Phase transitions in fullerenes: Fragmentation and reassembly of the carbon cage

TL;DR

This study uses molecular dynamics simulations to investigate phase transitions in fullerenes, revealing oscillations between cage and gaseous states and identifying a temperature range for the transition consistent with experiments.

Contribution

It introduces a new forcefield for fullerene simulations and combines simulation with statistical mechanics to accurately determine phase transition temperatures.

Findings

Oscillations between fullerene cage and gaseous states observed.

Peak in heat capacity indicates first-order phase transition.

Transition temperature estimated at 3800-4200 K, matching experimental data.

Abstract

Phase transition in fullerenes C60 and C240 are investigated by means of constant-temperature molecular dynamics simulations. In the phase transition region, the assembly (and fragmentation) of the C60 cage from (and to) the gaseous state is demonstrated via the dynamical coexistence of two phases. In this critical region, the fullerene system is seen to continuously oscillate between the carbon cage (the solid phase) and the state of carbon dimers and short chains (the gas phase). These oscillations correspond to consecutive disintegration and formation of the fullerene. Furthermore, the temperature-dependent heat capacity of the fullerene features a prominent peak, signifying the finite system analogue of a first-order phase transition. The simulations were conducted for 500 ns using a topologically-constrained pairwise forcefield which was developed for this work. Results of the simulations were supplemented by a statistical mechanics analysis to account for entropy and pressure corrections, corresponding to experimental conditions. These corrections lead to a phase transition temperature of 3800-4200 K for pressure 10-100 kPa, in good agreement with available experimental values.