Influence of Softness on the Stability of Binary Crystals

Binary systems of spherical colloids have shown the ability to self-assemble into many more superlattice structures than comparable monodisperse systems. Understanding the nature of the interparticle forces in such self-assembly is critical to target specific structures for self-assembly. Here we computationally investigate the influence of “softness”, the steepness of the interaction potential, on the solid phase behavior of particles interacting with a purely repulsive, isotropic pair potential. We determine the ground state phase diagram for our model using a basin-hopping algorithm to optimize candidate structures for low enthalpy. The phase behavior of repulsive particles is often understood by looking at the packing fraction of different phases; however, we find that a very small amount of softness can change the stable ground state phase of a system away from the densest packing structure. We also find softness increases the stability of many experimentally observed structures. Our results provide further insight into why particular structures self-assemble and will be useful as a reference for experimentalists working with softly repulsive particles.