Fullerene rotational dynamics generate disordered configurations that suppress thermal conductivity in superatomic crystals
Q Liang and M Bartnof and YL He and JA Malen and AJH McGaughey, NANOSCALE HORIZONS, 5, 1524-1529 (2020).
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 C-60 molecule rotational dynamics and orientation, which are quantified using the root mean square displacements of the carbon atoms and the relative orientations of the C(60)s. At low temperatures, the C(60)s exhibit small rotations around equilibrium positions (i.e., librations). When the librating C(60)s are orientationally-ordered, as in the C-60 and Co6Se8(PEt3)(6)C-60(2) SACs, thermal conductivity decreases with increasing temperature, as is typical for a crystal. When the librating C(60)s are orientationally-disordered, however, as in the Co6Te8(PEt3)(6)C-60(2) SAC, thermal conductivity is lower and temperature independent, as is typical for an amorphous solid. At higher temperatures, where the C(60)s in all three SACs freely-rotate and are thus dynamically disordered, thermal conductivity is temperature independent. The abrupt changes driven by the C-60 dynamics suggest that fullerene-based SACs can be designed to be thermal conductivity switches based on a variety of external stimuli.
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