High resolution electron microscopy of Al-Cu-Fe quasicrystals: atomic structure and modeling

Abstract

High quality Al-Cu-Fe quasicrystals have been studied at the atomic level with a high resolution microscope operating at 400 kV. When the incident beam is parallel to the fivefold axis, experimental bright-field images in thin regions of the specimen are found to be of two types, depending on the electron optical parameters employed. One of these images is of greater contrast than the other, but both types yield decagon-like image features. Close agreement is found between both types of contrast-enhanced micrographs and microscope image simulations of a realistic atomic model. This model involves the placement of overlapping Mackay icosahedra on a perfect quasicrystalline network. The limitation of resolving the projected atomic structure is discussed in terms of the contribution of different diffraction orders, considerations of specimen thickness, and optimal adjustment of the microscope objective lens defocus. A similar treatment is also applied to the threefold orientation where micrographs were taken under coherent imaging conditions. For the threefold pattern, fine modulations iof the quasicrystalline lattice are obtained which are not observed under less coherent conditions.