Raman-scattering study of isotopically engineered crystalline C
60
P. J. Horoyski and M. L. W. Thewalt
Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6
T. R. Anthony
GE Corporation Research & Development Center, General Electric Company, Schenectady, New York 12301
Received 10 January 1996
A high resolution Raman scattering study of the intramolecular phonons of crystalline C
60
is presented.
Many of the Raman-active vibrations are richly structured, revealing a crystal field splitting both above and
below the orientational ordering transition at 260 K. Much of the vibrational fine structure is also shown to be
strongly influenced by the small amount of orientational, or merohedral, disorder which persists in the low-
temperature phase. The impact of isotopic disorder is examined by the comparison of Raman spectra of single
crystals made from naturally abundant carbon, 99.95%
12
C, and 99.7%
13
C, as well as a range of intermediate
13
C concentrations. The
12
C
60
and
13
C
60
spectra are identical, apart from a uniform softening of the
13
C
60
vibrational energies by the factor
12
13
1/2
. As the
13
C concentration is increased from approximately zero, the
bulk of the vibrational modes display only a softening in energy and a broadening in width. In sharp contrast
to this expected behavior is the A
g
2-derived band which shows both a softening in energy and additional
splittings with increasing
13
C content. S0163-18299605218-6
INTRODUCTION
The study of vibrational modes in crystalline C
60
has at-
tracted considerable theoretical
1–6
and experimental
7–18
in-
terest for the past several years. Crystalline C
60
is a proto-
typical molecular solid; the intramolecular bonding is much
stronger than the intermolecular bonding, allowing the
phonons of the crystal to be divided into several distinct
types. The phonon spectrum contains high energy modes in
the range of 260–1600 cm
-1
, phonons which originate
from vibrations within the molecular units of the crystal. The
spectrum of these intramolecular phonons coarsely mirrors
the vibrational spectrum of an isolated molecule, although
many optically silent vibrations of the isolated molecule are
active in the crystal,
14,15
and some splittings of the degener-
ate molecular vibrations have been reported for the
solid.
11,13,17
Having no counterpart in an isolated molecule
are the low-energy 20–60 cm
-1
lattice modes of the solid
which consist of translational motions of the rigid molecular
units intermolecular phonons and librational excitations of
the molecules librons. The intermolecular phonons and li-
brons are visible at temperatures below 260 K, the order-
disorder phase transition temperature of the solid. At room
temperature, the C
60
molecules undergo isotropic rotation
19
while positioned at face centered cubic fcc lattice sites,
20
but upon cooling below 260 K, the molecules assume pre-
ferred orientations, forming a simple cubic sc structure
with four molecules per unit cell.
21
While long range orientational order exists in the crystal
below 260 K, a significant fraction of the molecules remain
misoriented. This orientational, or merohedral,
22
disorder is
reduced as the temperature is lowered to 90 K, below
which a second, ‘‘glassy’’ transition freezes in a small
amount of static disorder.
22
Surprisingly, this orientational
disorder has recently been shown to lead to a splitting of
some of the intramolecular vibrations which otherwise show
no splitting due to the static crystal field.
17
Another form of disorder arises from isotopic substitu-
tions of
13
C for
12
C within the molecules. The 1.1% natural
abundance of
13
C implies that nearly half of the molecules in
solid C
60
contain at least one
13
C atom. In a ‘‘conventional’’
crystal such as diamond, the phonons sample a large number
of unit cells, leading to a linear averaging of the isotopic
masses. In this case, the isotopic disorder simply leads to a
shift of the phonon frequencies by m ¯
-1/2
, where m ¯ is the
average atomic mass, along with a broadening of the phonon
linewidths due to the mass fluctuations.
23
A more compli-
cated dependence upon isotopic content could be expected
from the intramolecular phonons of solid C
60
given the mo-
lecular origin of the modes. A Raman-scattering study
17
of
natural and
12
C enriched C
60
has in fact shown that dramatic
isotopic-disorder induced changes occur within the
A
g
2-derived phonon band; in addition to energy shifts, very
pronounced splittings were seen from
12
C
60
to natural C
60
.
In this work we present a high resolution Raman-
scattering study of the intramolecular phonon spectrum of
crystalline C
60
. Fine structure is evident in most of the
Raman-active molecular modes, as well as in two intramo-
lecular modes which are seen in the solid state but are opti-
cally silent in an isolated molecule. The small splittings of
the phonons are resolvable due to the narrow vibrational
linewidths seen in good quality single crystals. The bulk of
the observed splittings are determined not to arise from iso-
topic substitutions by comparing spectra of crystalline C
60
made from naturally abundant, 99.95%
12
C, and 99.7%
13
C
carbon. Several modes do show slightly reduced linewidths
in the
12
C
60
enriched samples, while other modes show vir-
tually no change from the naturally abundant material to the
‘‘pure’’
12
C
60
material a straightforward
12
13
1/2
energy soft-
ening is, however, seen between
12
C
60
and
13
C
60
for the en-
tire phonon spectrum. Splittings due to isotopic disorder
PHYSICAL REVIEW B 1 JULY 1996-II VOLUME 54, NUMBER 2
54 0163-1829/96/542/92010/$10.00 920 © 1996 The American Physical Society