On the stability of orientationally disordered crystal structures of colloidal hard dumbbells

We study the stability of orientationally disordered crystal phases in a suspension of colloidal hard dumbbells using Monte Carlo simulations. For dumbbell bond length L/σ < 0.4 with L the separation of the two spheres of the dumbbell and σ the diameter of the spheres, we determine the difference in Helmholtz free energy of a plastic crystal with a hexagonal-close-packed (hcp) and a face-centered-cubic (fcc) structure using thermodynamic integration and the lattice-switch Monte Carlo method. We find that the plastic crystal with the hcp structure is more stable than the one with the fcc structure for a large part of the stable plastic crystal regime. In addition, we study the stability of an orientationally disordered aperiodic crystal structure in which the spheres of the dumbbells are on an random-hexagonal-close-packed (rhcp) lattice, and the dumbbells are formed by taking random pairs of neighboring spheres. Using free energy calculations, we determine the fluid-aperiodic crystal and periodic-aperiodic crystal coexistence regions for L/σ > 0.88.