Rotational dynamics of C-60 in (ND3)K3C60

The orientational dynamics of C-60 in the ammoniated fulleride (ND3)K3C60 a re studied by the neutron inelastic-scattering technique. Well-defined exci tations are observed in the low-energy spectra with energies varying betwee n 3.11(7) and 2.38(5) meV, as the temperature increases from 30 to 310 K. T he dependence of their intensities on the scattering Vector is consistent w ith their assignment to small-amplitude librational motion of the C-60(3-) ions. The librations in (ND3)K3C60 are both softer and broader than;those i n the parent fulleride K3C60, reflecting the existence of a weaker and more anisotropic orientational potential upon ammoniation. The estimated barrie r of the hindrance potential, E(a)similar to 250 meV is approximately half the size of that in K3C60 and comparable to that found in the primitive cub ic fulleride Na2RbC60. The temperature dependence of the energies and width s of the librational peaks in (ND3)K3C60 is also more reminiscent of the be havior of Na2RbC60 rather than K3C60 The result has been rationalized by in volving crystal-packing arguments, related to the increase in size of the t etrahedral interstices upon ammoniation. [S0163-1829(99)13401-5].