Structure of nu-Al80.61Cr10.71Fe8.68, a giant hexagonal approximant of a quasicrystal determined by a combination of electron microscopy and X-ray diffraction

nu-Al80.61Cr10.71Fe8.68, P6(3)/m (No. 176), a = 40.68 (7), c = 12.546 (1) A ngstrom, V = 17 983 (8) Angstrom(3), atoms/cell = 1184.56, D-x = 3.518 g cm (-3), lambda(Mo K alpha) = 0.71069 Angstrom, mu = 5.032 mm(-1), F(000) = 18 433, T = 293 K, final R = 0.075 for 3854 reflections with F-o > 4 sigma(F- o). The [001] high-resolution electron-microscopic image of the nu-AlCrFe p hase clearly shows similar local characteristics to those given by the comp lex icosahedral cluster found in somewhat smaller hexagonal approximant str uctures, such as kappa-Al76Cr18Ni6 [a = 17.674 (3), c = 12.516 (3) Angstrom ; Sato et al. (1997). Acta Cryst. C53, 1531-1533; Marsh (1998). Acta Cryst. B54, 925-926] and lambda-Al4.32Mn [a = 28.382 (9), c = 12.389 (2) Angstrom ; Kreiner & Franzen (1997). J. Alloys Compd. 261, 83-104]. Using the known atomic distribution of this icosahedral cluster in the kappa and lambda pha ses as the starting point, the structure of the nu phase, a hexagonal inter metallic compound with probably the largest a parameter, was solved by X-ra y single-crystal diffraction using direct methods. As in kappa and lambda p hases, almost all TM (transition metal) atoms in the complex icosahedral cl uster are icosahedrally coordinated. However, contrary to the lambda struct ure in which about 98% of the TM atoms have icosahedral coordination, the T M atoms in the nu structure also form capped pentagonal prisms in the regio n between these complex icosahedral clusters, yielding an average icosahedr al coordination of about 70% for TM atoms. After rapid solidification, the nu phase occurs together with a decagonal quasicrystal with a periodicity o f about 12.5 Angstrom along its tenfold axis and thus also consists of six layers, two flat ones each sandwiched between two puckered layers in mirror reflection, stacked along the c axis.