Structural disorder of the atom cluster in a highly-ordered decagonal quasicrystal of Al₇₂Ni₂₀Co₈

Spherical-aberration-corrected high-resolution transmission electron microscopy observation has been carried out for a highly ordered decagonal quasicrystal of water-quenched Al72Ni20Co8. Atom clusters with a 2 nm diameter, which are considered to be basic structural motifs of the present alloy, show triangular shaped atomic arrangements in the central part, as observed previously by high-angle annular dark-field scanning transmission electron microscopy. The triangular arrangement is not identical for all the atom clusters but varies from cluster to cluster, although the orientational order of the triangles agrees well with the quasiperiodic order. The present results indicate that decagonal Al72Ni20Co8 with a highly perfect quasiperiodic order contains a relatively large amount of structural disorder inside the atom cluster, which may be caused by a local relaxation to form the highly-ordered quasiperiodic arrangement of the atom-cluster.