Clustering in nondemixing mixtures of repulsive particles.

Using classical density functional theory (DFT), we investigate the phase behavior of binary mixtures, in which the interactions between all particles are described by ultrasoft, repulsive potentials. In the pure case, one of the species, species 2, forms clusters, and freezes into a cluster crystal at sufficiently high density, while the other, species 1, does not cluster and remains liquid at all densities of interest. For some mixtures, DFT predicts two instabilities in the liquid with respect to modulations of differing periodicities. One instability results from the cluster-crystal forming tendency of species 2. In concentrated species 2 mixtures, we find species 1 clusters in response to species 2 cluster formation, eventually freezing either on, or between the species 2 lattice sites. The second instability arises when the interaction between unlike species is either more favorable, or less favorable, than the interaction between like species; when less favorable, the particles form a highly delocalized cluster crystal. We examine the structure of the liquid and crystal phases. In addition, we explore the effect of the cross-interaction potential on the structure of the cluster crystal.