DISORDER AND COMPLEXITY IN THE ATOMIC-STRUCTURE OF THE PERFECT ICOSAHEDRAL ALLOY AL-PD-MN

The atomic structure of the perfect Al-Pd-Mn icosahedral phase has bee n studied on single grain samples. Using anomalous x-ray diffraction c lose to the Pd edge, the partial structure factor F(Pd) has been extra cted. In the six-dimensional description of its structure, we find tha t the atomic surface cannot be described by an object with a sharp bou ndary. A phason Debye-Waller term has been introduced to fully account for the data. It is interpreted as resulting from random phason disor der and nonsphericity of the atomic surfaces.