Atomic structure of the Al-Cu-Fe-Si 1/1-cubic approximant

By combining the X-ray Rietveld analysis with the high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), we have made a detailed structural analysis for the Mackay-type Al₅₃Si₉Cu_25.5Fe_12.5 1/1-cubic approximant. Electron diffraction measurements identified it as a simple cubic with a space group of Pm3, being the same as that of the Al-Mn-Si 1/1-cubic approximant (α-phase). Both atomic position and occupancy in every atomic site are refined by means of the Rietveld analysis with a choice of the Al-Mn-Si α-phase as the starting structure. The factors R_wp and R_I, which are taken as measures for the precision of the Rietveld analysis, are reduced to 2.32 and 3.87%, respectively. Two different icosahedral clusters consisting of triply stacked atomic shells are formed at the vertex and body-centered positions in the CsCl-type unit cell. The number of transition metals involved in one of the clusters is much higher than in the other. In addition, we found that the R-factors are substantially reduced by introducing a `structural disorder' in the first shell of the cluster in one of the sublattices containing more transition metals. Since the occupancy in the first shell is reduced significantly, the structural disorder is most likely introduced by vacancies in that shell. The HAADF images taken along the [100], [110] and [111] directions agree well with the projection images calculated from the Rietveld-refined structure, lending a strong support to the validity of the present atomic structure model.