Structural studies of decagonal quasicrystals and aproximants using the high-angle annular dark-field method

摘要

High-angle annular dark-field (HAADF) images of an Al_72Ni_20Co_8 decagonal quasicrystal and monoclinic Al_13Fe_4 and #tau#~2-inflated Al_13Co_4 approximants have been presented.We found that a 2 nm atom cluster of Al_72Ni_20Co_8,which have been believed to be tenfold so ar,has no tenfold symmetry.A new structural model of Al_72Ni_20Co_8 has bene constructed on the basis of the HAADF images and the structure of monoclinic Al_13Fe_4 approximant.We have found that the monoclinic Al_13Fe_4 and #tau#~2-inflated Al_13Co_4 approximants are composed of the same basic strucutre of a pentagonal atom cluster with an edge length of 0.47 nm.We have also found that a fivefold atom-cluster with a 2 nm diameter exists in #tau#~2-Al_13Co_4,which was observed in decagonal quasicrystals of Al-Ni-Fe,Al-Ni-Co,Al-Cu-Co and Al_Co by high resolution electron microscopy.A structural model of #tau#~2-inflated Al_13Co_4 is constructed on the basis of the convergent-beam electron diffraction symmetry,HAADF images and the atructure of monoclinic Al_13Fe_4.