A structural change of an Al72Ni20Co8 decagonal quasicrystal into the S1-type superlattice ordered phase has been observed at about 450°C by in-situ high-resolution electron microscopy. The Al72Ni20Co8 alloy, prepared by water quenching from 900°C, is composed of mirror-symmetric atom-clusters (m-cluster) with a 2-nm diameter, which form the Penrose pentagon tiling with an edge length of 2.0 nm. It has been found that above 400°C, most of the m-clusters transformed into fivefold-symmetric cluster (f-cluster) and the arrangement of the f-cluster formed the S1-type superlattice. It is considered that the formation of the S1 superlattice order is attributed to the disappearance of the 1.2-nm intercluster bond and to the preferential order of the sense of the f-cluster, which takes opposite senses between the neighboring clusters with a 2-nm intercluster distance. The phason flip of the positions of the 2-nm atom-clusters, which was first reported by Edagawa et al. [Phys. Rev. Lett. 85 (2000) 1674], has been observed with an atomic scale resolution. It has been found that the phason flip is caused by a flip between the fivefold-symmetry cluster and the mirror symmetric cluster. Structural models of the m- and f-clusters were reexamined.
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