Structural complexity of the intermetallic compound o-Co4Al13
High-resolution TEM, STEM, and X-ray diffraction experiments on orthorhombic o-Al13Co4, a quasicrystal approximant, confirmed the symmetry of space group Pmn21, reported by the first study of this crystal structure [1,2]. HRTEM and HRSTEM images revealed a structural arrangement with strong local violations of translational symmetry, in particular by aluminum positions located on the mirror plane, see Fig. 1a,b.
In crystallography, there is a constant struggle to understand the real structural organization of complex intermetallic compounds. The compound o-Al13Co4 has attracted attention of several research groups because of its unusual physical and chemical properties. In addition, it is an efﬁcient and highly selective hydrogenation catalyst for alkynes.
Single crystal of o-Al13Co4 was grown by the Czochralski technique. For TEM investigations focused ion beam (FIB) lift-out lamella were prepared with cuts parallel to (100) plane. The lamellae were further treated by the milder argon ion polishing at lower energies in order to obtain even thinner samples.
In order to verify the assumption of local atomic disorder, the study was continued by means of high-resolution transmission electron microscopy. A comparison with the structure model, obtained from the X-ray diffraction data, shows that the main contrast features of the high-resolution TEM (HRTEM) micrograph along  direction are created by vertex- and edge-connected Co pentagons. Within the large pentagon, a smaller-sized pentagon shows up, formed exclusively by aluminum atoms. In the center of each of these double-pentagons, a column consisting of cobalt and aluminum atoms is situated. The aluminum atoms within the pentagons are disordered. Bonding interactions between the three-dimensional framework and three-atomic groups Co-Al-Co located in the cavities of the latter in combination with large distances between the cages and cage caps are one possible reason for the extended disorder. The local twinning in different directions together with stacking faults is another possible origin of the experimentally observed crystallographic features .
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|Extended Abstract||(a) Projection of the crystal structure along . (b) HRTEM with overlaid model of disorder of Al atoms (blue) within the pentagonal pattern. Large pentagons correspond to Co-containing columns.||333 KB||Download|