Fullerene rotational dynamics generate disordered configurations that suppress thermal conductivity in superatomic crystals

Abstract

The thermal conductivity of fullerene-based superatomic crystals (SACs) is investigated using molecular dynamics simulations. The temperature-dependent predictions agree with the trends of previous measurements. The thermal conductivity behavior emerges as a result of the C₆₀ molecule rotational dynamics and orientation, which are quantified using the root mean square displacements of the carbon atoms and the relative orientations of the C₆₀s. At low temperatures, the C₆₀s exhibit small rotations around equilibrium positions (i.e., librations). When the librating C₆₀s are orientationally-ordered, as in the [C₆₀] and [Co₆Se₈(PEt₃)₆][C₆₀]₂ SACs, thermal conductivity decreases with increasing temperature, as is typical for a crystal. When the librating C₆₀s are orientationally-disordered, however, as in the [Co₆Te₈(PEt₃)₆][C₆₀]₂ SAC, thermal conductivity is lower and temperature independent, as is typical for an amorphous solid. At higher temperatures, where the C₆₀s in all three SACs freely-rotate and are thus dynamically disordered, thermal conductivity is temperature independent. The abrupt changes driven by the C₆₀ dynamics suggest that fullerene-based SACs can be designed to be thermal conductivity switches based on a variety of external stimuli.