Monte Carlo simulations identify cI16 as mechanically metastable for classical hard spheres

Among the simplest models to exhibit a solid-fluid phase transition are the single phases of hard spheres, and they provide fundamental theoretical insight into atomic, molecular and colloidal solids. In The Journal of Chemical Physics, the authors examine the mechanical stability of the body-centered cubic (bcc) structure for classical hard spheres using Monte Carlo computer simulations. They simulated systems comprised of several hundred spheres initiated in a bcc structure, and followed their structural evolution over hundreds of thousands of moves. The results were consistent with the established notion that the bcc lattice is unstable for hard spheres, possibly to a greater degree than previously believed. The authors further suggest that the bcc structure may not even qualify as a metastable phase that can be maintained through constraints.

During the simulation, the bcc structure underwent rapid transformation into one of four non-bcc structures, which include three amorphous structures and the cI16 structure. cI16, a cubic crystalline structure with 16 particles in each unit cell, has been experimentally demonstrated in lithium and sodium solids under high pressure. The study finds the cI16 structure to be mechanically stable for hard spheres. Free energy calculations further revealed the cI16 structure to be thermodynamically metastable relative to the face-centered cubic (fcc) or the hexagonal close packed (hcp) structures.

The authors look forward to seeing if the addition of other interactions or restrictions can make the cI16 structure thermodynamically stable under certain conditions. Another potential study would repeat the exercise for soft repulsive spheres, since the bcc structure can be thermodynamically stable for such systems.

Source: “On the mechanical stability of the body-centered cubic phase and the emergence of a metastable cI16 phase in classical hard sphere solids,” by Vadim B. Warshavsky, David M. Ford, and Peter A. Monson, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5009099 .