We use the model of hard spheres to analyse the packing effect and its contribution to stability or instability of nanoparticles embedded in disordered structures. For each simulation the initial structure is generated in a cubic box including 10⁴ to 10⁵ spheres. The system is subjected to a densification procedure (force biased algorithm) under periodic boundary conditions. The structural analysis is based on the S-Voronoi tessellation. Each sphere and its neighborhood is characterised by its local packing fraction defined by the ratio of sphere volume and volume of the corresponding Voronoi cell.
If the initial coordinates of the sphere centres are chosen at random according to the Poisson distribution and if all sphere diameters are equal, the structure obtained after densification to the mean packing fraction of 0.64 corresponds to the well-known homogeneous dense random packing model of hard spheres (DRPHS). Enhancement of the mean packing fraction of monodisperse systems to values above 0.64 leads to the formation of nanocrystals embedded in the DRPHS matrix. The results point to the existence of a critical radius of the nanocrystals having face-centred cubic order and the local packing fraction of 0.74. If a sphere is surrounded by 12 neighbours touching the central sphere and being situated on the vertices of an icosahedron, the central sphere has a local packing fraction of 0.76. The simulation of systems containing such icosahedral clusters shows that maximum local and global packing density are competing principles.
The results of our computer simulations are discussed in connection with recent experimental results on the structure of liquid metals and alloys.

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