The Effect of O 2 Intercalation on the Rotational Dynamics and the Ordering Transition of C 60
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solid C60 . The rotational dynamics and the associated orientational phase transition of pure C60 in the solid state have been the subject of considerable interest. The primary tool for probing the rotational dynamics of C60 has been 13 C nuclear magnetic resonance spectroscopy (NMR). Studies of pure C60 have recently been reviewed by Johnson, Bethune and Yannoni [4]. Intercalated oxygen causes a downfield shift of the 13 C resonance of C60 due to the Fermi-contact interaction between paramagnetic oxygen molecules and the carbon atoms of adjacent C60 molecules. Thus, we observe individual resonances for C 60 molecules surrounded by various numbers of oxygen molecules. Characterization of the relaxation properties of these resonances enables us to study the dynamics of the C60 molecule as a function of the number of surrounding oxygens. The behavior of the total sample is dependent on the loading level of oxygen. However, for a sample with a given loading level, no difference in the dynamics is observed for C 60 molecules surrounded by 1 to 5 oxygen molecules. These observations emphasize the fact that the dynamics and phase transition are cooperative phenomena dependent on the overall rather than local state of the system. This work is supported by the United States Department of Energy under Contract DE-AC0494AL85000.
505 Mat. Res. Soc. Symp. Proc. Vol. 359 e 1995 Materials Research Society
RESULTS AND DISCUSSION Figure 1 shows the 13C solid state spectrum of C60 exposed to 1 kbar of oxygen for 48 hours. The spectrum was recorded at 100.1 MHz on a Bruker AMX-400 using direct polarization and MAS at 6 kHz. Although the relaxation time of the unloaded C60 is approximately 28 s at 298 K and 9.4 Tesla, the relaxation times of all of the resonances of the loaded sample are considerably shorter. A delay time of 60 s between each of the 32 pulses allowed for the complete recovery of the magnetization and results in a quantitative spectrum. The resonance at 143.7 ppm corresponds to C60 without any oxygen molecules in the surrounding octahedral sites. As each of the six surrounding sites are occupied, the resonance undergoes a Fermi-contact shift of 0.7 ppm downfield [5]. The seven resonances correspond to n = 0 to 6 of the octahedral sites being occupied by an oxygen molecule. The distinct resonances permit the investigation of the magnetic relaxation behavior of C60 surrounded by 0 to 6 oxygen molecules.
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Figure 1. 13C MAS NMR spectrum of C 60 exposed to oxygen at 1 kbar for 48 hrs. Drot is 6 kHz. Since the relaxation of the 13 C resonances is dominated by the interstitial paramagnetic oxygen molecules, we first wish to establish that the relaxation still reflects the dynamic behavior of the C60 molecule rather than the spin properties of the oxygen. A paramagnetic species interacts with nuclear dipoles by two mechanisms: (1) direct dipole-dipole coupling and (2) hyperfine exchange coupling via the Fermi-contact interac
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