ONE-DIMENSIONALLY PERIODIC FIVEFOLD AL-CO-NI QUASI-CRYSTALS

One-dimensionally periodic fivefold quasicrystals have been found in t he Al-Co-Ni system. They are characterized by a fivefold rotation axis as a unique symmetry element in selected-area electron diffraction (S AED) patterns which is consistent with contrast features in high-resol ution transmission electron microscopy (HRTEM) images. The composition s of the samples are Al72.5Co20Ni7.5 and Al72.5Co19Ni8.5 respectively. The break of the tenfold symmetry in the SAED patterns taken normal t o the periodic axis, which contradicts Friedel's law, can be understoo d as being caused by dynamical scattering effects. In corresponding HR TEM images a homogeneously distributed pentagonal contrast feature due to an identical cluster is found in only one of the two orientations compatible with a tenfold aperiodic tiling. Based on a model with this fivefold-symmetric cluster in only one constant orientation, dynamica l calculations of the electron diffraction patterns were made for a sm all patch of the structure. They corroborate the observation of a brea k of the tenfold symmetry in the SAED patterns for a specimen thicknes s of more than 30 nm.