New High-Density Packings of Similarly Sized Binary Spheres

Binary sphere packings with a density greater than that of phase-separated face-centered cubic (fcc) packings are important structural candidates for the self-assembly of mixtures of stabilized, hard, spherical particles. Details are presented of an A(3)B binary hard sphere packing obtained by simulated annealing in an isopointal set 62cd/c. The structure has a higher density than all known others when the sphere radius ratio r(A)/r(B) is between 0.619 and 0.659786. This extends the known range over which compound binary sphere packings can exceed the density of phase-separated fcc packing. Previously, no packings were known to exceed this mark between a radius ratio of 0.62 and 1.0. The structure of interest has a peak density of 0.747857, which occurs at a radius ratio of 0.647989. A related dense packing is also discussed, which has a peak density of 0.7573 at a size ratio of 0.5147. This latter structure is the densest known noninterstitial compound packing when the mixture has three small spheres for every large sphere, at any size ratio. Both structures are likely candidates for stability in binary hard sphere phase diagrams at this composition, but neither has so far been observed experimentally.