Structural defects which resulted from the plastic deformation of approximant phases were examined, at the atomic scale, on the basis of the crystallographic structure of one approximant phase. High-resolution electron microscopic images revealed a simple transformation mechanism, based upon the motion of specific defects, which gave the impression of collective 0.48nm jumps of clusters. In order to furnish a microscopic description of this transformation, a 6-dimensional description of the structure was given in terms of atomic surfaces, in orthogonal space, within the framework of the oblique projection. As a result, it was possible to resolve the motions of the defects into correlated individual atomic phason jumps of less than 0.3nm. These partially destroyed the initial clusters, and reinstated them on their new sites. It was noted that the 6-dimensional description of an approximant structure was useful for the theoretical modelling of atomic surfaces and for fitting the approximant structure. It was also necessary to expand the theoretical approximant windows in order to account for some atomic positions which were observed in the structure. Such ideal atomic sites were ruled out by current theoretical models of icosahedral phases because they involved too short a nearest-neighbour distance. They could occur in real structures because of small relaxations of the positions. The refinement procedure could be facilitated by favouring atomic positions which corresponded to nodes that lay within the approximant windows.

Correlated Phason Jumps Involved in Plastic Deformation of Al-Pd-Mn Approximant Phases L.Beraha, M.Duneau, H.Klein, M.Audier: Philosophical Magazine A, 1997, 76[3], 587-613