A LARGE DODECAHEDRAL CLUSTER CONTAINING ABOUT 480 ATOMS IN A 2 1 CUBIC CRYSTALLINE APPROXIMANT/

A dodecahedral atom cluster, which is the largest atom cluster with ic osahedral symmetry found in crystalline alloy phases, was observed in the cubic crystalline phase of beta-(AlPdMnSi), referred to as a 2/1 a pproximant of an icosahedral quasicrystal, with a lattice constant of 2.0211 nm. The large dodecahedral cluster has a diameter of about 2.6 nm and contains about 480 atoms. The atomic arrangement of the dodecah edral cluster is divided into atomic shells with icosahedral symmetry, and most of the atoms in the shells are located at special positions associated with the golden mean in the three-dimensional Penrose latti ce. A small icosahedral atom cluster, formed with twelve Al atoms at t he vertices and a Pd atom at the center, is located at each vertex of a dodecahedral shell with a diameter of about 2 nm. The large dodecahe dral atom cluster is considered to be an important cluster for underst anding the structure of an Al-Pd-Mn icosahedral quasicrystal.