering

in this

_compound

above 4.2 K

_{(Fig. 6a).}

This is in

_agreement

with

_previous

results which show

no

_ordering

_above

4.2 K in this

_compound

_[17].

x- 1 : An ordered

phase

appears, evidenced

by

the thermal

_hysteresis

between 10 K and 60 K in

_good

_agreement

with the Môssbauer results

_(Fig.

_4).

The

_{magnetization}

tends to saturate at 4.2 and 20 K. In

_high

fields a linear behavior is observed and the

_{magnetization}

M can be written as : M =

Ts + _{X hf .} h. A small saturation exists also at

_high

_temperatures

(200 K)

which is due to _{the presence of} a small amount of a

_{ferromagnetic phase (less}

than

1

_{%) (Fig. 6b),}

not detected in the Môssbauer

_spectrum.

x = 2 and x = 3 : The

susceptibility

is

considerably

increased with

_respect

_to the

previous

cases

(Fig. 5).

If the

compounds

are cooled down to 4.2 K in zero field and then heated up to

300 K in the

_measuring

field one observes first a decrease of _{X st up} to around 20

_K,

then an

increase and a very broad maximum near 200 K above which _Xst

_again

decreases. If the

samples

are cooled under the

_measuring

field a very

large hysteresis

can be seen. The field

cooled

_{susceptibility}

is much

_higher

than the zero field cooled one due to _{the appearence of} a

remanent

_{magnetization}

around 250 K. In

fact,

a

_part

of the

_samples

is ordered well above room

_temperature.

As shown in the

_figures

the

_{susceptibility}

curves

_{XSt ( T)}

for x = 2 and

x = 3 are almost

_parallel

above 50 K

_indicating

that the

only

effect

_{of increasing}

the reduction from x = 2 to x = 3 is to increase the amount of

Fe°.

Fig.

6. -

Magnetization

curves up to 11 T at various _temperatures for the four

compounds. Starting

from the

highest magnetization

curve :

_{a) x = 0 ;}

4.2 K, 10.1 K, 20.4 K, 50.5 _K, 101.3 K, 199.5 K,

300 K.

_{b) x = 1 ;}

4.2 _K, 10.24 _K, 19.9 K, 29.98 K, 40.2 K, 50 K, 101.5 K, 200 K.

_{c) x = 2 ;}

4.2 _K, 10.35 _K, 20.3 K, 50.9 K, 100.5 K, 200 K, 300 K.

_d)

x = 3 ; 4.2 K, 101.5 K, 50.9 K, 300 K. Note that, in

the last case, the

magnetization

does not _vary

_monotonously

with temperature.

and

6b).

In addition the

_high

field

_part

of the curves,

corresponding

to the saturated

moments, does not _{vary very much with}

_temperature

between 100 and 300 K

_indicating

that a

magnetic

order _appears below room

_temperature.

We attribute it to the

_{Fe° phase}

detected

_by

Môssbauer

_spectroscopy

_{and which also appears in the}

_{susceptibility (plateau}

around 200

_K).

For x = 3 the saturation is much easier _{even at} 4.2 K

(Fig. 5d),

but the overall behavior is the

same as for x = 2.

Finally,

we see that the chemical reduction tends to

_produce

a

_{ferromagnetic phase}

in the

compound,

which we attribute to

Fe°

atoms in

_agreement

with the Môssbauer

_experiments.

These

Fe°

atoms are

_gathered

in domains of very different sizes and form an

_{inhomogeneous}

assembly

of clusters

_exhibiting

a

_spin-glass

like behavior : the fact that the

_{magnetization}

saturates more

_easily

at 50 K than at 4.2 K and the absence of

_divergence

of the

_{susceptibility}

at a well defined

_temperature

show that the

_{magnetic properties}

are driven

_by

_strong

ferromagnetic

interactions at short range, still

existing

in the

pristine

FeCl3

and

_FeCl2

_(in

these

_compounds

there exist a

_{ferromagnetic}

interaction at short range, in the

planes,

between the first

_neighbours,

and an

antiferromagnetic

interaction at

_longer

_{range between}

the

_{adjacent layers)}

and also in metallic

iron,

coexisting

with

_{antiferromagnetic}

interactions at

long

range. The Môssbauer

experiments, performed

in zero external

field,

cannot

_distinguish

The distinction can be made with the use of

_magnetic

measurements. The

_{antiferromagnetic}

interaction _may be of two

_{origins : dipolar}

between the

_supermoments

of the

_{ferromagnetic}

domains or of the RKKY _type via the conduction electrons. The overall behavior of the

susceptibility

can be

interpreted

as a

_{gradual freezing}

of the

_{ferromagnetic}

domains when the

temperature

is decreased. For a domain

containing

N atoms, with an internal field

Ho,

the

_ordering

will occur at

_{T(N) =}

(N)1/2

_{li 0}

_HolkB,

where u 0 is the individual moment of each atom

_[18].

_Thus,

the

_biggest

domains order first while the smallest ones order

_only

at low

temperatures

or do not order. If the domain size distribution is narrow, the

_{susceptibility}

has a

well

_{pronounced peak,}

and the

_ordering

process looks like a

_phase

transition. If the

distribution is

_broad,

no well defined

_ordering

_temperature

can be found. This is the case of

the

_compounds

with x = 2 and x = 3. When the

_temperature

is decreased well below

T(N ),

the

_{antiferromagnetic}

interactions at

_long

_range become

_important

and _Xst tends to

decrease.

The increase of _{X st} below 10 K can be attributed to the

_remaining

_FeCl3

and

FeCl2

into

_graphite

and also to the domains

_containing

a _very small number of

Fe°

atoms which do not order

_(«

frustrated

_{spins » [ 19]).}

It can be shown

[ 18]

that the _average

number of

Fe°

atoms per domain is

given by

the

_{equation :}

where Us and o,, are the saturation and the remanent

_{magnetization respectively.}

At 200 K

one has Us = 21

emu/g

and Ur = 0.48

emu/g

which

gives

N = 300 atoms.

According

to the

X-Fig.

7. -

Comparison of Xst

measured

_{(black dots)}

and _Xs, calculated

(open dots)

for x = 3. The vertical

ray results

(a

= 0.415 nm and

_c/2

= 0.544

nm)

we find that a

_{hypothetical spherical}

cluster

containing

300 atoms should have a radius of 2.6 nm. The same calculation made at 4.2 K

gives

an _average number of 9 atoms _{per cluster. This confirms} our

_simple

model and shows

that the

Fe°

atoms are

_mainly

included in very small domains.

The

_{susceptibility}

for x = 3 _can be written as follows :

where x2, x3

and %

are the

_respective

concentrations of

_{Fe2 + ,}

Fe3 +

and

Fe°

deduced from Môssbauer

_experiments.

_{X2, X3} and _Xo are the

corresponding susceptibilities.

X3 is deduced

from the curve for x =

0,

X2 is deduced from the curve for x =

1,

X ° is calculated

by

subtracting

the contributions _{X 2} and _X3 from the curve for x = 2.

Figure

7 shows the calculated

_{(open dots)}

and the measured

_{(black dots) susceptibilities.}

Within our crude

approximations

the

_agreement

between the two curves is

_{fairly good (better}

than 25

%).

We can also

_apply

this

_analysis

to the saturation

_{magnetization}

at 4.2 K

_{by writing}

for x = 3 :

M2,

M3,

the saturation

_{magnetizations}

of the

Fe2 +

and

_{Fe3 + phases respectively}

are deduced

from the

_{magnetization}

curves at 4.2 K for x = 0 and 1 in the _{same way as} described above.

We find

_M3

=

0, M2

= 37.5

emu/g. Assuming

a 2.2 _uB moment _per

Fe° (as

in

_bulk)

we

calculate

_M°

= 32

emu/g.

Finally

the calculated value of U3 is 29

emu/g

to be

_compared

with the measured value 32

_emu/g.

Fig.

8. - Saturation

In

_figure

8 we have

_plotted

the thermal variation of the saturation

_{magnetization}

a-, and of the

high

field

susceptibility

Xhf between 4.2 and 300 K of the most reduced

compound (x

=

3).

We find for

MS

the same behavior as for X st : a broad maximum around 200 K due to the

Fe°

and a low

temperature

tail due to the

Fe2 +

and

_{Fe3 + ions (their}

_average

moments are

_respectively

6 and _{5 fl.B,}

_higher

than the

_{Fe° moment).}

The

_{susceptibility}

X hf exhibits a well marked

peak

at 50 K

_indicating

that some

_phase

orders at this

_temperature,

in

_good

_agreement

with the Môssbauer results

(x

=

1).

Conclusion.

The

_improvement

of the chemical reduction of the

_stage-1

_FeC13-GIC

leads to a

_progressive

and controlled reduction of the iron

_valency

from 3 to 0. In the most reduced

_compound

(x

=

3),

the

major phase

consists in _a

stage-1

Fe°-GIC.

Its

crystalline

structure has been

described within the framework of a new model

_{(hexagonal lattice), differing}

from the

previous

ones

_{[5, 7].}

Figure

9 summarizes the reduction process

by showing

the relative

amounts of the different

_species

at 4.2 K.

It should be noted

_that,

for x >

1,

the

quantity

of

Fe3 +

is almost

independent

of the number

of intercalated lithium atoms. _{This may be related} to the island

_picture.

In

_large

islands the

diffusion of lithium is

_long

and the core cannot be reduced. Another

_explanation,

not

contradictory,

should be the _presence of defects in the

_parent

_compound,

such as Daumas-Herold defects

_{[20]. Figure}

9 _{also shows that the reduction process}

_begins

with the reduction of the

Fe3 +

ions to

Fe2 + (x

=

1).

The reduction to

Fe°

_{occurs at} the _expense of the

Fe2 +

ions. For x = 3 two different kinds of

Fe°

exist. The first _one is ordered at _room

temperature

while the second one consists in small

superparamagnetic particles.

The Môssbauer

_experiments

and the

_{magnetization}

measurements

_{give complementary}

information and are in

_good

_agreement.

_X-ray

_analysis

allows a determination of the

Ic

parameter

which is intermediate between pure

graphite

and first

stage

FeCl1-GIC.

Fig.

9. -

In addition neither the value of the

_hyperfine

field measured on the

Fe°

sites nor its thermal

variation nor the

_magnetic

behavior look like those of bulk iron. We thus conclude that the

Fe°

islands are _{intercalated between the}

_{graphene layers.}

This is not in contradiction with the fact that

they

exhibit 3D

_{magnetic properties.}

In order to

_get

more information about the local

_{magnetic properties,}

Môssbauer

experiments

under an external

_magnetic

field should be of

great

interest. The

_synthesis

of a

reduced Fe-GIC from HOPG

_(Highly

Oriented

_{Pyrolitic Graphite)}

should also be very useful

and should

_give

information about the

_anisotropy

of the interactions. Some

_anisotropy

of the Môssbauer

_absorption

factor is

_expected

for the intercalated

_species.

Therefore,

it should be

possible

to

_distinguish

which

_species

are

_really

intercalated and which ones are not if

_they

exist.

The

_problem

of the

_stability

of such

_{compounds containing}

mixed Fe valencies remains open. The presence of

Fe2 + and/or Fe3 +

even

_though

difficult to eliminate may be necessary

to stabilize the

_{Fe° layers}

via

_strong

electron

_exchanges.

Acknowledgements.

R. Tur is thanked for

_{having performed}

the

_magnetic

measurements.

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references can be found in the literature

concerning

this

point.

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_Physics

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