Neutron diffraction on bromine intercalated in graphite

Synthetic Metals, 23 (1988) 147 - 153

147

NEUTRON DIFFRACTION ON BROMINE INTERCALATED IN GRAPHITE

Ch. SIMON, I. ROSENMAN, F. BATALLAN Groupe de Physique des Solides de l'Ecole Normale Sup~rieure, Laboratoire associ~ au Centre National de la RechercheScientifiqu~Universit~ Paris VII, Tour 23, 2, place Jussieu, 75251 Paris C~dex 05. G. PEPY Laboratoire L~on Brillouin, C.E.N.-Saclay, 91190 Gif/Yvette, France H. LAUTER Institut Laue Langevin, 156 X, 38042 Grenoble, France ABSTRACT We present in-situ neutron diffraction studies on graphite intercalated with bromine. We have made kinetics studies and determined the phase diagram of the system versus temperature and bromine pressure. We have observed in high stages the commensurate-incommensurate transition and the melting of the bromine layer. Even in these stages, the stacking of the bromine layers is three-dimensional so the two-dimensional theory cannot be applied. We also present some inelastic resuits which confirm the registered nature of the incommensurate and liquid phases.

INTRODUCTION The study of the commensurate-incommensurate (CI) transition was the subject of many experimental and theoretical studies but the relation between them seems to be systematically impossible /I/ . For this reason, the success of the Pokrovski Talapov (PT) /2/ theory in the CI transition of bromine intercalated in graphite /3/ appears to be a miracle. And it was. The system graphite-bromine was extensively studied by X-rays diffraction by many groups /4/ but many problems remained unsolved since no in-situ experiments were performed in which 001, hkO and khl diffraction patterns were measured at the same time. For this reason, Erbil et al /3/ did not know at the CI transition the stage of their compound nor the stacking of the bromine layers. The PT theory is a two-dimensional (2D) theory at a constant chemical potential and predicts a misfit variation in (T-Tc) I/2. The knowledge of the phase diagram gives information about the chemical potential and we have found that a constant-pressure experiment does not correspond to a constant chemical potential measurement. We present also the hkl diffraction pattern for the superstructure peaks and show

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that the compound is three dimensional,

© Elsevier Sequoia/Printed in The Netherlands

148 even in the incormnensurate phase. Then we present the inelastic results, ming that the incommensurate

phase is registered

confir-

and showing that the liquid phase

is also registered.

EXPERIMENTAL

DETAILS

A two zones aluminium furnace holds a quartz container (made of HOPG) was maintained temperature.

Temperatures

in a controlled

in which the sample

pressure of bromine at a controlled

below room temperature were reached by

The furnace was mounted on a triple axis neutron spectrometer IT at the Laboratoire Laboratoire

LEon Brillouin

Peltier

effect.

: IN3 at the ILL and

for thermal neutrons and 4FI, 4F2 at the

LEon Brillouin for cold neutrons.

KINETICS The difficulty

to determine

the phase diagram is in obtaining

the equilibrium.

Our sample was Ix2.5x0.5 cm 3 before insertion and the characteristic

times are very

long (between hours and days). For this reason, we have not obtained many points on the phase diagram and powder experiment will be performed we have performed a kinetics librium.Figure

I presents

of intercalation

two different ways to obtain a second stage compound

from a high stage compound.

In the first one, the intercalation was performed

bromine vapor and the sample decreased axis coherence

later. For each point,

to be sure of the stage at the equi-

its stage regularly

length does not vary in this process.

in

in the bulk. The c-

On the contrary,

the inter-

calation in the liquid goes much more fast, hours instead of days, but the sample is obtained directly tion destroys necessary

in second stage from the edge of the sample. This intercala-

the c-axis coherence

length and a long annealing of the sample is

to obtain a good quality sample.

THE PHASE DIAGRAM AND THE CHEMICAL POTENTIAL Figure 2 presents

the phase diagram of bromine intercalated

have got it in these experiments. previously

assumed

/3,5/ but we did not observe any contradiction

X-ray and thermogravimetric

data. The staging transitions

sical law in the domain we have studied bromine pressure,

in graphite as we

This phase diagram is quite different

from that

with the actual

can be fitted by a clas-

: ~ = AHo - TASo = ~o + RT InP (P is the

T the temperature of the sample, N is the chemical potential).

Since R TBr in P = A-B TBr (P is given by the equilibrium with the liquid at TBr in this method),

(AH o- ~o)/T - AS ° = A/TBr - B

(I)

149 !

!

2

4

2

'

!

35(

g •

~

3

"

'

~

5

6

7

8

300

C

t

l

2D

1

0 0

10 20 time ( hours )

3~

30(

350

400

T (K)

Fig. I. Kinetics of intercalation of bromine into graphite. (a) in vapor phase at 310 K (b) in liquid phase at 310 K The curves represent the respective intensities of the (O 0 s+2) peak of each stage s versus time.

Fig. 2. Domains of stability of different stages versus temperature and bromine pressure. The dots are the experimental points and the curves are given by equation (I).

The constant chemical potential curves are given by the same equation with AS taken to zero. Their shape is similar to that of the staging transitions it is clear that the chemical potential varies especially

o and hence

in a constant pressure experiment,

at low pressures.

THE IN-PLANE STRUCTURES We have studied the in-plane hkO scans.

structure of the intercalated

In HOPG, these are powder scans. However,

tions are very clear (figure 3). The commensurate ~r3x~3 superstructure ( ~

bromine by performing

the CI and melting

= (I ,I) and /[3 = (3,1)) . The numher of bromine atoms

per unit cell is unknown and subject to controversy but preliminary measurement

of the bromine quantity

mine outside of the sample) phase diagram.

(by measuring

results of

the remaining quantity of bro-

seems showing 4 atoms per unit cell everywhere

The incommensurate

phase was described /3/ in detail.

to a striped domain phase. Figure 4 presents domain-wall.

transi-

phase is the well-known /4/

in the

It corresponds

the in-plane structure with one

150

)

c

~n °~

=

o

371K

376K

o

-

o

O

o i

I .42

1.44

!

Q

(~-11i

46

I

1.48

Fig. 3. The (2 0 0) Bragg peak at three different temperatures in the commensurate, incommensurate and liquid phases (TBr = 270 K, stage 6 for C and I, stage 8 for the liquid phase).

Fig. 4. The in-plane unit cell and a schematic position of the bromine atoms on graphite lattice. A domain wall is presented with its translation vector ~ between the two domains.

Stage 2 is incommensurate between 300 and 400 K and the misfit does not vary with temperature. This is similar to previous experiments /6/ . This incommensurability corresponds to a domain wall every two cells. The relaxation of the atoms outside of the atoms outside of the commensurate positions is very important in this stage

6

On the contrary, the incommensurate phase of A. Erbil et al /3/ is registered, i.e. the relaxation of the atoms is very small and the domain walls are very narrow, as it is shown by the respective intensities of the satellites. The surprising result is that this phase does not exist in stage 3, 4 or 5, which are always commensurate, but only in stage 6.

THE STACKING OF THE INTERCALATED LAYERS In the second stage compound, F. Aberkane had shown that the stacking of the intercalated layers is regular. We call this a 3D compound. In their experiment, A. Erbil et al. /3/ have claimed that a crossover from a such 3D behavior to a 2D compound (random stacking of the layers) occurs at 310 K. But the change of the observed hkl can now be interpreted as a change of the stage (4 to 5). Figure 5 shows that the compound remains 3D in stage 6 above CI transition. One can s e e on this figure the AIIABABABI2BABABAI I stacking which gives a 47.4 A cell. Such

151 a 3D behavior was obser~ed in stage 8 before melting at 370K, but the period o

60.8 A is close to our experimental resolution. Because the compound is 3D at the CI transition, it is impossible to use the PT results to interpret

them and the

success of Erbil et al. is very surprising.

,.,i= E,~

0

0.1

0.2

E (Thz)

= 2

=

I

0 cJ

0

I

0

I

Q (~-I)

m

2

Fig. 5. The (2 0 i) scan at 360 K. The unit I c is that of a stage 6 compound (TBr = 270 K).

0

0

0.2

0.4

0.6

Q(~-I) Fig. 6. The in-plane phonons of bromineintercalated graphite : L : longitodinal ones, T : transverse ones. In insert, the phonon marked by an arrow.

THE MELTING At 373 K, the hk0 superstructure peaks disappear. This was interpreted as a 2D melting /3 /. We already know that the compound is actually 3D in the solid phase. A possible interpretation of this transition is a 3D to 2D crossover. This also explain /7/ the strange b~havior of the respective intensities of the satellites versus temperature already observed /3 / • This crossover is associated to a loss of in-plane coherence which is usual in 2D systems. Since the bromine contribution to the graphite in-plane Bragg peaks does not vary at the transition, the liquid is actually a lattice liquid. We have tried to perform quasi elastic coherent neutron scattering similar to incoherent ones performed on intercalated nitric acid /8 /but we did not observe any broadening in energy at the transition. It is not sure that the "liquid" phase is really

152

liquid, but may be only a disordered solid phase. However, our resolution on 4FI was "only" 0.02 THz and a lattice liquid is never very broad in energy.

THE INELASTIC RESULTS We have studied by inelastic coherent neutron scattering the phonons of the system which should be modified by the transitions /9/ i.e. the in-plane transverse and longitudinal modes. H. Zabel et al. /10/ has shown that the in-plane transverse modes polarized along c-axis are sensitive to the CI transition in intercalated potassium. On the contrary, we did not observe any effect in bromine. Different explanations are possible. First, the stage is here six instead of one or two in potassium. Since these modes are mainly movements of graphite atoms, the contribution of bromine is here smaller. Moreover, since the relaxation of bromine atoms is very small, the role of the domain wall is also very small. At last, the incon~ensurate phase is here one dimensional, so the powdered averaging is more important than in potassium which is two dimensional. For these very reasons, we did not observe any modification at the melting transition neither. This confirms the lattice "liquid" nature of the high temperature phase. We have also observed the in-plane longitudinal modes. The phonons of high o

wave vectors are observed in the neighborhood of the (00 2~/3.35 A) Bragg peak o

and the phonon at the zone center on the (I 0 0) G Bragg peak (q = 2.95 A-I), for experimental reasons (powder averaging). The zone center phonon corresponds to the shear mode of bromine in the graphite wells. This value (0.15 THz) is very low and corresponds to 7 K. This is not the potential modulation, but the face in the bottom of the well. We did not observe any modification at the CI transition, indicating that the domain walls are very narrow, as it was claimed from the X-ray results /3/ . The in-plane dispersion of this mode is very large as it is seen on figure 6, showing that bromine-bromine coupling is strong, larger than the graphite potential itself. This seems a strange result, since the bromine atoms are in the bottom of the wells, but S. Aubry have shown /11/ that such a 7-fold striped-domain phase remains registered, even with a very weak substrate potential. In this case, there is no phason in the incommensurate phase.

ACKNOWLEDGMENTS We are indebted to G. Furdin

who

has prepared the experimental setup and

had made available to us many unpublished results on intercalated bromine.

REFERENCES I

J. Villain, "Structures et instabilit4s", C. Godr~che ed., Editions de Physique Paris (1986).

2

V.L. Pokrovsky, A.L. Talapov, Phys. Rev. Lett. 42 (1979) 65.

153 3

A. Erbil, A.R. Kortan, R.J. Birgeneau, M. Dresselhaus, Phys. Rev. B28 (1983) 6329 and S.G.J. Mochrie, A.R. Kortan, R.J. Birgeneau, P.M. Horn, Phys. Rev. Lett. 53 (1984) 985 and S.G.J. Mochrie, A.R. Kortan, P.M. Horn, R.J. Birgeneau, Phys. Rev. Lett. 58 (1987) 690.

4

See for example Synthetic Metals 8 (1983) and W.T. Eeles and J.A. Turnbull,

5

G. Furdin, Synthetic Metals 8 (1983) 101.

6

F. Aberkane, th~se (unpublished).

7

To be published.

8

I. Rosenman, Ch. Simon, F. Batallan, A. Magerl, Europhysics Letters 3 (1987)

9

Ch. Simon, F. Batallan, I. Rosenman, H. Lauter, G. Furdin, Phys. Rev. B27

Proc. R. Soc. London A 283 (1965) 179 and ref. 3, 5 and 6.

1013.

(1983) 5088. 10 H. Zabel, S.E. Hardcastle, D.A. Neuman, M. Suzuki, A. Magerl, Phys. Rev. Lett. 57 (1986) 2041. 11S.

Aubry, same as ref. I.