STRUCTURE ANALYSIS OF DECAGONAL QUASICRYSTALS USING THE HIGH-ANGLE ANNULER DARK-FIELD SCANNING TRANSMISSION ELECTRON MICROSCOPE METHOD.

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○Al_<72>Ni_<20>Co_8, Al_<73>Ni_<22>Fe_5 decagonal quasicrystalsWe, using the HAADF method, revealed that the atom cluster of an Al_<72>Ni_<20>Co_8 decagonal quasicrystal, which had been believed to be of tenfold rotation symmetry, is not of tenfold but of mirror symmetry for the first time, Furthermore, we found that Al_<72>Ni_<20>Co_8 is composed of basic atom clusters, which exist in a basic quasicrystalline approximant of monoclinic Al_<13>Fe_4. On the basis of the approximant structure, a new structural model of Al_<72>Ni_<20>Co_8 was constructed. It was also found using the HAADF method that an Al_<73>Ni_<22>Fe_5 decagonal quasicrystal has a similar structure to that of Al_<72>Ni_<20>Co_8.○Al-Ni-Fe non-centrosymmetric decagonal quasicrystalWe previously found, by conventional high-resolution electron microscopy, that an Al_<70>Ni_<15>Fe_<15> quasicrystal is formed by an atom cluster with fivefold rotation symmetry. We applied the HAADF method to the quasicrystal for the first time … More , and verified that the atom cluster is truly of fivefold symmetry. Furthermore, a structural model was constructed on the basis of the HAADF image and an approximant structure. The direct observation of an antiphase shift at an inversion domain boundary, which had not been carried out, was successfully performed by the HAADF method.○Al-Fe and Al-Co crystalline approximantsFrom the HAADF observations of an Al_<13>Fe_4 crystalline approximant, whose structure was already solved, it was confirmed that the HAADF images exhibit transition-metal-atom positions selectively. We found that HAADF images of a O^2-inflated Al_<13>Co_4 approximant show a similar image of the 2nm-diameter atom-clusters of decagonal quasicrystals. On the basis of the HAADF images and the structure of Al_<13>Fe_4, a structural model of O^2-inflated Al_<13>Co_4 approximant was constructed. Furthermore, by the comparison between the decagonal quasicrystal and its approximant structures, a formation model of decagonal quasicrystals from the O^2-inflated Al_<13>Co_4 was proposed. Less