29. Direct Observations of Aperiodic Arrangements of Transition-Metal Atoms in Al–Co–Ni Decagonal Quasicrystals by Cs-Corrected HAADF-STEM

HAADF (high-angle annular detector dark-field) images with Cs-corrected scanning transmission electron microscopy (STEM) have been observed for a W-(AlCoNi) crystalline phase and two-types of decagonal quasicrystals in Al_71.5Co_25.5Ni₃ and Al_72.5Co_17.5Ni₁₀ alloys. We have secured positive evidence that three-dimensional arrangements of transition-metal (TM) atoms of decagonal quasicrystals can be directly derived from the arrangements of bright dots in HAADF-STEM images, which correspond to individual TM atoms, by reference to results on HAADF-STEM observation and the structure of W-(AlNiCo) determined by X-ray diffraction analysis. We could conclude that pure TM atomic sites and mixed TM sites (with Al atoms) on A and B planes stacking along the periodic axis are located at the lattice points of a Penrose lattice with a bond length of 0.25 nm. In both planes atomic sites form pentagonal tilings with bond lengths of 0.47 nm and 0.76 (=0.47⋅τ) nm, respectively, in both the Al_71.5Co_25.5Ni₃ and Al_72.5Co_17.5Ni₁₀ decagonal quasicrystals, whose structures were formally characterized as rhombic and pentagonal tilings of atom columnar clusters with a bond length of 2 nm.