Rotational dynamics of C60 in superconducting K3Ba3C60

The orientational dynamics of C60 in superconducting K3Ba3C60 are studied by neutron inelastic scattering. Low-energy excitations are present near ∼6 meV between 150 and 320 K and are assigned to small-amplitude librations of the C609− ions. They are harder and broader than those measured in K3C60, reflecting the existence of a stronger and more anisotropic orientational potential. The anisotropy and dispersion effects are also larger than those in isostructural Rb6C60, as a result of positional disorder of the Ba2+ and K+ ions, which reside in distorted tetrahedral interstices. The estimated barrier of the hindrance potential, Ea, is ∼1.4 eV.