Rotational dynamics of ${\mathrm{C}}_{60}$ in $({\mathrm{ND}}_3{)\mathrm{K}}_3{\mathrm{C}}_{60}$

The orientational dynamics of ${\mathrm{C}}_{60}$ in the ammoniated fulleride $({\mathrm{ND}}_3{)\mathrm{K}}_3{\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}^{3-}$ ions. The librations in $({\mathrm{ND}}_3{)\mathrm{K}}_3{\mathrm{C}}_{60}$ are both softer and broader than those in the parent fulleride ${\mathrm{K}}_3{\mathrm{C}}_{60},$ reflecting the existence of a weaker and more anisotropic orientational potential upon ammoniation. The estimated barrier of the hindrance potential, $E_a\sim 250\hspace{0.5em}\mathrm{meV}$ is approximately half the size of that in ${\mathrm{K}}_3{\mathrm{C}}_{60}$ and comparable to that found in the primitive cubic fulleride ${\mathrm{Na}}_2{\mathrm{RbC}}_{60}.$ The temperature dependence of the energies and widths of the librational peaks in $({\mathrm{ND}}_3{)\mathrm{K}}_3{\mathrm{C}}_{60}$ is also more reminiscent of the behavior of ${\mathrm{Na}}_2{\mathrm{RbC}}_{60}$ rather than ${\mathrm{K}}_3{\mathrm{C}}_{60}.$ The result has been rationalized by involving crystal-packing arguments, related to the increase in size of the tetrahedral interstices upon ammoniation.