Novel mesophase behavior in two-dimensional binary solid solutions*

Towards the goal of designing new complex materials, Monte Carlo simulations were used to study the entropic-driven assembly of binary mixtures of hard polygons and disks. Two distinct types of mixtures were studied, such that individual components either form distinct crystal lattices, like squares and disks, or similar crystal lattices, like hexagons and disks. Our focus was the 2D phase behavior of mixtures where the components have size ratios that optimize their co-assembly into solid solutions, and over the full range of compositions and concentrations to further detect any partially ordered phases (mesophases). Besides the enhanced regions with solid miscibility, a novel mosaic/polycrystalline phase was found for the disk+square mixtures and a rotator-plastic solid phase for the disk+hexagon mixtures. The mosaic phase has interspersed clusters of locally ordered 4-fold squares and 6-fold disks, distributed throughout the domain with random orientations. The plastic-solid rotator mesophase of hexagons and disks exhibits long-range translational and short-range orientational order. Changes in phase behavior for different component size ratios were also investigated to assess the importance of the optimal size ratio in promoting mesophase behavior.