INVESTIGATION OF THE CLUSTER SUBSTRUCTURE OF ICOSAHEDRAL AL-PD-MN QUASI-CRYSTALS BY MEANS OF EXIT-PLANE WAVE-FUNCTION RECONSTRUCTION IN HIGH-RESOLUTION ELECTRON-MICROSCOPY

The exit-plane wave reconstruction technique has been used for the fir st time to investigate the structure of icosahedral Al-Pd-Mn quasicrys tals. The complex-valued exit-plane wavefunction along a fivefold dire ction was reconstructed with a resolution of 0.135 nm on the basis of a series of high-resolution electron microscopy images taken at differ ent defocus values of the objective lens. In contrast with the origina l electron microscopic images, the exit-plane wavefunction is free fro m imaging artefacts. The result fits well with that expected from the Mackay-type cluster model for the Al-Pd-Mn icosahedral quasicrystal de rived from X-ray and neutron diffraction structure investigations. Dif ferent columnar arrangements of clusters were identified; clusters who se projections barely interfere with those of other clusters display a lmost perfect tenfold symmetry, while the local symmetry of strongly o verlapping projected clusters is degraded.