Localised plasticity in crystals formed by wireframe particles

Particles with wireframe geometry, i.e. hollow, open structures comprised by interconnected edges, were first realised experimentally in 1991 when Chen et al. synthesised a cubic nanoframe from DNA strands. By today a vast... [ view full abstract ]

Particles with wireframe geometry, i.e. hollow, open structures comprised by interconnected edges, were first realised experimentally in 1991 when Chen et al. synthesised a cubic nanoframe from DNA strands. By today a vast library of DNA wireframe geometries have been reported, alongside a growing number of metal nanoframes, ceramic networks, and open protein structures.

In this work, we study the phase behaviour and packing of a non-convex particle model that captures the main geometrical features of the experimental frame-like particles. In particular, we consider the case of cubic frame-like particles. The edges of the cubic frame are represented by twelve permanently bonded hard spherocylinders of varying thickness t ^∗ = σ/L, where σ and L are the diameter and length of an edge respectively. Using Monte Carlo simulations and a tailored overlap algorithm, we have determined the unique phase behaviour of the model observing that thin cubic frames spontaneously form partially-plastic crystals upon increasing concentration. While these systems exhibit long-range orientational order typically characteristic of crystals, a dynamic fraction of particles occupy orientationally disordered states on a fixed lattice. This behaviour is in clear contrast to the observations reported in convex cubic particles [6]. As may be expected, the fraction of orientationally disordered particles, f_d , increases as the frames are made thinner. This behaviour enables the plastic crystals to exist at unusually high concentrations. To further assess the role of the symmetry on the phase behaviour of frame-like particles,we also report simulation results of systems of rhombic dodecahedra and triangular prisms - shapes with respectively lower and greater asphericity - to find that they too form partially plastic crystals. We argue that this novel phase behaviour is indicative of a wider trend for wireframe particles.