Cluster self-organization of intermetallic systems: Nanocluster precursors (i-K8, i-K6, K4) and self-assembly of crystal structures Ir₈Mg₅₈(cF396), Ir₇Mg₄₄(cF408), and Ir₆Mg₂₆ (hR96)

Abstract

Suprapolyhedral cluster precursors Kn with a hierarchical structure (with the number of polyhedra n = 4, 6, 8) have been derived and their topological representation as bichromatic graphs has been performed. With the use of computer methods (the TOPOS program package), combinatorial-topological analysis of icosahedral monster structures of close compositions Ir₈Mg₅₈ (cF396, F $\bar 4$3m, V = 8139 ų) and Ir₇Mg₄₄ (cF408, F $\bar 4$3m, V = 8117 ų) has been performed. Suprapolyhedral nanoclusters consisting of eight and six Ir icosahedra (nanoclusters i-K8 and i-K6 comprising 86 and 50 atoms) have been identified by the method of complete decomposition of the 3D factor graph of crystal structures into cluster substructures in Ir₇Mg₄₄. In Ir₈Mg₅₈, the i-K8 nanocluster is retained, whereas the i-K6 cluster is substituted by the K4 nanocluster composed of four Ir polyhedra with CN = 9 comprising 34 atoms. Complete 3D reconstruction of the self-assembly mechanism of crystal structure has been carried out by the scheme nanocluster precursor-primary chain-microlayer-microframework. It has been demonstrated that the voids in the Ir₈Mg₅₈ and Ir₇Mg₄₄ frameworks accommodate tetrahedral cluster spacers T-Mg(Mg₄) and T-Mg₄, respectively. The Ir₆Mg₂₆ structure (hR96, R $\bar 3$ c, V = 1890 ų) is made of the suprapolyhedral cluster K6 formed by six Ir polyhedra with CN = 11 and comprising 50 atoms; in this case, the structure contains no cluster spacers. In all structures, nanoclusters retain their own symmetry ($\bar 4$3m for i-K8, i-K6, and K4 and $\bar 3$ for K6).