A NEW CLUSTER CONCEPT AND ITS APPLICATION TO QUASI-CRYSTALS OF THE I-ALMNSI FAMILY AND CLOSELY-RELATED CRYSTALLINE-STRUCTURES

A number of complex metal alloys, including the structure types of gamma-Al4Mn, Al10Mn3, Al5Co2, BHf9Mo4, C7Co6W20, Zr6Ni6TiSiOx, Al23V4, mu-Al4.12Mn, Al13Cr4Si4, NiTi2, CFe3W3, Al10V, Al18Cr2Mg3, Al45V7, Al31Mn6Ni2, Al11Mn3Zn2, Al11Mn2Si, Hf54OS17, Rh13Sc57, Ag17Mg54, GaMnSc9, Mg44Rh7, Na6TI, Mg(6)fd and Fe11Zn39, representing several hundred different intermetallic structures, are discussed by means of a new cluster concept. The basic building block of the cluster concept consists of 36 atoms in three vertex-connected icosahedra with bi-capped pentagonal coordination polyhedra for the link atoms. Together with a small set of connection types, the structures mentioned above can then be described as being composed of only one building block. This approach enables a good visualization of the structures, points out the similarities of the structural principles between different structures, and is helpful in the structure solution of complex metal alloys. Quasi-crystals for the icosahedral i-AlMnSi and i-TiTM (TM, transition metal) classes are closely related to phases of the above family by similarities in the diffraction pattern and by phase formation. Therefore, we suggest that the structures of the quasi-crystals of both classes have similar underlying structural principles, so contain the basic building block mentioned above. A model for the structure of the quasi-crystals, based on a zonohedral tiling of the Penrose local isomorphism class with four different unit cells, is discussed. The application of the rules of the duster concept yields atomic decoration of the unit cells with reasonable chemistry in terms of local coordination polyhedra and their three-dimensional arrangement.