Crystal structure of the orthorhombic Al4(Cr,Fe) approximant of the Al–Cr–Fe decagonal quasicrystal

The crystal structure of the orthorhombic phase Al 4 (Cr,Fe) with a composition of Al 80.6 Cr 10.7 Fe 8.7 has been determined by single-crystal x-ray diffraction (table 2). There are 11 crystallographically independent Cr/Fe sites, of which 10 are icosahedrally coordinated. Of the 28 crystallographically independent Al sites 5 are icosahedrally coordinated. However, owing to the presence of many partial occupancies, caused by split atoms, some of these coordinations are only pseudo-icosahedral with effective coordination numbers (section 4.3). The icosahedral and pseudo-icosahedral clusters are oriented with a two-fold axis parallel to [100]. They are linked either by vertex-, edge- or face-sharing or by mutual interpenetration, thus forming large clusters of icosahedra in the (100) layers with one of the five-fold axes in the [010] direction. Four of the 12 atoms defining the vertices of an icosahedral cluster lie in a mirror plane (F plane), while the other atoms are in puckered layers below and above the mirror plane. Thus, the icosahedral cluster consists of layers of three atoms thick. Two such layers stack within one lattice constant nm. The interconnection of icosahedra in the layer block forms large triangular and hexagonal structural motifs, though they have only two-fold rotational symmetry. Similar geometrical motifs but displaying three- or six-fold symmetry do exist in the structure of the hexagonal μ - Al 4 Mn, κ-(AlCrNi), λ - Al 4 Mn and ν - Al 12 Cr 2 Fe phases.