Arrangements of Transition-Metal Atoms in Three Types of Al–Co–Ni Decagonal Quasicrystals Studied by Cs-Corrected HAADF-STEM

抄録

Three types of Al–Co–Ni decagonal quasicrystals in Al_72.5Co₁₁Ni_16.5, Al₇₁Co_14.5Ni_14.5 and Al_71.5Co₁₆Ni_12.5 alloys have been examined by Cs-corrected high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM), which has an enough resolution to represent individual TM atoms as bright dots in observed images of decagonal quasicrystals. On the basis of the structure of a W-(AlNiCo) crystalline approximant, transition-metal (TM) atoms and mixed sites (MSs) of Al and TM atoms are separately distinguished in HAADF-STEM images. Most of TM atoms are arranged with pentagonal tilings of a bond-length of 0.76 nm, and MSs are located inside of pentagonal frames with definite orientations. All of the TM atoms and MSs are located at lattice points of a Penrose lattice of an edge-length of 0.25 nm, and arranged with a bond-orientational order. The present result shows that the structures of Al–Co–Ni decagonal quasicrystals should be characterized as BOO arrangements of transition-metal atoms, instead of previous models of BOO arrangements of 2 nm atom clusters.