Magnesium Doping Enhances Thermal Conductivity of Polymerized Fullerene Crystals
CJ Dionne and A Giri, JOURNAL OF PHYSICAL CHEMISTRY C, 126, 17406-17414 (2022).
The polymerization of C60 molecular crystals through external stimuli, such as the inclusion of alkali earth metals, offers tunable control of their physical properties through replacing weak van der Waals interactions between the fullerenes with strong covalent bonds. Herein, we use systematic atomistic simulations to uncover the drastic enhancement in thermal conductivity of two-dimensional C60 polymers along the plane forming a periodic nanocluster network of covalently bonded fullerene molecules as compared to the nonbonded fullerite structure. Moreover, we find that the thermal conductivity along the direction with two covalent bonds connecting the adjacent fullerene molecules is higher than the direction with only a single bond between the molecules, which provides a tunable knob to tailor their thermal properties. The addition of magnesium atoms in the polymerized C60 structure to form a Mg2C60 crystal is shown to reduce this anisotropy through impurity scattering. More interestingly, while the thermal conductivity is reduced at lower temperatures due to magnesium doping, we find that the thermal conductivity of the metal-doped structure can be higher than the pristine polymer C60 at relatively higher temperatures. We attribute this to the reduced temperature dependence of thermal conductivity in our Mg2C60 structure due to impurity scattering, while the dominance of anharmonic phonon scattering processes drives the reduction in thermal conductivity of the pristine polymer C60 crystal at high temperatures. Spectrally decomposed thermal conductivity calculations show that the heat-carrying vibrations in the metal-doped structure are shifted to lower frequencies due to the preferential scattering of higher frequency vibrations driven by the impurity atoms. An improved understanding of the heat transfer mechanisms in polymerized C60 molecular crystals offers valuable insights into their potential use in various applications such as C60-based electrodes for batteries.
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