Optimizing the formation of colloidal compounds with components of different shapes.

By introducing favorable inter-species interactions, stoichiometric compound phases (C*), akin to intermetallic alloys, can be formed by binary mixtures of nanoparticle components of different shapes. The stability of such C* phases is expected to be affected by asymmetries in both the energetics of like vs. unlike species contacts, and the packing entropy of components, as captured by their shapes and relative sizes. Using Monte Carlo simulations, we explore the effect of changes in size ratio (for fixed contact energy) and in binding energy (for fixed size ratio) in the stability of the CsCl compound phase for equimolar mixtures of octahedra and spheres and of the NaCl compound for equimolar mixtures of cubes and spheres. As a general design rule, it is proposed that enhanced compound stability is associated with inter-species interactions that minimize the free-energy of the C* phase at coexistence with the (disordered) phase that is stable at lower concentrations. For the systems studied, this rule identifies optimal relative particle sizes and inter-species binding energies that are consistent with physically grounded expectations.