2 DIFFERENT SUPERSTRUCTURES OF THE DECAGONAL AL-CO-NI QUASI-CRYSTAL

In a decagonal quasicrystal with composition Al70Co15Ni15 two differen t types of superstructure have been found by electron microscopy techn iques. One of these superstructures has previously been observed. The other newly characterized superstructure causes a ring-like zone of lo w diffraction intensities and, additionally, reflections at positions having half-integer indices. Both types of superstructure have a doubl ed period of 8.2 angstrom along the unique tenfold axis, which is sugg ested here to be due to Al/TM (TM = Co, Ni) ordering on different laye rs. The main structural elements are columns with a diameter of approx imately 20 angstrom. During irradiation with a 300 keV electron beam b oth superstructures are transformed to a 'basic' decagonal structure w ith a 4.1 angstrom period, in which wheel-like contrast features with 20 angstrom diameter are absent. For the more disordered basic decagon al structure no conspicuous tiling can be found which can be superimpo sed onto the electron micrographs, while the new superstructure corres ponds to a 'pentagon tiling' and the other superstructure to a tiling composed of the Penrose rhombs. The relation between the superlattice reflections and the real space order is discussed on the basis of thes e tilings.