Compositional order of a symmetric hard-sphere mixture with cross attraction: Role of concentration

We investigate a binary mixture of identical hard spheres, interacting through a square-well cross attraction. Recently, we have shown that the equimolar mixture exhibits stripe order (i.e., a regular alternation of layers filled with particles of mostly one species) in both solid and liquid phases [Prestipino et al., J. Chem. Phys. 159, 204902 (2023)]. Here, we extend our study to nonequimolar conditions. Using Monte Carlo simulations, we find a rich self-assembly behavior with several forms of compositional order, depending on the departure from equimolarity and the range of attraction. While stripes are still seen in the solid phase of the 30%/70% mixture, for a larger concentration asymmetry we rather observe fcc ordering of the minority species, provided that the range of attraction is long enough. Due to the possibility to adjust the concentration in the two phases, compositional order is generally richer at solid-vapor coexistence than in the bulk solid: we observe solid droplets with stripes, displaying either square or triangular in-plane order, as well as droplets with bcc structure and honeycomblike order of the minority species in the matrix of the other species. Finally, at sufficiently low temperature the increased concentration of minority particles in the solid droplet makes the cohesive energy at coexistence systematically larger than in the bulk solid.