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