  A structure analysis of Zn-Mg-Ho icosahedral quasicrystal was carried out by
the powder X-ray diffraction method using synchrotron radiation (Lambda=0.73490
A) at SPring-8. The intensity distribution was analyzed by the Rietveld method
modified for an icosahedral quasicrystal, in which simplified models were
assumed: ellipsoid and spherical windows were assumed at five crystallographic
sites in a F-type hypercubic unit cell. The analysis revealed the presence of
an almost perfect Penrose tiling with edge length 5.20 A. The vertices are
occupied alternatively by Zn and Mg, and almost all of the edge centers of the
Penrose rhombohedra are occupied by 0.8Zn and 0.2Mg. Ho and Mg atoms tend to be
present on the body diagonal of the prolate rhombohedra. Good agreement between
the measured and calculated intensity distribution using the simplified model
suggests the applicability and the limitation of structure analysis using the
powder X-ray diffraction method.
