Phase diagram of colloidal hard superballs: from cubes via spheres to octahedra

TL;DR

This study maps the phase diagram of colloidal superballs, revealing stable plastic crystal phases and a two-step melting process, highlighting shape effects on phase stability.

Contribution

It provides the first detailed phase diagram of superballs interpolating between cubes and octahedra, identifying new stable plastic crystal phases and melting behaviors.

Findings

Stable fcc plastic crystal near spheres

Stable bcc plastic crystal near octahedra

Two-step melting of octahedra into fluid

Abstract

The phase diagram of colloidal hard superballs, of which the shape interpolates between cubes and octahedra via spheres, is determined by free-energy calculations in Monte Carlo simulations. We discover not only a stable face-centered cubic (fcc) plastic crystal phase for near-spherical particles, but also a stable body-centered cubic (bcc) plastic crystal close to the octahedron shape, and in fact even coexistence of these two plastic crystals with a substantial density gap. The plastic fcc and bcc crystals are, however, unstable in the cube and octahedron limit, suggesting that the rounded corners of superballs play an important role in stablizing the rotator phases. In addition, we observe a two-step melting phenomenon for hard octahedra, in which the Minkowski crystal melts into a metastable bcc plastic crystal before melting into the fluid phase.