Highly-perfect decagonal quasicrystalline Al(64)Cu(22)Co(14) with non-centrosymmetry

The structure of a decagonal quasicrystal of Al(64)Cu(22)Co(14), which was a single-grain specimen (a few mm in size) grown by slow-cooling from a melt and subsequently annealed at 1073 K, was investigated using electron diffraction and scanning transmission electron microscopy (STEM). Electron diffraction patterns of the Al(64)Cu(22)Co(14) revealed a large number of sharp reflections that indicate a good long-range quasiperiodic order, and convergent-beam electron diffraction (CBED) analysis identified the space group of the decagonal Al(64)Cu(22)Co(14) as being non-centrosymmetric P (10) over bar m2. Atomic-resolution Z-contrast STEM imaging showed that the structure can be described based on a fivefold-symmetric decagonal cluster 2 nm across, most of which is found to be located at ideal positions in the pentagonal Penrose tiling with significantly less phason disorders. Therefore, to our knowledge, the present decagonal Al(64)Cu(22)Co(14) provides one of the best quasiperiodic ordered structures among those belonging to non-centrosymmetric P (10) over bar m2. Further details of atomic configurations within the fivefold-symmetric cluster were also examined using ultrahigh-resolution Z-contrast STEM with an aberration-corrector for the objective lens. This clearly revealed both the Al and Cu/Co sites at resolution close to 1 . Atomic arrangements within the cluster are found to be fairly well described based on the fivefold-symmetry cluster that exists in the W-(AlCoNi) approximant crystal, except for the atomistic disorders localized around the cluster centres.