Graphite intercalation compounds with transition metal fluorides

Synthetic Metals, 23 (1988)435-439

435

G R A P H I T E IMTERCALATION COMPOt~NDS W I T H TRANSITION M E T A L FLUORIDES

S.

FLANDRDIS and B.

HUN

Centre de Recherche Paul Pascal, Domaine Universitaire de Bordeaux I, 33405 Talence C e d e x - France J.

GRANNEC and A.

TRESSAUD

Laboratoire de Chimie du Solide, Universite de Bordeaux I, 33405 Talence C e d e x - France C.

HAUW

Laboratoire de CristallographLe, Universita de Bordeaux I 33405 Talence C ~ e x

- France

ABSTRACT The synthesis of fluorides

has

graphite

intercalation

b e e n investigated.

fluorides, TiF 4

can

be

easily

presence of fluorine gas.

compounds

with

transition

metal

It is shown that, contrary to other 3d metal intercalated

as

second-stage

compounds,

in

F r o m X-ray diffraction studies, evidence is given for

the presence in the layers of ionic T i F 6

and neutral TiF 4

species.

Finally,

the intercalation of metal chlorides and fluorides is co~pared.

INTRODUCTION Although numerous t r a n s i t i o n metal c h l o r i d e s can be i n t e r c a l a t e d i n g r a p h i t e , only

a

few

intercalation

compounds Containing homologous fluorides have b e e n

synthesized, especially with 3d elements. great

interest

in

connection

with

However these materials could

be

of

high electrical conductivity (the highest

conductivities h a v e b e e n obtained w i t h

SbF 5

or

AsF 5

intercalated

graphite},

low-~imensional m a g n e t i s m (metal fluorides h a v e various magnetic behaviours) and electrochemistry (they could give rise to rechargeable lithi ~m batteries). In a first step, w e have investigated the intercalation of 4~ and 5~ fluorides

/I/,

namely

points (86"C and 70"C Containers

under

RUF 5

and

OsF 5 .

resp. )

and

can

vacuum,

leading

be

to

element

These pentafluorides have low melting intercalated

first

stage

around

compounds

100ec with

in

Ni

a C-axis

periodicity o f 8.4 A.

This interplane distance ~eans that the structure o f the

intercalated

differs

species

from

the

pristine fluorides:

instead o f M 4 F 2 0

tetramers, the layers are formb~ of ~F 6 octahedra w i t h two faces parallel to the carbon

layers.

Moreover from magnetic susceptibility experiments, it was shown

that, b y intercalation, oxidation

state

+V

of

Ru 5÷

ions

are

reduced

o s m i u m is maintained.

conclusion that intercalated

species

are

RuF 6

to

Ru 4~

ions,

whereas

the

Analysis of the d a t a led to the (20%)

and

RuF 4

(SO%)

for

rutheni~a and probably a mixture o f OsF 6 and OsF 5 for osmium.

0379-6779/88/$3.50

© Elsevier Sequoia/Printed in The Netherlands

436

The

poor

stability

intercalation

of

been successful up to now. under

these

materials

of 3d element fluorides.

chlorine

incited

on

the

of

results

several

obtained

fluorides of 3d elements: second-stage

compounds,

to

stage

with

TiF4,

In

which

c(~toourN~

this

paper,

fluorides

of

we

have

is a special case among

it can be intercalated under fluorine whereas

the

atmospheres /2/, and NiP 2 was claimed to

atmosphere

as

Mn, Fe, Co, Ni, Cu do not seem

actually to react with graphite either in vapour phase or experiments

investigate

very few attempEs have

TiF 4 has been intercalated as third

pressure

intercalate graphite from K2NiF 6 solution in HF /3/. focused

us

To our knowledge,

in

solution.

X-ray

will raise the evidence of different intercalated species.

Finally

we will compare metal chloride and metal fluoride intercalation.

SYNTHESIS PROCEDURES A number of intercalation methods have been carried out: i/ Reaction graphite-l~

under fluorinating atmosphere. x By analogy with the intercalation of chlorides, powdered fluorides were mized

with

graphite flakes in a nickel reactor filled with one atmosphere of F 2 or

and heated at various te~oeratures. ii/ Reaction graphite-44F x under Cl 2 atmosphere. This method has already proved to

be

valid

for

TiF 4

/2/

and

TaF 5

/4/.

Chlorine pressures up to 15 bar and temperatures up to 850oc were used. iii/ Cl

F exchange reaction from metal chloride GIC.

Fluorine or

HP

gas

(one

bar)

have

been

allowed

to

react

with

metal

chloride-graphite compounds at temperatures up to 500°C. iv/ Reaction in molten salts. The NaCI-ZnCI 2 system, often used for the crystal growth of fluorides, has an eutectic point at 260oc for 30% of NaCI. oCcurs when this fluorides

were

mixture

is

heated

We first checked that no intercalation

alone

in

presence

of

graphite.

Then

dissolved and heated with graphite at temperatures in the range

270 oC-500oc. v/ Reaction graphite--AxMFy__ in anhydrous HF solution. Nikolasv et al. into

graphite

/3/ have claimed that nickel fluoride

from

K2NiF 6 dissolved in ~ .

can

be

intercalated

From elemental analysis,

spectroscopy and magnetic susceptibility measurements, they obtained a of

formula

ClgNiF3,

5HF.

infrared compound

We carried out similar experiments using K21q~6r

K214r~6 and MnF 3 as reactants.

Graphite ~ e r

was allowed

to

react

with

the

solutions during 4 to 20 days at room temperature, For fluorides of Mn, Fe, Co, intercalation.

Only

TiF4

Ni

could

and

first one gave second-stage compounds. Before intercalation, under

fluorine

Cu,

none

of

these

methods

to The

They were prepared in the following way:

commercial TiF 4 was purified by distillation

gas (4 bar).

led

be intercalated using method i/ or ii/.

at

220oc

Then the mixture gra~hite-TiF 4 was heated under a

fluorine pressure of i to 2 bar at temperatures between 200 and 250oc

during

5

437 to 6 hours.

Excess of TiF 4 was eliminated by sublimation onto the cold parts of

the reactor. The weight uptake can reach 105%. TiO 2

after

calcination

under

composition deduced f r c ~ w e i g h t CI6TiF 5 • The products atmosphere~ spectra.

are

more

The ratio F/Ti, obtained by gravimetry

oxygen at 700oC,

is equal to 5 • 0.3.

uptake measurements hie in between

hygroscopic

than

those

prepared

CI2TiF 5

under

of

Thus the and

chlorine

after 2 days in air the (002) peak of graphite appears on the X-ray

The compounds are destroyed after being exposed two weeks in air.

Ci~ARACTERIZAT~ON OF TiF%-GZC Two asstm~tions can be made to explain the ratio F/Ti : 5 instead of 4: fluorine

excess

can

be either due to a partial fluorination of graphite or to

the presence of TiF 6 generally

of

the

ions besides

species

based

TiF 4 .

The

presence

of

TiF3,

and

more

on Ti 3÷ ions, can be excluded, since our magnetic

susceptibility measurements showed the compounds to be diamagnetic. AS graphite fluorides CF and C2F have characteristic ~R absorption peaks /5/, transmission

spectra

were examined on powder samples dispersed in KBr pellets.

A typical spectrum, obtained with a sample of nominal cc~¢Dosition C14TiFs.3, shown

on

Figure i.

of C2F whose spectrum is also shown on Figure I for comparison. a

small

amount

is

A weak absorption band occurs at 1180 cm -I, characteristic This means that

of carbon is fluorinated, even at temperatures between 200 and

250oC.

transmittance

C2 F

I

l

I

1600

I

~

1200

I

I

9 (cm-i)

-

J

800

Fig. 1. IR s p e c t r a of C14TiF5. 3 and C2F. The a b s o r p t i o n band at about 1200 cm-1 is c h a r a c t e r i s t i c of C-F groups. X-ray

diffra~ogra-m

gave values

11.47 ~ ac~oz~ing t o t h e s a a ~ l e s . compounds.

of

the

C axis

This periodicity

p~armaeter I

c

between

11.31

and

c o r r e s p o n d s t o second-sta~e

The thickness of the intercalated layer is

thus

between

4.61

and

438

4.76 A,

in

good

agreement

with

the height calculated for a TiF 6 octahedron

(4.83 A) having two faces parallel to the carbon layers.

No (ool) peak

of

CF

or C2F could be observed on the spectra. Precession photographs at room temperature composition

CI6TiFs. 1

spots characteristic of an ordered phase. hexagonal

lattice

a G = 2.%6 ~.

single

crystals

of

The

nominal

These spots can be attributed

of Parameter a = 4.92 A, i.e.

corresponding

to

an

twice the graphite parameter

The two lattices have the same orientation (Figure 2):

coam~nsurate. TiF 6

on

showed, besides the graphite lattice, a diffuse ring and

they

are

intercalated layer could be made of isolated

ions.

The diffusion ring corresponds to a mean Parameter of 4.7 A. different

from

(a = 2.57 A)

the

and

parameters

C2F

determined

(a = 4.92 A

and

by

Watanabe

b = 5.01 A).

This value

et

al.

Thus

is

/5/ for CF

the

observed

diffusion must be due to a disordered phase of titaniua fluoride.

Orap hoi 2.46 A

'

'...//

I I

m I

'

Fig.

2.

It

hu

been

intercalation

show,

of

I

#

j.

Unit cells of TiF 6

/6/

electron

'

and graphite sublattices (direct space).

that

the

accei~ors

in-plane and

charge transfer between carbon atoms and distances

have

been

4.92 A

that

C-C

distance

decreases

by

its value gives the amount of

intercalated

species.

In-plane

C-C

measured using an automatic diffractoweter from the (200)

peak of graphite lattice.

The charge transfer estimated from these measurements

is equal to 0.25 electronic charge per TIF5.1 entity. • hese crystallographic data allow us to obtain information on the composition of

the products.

Ti 4+ ions must be present as T i F ~

be assumed that the charge transfer is due ratio

between

to TiF 6

to

ionic

an(] TiF 4 species. si~ecies.

It can

Moreover,

the

the graphite unit cell area and that of the unit cell attributed

leads to C/Ti - 16 for a second-stage compound.

Thus,

nominal composition CI6TIFs. 1 contains 0.125 mole of CIETiF 6 .

a

sample

of

Besides, assuming

that the diffuse ring is due to TiF 4 domains with an hexagonal lattice

of

mean

439

parameter

4.7 ~,

the C/Ti ratio is equal to 14.2 for a second-stage compound.

Therefore the compound would contain 0.875 mole of with

the

nominal

~sition

C16TiFs. 1

C14.2TiF4.

could

he

due

Referred to masses, the composition would be as follows:

The

difference

to 0.85 mole of C2F. 13% of CI6TiF 6, 76% of

CI4.2TiF4 and 11% of C2F.

CONCLUSION

COMPARISON WITH METAL CHbORIDE GIC'S

-

This study shows the complexity of the intercalation of comparison

w i t h metal chlorides.

generally of o n l y one kind and have a chlorides

(with

the

same

metal

fluorides

in

Whereas the intercalated chloride species are structure

oxidation

state

close

of

to

the

that

metal ),

of

the

the

free

layers

of

intercalated fluorides can be made of several species whose arrangement does not reproduce the structure of the free fluorides.

The intercalation of RuF S occurs

w i t h reduction of ruthenium from Ru S÷ to Ru 4~, w i t h ionic RuF 6

two

intercalated

(isolated ions) and neutral RuF 4 (chain structure).

species,

Similarly, the

tetrameric structure of pristine OsF 5 does not persist after intercalation. this

case,

TiF 4

also

however, occurs

the oxidation state 085+ is maintained.

without

change

in

titanium

intercalated layers are made of two species: neutral

oxidation

ordered TiF 6

In

Intercalation of state,

but

the

ions and disordered

TilF 4 .

AS a result, the charge transfer mechanism differs from that of chlorides.

For

fluorides,

ionic

species

transfer, whereas for chlorides it is

are

generally

responsible due

to

intercalated

for

the

the

charge

chlorine

excess

resulting from the island structure /6/. Another consequence o f the presence of neutral species is the

poor

chemical

stability of intercalated fluorides compared to chloride-graphite compounds. O n the other hand, intercalated fluorides and chlorides common

characteristic:

our

the intercalates consist of octahedra w i t h two

faces

layers.

for

The

resulting

have

an

unexpected

crystal structure studies show that in both cases

C-axis

periodicity,

parallel a

to

the

between 8.1 and 8.4 A (fluorides) and 9.3-9.6 A (chlorides) according size of the cation contained in the octahedral cavity.

carbon

first-stage compound, is to

the

This difference of about

I. 2 A between fluorides and chlorides corresponds to the size difference of and C1

F-

ions.

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B.

2

E.Bu~=azlet, P.

3

A.V.

Nikolaev, A.S.

Otd.

Akad.

4

D.

Run et al., to be published. Touzain and L.

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Yudanov and V.N.

Ikorsky, Izv.

sib.

Nauk SSSR, 7 (1972) 1366.

Ravaine, J.

Boyce, A.

Hamuri and P.

Touzain, Synth.

Met. r 2 (1980)

249. 5

Y.

Kita, N.

6

F.

Baron, S.

(1982) 759.

Watanabe and Y. Flandrois, C.

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