Cluster Self-Organization of Crystal-Forming Systems: Suprapolyhedral Cluster Precursors and Self-Assembly of the Icosahedral Structure of ZrZn22 (cF184)

The mechanism of self-assembly of symmetrically and topologically different chains and micro-layers ( in the form of planar nets) from cyclic three-node clusters A(3) is considered in the model system. The obtained nets correspond to the uninodal Shubnikov nets N 3 12 12 and N 3 6 3 6 and the new binodal net N1 3 6 3 6 + N2 3 3 6 6 ( 1 : 2). A complete three-dimensional reconstruction of the self-assembly of the icosahedral structure of the ZrZn22 compound ( cF184) is performed using computer methods ( with the TOPOS program package) according to the following scheme: cluster precursor -> primary chain -> microlayer -> microframework (supraprecursor) ->...-> framework. It is revealed that the suprapolyhedral cluster precursor ( nanocluster similar to 12 angstrom in size) of the AB(2) composition is formed by three polyhedra shared by vertices in a cyclic manner: the A-ZrZn16 polyhedron ( sixteen-vertex polyhedron with the point symmetry (4) over bar 3m) and two B-ZrZn12 polyhedra ( icosahedra with the point symmetry (3) over barm). The AB2 cluster precursor in the structure retains the symmetry m. It is established that the structural mechanism of self-assembly of the two-dimensional layer in the ZrZn22 structure is described by the binodal net N1 3 6 3 6 + N2 3 3 6 6 ( 1 : 2) constructed in the modeling.