Structural characteristics of Al-Co-Ni decagonal quasicrystals and crystalline approximants

The structural characteristics of several modulations of Al-Co-Ni decagonal quasicrystals and crystalline approximants, all of which are found in and around an Al-Co-Ni decagonal phase with a wide compositional range, are discussed systematically. The structures of a Ni-rich basic structure and the other modulations are characterized as quasiperiodic arrangements of columnar clusters of atoms with decagonal sections of 3.2 and 2.0nm in diameter, respectively. Various types of electron diffraction patterns including so-called superlattice reflections are interpreted by tiling features of the atomic clusters as well as ordered arrangements of the atomic clusters with pentagonal symmetry in the tilings. From the structure of a crystalline approximant (W-(AlCoNi) phase), precise models of the atomic clusters are proposed, and also the origin of diffuse reflections often observed in the Al-Co-Ni decagonal quasicrystals may be interpreted.