Structures of the ZrZn_(22)family: suprapolyhedralnanoclusters, methods of self-assembly and super-structural ordering

摘要

A combinatorial topological analysis is carried out by meansof the program package TOPOS4.0 [Blatov (2006), IUCrComput. Commun. Newsl. 7, 4-381 and the matrix self-assembly is modeled for crystal structures of the ZrZn,, family(space group Fd3 m, Pearson code cF184), including thecompounds with superstructural ordering. A number of strictrules are proposed to model the crystal structures ofintermetallics as a network of cluster precursors. Accordingto these rules the self-assembly of the ZrZn_(22)-like structureswas considered within the hierarchical scheme: primarypolyhedral cluster —> zero-dimensional nanocluster precursor one-dimensional primary chain —> two-dimensionalmicrolayer -± three-dimensional microframework (three-dimensional supraprecursor). The suprapolyhedral clusterprecursor AB2X37 of diameter — 12 A and volume — 350 A3consists of three polyhedra (one A Xi6 of the 43m pointsymmetry and two regular icosahedra BX12 of the 3m pointsymmetry); the packing of the clusters determines thetranslations in the resulting crystal structure. A noveltopological type of the two-dimensional crystal-forming 4,4-coordinated binodal net A B2, with the Schlafli symbols 3636and 3366 for nodes A and B, is discovered. It is shown that theZrZn22 superstructures are formed by substituting some atomsin the cluster precursors. Computer analysis of theCRYSTMET and ICSD databases shows that the clusterAB2X37 occurs in 111 intermetallics belonging to 28 structuretypes.