Lithium diffusion and C-60 dynamics by quasielastic and inelastic neutron scattering in Li12C60 fulleride

We have performed inelastic and quasielastic neutron-scattering experiments (IQENS) on the highly lithium doped fulleride Li12C60 covering the T range from 150 to 600 K, across a tetragonal to cubic fee phase transition which was shown to occur around T = 550 K by synchrotron x-ray diffraction. In t his work we focus on the dynamics of the Li ions and C-60 molecular units. The QENS data suggest a dynamical model in which the Li ions diffuse in the confined environment of the octahedral void of the pristine C-60 molecule, in agreement with the structural information that we have obtained on the same compound. These results are discussed together with our previously rep orted analysis of the changes in the Vibrational density of states across t he phase transition. The emerging picture is that Li12C60 at high temperatu re (fce phase) has the same dynamical features as pure C-60 With the additi on of Li localized diffusion within the octahedral void, while the low-temp erature tetragonal phase is characterized by a broadening and disappearance of many dynamical features, i.e., of the Li diffusion and of the C-60 libr ations and molecular modes.