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: cF 444-Al 63.6 Ta 36.4 (AT-19), a = 19.1663 (1) Å, V = 7040 Å 3 , cF (5928 − x )-Al 56.6 Cu 3.9 Ta 39.5 , x = 20 (ACT-45), a = 45.376 (1) Å, V = 93 428 Å 3 and cF (23, 256 − x )-Al 55.4 Cu 5.4 Ta 39.1 , x = 122 (ACT-71), a = 71.490 (4) Å, V = 365 372 Å 3 . The space group is F\bar 43m 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 Å 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 μ phase.