Critical densities in hard sphere packings. Delaunay simplex analysis.

A large set of computer models (more then 200 models) of hard sphere packings with packing fraction eta between 0.52 - 0.72 is examined. Every packing consist of 10.000 identical spheres in the model box with periodic boundary conditions. Delaunay simplexes (quadruples of mutually closest spheres) with shape resembling to perfect tetrahedron or quartoctahedron are studied. Fraction of such simplexes is studied as a function of packing density. Structure changes at the transition from disordered to crystalline phase are discussed. A limited packing fraction of the non-crystalline packing is estimated as 0.6455plusmn0.0015. The ratio of tetrahedral to quartoctahedral simplexes (T/Q) in the packing at this density provided to be close to 2/3. We pay attention to one more critical interval of density at around eta=0.665 plusmn0.005. At this density the crystalline nuclei which were in the packing run into unified crystal and the ratio T/Q reaches a crystalline value 1/2.