Congruent Phase Behavior of a Binary Compound Crystal of Colloidal Spheres and Dimpled Cubes

We performed Monte Carlo simulations to study the phase behavior of equimolar mixtures of spheres and cubes having selective inter-species affinity. Such selectivity was designed to promote the formation of the substitutionally ordered NaCl compound, the C* phase, and to be driven by energetic bonds and also by entropic bonds generated by dimples on the cube facets. The spheres' nestling in the cube indentations can promote negative nonadditive mixing and increase the C* phase packing entropy. The focus is on congruent phase behavior wherein the C* phase directly melts into the disordered state. We used a thermodynamic integration scheme to trace the coexisting curves for varying values of the interspecies contact energy, ε^*, the relative indentation size, λ, and the sphere-to-cube size ratio, ζ. By starting from a known coexistence point, we find that increasing λ (at fixed ε^* and ζ) reduces the free-energy and pressure of the C* phase at coexistence. Remarkably, a purely athermal C* phase formed for λ>0.7 and specific ζs. We suggest a metric of a nonadditive volume of mixing as an approximate predictor of athermal C* phase stability. We expect the principles developed in this study to apply to other particle shapes and crystalline phases.