The structural characteristics of Al-Co-Ni decagonal quasicrystals and crystalline approximants

The structural characteristics of four modulations of Al-Co-Ni decagonal quasicrystals, called S1-type and type-I superstructures, Co-rich basic structure and 5-fold superstructure, and crystalline approximants, W-(AlCoNi) and PD_3c phases, all of which are found in and around an Al-Co-Ni decagonal phase with a wide compositional range, are systematically discussed on the basis of results obtained from high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) observations. All the structures of the modulations of decagonal quasicrystals and crystalline approximants are described as arrangements of columnar atom clusters, which have a decagonal section of 2.0 nm in diameter and pentagonal symmetry. The structures of the S1-type and type-I superstructures can be characterized as ordered arrangements of two types of the atom clusters with two orientations of pentagonal symmetry in pentagonal and rhombic quasiperiodic lattices, respectively. On the other hand, the structures of the Co-rich basic state and 5-fold superstructure are described as arrangements of the atom clusters with one orientation of pentagonal symmetry. The PD_3c and W-(AlCoNi) crystalline phases can be concluded to be approximants of the type-I and pentagonal superstructure, respectively.