TILING-DECORATION MODEL OF THE QUASI-CRYSTAL I-AL73MN21SI6

We propose a tiling-and-decoration model of the icosahedral phase of A l-Mn-Si, in which a 126-atom ''double'' Mackay icosahedron and a 54-at om ''simple'' Mackay icosahedron alternate on the even and odd nodes o f the tiling of four canonical cells. The model structure exhibits wea k six-dimensional fcc ordering introduced by Al-vacancy alternation, s imilar to that observed in the related a phase. The density of the mod el is consistent with experimental data and calculation of the x-ray- and neutron-diffraction intensities leads to good agreement with exper imental data in both cases: R factors are 0.09 and 0. 16, respectively .