Atom clusters in A 2/1 cubic approximant phase for understanding the structures of icosahedral phases

摘要

Two types of atom clusters found in the #beta#-(AlPdMnSi) cubic phase,referred to as a 2/1 crystalline approximant,with a composition of approximately Al_70Pd_23Mn_6Si_1 which is near to the composition Al_72Pd_20Mn_8 fo the icosahedral phase,are discussed in detail for understanding the strucutre of the Al-Pd-Mn icosahedral phase.A large dodecahedral atom cluster located at the body-centered position cna be divided into 19 atom shells with approximately icosahedral symmetry,and a dodecahedron of the 12th shell internally touches the surface of the cubic unit cell with a lattice constant of 2.0211 nm.At each vertex of the dodecahedron,a small icosahedral atom cluster consisting of 12 Al atoms surroundifng a central Pd atom is located.The deodecahedron is ocnnected to each other by edge-sharing,namely by sharing two small icosahedral atom clusters,along the twofold frotational direction,and ofrms a simple-cubic packing of hte atom cluster in the #beta#-(AlPdMnSi) cubic phase.Another atom cluster located at the origin fills up gaps of the simple-cubic packing of the large dodecahedral atom cluster.