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

Abstract

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) Å, V = 7040 Å3, cF(5928 − x)-Al56.6Cu3.9Ta39.5, x = 20 (ACT-45), a = 45.376 (1) Å, V = 93 428 Å3 and cF(23, 256 − x)-Al55.4Cu5.4Ta39.1, x = 122 (ACT-71), a = 71.490 (4) Å, V = 365 372 Å3. 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.