Structural Study of an Al-Co Approximant by High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy

It has been discovered by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) that a melt-quenched Al ₇₃Co₂₇ alloy forms a new approximant. The approximant is composed of the Penrose skinny and fat rhombi with a 2 nm edge length, which are arranged periodically in one direction but disordered in the direction perpendicular to the periodic direction. It has been clarified that rocket- and butterfly-shaped tiles, which are constructed by the basic structural units of τ²-Al₁₃Co₄ (the 1-nm column pairs) [K. Saitoh et al.: J. Phys. Soc. Jpn. 68 (1999) 2886], appear at the skinny tile and over the skinny and fat tiles, respectively. The structural relationship between the skinny and fat tiles and the 1-nm column pairs has been clarified for the first time. It has been revealed that the tiling of the skinny and fat rhombi in the present approximant, including the disordered feature of the tiling, is formed by a network of the 1-nm column pairs.