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

Abstract

The crystal structure of the orthorhombic phase Al4(Cr,Fe) with a composition 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 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-Al4Mn, κ-(AlCrNi), λ-Al4Mn and ν-Al12Cr2Fe phases.

Surface study of the (100) and (010) faces of the quasicrystalapproximant Al4(Cr, Fe)

“…The pseudo-decagonal symmetry is clearly manifested in the ten-fold rings observable in the figure, and the large unit cell is evidenced by the dense two-fold mesh. The aspect ratio of the smallest feature visible in the LEED patterns matches that expected from the X-ray structure solution [16].…”

“…A structural model for orthorhombic Al 4 (Cr,Fe) was proposed [14,10] by comparing high-resolution electron microscopy images with the well-known structure of m-Al 4 Mn [15]. It has since been the subject of an X-ray diffraction study [16]. Its physical properties are also of interest [17,18].…”

Formation of a quasicrystalline phase in Al–Mn base alloys cast at intermediate cooling rates

“…Interestingly, this phase does not occur in the equilibrium or metastable Al-Mn phase diagram [27,28]. Nevertheless, it was found to coexist with the decagonal phase in a number of transition-modified aluminium alloys (mostly of ternary compositions) [49][50][51][52], including rapidly solidified Al-Mn alloys with Zn and Ni, as a product of metastable I-phase decomposition. The occurrence of an L-phase in binary Al-Mn alloys during casting at intermediate cooling rates may confirm that nucleation of this crystalline structure is a common step in the formation of particles characterised by quasicrystalline arrangement in Al-TM alloys.…”

Al4(Cr, Fe): single crystal growth by the Czochralski method and structural investigation with neutrons at FRM II

“…As a first result the neutron data suggests a body-centered space group Immm with a ¼ 1:24985ð3Þ nm, b ¼ 1:25500ð3Þ nm and c ¼ 3:0512ð2Þ nm as proposed by Deng et al [1] obtained from a single crystal X-ray diffraction experiment. Nevertheless a renewed analysis of the structure (¼ "substructure" cf.…”

“…Taking the structure model derived by Deng et al [1], the iron seems to occupy the positions TM 10 and the sites Al/TM 5 and Al/TM 15 (notation according to [1]) with significant contributions (Occ: Fe10 ¼ 0:26, Occ: Cr10 ¼ 0:36, Occ: Al15=Cr5 ¼ 0:24, Occ: Fe5 ¼ 0:12, Occ: Al15=Cr15 ¼ 0:0, Occ: Fe15 ¼ 0:2). Due to the almost identical scattering lengths, Al and Cr were restricted to the same occupancy on the triply mixed sites.…”