Published online by Cambridge University Press: 10 February 2011
By combining the powdered x-ray Rietveld analysis and the High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM), we have made the detailed structural analysis for the Mackay-type Al55Si7Cu25.5Fe12.5, 1/1-cubic approximant. Electron diffraction measurements revealed its space group to be Pni3. Both position and occupancy in every atomic site are refined by means of the Rietveld analysis. The factors Rwp and Riwhich are taken as measures for the precision of the Rietveld analysis, are reduced to 2.57 and 3.89 %, respectively. The refined structure of this approximant is a simple cubic with the lattice constant of 12.31916(2) Å. Two different icosahedral clusters consisting of triply stacked atomic shells exist at the vertex and body centered positions in the CsCl-type unit cell. In addition, we propose that there exists most likely a “structural disorder” in the first shell of the cluster in one of the sublattices. The HAADF images taken along [100] and [111] directions agree with the projection images calculated from the Rietveld-refined structure, lending a strong support to the validity of the present atomic structure model deduced from the Rietveld analysis.