AN ATOM CLUSTER MODEL OF F-TYPE ICOSAHEDRAL QUASI-CRYSTALS, DERIVED FROM THE STRUCTURE OF 1 1 CUBIC APPROXIMANT PHASE (ALPHA-(ALPDMNSI))/

We propose an atom cluster model of F-type icosahedral quasicrystals b ased on the structure of an alpha-(AlPdMnSi) phase, which is a p/q = 1 /1 cubic approximant of icosahedral quasicrystals. In the model, large dodecahedral and small icosahedral atom clusters, which are located a t the (1/2, 1/2, 1/2) and (0, 0, 0) positions in the alpha-(AlPdMnSi) structure, respectively, are placed at twelvefold sites classified int o two groups in the three-dimensional Penrose lattice. A diffraction p attern of this model reproduces well the characteristic features of an electron diffraction pattern of the F-type Al-Pd-Mn icosahedral quasi crystal.