Large, larger, largest - a family of cluster-based tantalum copper aluminides with giant unit cells. I. Structure solution and refinement

This is the first of two parts, where we report the structure determination 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. These cluster-based structures are closely related to the class of Frank-Kasper phases. It is remarkable that all three structures show the same average structure that resembles the cubic Laves phase.