Structure of an Al-Cu-Co Decagonal Quasicrystal Studied by Cs-Corrected STEM

The structure of an Al-Cu-Co decagonal quasicrystal (DQC) with two quasiperiodic planes in an annealed Al64Cu22Co14 alloy has been studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM) with high-angle annular detector dark-field (HAADF) and annular bright-field (ABF) techniques. The observed HAADF-STEM images taken with the incident beam parallel to the periodic axis clearly represent individual transition-metal (TM) Cu/Co atoms and mixed sites (MSs) of Al and TM atoms as separated bright dots, and consequently arrangements of TM atoms and MSs on the two quasiperiodic planes can be directly determined. The TM atoms on the two quasiperiodic planes are arranged in pentagonal tiling with an edge-length of 0.76nm, and also the TM atoms and MSs are located at vertices of Penrose tiling with an edge-length of 0.25 nm, and so they are arranged with a bond orientational order (BOO). Pentagonal frames with definite directions in the pentagonal tiling of TM atoms are also arranged in tau(2)-inflated pentagonal tiling with an edge-length of 2 nm. Arrangements of TM atoms derived from the observed HAADF-STEM image are placed in ideal pentagonal Penrose tiling with an edge-length of 2 nm, which is generated by the projection of five-dimensional (5D) hyper-cubic lattice, and are projected on occupation domains (ODs) in the perpendicular space. The arrangement of Al atoms and TM atoms and MSs in a well-symmetric region is interpreted from the observed HAADF- and ABF-STEM images.