Lithium diffusion and ${\mathrm{C}}_{60}$ dynamics by quasielastic and inelastic neutron scattering in ${\mathrm{Li}}_{12}{\mathrm{C}}_{60}$ fulleride

We have performed inelastic and quasielastic neutron-scattering experiments (IQENS) on the highly lithium doped fulleride ${\mathrm{Li}}_{12}{\mathrm{C}}_{60}$ covering the T range from 150 to 600 K, across a tetragonal to cubic fcc phase transition which was shown to occur around $T=550\mathrm{}\mathrm{K}$ by synchrotron x-ray diffraction. In this work we focus on the dynamics of the Li ions and ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$ molecule, in agreement with the structural information that we have obtained on the same compound. These results are discussed together with our previously reported analysis of the changes in the vibrational density of states across the phase transition. The emerging picture is that ${\mathrm{Li}}_{12}{\mathrm{C}}_{60}$ at high temperature (fcc phase) has the same dynamical features as pure ${\mathrm{C}}_{60}$ with the addition of Li localized diffusion within the octahedral void, while the low-temperature tetragonal phase is characterized by a broadening and disappearance of many dynamical features, i.e., of the Li diffusion and of the ${\mathrm{C}}_{60}$ librations and molecular modes.