Pressure dependence of the structural phase transition in C60

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Pressure dependence of the structural phase transition in C60

G. Kriza, Jean-Claude Ameline, D. Jérome, A. Dworkin, H. Szwarc, C. Fabre, D. Schütz, A. Rassat, P. Bernier, A. Zahab

To cite this version:

G. Kriza, Jean-Claude Ameline, D. Jérome, A. Dworkin, H. Szwarc, et al.. Pressure dependence of the structural phase transition in C60. Journal de Physique I, EDP Sciences, 1991, 1 (10), pp.1361-1364.

�10.1051/jp1:1991212�. �jpa-00246418�

J

Phys.

Ifmnce 1

(1991)

1361-1364 CK3DBRE1991, PAGE 1361

Classification

lfiysicsAbstnwtr

64.70K

Short Communication

Pressure dependence of the structural phase transition in C60

G.

Kdza(I)(*),

J.-C, Amefine

(I),

D.

J6rome(1),

A~

Dworkin(~),

H.

Szwarc(~),

C.

Fabre(3),

D,

Schiitz(3),

A~

Rassat(3),

R

Bemier(4)

and A~

Zahab(4)

(1)

Laboratoire de

Physique

des

Solides,

Universit6

Paris~ud,

91405

Orsay,

France

(~)

Laboratoire de Chimie

Physique

des Mat6riaux

Amorphes,

Universit6

Paris~ud,

91405

Orsay,

France

(3)

Laboratoire d'Activation

Mo16culaire,

Ecole Normale

Sup6rieure,

24 _rue

Lhomond, 75231Paris,

France

(4) Groupe

de

Dynamique

des Phases

Condens6es,

Universit6 de

Montpellier,

34k0

Montpellier,

France

(Received

5

August

1991,

accepted

7August

1991)

R4kum6. L'eflet de la

pression hydrostatique

_sur la

temp6rature

de la transition structurale du

Cw

cdstallin _est 6tud16 par

analyse thermique

difl6rentielle dans la gamme de

pressions

I bar-5 kbar. La

temp6rature

de transition

augmente

lin6airement suivant la

pression

avec une

pente

^de

I1,7

K/kbar.

Une limite

supdrieure

de la variation relative du

parambtre cristallin,

obtenu h

partir

de la relation de

Clausius-Clapeyron,

est

bala

= 2,5 _x

10~3

Abstract. The effect of

hydrostatic

_pressure on the

temperature

^{of the structural}

phase

^transition

in

crystalline

C6ois

investigated by

differential thermal

analysis

in the pressure range ^{of I bar} to 5 kbar.

The transition temperature ^increases

linearly

vith pressure ^at a rate of I1.7 K/kbar. An upper limit of the relative

change

of the lattice parameter, obtained ~om the

Clausius-Clapeyron equation,

is /ha

la

= 2.5 _X

10~~

One of the first

problems

related _to

crystalline cw

attacked

by

various

experimental investigations

is the nature of the structural

phase

transition

taking place

at about 260 K This transition has been detected

[I] recently by

differential

scanning calorimetry (DSC),

and later characterized [2]

by high-resolution powder

x-ray diffraction measurements.

At room

temperature

^the

^Cw

^molecules are centered at the vonices of _a face-centered _cu- bic

(fcc)

Bravais lattice

[2, 3].

^{Below the}

phase

transition the molecules in the fcc unit cell be- come

inequivalent

and the

low-temperature phase

has _a

simple

^cubic

(sc)

^{Bravah lattice}

_[2].

^The

room-temperature shape

of the

13C

NMR line

[4, 5] suggests

a

high-speed isotropic

rotation of

molecules,

and the

signs

of molecular motion

gradually disappear

when

decreasing

the

tempera-

ture to 77 K

connecting

all these

observations, Heiney

_et al. [2] identified the transition with the

(*)

Permanent address: Central Research Institute for

Physics,

PO. Box

49,

H-1525

Budapest, Hungary.

1362 JOURNALDEPHYSIQUEI N°10

freezing

of molecular motion, The NMR

lineshapes

for

temperatures

as low _as 123

Kj however,

stiff indicate _a

nearly botropic rotation,

which is difficult _to reconcile with the above

interpreta-

tion.

In this

paper

we

investigate

the effect of

hydrostatic _pressure

_on the

temperature

of the

phase

transition in the range ^{of I bar to} 5 kbar. We find that the transition

temperature

^increases

linearly

with

pressure

at a rate of

dTc /dP

₌ 11.7

K/kbar,

16

prepare

our

sample,

soot

containing

different fullerenes _was obtained in _a

procedure

similar to that described

by

^Haufler et al

[6].

^{A mixture of}

approximately

809b of

cw

and 209b of

Cm

was

extracted from the soot with

boiling

toluene.

Cto

was then

separated

from the mixture

by

column

chromatography

on neutral alumina

[7~ followed

by

solvent

drying.

The

purity

of

cm

was checked

by high-performance liquid chromatography, infrared-, ultraviolet-,

and _mass

spectroscopy.

No

signal

attributed _to

impurities

was detected. Before the thermal

analysis,

the

sample

_was

subjected

to a further heat treatment in _{vacuum at} 440 K for 72 hours.

First,

ambient

pressure

^DSC measurements have been

performed

_{[8] on} a

5-mg sample

in a Perkin Elmer DSC-2C calorimeter. The measured onset

temperature

^and

enthalpy change

of the

transition is

Tc

₌ 256.8 K and 257.6 _{K~ and} AH

= -6.8

kJ/mol

and 7.I

kJ/mol

_upon

cooling

and

heating, respectively.

The value of

enthalpy change

is

significantly higher

and the transition is

sharper

than in

previous

measurements,

indicating

_a

good crhtallinity

of the

sample _[8].

For the differential thermal

analysh (DTA)

under

pressure,

⁸ mg of

Cto powder

was loaded in _a

cylindrical

teflon

capsule (inner

diam _x

height

₌ 2.5 _{mm x} 3 _mm, wall thickness ₌ I

mm).

One reference

junction

of _a Chromel-Constantan

thermocouple

was immersed into the

c6o

_pow-

der,

while the other _{one was} anchored to the outside surface of the

capsule.

The

capsule

_was

mounted _on one of the

opturators

of a

He-gas-pressure

^cell, lb increase the

accuracy

^of tem-

perature determination,

a second

thermocouple

measured the

temperature

^{difference between}

the

sample

chamber and a silicon-diede thermometer situated _on the outside wall of the

pressure

cell. The cell _was

cooled/Warmed by nitrogen

_{gas flow.} The helium gas

pressure

was monitored

by measuring

the reshtance of _a

manganin

Mire

kept

at room

temperature.

bar

~ ^{4 kbar}

~

_{2 kbar}

_j__/_

f __j__

~ ^-

f

2

-~-- ~

~

~j~

S kbar

~

~

3 kbar I kbar

Temperature (K)

Fig.

I. DTA _{scans at} the

phase

transition under various pressures. AT is the temperature ^difference

between the

Cw sample

and the reference

point.

Arrows indicate the direction of temperature scan. The onset temperature of the transition is defined _as the

intercept

of _a

straight

line fitted to the

leading edge

of the

peak

with _a horizontal baseline measured far from the

peak.

Results of

temperature

scans under various

pressures

are shown in

figure

I. The _{scan rate was}

approximately

I

K/min

in all _cases. The

hysteresis

of the onset

temperatures

^is

always

^smaller

N° lo PRESSURE DEPENDENCE OF THE PHASE TRANSI'lTON IN C6o 1363

than 2 K We _cannot rule out the

possibility

^{that the}

hysteresis originates

from unknown

temper-

ature

gradients

in the

high-thermal-inertia _pressure

cell. The _onset

temperatures

as a function of

pressure

are shown in

figure

2. The transition

temperature

^increases

linearly

with

pressure

^at a rate of

dTc /dP

= 11.7 K

/kbar.

No reliable estimate can be made on the

pressure dependence

of the

enthalpy

^of the transition since _the

response

^{of the} measurement

setup

varies with

pressure

in _an unknown way.

Pressure

Fig.

Z Pressure

dependence

of the transition

temperature.

^The error bars indicate the

hysteresis

of the onset temperatures. ^The

straight

line is a linear

fit;

the

slope

is

dTc/dP

= I1.7

K/kbar.

The absence of

hysteresh

in

early

DSC measurements

[1, 2]

as well as the smooth decrease of the

simple

^cubic order

parameter upon approaching Tc

from below

suggest

a second -order transition.

Our

ambient-pressure

DSC measurements,

however,

indicate the existence of _a small

hysteresh

in the order of IK~ so we believe that the transition is of

(weakly)

first order. An

upper

^{limit can be}

given

for the

change

of the molar volume

assuming

that the

enthalpy

^of the transition

originates entirely

from the latent heat: from the

Clausius-Clapeyron equation

Av ₌ 3.2 _x

10~~ m3/mol

is

inferred, corresponding

to a relative

change

in the lattice

parameter

^{of ha}

la

₌ 2.5 _x

10~~.

Our results

suggest

^{that in the}

x-ray-diffraction compressibility

measurements of reference [9]

lattice

parameters

^{of the two different}

phases

were

compared.

At the

temperature

^{of these} mea- surements

(293 K)

we find the

phase

transition at 3

kbar,

while the data of reference [9] were collected at I bar and 12 kbar. A

possible

reason of the failure of

detecting

the

low-temperature phase

at 12 kbar is that marked differences in the diffraction

patterns

^{of the two}

phases _appear only

at wave numbers

higher

than those

investigated

in the

high-pressure

cell. Our

upper

^estimate of the

change

in the lattice

parameter

at the transition

gives

a 9ib maximal downward correction to the overall

compressibility.

Further structural studies and the use of local

probes

_may

clarify

the nature of the transition.

References

Ill

DwoRnN A. et

aL,

C. R. Hebd Acad Sci Pa1i§ Ser II 312

(1991)

W9.

[2] ^{HEINEY PA.} et

al, Phys.

Rev Lett. 66

(1991)

^2911.

[3] FWWNG R.M. etaL, Mater Res. Sac.

Spqp.

Pnc.

(to

be

published);

this work is

only

known to the authors ~om references.

1364 JOURNAL DE PHYSIQUE I N°10

[4] TYcKo R. et

al,

f

Phys.

^Chew 95

(1991)

518.

[5l ^{YANNONI C-S-} etaL, ibid. 95

(1991)

9.

[fl

HAUFLER R.E. et

al,

ihd. 94

(1990)

8634.

Ii

ALLEMAND PM. et al, L Am Chew Sac. l13

(1991)

1050.

[8] DwoRKlN A. et aL, C. R. Hebd Acad Sci Pani Ser II

(to

be

published).

[9] FISCHER J.E. et aL, Science 252

(1991)

1288.