The local atomic quasicrystal structure of the icosahedralMg25Y11Zn64 alloy

Abstract

A local and medium range atomic structure model for the face centred icosahedral (fci)Mg25Y11Zn64 alloy has beenestablished in a sphere of r = 27 Å. The model was refined by least squares techniques using the atomic pair distribution(PDF) function obtained from synchrotron powder diffraction. Three hierarchies of theatomic arrangement can be found: (i) five types of local coordination polyhedra for thesingle atoms, four of which are of Frank–Kasper type. In turn, they (ii) form a three-shell(Bergman) cluster containing 104 atoms, which is condensed sharing its outer shell with itsneighbouring clusters, and (iii) a cluster connecting scheme corresponding to athree-dimensional tiling leaving space for a few glue atoms. Inside adjacent clusters,Y8 cubes are tilted with respect to each other and thus allow for overall icosahedralsymmetry. It is shown that the title compound is essentially isomorphicto its holmium analogue. Therefore, fci-Mg–Y–Zn can be seen as therepresentative structure type for the other rare earth analogues fci-Mg–Zn–RE(RE = Dy,Er, Ho, Tb) reported in the literature.