Infrared Studies on C 60 Polymers
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K. KAMARAS*, L. FORRO**, Y. IWASA***
Research Institute for Solid State Physics, P. 0. Box 49, H 1525 Budapest, Hungary
*
Dpartement de Physique, Ecole Polytechnique F~derale de Lausanne, CH 1015 Lausanne, Switzerland *** JAIST, Tatsunokuchi, Ishikawa 923-12, Japan **
ABSTRACT
We present infrared spectra of linear and planar polymeric structures consisting of C 6 0 balls. The splittings of infrared lines can be explained fairly well on the basis of symmetry reduction, indicating that the structures are ordered. Increasing the temperature results in breaking of intermolecular bonds. This process is reversible in the alkali salts but irreversible in the neutral rh-C6 o polymer; we conclude that charged C 60 balls are more stable in polymer form than as monomers, while for neutral C 60 the situation is reversed.
INTRODUCTION Among the many intriguing features of C6 0 is the ability to form several one- and twodimensional polymers. These can be obtained by photoreaction [1], at high temperature and high pressure [2], or following ionization by alkali metals [3]. Infrared spectroscopy is intimately related to the symmetry changes on polymerization and is therefore a very good indicator of the nature of the bonds forming [4]. In this paper, we compare the infrared spectra of the one- and two-dimensional polymers RbC 60 and rh-C6 0 , respectively, with the expected changes based on reduction of symmetry from C 6 0. We also follow the thermal breaking of the intermolecular bonds and the reversibility of these processes.
EXPERIMENTAL
RbC6 0 and rh-C 6 0 have been prepared as discussed previously [2, 5]. Infrared spectra 1 were taken in KBr pellets on a Bruker IFS 28 infrared spectrometer with 1 cm- resolution. For the temperature dependence, we employed a cold finger cryostat with liquid nitrogen as coolant and KBr windows.
RESULTS AND DISCUSSION
Room temperature spectra are shown in Fig. 1 and the positions of the lines are listed in Table 1. For the explanation of the splittings we have to apply group theory and construct a correlation table as in Table 2. According to group theory, the principal FI• lines allowed in Ih symmetry should show a twofold splitting in rh-C6 0 (symmetry Dad) and a threefold 937 Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
Table 1: Infrared lines of C60 , rh-C 60 and RbC 60 at room temperature. rh-C 6o RbC 60 C60 rh-C 60 RbC 60
C6 o 526
576
509 524
509 517 527
550 555
541 554 570
609
607
1121 1206
1128 1195 1210
697 701
1306
1290
1383 1406
1340 1386 1405
775 965 997
1016 1182
707 718 743 763
722 726 732 747 755 760
774
1429
splitting in RbC 60 (symmetry D 2h), respectively. The modes activated by reduced symmetry are also expected to appear as doublets in rh-C6o and triplets in RbC 60 . Indeed, from Table 1 it is apparent that most lines behave according to these predictions, indicating little or no disorder in both materials.
"60
RbC6 0
0_
E CD, CU
I'Y"
600
•
800
rh-Co
1000
1200
1400
1600
Frequency (cm- 1) Figure 1. Infrar
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