Journal of Superconductivity, VoL 8, No. 5, 1995
Optical spectroscopy on Monomeric and Polymeric 1:1
Fulleride Salts
K. KamarAs, 1 D.B. Tanner, 2 L. ForrS, 3 M.C. Martin, 4 L. Mihaly, 4 H. Klos, 5 and
B. Gotschy 5
Received 5 January 1995
We compare infrared spectra of the C60 monoanion in different solid-state structures with
each other as well with that of the neutral molecule. We relate the
shift and
splitting of
the Tu infrared modes to the strength and anisotropy of electron-phonon coupling in those
environments.
KEY WORDS: Fullerenes; optical properties; polymers
1. INTRODUCTION
C60, the prime example of the recently discov-
ered fullerenes, tends to form ionic salts by attract-
ing up to 6 electrons. Of these, the trianions are the
most extensively studied due to the A3C60 super-
conducting compounds, where A is an alkali atom.
However, intriguing solid-state effects can be found in
other phases as well. The most recent examples are
the alkali salts with 1:1 composition. These materials
undergo several phase transitions depending on ther-
mal history. There is growing evidence for an orthog-
onal phase (ortho-I) consisting of covalently bound
polymers [1,2] and another one (ortho-II) of dimers
[3]. Vibrational spectroscopy has been used to iden-
tify the valence state of anions in C60 salts [4,5] and
it was shown early on that electron-phonon interac-
tions play a significant role in the infrared properties
of the salts [6]. In this study, we compare the infrared
spectra of different phases of RbC60 to (Ph4)2C60I,
a charge transfer salt containing C60 monoanions.
1Research Institute for Solid State Physics, Budapest, Hun-
gaxy H 1525
2Dept. Physics, University of Florida, Gainesville, FL 32611
3Depaxtement de Physique, ]~cole Po]ytechrtlque Fdderale de
Lausanne, Lausanne, Switzerlmld CH-1015
4Department of Physics, SUNY
at
Stony Brook, Stony Brook,
NY 11794
5Urtlversity of Bayreuth, Germany D 95440
621
2. EXPERIMENTAL
(Ph4)2C60I crystals were grown electrochem-
ically [7]. The process results in a composi-
tion which ensures -1 charge on the fullerene ball.
RbC60 powder was prepared by reacting C60 and
Rb at high temperature. Infrared spectra of the
tetraphenylphosphonium salt crystals were taken un-
der an infrared microscope in reflectance mode at
room temperature; the rubidium compound was
ground into KC1 pellets and the transmission mea-
sured against a pure KCI reference at each tem-
perature. The room-temperature stable phase of
l~bC60 is the ortho-I structure; it transforms to
fcc above 400 K and from there can be quenched
into the ortho-II form which is metastable but has
a long relaxation time when kept under 250 K.
Unlike the superconducting salts or the fer-
romagnetic TDAE-C60, both our materials are
stable in air thus requiring no special precau-
tions during measurements; a slight decomposi-
tion into C60 is observed when heating RbC60
but this does not affect our main conclusions.
3. RESULTS AND DISCUSSION
Fig. 1. shows the section of the infrared spec-
trum containing the two high-frequency infrared-
active vibrations of Cr (To facilitate the compar-
ison, we present the data as absorption, calculated
by Kramers-Kronig analysis of the reflectance and as
0896-1107/95/1000-0621507.50/0 9 1995 Plenum PubLishing Corporation
622 Kamarfis
et al.
A = -logT from transmission.) The two modes in
question are found at 1183 and 1429 cm -1, respec-
tively, in pure C60.
I ' ' ' ' I ' ' ' ' I
<
(Ph4P)2C6ol
1200 1300 1400
Frequency
(cm "1)
Fig. 1. Infrared spectra of C60 , (Ph4)2C60I and
the fcc, ortho-I and ortho-II phases of RbC60.
The somewhat complicated frequency shifts of
the two T,~ modes can be understood taking into
account two solid-state effects: the change in lat-
tice constant and the electron-phonon interaction.
If the distance between fulleride ions gets shorter,
the modes should harden in absence of other per-
turbations; electron-phonon interaction, on the other
hand, causes softening whose magnitude depends on
the strength of the coupling. As shown both ex-
perimentally [4,5] and theoretically[6], the highest-
frequency mode has a much larger electron-phonon
coupling constant than the 1183 cm -1 mode. There-
fore, the former is indicative of the coupling, while
the latter reflects the changes in lattice constant.
In (Ph4)2C60I the C60 ions are separated from
each other by the bulky organic cations. Thus this
compound can be regarded as a prototype of an iso-
lated monoanion in a crystalline environment. The
shift in the T~(4) mode from 1429 to 1394 cm -1
results from the on-ball electron-vibrational interac-
tion. (Note that there are several modes of the coun-
terion present, however, the fullerene modes can be
clearly identified.) This mode does not shift appre-
ciably on going to the fcc phase, indicating that the
fcc phase can also be regarded as an ensemble of C60
ions, interacting with the electron on the ball but
not with each other. The ortho-I phase shows in-
creased softening to 1385 cm -1, along with a splitting
into three components, at 1340, 1385 and 1406 cm -1
[5]. The softening can be related to an increased in-
teraction with the collective electronic system and
the splitting to the one-dimensional character of the
polymeric chains, leading to the lifting of the three-
fold degeneracy of the Tu modes. Finally, the ortho-
II phase has a definite reduction in symmetry, with
many more modes showing, but the vibration in ques-
tion shifting up in frequency, indicative of the lattice
contraction being more important than the electron-
phonon coupling.
In contrast, the position of the T~,(3) mode at
1183 em -1 is determined by structural effects. In the
fcc phase this peak is at the same position as in neu-
tral C60. The slight downshift in the phosphonium
salt reflects the increased distance between fullerenes
in that lattice. The significant contraction in the or-
thorombic phases leads to a hardening in both ortho-I
and ortho-II RbC60. As expected, the hardening is
more pronounced in the ortho-I phase, which shows
a stronger compression along the [110] direction.
ACKNOWLEDGEMENTS
Work supported by the U.S.-Hungarian Joint
Fund Grant No. 271, the Bundesminister fiir
Forschung und Technologie (Berlin, Germany) and
by the European Community Grant No. CIPA-CT-
93-0032.
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