Composition dependence of the atomic structure of Al_xMg_(39.5)Zn_(60.5-x) (20.5≤x≤50.5) 1/1-1/1-1/1 approximants determined by the Rietveld method

摘要

The atomic structure of the Al Mg Zn (20.5 x 50.5) 1/1- 1/1 - x 39.0 60.5- x 1 /1 approximants has been determined as a function of the Al concentration x by applying the Rietveld method to the X-ray powder diffraction data. The refinement was accomplished by using the data due to Bergman et al . as starting parameters. It is shown that the centre of the cluster is only 10% occupied by the Al atom at x = 20.5 but becomes essentially vacant when x exceeds 30. The sites corresponding to the vertex of the icosahedral cluster in the first shell are 80% occupied by the Zn atoms and the remaining sites by the Al atoms in the concentration range x 40. We also point out, from the Al concentration dependence of the interatomic distance of various atomic pairs, that the freeelectron-like bonding mechanism dominates the Al-Mg-Zn approximants. This is consistent with previous studies of the electronic structure for the present approximants.