Structural themes in approximant and decagonal quasicrystalline phases in Al based alloys

The structure of two periodic phases in the Al-Pd-Mn system showing the importance of chemical ordering in the formation of decagonal quasicrystalline and approximant crystalline structures is reviewed. Different descriptions of the periodic phases in terms of clusters, dense planes and pentagonal chains of transition metals are presented. We also review a structural model for AlPdMn decagonal quasicrystals derived from the general schematic distribution of TM existing in the approximant phases. This model is discussed in relation of an unique decagonal overlapping cluster in a Penrose pentagonal tiling.