30. Arrangement of Transition-Metal Atoms in an Approximant Crystal Related to Al–Cu–Co Decagonal Quasicrystals Studied by Cs-Corrected HAADF-STEM

A crystalline approximant, which is related to Al–Cu–Co decagonal quasicrystals with two aperiodic planes stacking along the periodic axis, in an Al₆₆Cu₁₅Co₁₉ alloy annealed at 900 ^∘C for 36 h has been studied by high-angle annular detector dark-field (HAADF) observations with Cs-corrected scanning electron microscopy (STEM). Observed HAADF-STEM images represent individual transition-metal (TM) atoms as bright dots, and so a three-dimensional arrangement of TM atoms in the approximant can be derived from the arrangement of bright dots. The structure has an orthorhombic unit cell with a ₀=10.1 nm, b ₀=0.4 nm and c ₀=6.7 nm, formed by an ordered arrangement of two types of atom columnar clusters in a τ ³-inflated monoclinic Al₁₃Co₄ structure formed by a network of pentagons with an edge-length of 2 nm. The TM atoms in the two planes stacking along the b-axis are located at lattice points of a Penrose lattice with a bond length of 0.25 nm and pentagonal tilings with bond lengths of 0.47 and 0.76 nm.