Molecular motion and phase transition in K 3 C 60 and Rb 3 C 60 by nuclear magnetic resonance

Nuclear magnetic resonance (NMR) has been performed to study the molecular dynamics of the ${\mathrm{C}}_{60}$ molecule in ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$. It is shown that the hypothesis of a uniaxial rotational motion around an axis fixed in the crystal can satisfactorily explain the observed $^{13}\mathrm{C}$ NMR results. A resonance line is detected in the $^{39}\mathrm{K}$ and $^{87}\mathrm{Rb}$ NMR spectra below a temperature ${T}^{*}$ of 200 and 380 K in ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$, respectively. In particular, this ${T}^{*}$ in ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ coincides with the temperature at which we observed a phase transition. This provides some clue about the origin of the phase transition. The discrepancy between the observed and calculated $^{13}\mathrm{C}$ line shapes at low temperature suggests that the freezing of the molecular motion and a structural change at the phase transition induce a significant modification of the electron density on the ${\mathrm{C}}_{60}$ molecule and likely on the conduction band electronic state.