The δ-Al4Cu9phase as an approximant of quasicrystals

This paper analyses the structural properties of the δ-Al4Cu9 phase as an approximant to quasicrystals. The discussion is developed along two directions. One is the valence electron concentration (VEC) correlation; both the δ-Al4Cu9 and Al-Cu-Fe icosahedral phases are Hume-Rothery intermetallis with similar values of VEC per unit volume. The other is atomic structural resemblance analysis; δ-Al4Cu9 can be viewed either as six-layer stacking of flat and puckered layers along one 〈110〉 direction or as parallel pentagonal columns aligned along the same direction. It is also pointed out that vacancy sites play two important roles: to regulate a relatively constant VEC per unit volume and to facilitate the formation of basic pentagonal configurations.