Icosahedral nanoclusters-precursors and self-assembly of crystal structures of the WAl12 (Im-3, cI26) family and sillenite Bi12 SiO20 (I23, cI66)

Abstract

The self-assembly of crystal structures of the family WAl12 (Im-3, cI26) and sillenite Bi12SiO20 (I23, cI66) has been simulated. The methods of the combinatorial and topological analysis (TOPOS program package) have been used based on the determination of the type of the cluster-precursor and building the basic 3D net of the structure in the form of a graph, the nodes of which correspond to the position of the centers of gravity of the clusters-precursors, and the edges characterize the bonds between them. The topological type of the basic 3D net corresponds to the body-centered Fe structure (with the 3D net of the BCC type). The symmetric-topological code of the processes of self-assembling 3D structures from nanoclusters-precursors—primary chain → microlayer → micronetwork—has been completely reconstructed. The chemical composition of the icosahedral nanoclusters-precursors of 13 atoms for WAl12 and 33 atoms for Bi12SiO20 corresponds to the chemical composition of the compounds on the whole. The cluster-precursor of the crystal structure WAl12 is an icosahedron of 12 Al atoms with the W atom in the center. The icosahedron occupies position 2a in the unit cell with the highest crystallographically possible symmetry of m-3. In the cluster-precursor of the Bi12SiO20 structure, the location of the 32 atoms in the shell of the nanocluster Bi12SiO20 corresponds to the Bergman icosahedral cluster: the 12 vertices of the icosahedron are occupied by Bi atoms, oxygen atoms, of which four are also coupled with the Si atom located in the center of the cluster, are located above and below the centers of the 20 faces of the icosahedron. Nanoclusters Bi12SiO20 have the symmetry 32 and occupy positions 2a in the unit cell. The crystal structure of WAl12 can be obtained from Bi12SiO20 by the removal of O atoms, and the system of bonds between icosahedra is completely conserved.