By chemically modifying the nucleation burst that generates monodisperse FePt nanocrystals, a mixture of Pt and Fe(x)Pt(1-x) nanoparticles forms during a one-pot reaction that includes a small amount of Cu as a catalyst; size-selective precipitation yields a bi-disperse population of Fe(x)Pt(1-x) nanoparticles, which can assemble into high-quality AB2, AB5, and AB13 superlattice structures.