THE STRUCTURE OF DECAGONAL AL-CU-CO-SI - AN ELECTRON CHANNELING STUDY

The planarity of the (00001) planes in the decagonal quasicrystal Al65 Cu16Co15Si4 is investigated by a further developed electron channellin g technique, which is based on the dependence of the X-ray yield on th e direction of the incident electron beam. It is found that Si atoms a re located midway between the (00001) planes containing the Co, Cu and Al atoms. This results is discussed in terms of the electronic proper ties of Al and Si. A straightforward explanation is given for the phas e transition of Al65-xCo20Cu15Six from decagonal (x<4.5) to a one-dime nsional quasicrystal (x>4.5).