ON THE STABILITY OF THE PLASTIC CRYSTAL PHASE OF HARD DUMBBELL SOLIDS

We consider the thermodynamic properties of the solid phase and solid-fluid phase equilibria of hard dumbbells. The focus of the present work is a more detailed study of the stability of the orientationally disordered (plastic crystal) solid phase relative to other solid phases and to the fluid. Our results indicate that the plastic crystal is not stable for values of L* (the separation of the two spheres of the dumbbell reduced by the sphere diameter) greater than about 0.38. For larger anisotropies than this the hard dumbbell fluid freezes into an orientationally ordered structure. The disappearance of the plastic crystal phase is accompanied by the approach to zero of the density difference between the fluid and the plastic crystal at coexistence and sharp increases in the coexistence pressure and densities. At high pressures the plastic crystal phase transforms into an orientationally ordered structure. These results and those of previous studies allow us to present a fairly complete description of the phase diagram of hard dumbbells.