The local atomic quasicrystal structure of the icosahedral Mg25Y11Zn64 alloy

TL;DR

This paper models the local and medium-range atomic structure of the icosahedral Mg25Y11Zn64 alloy using synchrotron diffraction data, revealing hierarchical arrangements and symmetry properties.

Contribution

It provides a detailed atomic structure model of the alloy, identifying hierarchical clusters and their arrangements, and relates it to similar rare earth alloys.

Findings

Identifies five types of local coordination polyhedra.

Reveals a three-shell Bergman cluster structure.

Shows the alloy's isomorphism with its holmium analogue.

Abstract

A local and medium range atomic structure model for the face centred icosahedral (fci) Mg25Y11Zn64 alloy has been established in a sphere of r = 27 A. The model was refined by least squares techniques using the atomic pair distribution (PDF) function obtained from synchrotron powder diffraction. Three hierarchies of the atomic arrangement can be found: (i) five types of local coordination polyhedra for the single 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 its neighbouring clusters and (iii) a cluster connecting scheme corresponding to a three-dimensional tiling leaving space for few glue atoms. Inside adjacent clusters, Y8-cubes are tilted with respect to each other and thus allow for overall icosahedral symmetry. It is shown that the title compound is essentially isomorphic to its holmium analogue. Therefore fci-Mg-Y-Zn can be seen as the representative structure type for the other rare earth analogues fci-Mg-Zn-RE (RE = Dy, Er, Ho, Tb) reported in the literature.