Large, larger, largest - a family of cluster-based tantalum copper aluminides with giant unit cells. II. The cluster structure

This is the second of two papers, where we discuss the cluster structures of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al63.6Ta36.4 (AT-19), a = 19.1663 (1) angstrom, V = 7040 angstrom(3), cF(5928 - x)Al56.6Cu3.9Ta39.5, x = 20 (ACT-45), a = 45.376 (1) angstrom, V = 93 428 angstrom(3) and cF(23, 256 - x)-Al55.4Cu5.4Ta39.1, x = 122 (ACT-71), a = 71.490 (4) angstrom, V= 365 372 angstrom(3). The space group is F (4) over bar 3m in all three cases. The structures can be described as packings of clusters such as fullerenes, dodecahedra, pentagonal bifrusta and Friauf polyhedra. A characteristic feature of the two larger structures are nets of hexagonal bipyramidal Ta clusters (h.b.p.). The extremely short distance of 2.536-2.562 angstrom between their apical Ta atoms indicates unusually strong bonding. The large h.b. p. nets are sandwiched between slabs of Friauf polyhedra resembling the structure of the mu phase.