Crystal structure of the orthorhombicAl4(Cr,Fe) approximant of the Al–Cr–Fe decagonal quasicrystal

Abstract

The crystal structure of the orthorhombic phaseAl4(Cr,Fe) with acomposition of Al80.6Cr10.7Fe8.7 has been determined by single-crystal x-ray diffraction (table 2). There are 11 crystallographically independent Cr/Fe sites, of which10 are icosahedrally coordinated. Of the 28 crystallographically independent Alsites 5 are icosahedrally coordinated. However, owing to the presence of manypartial occupancies, caused by split atoms, some of these coordinations are onlypseudo-icosahedral with effective coordination numbers (section 4.3). The icosahedral and pseudo-icosahedral clusters are oriented with a two-fold axisparallel to [100]. They are linked either by vertex-, edge- or face-sharing or by mutualinterpenetration, thus forming large clusters of icosahedra in the (100) layers with one of thefive-fold axes in the [010] direction. Four of the 12 atoms defining the vertices of anicosahedral cluster lie in a mirror plane (F plane), while the other atoms are in puckeredlayers 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, thoughthey have only two-fold rotational symmetry. Similar geometrical motifs butdisplaying three- or six-fold symmetry do exist in the structure of the hexagonalμ-Al4Mn,κ-(AlCrNi), λ-Al4Mn and ν-Al12Cr2Fe phases.