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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