Nucleation of pseudo hard-spheres and dumbbells at moderate metastability: appearance of A15 Frank-Kasper phase at intermediate elongations.

Crystal nucleation of repulsive hard-dumbbells from the sphere to the two tangent spheres limit is investigated at moderately high metastability by brute-force molecular dynamics simulations. Nucleation rates are in good agreement with previous simulations of hard-spheres and dumbbells. Icosahedral structures formed by twinned face-centred-cubic tetrahedra sharing five-fold symmetry axes and icosahedral centers are often found in spheres and dumbbells with either small (L/σ = 0.1 and 0.2) and large (L/σ = 1) elongations. These structures are incompatible with long range crystalline order but are able to survive up to quite large sizes. In contrast, at intermediate elongations (L/σ = 0.3), corresponding roughly to the bond length of molecular nitrogen, the fluid crystallizes into three distinct solid structures, namely, a low density plastic crystal, a hexagonal close-packed plastic crystal (with the same structure as β-N2), and an A15 Frank-Kasper phase (cP8 structure corresponding to δ-N2). At the lower pressures studied the hexagonal close packed plastic crystal is the most stable phase, but at the higher pressures the stable phase is an orientationally ordered solid designated as CP1 that is never spontaneously formed in our crystallization simulations.