HAADF-STEM study of chemical ordering in Al–Pd–Sc 1/1-crystal approximant

The atomic structure of the Al54Pd31Sc15 1/1-crystal approximant to the icosahedral quasicrystal has been studied using high-angle annular dark-field (HAADF) imaging in a scanning transmission electron microscopy (STEM). HAADF imaging directly reveals that the 1/1-phase has a periodic arrangement of Tsai-type atomic clusters, which consist of three successive shells with icosahedral symmetry as well as a disordered tetrahedron at the cluster center. In the 1/1-phase, the chemical ordering of Al and Pd is observed in the glue sites between the clusters, similar to that of Al and Au in the recently discovered Al35Au51Yb14 1/1-phase, indicating a common structural feature of ternary Tsai-type approximants. Moreover, the preferential introduction of structural disorder into the second dodecahedral shell is observed in the HAADF images, which causes symmetry breaking of the 1/1-phase. On the basis of a comparison with the experimental and simulated images, a preliminary model for the atomic structure of the 1/1-phase is presented.

► We presented the atomic structure model for the Al–Pd–Sc 1/1-crystal approximant. ► The approximant has a periodic arrangement of Tsai-type atomic clusters. ► Most of the Al and Pd occupy specific sites in the approximant. ► Structural disorder was introduced into the second shell of the Tsai-cluster.