Freezing of parallel hard cubes with rounded edges

TL;DR

This study investigates the freezing transition of parallel hard cubes with rounded edges using simulations and density functional theory, revealing how the phase behavior evolves with edge rounding from cubes to spheres.

Contribution

It provides the first detailed phase diagram for rounded parallel hard cubes, showing the persistence of second-order freezing and the transition to first-order phases as rounding increases.

Findings

Second-order freezing persists up to s=0.65

Fluid freezes into a simple-cubic crystal with high vacancy concentration

Transition becomes first-order with further rounding, leading to different crystal structures

Abstract

The freezing transition in a classical three-dimensional system of parallel hard cubes with rounded edges is studied by computer simulation and fundamental-measure density functional theory. By switching the rounding parameter s from zero to one, one can smoothly interpolate between cubes with sharp edges and hard spheres. The equilibrium phase diagram of rounded parallel hard cubes is computed as a function of their volume fraction and the rounding parameter s. The second order freezing transition known for oriented cubes at s = 0 is found to be persistent up to s = 0.65. The fluid freezes into a simple-cubic crystal which exhibits a large vacancy concentration. Upon a further increase of s, the continuous freezing is replaced by a first-order transition into either a sheared simple cubic lattice or a deformed face-centered cubic lattice with two possible unit cells: body-centered orthorhombic or base-centered monoclinic. In principle, a system of parallel cubes could be realized in experiments on colloids using advanced synthesis techniques and a combination of external fields.