Composition dependence of the atomic structure of Al_xMg_39·5Zn_{60· 5-x}(20·5 ≤ x ≤ 50·5) 1/1 - 1/1 - 1/1 approximants determined by the rietveld method

The atomic structure of the AlxMg39.0Zn_60.5-x(20-5≤x≤ 50-5) 1/1 - 1/1 -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 free-electron-like bonding mechanism dominates the Al-Mg-Zn approximants. This is consistent with previous studies of the electronic structure for the present approximants.