Cluster self-organization of intermetallic systems: Quasi-spherical nanocluster precursors with internal Friauf polyhedra (A-172) and icosahedra (B-137) in the Li19Na8Ba15 (hP842) crystal structure

A combinatorial and topological analysis of Li19Na8Ba15 (hP842, a = 20 angstrom, c = 93 angstrom, V = 33552 angstrom(3), P (3) over bar) has been performed using computer methods ( the TOPOS program package). Two types of crystal-forming quasi-spherical nanoclusters about 20 angstrom in diameter with internal Friauf polyhedra (A-172) and icosahedra (B-137) have been established by the complete decomposition of the 3D factor graph of the structure into cluster substructures. Each type of nanoclusters forms close-packed 2D layers 3(6), which alternate along the c axis. The B-137 and A-172 nanoclusters are composed of three layers and have shell compositions (1 + 12 + 32 + 92) and (1 + 16 + 59 + 103) with local symmetries 3 and (3) over bar, respectively; they were revealed for the first time in crystal structures as cluster precursors. The icosahedral B-137 nanocluster contains a 104-atom quasicrystal approximant (Samson cluster).