Structural study of an Al₇₂Ni₂₀Co₈ decagonal quasicrystal using the high-angle annular dark-field method

High-angle annular dark-field (HAADF) images of an Al₇₂Ni₂₀Co₈ decagonal quasicrystal have been taken for the first time. It has been found that the transition metals form pentagonal and star-shaped cluster columns with radii of about 0.4 nm. The atom-cluster columns with a 2 nm diameter of the alloy, which have been considered as the basic structural unit with decagonal symmetry so far, have been revealed to have no decagonal symmetry. These atom-cluster columns have been found to be decomposed into the pentagonal and star-shaped cluster columns, corresponding clusters existing in the Al₁₃Fe₄ monoclinic approximant. The tiling of the pentagonal and star-shaped clusters has been found to agree with the rhombic Penrose lattice for an area with a diameter greater than 50 nm.