Phason defects in Al-Pd-Mn approximant phases: Another example

A six-dimensional description is given for two approximants, R and T, of the decagonal Al-Pd-Mn phase. Both structures are shown to be entirely covered by two sorts of overlapping clusters, namely by 'Bergman clusters' and by 'pseudo-Mackay clusters'. On the basis of structural defects observed by high-resolution electron microscopy, a possible structural transformation between these two phases is studied. The mechanism of transformation involves the motion of linear defects which are not dislocations but lines of phasons. The atomic jumps related to the defect motion occur at distances less than 3 Angstrom and are shown to be phason jumps since they follow from translations of the windows (atomic surfaces) in perpendicular space. Moreover experimental observations suggest that the motion of phason defects can also be driven by a chemical composition gradient.