Structure of a metastable Al₃Ni decagonal quasicrystal: Comparison with a highly perfect Al₇₂Ni₂₀Co₈

We study the structure of a decagonal quasicrystalline phase formed in a rapidly-solidified Al₃Ni alloy, by using electron diffraction and atomic-resolution Z (atomic number)-contrast scanning transmission electron microscopy (STEM). Electron diffraction patterns of the metastable Al₃Ni phase reveal features quite similar to those of the stable Al₇₂Ni₂₀Co₈ phase; a large number of reflections in the tenfold symmetry plane and no significant diffuse streaks in the twofold patterns, indicating a good quasiperiodic order of the phase. Z-contrast STEM imaging shows that the structure of the Al₃Ni phase can be interpreted based on a decagonal atomic cluster column with broken tenfold symmetry, being basically isostructural with that of the Al₇₂Ni₂₀Co₈ phase. Structural disorders are found to be significant at the core of the decagonal atomic clusters in the metastable Al₃Ni decagonal phase.