Many crystals, such as those of metals, can transform from one symmetry into another having lower free energy via a diffusionless transformation. Here we create binary colloidal crystals consisting of polymer microspheres, pulled together by DNA bridges, that induce specific, reversible attractions between two species of microspheres. Depending on the relative strength of the different interactions, the suspensions spontaneously form either compositionally ordered crystals with CsCl and CuAu-I symmetries, or disordered, solid solution crystals when slowly cooled. Our observations indicate that the CuAu-I crystals form from CsCl parent crystals by a diffusionless transformation, analogous to the Martensitic transformation of iron. Detailed simulations confirm that CuAu-I is not kinetically accessible by direct nucleation from the fluid, but does have a lower free energy than CsCl. The ease with which such structural transformations occur suggests new ways of creating unique metamaterials having structures that may be otherwise kinetically inaccessible.

DNA is the premier material for directing nanoscale self-assembly, having been used to produce many complex forms. Recently, DNA has been used to direct colloids and nanoparticles into novel crystalline structures, providing a potential route to fabricating meta-materials with unique optical properties. Although theory has sought the crystal phases that minimize total free energy, kinetic barriers remain essentially unstudied. Here we study interfacial equilibration in a DNA-directed microsphere self-assembly system and carry out corresponding detailed simulations. We introduce a single-nucleotide difference in the DNA strands on two mixed microsphere species, which generates a free-energy penalty for inserting 'impurity' spheres into a 'host' sphere crystal, resulting in a reproducible segregation coefficient. Comparison with simulation reveals that, under our experimental conditions, particles can equilibrate only with a few nearest neighbours before burial by the growth front, posing a potential impediment to the growth of complex structures.

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A detailed computational study of compositional segregation during growth of colloidal binary solid-solution crystals is presented. Using a comprehensive set of Metropolis Monte Carlo simulations, we probe the influence of colloid size, interaction strength, and interaction range on the segregation process. The results are interpreted in terms of a simple, but descriptive mechanistic model that allows us to connect to studies of binary segregation in atomic systems. The validity of Metropolis Monte Carlo simulations for the nonequilibrium phenomena investigated in this work is established theoretically and by connections to Brownian dynamics and molecular dynamics simulations. It is demonstrated that standard Metropolis Monte Carlo, properly applied, can provide an efficient framework for studying many aspects of crystallization in colloidal systems.