Rotational dynamics of (formula presented) in (formula presented)

The orientational dynamics of (Formula presented) in the ammoniated fulleride (Formula presented) are studied by the neutron inelastic-scattering technique. Well-defined excitations are observed in the low-energy spectra with energies varying between 3.11(7) and 2.38(5) meV, as the temperature increases from 30 to 310 K. The dependence of their intensities on the scattering vector is consistent with their assignment to small-amplitude librational motion of the (Formula presented) ions. The librations in (Formula presented) are both softer and broader than those in the parent fulleride (Formula presented) reflecting the existence of a weaker and more anisotropic orientational potential upon ammoniation. The estimated barrier of the hindrance potential, (Formula presented) is approximately half the size of that in (Formula presented) and comparable to that found in the primitive cubic fulleride (Formula presented) The temperature dependence of the energies and widths of the librational peaks in (Formula presented) is also more reminiscent of the behavior of (Formula presented) rather than (Formula presented) The result has been rationalized by involving crystal-packing arguments, related to the increase in size of the tetrahedral interstices upon ammoniation.