Atom Clusters with Icosahedral Symmetry in Cubic Alloy Phases Related to Icosahedral Quasicrystals

Icosahedral symmetry can not be allowed to exist in crystalline phases. However, the structures of some crystalline alloy phases are characterized by packing of atom clusters with icosahedral symmetry, and their structures are considered to be closely related to the structures of icosahedral quasicrystals. For examples, a cubic α-(AlMnSi) crystalline phase with a lattice constant of 1.264 nm is formed by body-centered packing of so-called Mackay-type atom clusters with icosahedral symmetry, as shown in Fig. 1 [1]. The atom cluster consists of three atom shells with icosahedral symmetry, and an icosidodecahedron of the second shell is connected with each other through an octahedron along 〈111〉 directions. On the other hand, in cubic (Al,Zn)49Mg32 [2] and Al5CuLi3 [3] phases with lattice constants of about 1.4nm, larger atom clusters, called Bergman-type atom clusters, consisting of six atom shells with icosahedral symmetry form body-centered packing by sharing a hexagonal plane of the forth shell, as shown in Fig. 2. Those atom clusters have been considered to be structure units in Al-Mn and Al-Li-Cu icosahedral quasicrystals [4-7], respectively. Those crystalline phases are referred as “1/1 cubic crystalline approximants” produced by an 1/1 rational cut of a six-dimensional hypercubic lattice.