Acta Crystallographica Section B

Structural Science

Volume 56, Part 3 (June 2000)

research papers

Acta Cryst. (2000). B56, 392-401 [ doi:10.1107/S0108768199015761 ]

Structure of -Al_80.61Cr_10.71Fe_8.68, a giant hexagonal approximant of a quasicrystal determined by a combination of electron microscopy and X-ray diffraction

Z. M. Mo, H. Y. Zhou and K. H. Kuo

Abstract: [nu] -Al_80.61Cr_10.71Fe_8.68, P6₃/m (No. 176), a = 40.68 (7), c = 12.546 (1) Å, V = 17 983 (8) Å³, atoms/cell = 1184.56, D_x = 3.518 g cm^-3, [lambda] (Mo K [alpha] ) = 0.71069 Å, [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 phase clearly shows similar local characteristics to those given by the complex icosahedral cluster found in somewhat smaller hexagonal approximant structures, such as [kappa] -Al₇₆Cr₁₈Ni₆ [a = 17.674 (3), c = 12.516 (3) Å; Sato et al. (1997). Acta Cryst. C53, 1531-1533; Marsh (1998). Acta Cryst. B54, 925-926] and [lambda] -Al_4.32Mn [a = 28.382 (9), c = 12.389 (2) Å; Kreiner & Franzen (1997). J. Alloys Compd. 261, 83-104]. Using the known atomic distribution of this icosahedral cluster in the [kappa] and phases as the starting point, the structure of the [nu] phase, a hexagonal intermetallic compound with probably the largest a parameter, was solved by X-ray single-crystal diffraction using direct methods. As in [kappa] and [lambda] phases, almost all TM (transition metal) atoms in the complex icosahedral cluster are icosahedrally coordinated. However, contrary to the structure in which about 98% of the TM atoms have icosahedral coordination, the TM atoms in the [nu] structure also form capped pentagonal prisms in the region between these complex icosahedral clusters, yielding an average icosahedral coordination of about 70% for TM atoms. After rapid solidification, the [nu] phase occurs together with a decagonal quasicrystal with a periodicity of about 12.5 Å 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.

Keywords: Aluminium-chromium-iron; Electron microscopy; X-ray diffraction.