Disorder and complexity in the atomic structure of the perfect icosahedral alloy of Al-Pd-Mn

The atomic structure of the perfect Al)Pd)Mn icosahedral phase has been studied on single grain samples. Using anomalous X-ray diffraction close to the palladium edge, the partial structure factor F sub Pd has been extracted. In the six-dimensional description of its structure, we find that the atomic surface cannot be described by an object with a sharp boundary. A phason Debye)Waller term has been introduced to fully account for the data. It is interpreted as resulting from random phason disorder and nonsphericity of the atomic surfaces.