Thermal Stability of the Solidlike and Liquidlike Phases of (C60)n Clusters

Abstract

We report extensive simulation results on the finite temperature behavior of small clusters of C60 molecules. The interaction between C60 molecules is described by a simple all-atom model, as well as the Pacheco-Prates-Ramalho united-atom potential which includes 2- and 3-body terms. We perform parallel tempering Monte Carlo simulations in both the microcanonical and canonical ensembles. By enclosing the clusters into a hard-wall sphere of increasing radius R and by calculating the thermodynamical properties, we are able to distinguish the boiling phase change from the melting phase change in the case of (C60)7, but not in the case of (C60)13. Analyzing the isomers distributions for n = 7 shows that the liquidlike state of (C60)7 can be unambiguously defined in a range of temperature below 800 K. The influence of the internal degrees of freedom appears to be limited to an extra peak in the heat capacity near 80 K which characterizes the orientational phase change. Finally, we calculate the average kinetic energy release after evaporation from molecular dynamics simulations, and we compare these results with the predictions of the phase space theory (PST) for different values of R. We show that the PST results provide a clear definition of the boiling energy, and confirm that the largest 13-molecule cluster sublimes molecules as soon as it melts.