Short-range structure of Al-Mn and Al-Mn-Si aperiodic alloys

X-ray-absorption fine-structure results on the icosahedral Al-Mn and Al-Mn-Si, ±-(Al-Mn-Si), and decagonal Al-Mn alloys are presented. High-resolution first-shell Al radial distributions around Mn atoms were obtained using a newly developed, direct Fourier-transform method. The first and second Mn shells are analyzed in detail. The Al and Mn positions on the structural unit of the icosahedral phase are determined from these results. The result is in agreement with the speculation that the structural unit is a Mackay icosahedron (MI). Furthermore, it is found that about 40% of the MI units in the icosahedral phase have the same connection as that in ±-(Al-Mn-Si) and the average coordination number of the MI is about 7 in the icosahedral phase. The MI units also closely resemble those in the ± phase, including the distortion induced by the cubic packing of the MIs in ±-(Al-Mn-Si). These results suggest the presence of a short-range cubic environment in the icosahedral phase with an estimated correlation length of about 20 . These results are in disagreement with both the completely random accretion of oriented MI units and a quasicrystalline arrangement of the MI. Results on the decagonal Al-Mn phase and the role of Si in icosahedral Al-Mn-Si are also discussed.