Can Small Free Methane Clusters Exhibit Phase-like Transitions?

Abstract

Low-temperature (below 60K) phase changes of free clusters containing 50, 137, and 229 methane molecules have been observed in isoenergetic Molecular Dynamics computations. Bulk solid methane exhibits structural phase transformation at 20.4 K. However, clusters of 50 molecules already melt at about 25 K, which makes the observation of solid-solid transformations rather challenging. Molecular Dynamics calculations of isoenergetic free methane clusters show that they exhibit transformations from an orientationally-ordered (~ 10K) and a disordered phase (above 20K). There is no hysteresis in the cooling-heating regime - a signal for continuous transition.