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INTRODUCTION AjC6o was not discovered [1] until well after the other phases of alkali intercalated C 60 were known, and it was found to undergo a number of structural and electronic phase transitions.[1, 2, 3, 4, 5, 6, 7, 8] The existence of a phase consisting of linear polymer chains,[8] which is stable in air,[9, 10] has stimulated recent interest in this material and could help in developing applications for alkali-doped fullerenes. At high temperatures ( > 100°C), RblC 60 has an fcc rocksalt structure [4] and is electrically conducting.[2, 3, 5, 6] When slowly cooled, the specimen undergoes a first-order phase transition to an orthorhombic phase,[7] forming linear chains of C60 molecules along the a direction (the face-diagonal direction of the fcc phase).[8] These chains have been proposed to be [2 + 2] cycloaddition C 60 polymers.[1 1] When RbiC 60 samples are cooled extremely rapidly from the fcc phase to below -0°C a metastable phase is produced.[3, 6] This "quenched" phase is insulating [6] and relaxes back to the stable polymer chain phase, with a strongly temperature dependent timescale.[3, 6] Dimers forming from [2 + 2] cycloaddition or a single bond between two free radical C 60 molecules have been suggested as a possible structure.[12] Recent x-ray scattering structural studies confirm a dimerized structure, but are unable to determine the exact bonding between the fullerene molecules [13]. Vibrational spectroscopy can help determine the symmetry of the C6 0 molecules in this quenched phase by observing which vibrational modes are infrared- (IR) or Raman-active. In this paper we show that the IR and Raman vibrational spectra of the slowly cooled and quenched states are indeed consistent with polymerized and dimerized C 60 structures, respectively.[14] Using this C 60 dimer picture, we are able to show that many of the characteristic properties of this phase are understood.

EXPERIMENTAL The C 60 films used for the present study were evaporated and doped in vacuum-sealed pyrex sample chambers described in detail previously.[15] Si substrates were utilized for the 319 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society

IR measurements, and MgO substrates were used for the Raman measurements. During doping, IR spectra obtained in situ were used to monitor the stoichiometry of all samples by measuring the position of the Fl,(4) vibrational mode.[16, 17] Once Rb, C 60 samples with the majority phase x = 1 were produced, these rugged, portable chambers allowed a number of measurements under a variety of thermal conditions. A copper block with a heater resistor and thermocouple was attached to each chamber in order to regulate and measure the sample temperature by means of a temperature controller. Quenching was accomplished by pouring liquid nitrogen directly on the sample chambers. Typically, the sample temperature dropped from 1250C to -200'C in approximately 30 seconds. IR spectra were obtained at 2cm-1 resolution on a Bomem MB-155 Fourier transform spectrometer and ratioed to spectra o