X-ray diffuse scattering study of the orientational ordering in single crystal C60

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X-ray diffuse scattering study of the orientational ordering in single crystal C60

R. Moret, S. Ravy, J.-M. Godard

To cite this version:

R. Moret, S. Ravy, J.-M. Godard. X-ray diffuse scattering study of the orientational ordering in single crystal C60. Journal de Physique I, EDP Sciences, 1992, 2 (9), pp.1699-1704. �10.1051/jp1:1992237�.

�jpa-00246651�

Classification Physics Abstracts 61.50 64.70K

Short Communication

X-ray ^diffuse scattering study of the orientational ordering

in single crystal C60

R.

Moret,

S.

Ravy

and J.-M. Godard

Laboratoire de Physique des Solides, URA 02, ^Universit4 Parh-Sud, 91405 Orsay, France

(Received

23 June 1992, accepted 25 June

1992)

R6sumd. L'ordre orientationnel des moldcules de C60 _a dtd dtudid _{au moyen} de clich4s de diffusion diffuse des _rayons X. L'intensit6 de cette diffusion pr6sente des modulations anisotropes qui d6montrent l'existence de corr41ations orientationnelles £ tempdrature ambiante. Une _nou- velle transition structurale est mise _en 4vidence k T2 ₌ 255 K * 2 K, en-dessous de la transition

orientationnelle cubique k faces centrdes

(c.f.c.)-cubique

simple £ Ti

" 259 +1 K. La seconde

transition

(h T2)

conduit k basse tempdrature k _une surstructure de parambtre 2«, vraisemblable- ment c-f-c-- Des fluctuations assocides h _ces deux transitions persistent h tempdrature ambiante alors qu'une ^{diffusion diffuse} anisotrope est _encore prdsente £ 20 K.

Abstract. Single crystal X-ray diffuse scattering photography has been used to provide

insights

into the orientational ordering of the C60 molecules. The observation of anisotropic modulations of the diffuse scattering intensity in reciprocal _space demonstrates the existence of orientational correlations at _room temperature. A _new structural transition is observed at

T2 ₌ 225 + 2 K, below the face-centered cubic

(f.c.c.)

to simple cubic orientational ordering phase transition at Ti

= 259 +1 K. The transition at T2 ^leads to _a low temperature _super- structure, presumably f-c-c-, with _a doubling of the cell constants. Fluctuations associated to both transitions _{are seen} at _room temperature while _some anisotropic diffuse scattering is still present at 20 K.

At _room temperature solid C60 forms _a face-centered cubic lattice

(f.c.c.)

where the

nearly spherical C60

molecules

are

orientationally

disordered [1,

2].

^NMR [3,

4],

^inelastic neutron

scattering

[5] ^{and sound}

velocity

_[6] data indicate that this disorder is

dynamic

and consists in

rapid isotropic

rotations of the

molecules,

uncorrelated with their

neighbors

_[5].

The existence of _an orientational

ordering phase

transition at

Ti

₌ 250-260 K has been established

by

differential

scanning calorimetry (DSC)

_{[7, 2]} ^and diffraction

techniques [2, 8,

9,

10].

^The orientations of the four C60 molecules in the f-c-c- Bravais lattice become _non-

equivalent

at the

transition,

which leads to a

simple

cubic

(s.c.)

structure

(space

_group

Pal)

below Ti However

rapid

reorientations

persist

well below Ti The current

model,

substantiated

by

NMR, inelastic neutron

scattering,

muon

spin

rotation

(pSR),

sound

velocity

and attenu-

1700 JOURNAL DE PHYSIQUE I N°9

ation,

Raman

scattering

and thermal

conductivity

measurements, as reviewed

by Heiney [Il], suggests

^{that the molecules}

perform thermally

activated

jump rotations, presumably

about the

<ill> _axes of the Pal _{space group} and between _two

nearly degenerate

orientations. These orientations appear ^to

optimize

the _van der Waals

bonding

and the electrostatic

repulsions

of nearest

neighbor

molecules. Below about 85 K the orientational

dynamics

become

extremely

slow and _a substantial

degree

of disorder is frozen. It _was also found that the low temperature

s-c-

phase

is stabilized under _pressure

(dTi/dP

₌ II

K/kbar)

_so that it becomes stable at room temperature ^above 3-4 kbar

[12,

13].

We also mention that recent electron diffraction

experiments by

_van Tendeloo et al. indicate the existence of _{a new} 2a-f.c.c. superstructure at _a

slightly

lower temperature than the Ti f-c-c- to _s-c- transition

[14].

The present paper reports ^{the first diffuse}

scattering study performed

_on

C60 single crystals

and

gives

_new information _on the nature of the intermolecular orientational correlations in the 295 K-20 K temperature _range. ^A

supplementary

^transition near 255 K towards _a superstruc-

ture

(presumably f-c-c-)

with _a

doubling

of the lattice constant is

clearly revealed, confirming

the observations of _van Tendeloo et al.

[14].

C60

_was

prepared

and

purified using

standard

procedures [15]. Single crystals

with sizes up to 500 pm were grown

by

slow

evaporation (75 ^°C,

I

month)

from toluene solution of C60

powder.

Precession

photographs

confirmed the f-c-c- symmetry ^of these

crystals

with

a =

14.151,

which is characteristic of non-solvated C60 materials

(solvent adsorption

cannot be ruled out but it would not affect the bulk

properties probed by X-ray diffraction).

The

photographs

also exhibit diffuse lines which _are due to

stacking

faults relative to the f-c-c-

close-packing

of the C60

layers.

A small _amount of these

low-energy

defects _seems to be _very difficult to avoid in solid C60 [16].

Diffuse

scattering photographs

have been obtained

using

the X-ray monochromatic fixed-

crystal

fixed-film method. This method

gives

_a

projected image

of _a

spherical

section

(by

the Ewald

sphere)

of the

reciprocal

_space and it is _very

appropriate

^{for the} characterization of weak diffuse

scattering phenomena.

The

single crystal sample

was attached _to the cold end of _a cryogenerator

allowing

_mea-

surements to be made in the 295 K-20 K _range.

Temperature

_{accuracy was} estimated to be about I K. CuKa

X-ray

radiation from _a sealed tube _was selected

by (002)

reflection _{on a}

doubly-bent graphite

monochromator and

impinged

_on the

stationary sample.

A

cylindrical

film

(radius

30

mm)

and _exposure times of12 hours _were used.

Figure

I shows

X-ray photographs

taken with the _same

crystal.

All diffraction patterns show spots

corresponding

to

Bragg

reflections from the f-c-c- lattice

(a

number of such reflections

are excited because of the

focusing

^beam

produced by

the curved

graphite monochromator) together

with streaks

connecting

_some of the spots and which _are due to

stacking

faults. A

central diffuse

ring (arrow, Fig. la)

is _an artefact caused

by

the

glue holding

the

sample.

At 300 K

(Fig. la)

_a broad and intense diffuse

scattering ring (labelled

I in the

figure)

is observed It

originates

from _a

spherical

halo in

reciprocal

_space centred _{at a wave} vector qi ~J

3.5

l~~.

_The

_ring

_is

slightly elongated

in the vertical direction because of the

cylindrical

geometry ^{of the film holder which is also}

responsible

for the weaker

intensity

in its top and bot- tom parts. ^At

larger

_{q a}

second,

broader and weaker halo

(labelled 2)

is visible at q2 ~' 5.8

l~~

(this

value is

approximate

because the halo is broadened due to the

particular scattering

_geom-

etry).

Powder neutron diffraction

experiments

have

already reported

the existence of diffuse

scattering intensity

maxima at q = 3.5

l~~

_and

q = 5.8

l~~

_[8,

_17].

_{Such maxima have}

previously

been fitted

quite satisfactorily

with _a model that consists of

freely

and

isotropically

rotating

C60 ^molecules [17].

Obviously

this model should lead _{to an}

isotropic

diffuse

scattering

aj ^bi

_;

1'

« .

',: ~

300K §iK, _.)

C ^'

, ~

'.

'

f) ) ₎

i

j i

1

)

(

~ ~

( ~ j

~~~(~~

1702 JOURNAL DE PHYSIQUE I N°9

intensity,

_even in the _case of _a

single crystal.

Careful examination of the _qi

ring

in

figure

la shows

clearly

that this is not the _case. In the circled

regions

the diffuse

scattering intensity

varies

quite rapidly

as a function of _q and defines _a

peculiar

_pattern of maxima and minima.

These

intensity

modulations

imply

that the rotation of the C60 ^molecules is not

isotropic

_or

that there _are sizeable intermolecular

correlations,

_or both.

Information _on the

origin

of this

anisotropy

_can be

gained

from the low temperature data.

First,

at 264 K the diffuse

scattering

pattern has not

changed qualitatively

but the modulations

are more

prominent (Fig. lb).

^At 257 K

supplementary spots

_appear

(Fig. Ic, arrows). They

can all be indexed _as f-c-c--forbidden reflections

(open

circles in the

diagram

of

Fig. If)

in agreement ^{with the} characteristics of the orientational

ordering

at Ti [2, 8,

10].

It is

important

to note that these spots are located _{on a} set of diffuse

scattering

maxima which _are therefore

produced by

fluctuations of the

simple

cubic structure.

Figure

la shows that these fluctuations

are

already significant

at room temperature.

At 252

K, figure

Id reveals _anew set of spots

(arrows)

with reduced _wave vector components of the form

(1/2, 1/2, 1/2)

relative to the _s-c-

reciprocal

lattice

(some

of these spots are

represented

in

Fig.

if with

x's).

This

undoubtedly

shows that _a second transition towards _a

superlattice

with _a cell constant a'

= 2a _occurs at _a temperature T2 which is

approximately

4 K lower than Ti The _new superstructure spots are

mostly

situated in

place

of _a second set of diffuse

scattering

maxima that remained below

Ti

It is therefore clear that this

particular

diffuse

scattering

is _a

signature

of fluctuations of the low temperature

2a-superstructure.

Like the _{s-c- ones}

they

subsist _at least _up to _room temperature.

Most of the

high

temperature ^diffuse

scattering intensity

modulations constitute the two sets of maxima described above and

they

vanish at

Ti

and T2. However _some diffuse

scattering

remains below T2

(Fig. id)

and its

intensity

decreases with temperature but

persists

down to 20 K

(Fig. le).

Its modulations _{vary more}

slowly

_{in q space} than those described above.

Figure

le also shows _a few additional spots

(arrows)

which _{appear near}

T2

and _are broader than the other spots. ^Some of them

correspond

to reduced components ^{of the} type

(1/2, 1/2, 0) (such

_a spot ^{is indicated in}

Fig. If,

solid

triangle)

but _a clear

assignment

is not

possible given

the small number of such spots.

The above results have been obtained _upon

heating

from 20 K.

They

_are reversible but _we noticed

that,

_as for the Ti transition [10], a small

hysteresis

effect _occurs for the T2 _one.

Our

single crystal X-ray

observation of _a

2a-superstructure

at low

temperature

^{is consistent} with the results

by

_van Tendeloo et al. [14] ^and we find that this superstructure ^{is stabilized} below T2 _" 255 + 2 K. The observed

(1/2, 1/2, 1/2)

reduced _wave vector indicates that the

superlattice

is

presumably

f-c-c-- However, the additional spots ^{mentioned above} _may

correspond

to _a 2a-s.c. superstructure while _{a more}

complex

superstructure or a structural

modulation _cannot be excluded _at the moment. With these

reservations,

the low temperature superstructure ^{will be referred} to 2a-f.c.c. in the

following.

The T2 ^{transition had} not been detected in most

previous

studies

including powder

and

single crystal

diffraction work [2,

5, 8, 9,

10,

Iii.

This is

partly

^due to the closeness of the transition temperatures and the

relatively

weak 2a-f.c.c. superstructure reflections.

However,

several reports of _a double transition in the 250-260 K range were made from

thermogravimetric analysis

and DSC measurements [2, 7, 11,

13]. Although

the _presence of residual solvent casts some doubts _on the intrinsic character of the

reported

double DSC

peak [II]

_one is

tempted

to attribute this

doubling

to the

thermodynamic signature

of the Ti and T2 transitions.

Besides,

the

possibility

of several successive

phase

transitions has

recently

been

suggested

from theoretical arguments

[18].

The present observations

modify substantially

the current

understanding

of the orientational

ordering

in solid

C60.

Three types ^{of diffuse}

scattering

have been identified. Two of them _are

produced by

fluctuations of the two structural transitions at Ti and T2 and

they correspond

to s.c. and 2a-f.c.c. intermolecular

correlations, respectively.

The associated diffuse scatter-

ing intensity

has

a q

dependence

that matches the _s-c- and 2a-f.c.c.

reciprocal

lattices and thus varies

rapidly

with _q. It is worth

pointing

out that _a

large

fraction of the diffuse scatter-

ing disappears

below T2 which

implies

_a

significant

^{decrease of} orientational disorder at this temperature.

The third type of diffuse

scattering persists

to low temperature ^and

exhibits,

in contrast, a

smooth _q

dependence.

^{This indicates that it}

probably originates

from _a one-molecule disorder such _{as an} orientational disorder of uncorrelated C60 ^molecules. The

thermally

activated

uniaxial reorientations which freeze below 85 K [9,

10,

_{19] appear} to be

good

^candidates for

this _source of disorder. In this _case the

corresponding

diffuse

scattering intensity

should level off

below 85 K _{as was} observed for the

intensity

of _a f-c-c- forbidden reflection

[10].

Measurements of the temperature

dependence

of the diffuse

scattering intensity

below T2 should be undertaken to confirm this behavior and also _to search for

possible

effects _near 155 K where the lattice

constant [9] and the

Bragg intensity

^{show anomalies} [10].

Finally

the present ^{results show that} our

understanding

of the orientational

ordering

of the C60 molecules is still

incomplete.

The orientational correlations and the

phase

transitions _are

obviously

_more

complex

than _we thou

ght. Apart

from detailed studies of the above

phenomena

more work is also needed to

clarify

the 155 K

anomaly

[9, 10] ^{and the low} temperature extra reflections not yet identified. The _use of

high quality single crystals

will be crucial to tackle most of these

problems.

Acknowledgements.

We

gratefully acknowledge

R-L

Whetten,

F.Diederich and K.Holczer for

providing

_us with the

purified C60 Powder.

Note added in

prooE

Upon completion

of this work _we found that _some

crystals

behave

differently, though they

were taken from the _same batch and _are cubic with the _same cell constant. These

crystals

do not exhibit the lower transition at T2 and their diffuse

scattering intensity

modulations _are weaker. Somehow this _seems to be related to the

crystal

habit because the present

sample

_was in the

shape

of _a

trigonal plate

while the "different"

crystals

_are octahedral.

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