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/sigma < 0.4 with L the separation of the two spheres of the dumbbell and sigma 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 a random-hexagonal-close-packed 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/sigma>0.88.