STRUCTURE OF QUASI-CRYSTALLINE APPROXIMANT PHASE IN THE AL-PD-MN SYSTEM

The structure of an orthorhombic phase (xi' phase) with Al73.5Pd22.4Mn 4.1 composition was solved by applying the direct methods and refined by a least-squares procedure. The space group is Pnma and the magnitud es of cell parameters are a = 23.541 Angstrom, b = 16.566 Angstrom and c = 12.339 Angstrom. The final reliability factor is R = 0.072 for 34 19 independent reflections. The structure is made up of partial Mackay icosahedra in which the Pd atoms form icosahedra centred on Mn atoms. These central Mn atoms are located at the vertices of flattened hexag ons arranged in quincunx. The xi' phase is nearly isomorphic to the Al 3Pd structure and has a layer structure consisting of flat and puckere d layers similar to what is found in Al3Mn and Al13Fe4.