Rotational Dynamics of C 60 in Na 2 RbC 60

We have measured the low-energy neutron inelastic-scattering (NIS) spectra of superconducting Na 2 RbC 60 in the temperature range 50–350 K. Well-defined librational peaks are observed at 50 K at 2.83(17) meV (FWHM = 1.7(5) meV). They soften and broaden with increasing temperature. Their behaviour mimics that found in solid C 60 and differs markedly from K 3 C 60 . The rotational barrier for C 60 reorientations in Na 2 RbC 60 is somewhat higher than in pristine C 60 and approximately half as large as in K 3 C 60 . An order-disorder transition is anticipated at a temperature higher than that found in C 60 .