MOLECULAR-MOTION AND PHASE-TRANSITION IN K3C60 AND RB3C60 BY NUCLEAR-MAGNETIC-RESONANCE

Nuclear magnetic resonance (NMR) has been performed to study the molec ular dynamics of the C-60 molecule in K3C60 and Rb3C60 It is shown tha t the hypothesis of a uniaxial rotational motion around an axis fixed in the crystal can satisfactorily explain the observed C-13 NMR result s. A resonance line is detected in the K-39 and Rb-87 NMR spectra belo w a temperature T of 200 and 380 K in K3C60 and Rb3C60, respectively. In particular, this T in K3C60 coincides with the temperature at whi ch we observed a phase transition. This provides some clue about the o rigin of the phase transition. The discrepancy between the observed an d calculated C-13 line shapes at low temperature suggests that the fre ezing of the molecular motion and a structural change at the phase tra nsition induce a significant modification of the electron density on t he C-60 molecule and likely on the conduction band electronic state.