Abstract
In a decagonal quasicrystal with composition AI70Co15Ni15 two different types of superstructure have been found by electron microscopy techniques. One of these superstructures has previously been observed. The other newly characterized superstructure causes a ring-like zone of low diffraction intensities and, additionally, reflections at positions having half-integer indices. Both types of superstructure have a doubled period of 8·2 Å along the unique tenfold axis, which is suggested here to be due to Al/TM (TM= Co, Ni) ordering on different layers. The main structural elements are columns with a diameter of approximately 20 Å. During irradiation with a 300 keV electron beam both superstructures are transformed to a ‘basic’ decagonal structure with a 4·1 Å period, in which the wheel-like contrast features with 20 Å diameter are absent. For the more disordered basic decagonal structure no conspicuous tiling can be found which can be superimposed onto the electron micrographs, while the new superstructure corresponds 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 these tilings.
