Structural Studies Of Decagonal Quasicrystals And Approximants Using The High-Angle Annular Dark-Field Method

High-angle annular dark-field (HAADF) images of an A172Ni20Co8 decagonal quasicrystal and monoclinic Al13Fe4 and τ2-inflated Al13C04 approximants have been presented. We found that a 2 nm atom cluster of Al72Ni20Co8, which have been believed to be tenfold so far, has no tenfold symmetry. A new structural model of Al72Ni20Co8 has been constructed on the basis of the HAADF images and the structure of monoclinic Al13Fe4 approximant. We have found that the monoclinic Al13Fe4 and τ-inflated Al13Co4 approximants are composed of the same basic structure of a pentagonal atom cluster with an edge length of 0.47 nm. We have also found that a fivefold atom-cluster with a 2 nm diameter exists in τ-Al13Co4, which was observed in decagonal quasicrystals of Al-Ni-Fe, Al-Ni-Co, Al-Cu-Co and Al-Co by highresolution electron microscopy. A structural model of τ-inflated Al13Co4 is constructed on the basis of the convergent-beam electron diffraction symmetry, HAADF images and the structure of monoclinic Al13Fe4.