The local atomic structure of r-mg-zn (r = y, gd, dy and tb)

The rare earth element in the R-Mg-Zn system provides a unique probe of the atomic structure of a face-centered icosahedral (fci) quasicrystal (qc). Unlike many of the other ternary and higher component qc systems, the compound around R₈Mg₄₂Zn₅₀ allows for the substitution of one of its main components (R = Y, Er, Ho, Dy and Tb) without any appreciable change in its qc structure (space group or quasi-unit cell dimension). The large atomic number difference between Y and the heavy rare earth elements affords large differences in the scattering cross-sections for electrons, neutrons and X-rays. Using high quality flux grown single grains with resolution limited diffraction peaks, the local atomic structure was probed with a variety of complimentary techniques. The differences in the pair distribution function (PDF) due to loss of the anti-ferromagnetic ordering of the Tb between 4.2 and 30 K was used to probe the R site. High resolution TEM and single grain XRD have been used to determine the cluster structures. Results of this work suggest that there are local clusters in this fci compound consisting of five concentric polyhedra with 124 atoms: an icosahedron, dodecahedron, icosahedron, bucky-ball and a dodecahedron, similar to previous work. However, the HRTEM results indicate that the local structure consists only of these five shells which may edge share with the next cluster. Based on this atomic arrangement, the PDF results are consistent with R atoms partially occupying the vertices of the outer icosahedron and dodecahedron.