High-resolution TEM of Al-Cu-Fe quasicrystals

Abstract

High resolution electron microscopy and electron diffraction have always been key tools for the study of quasicrystals. The unique features of quasicrystals arc their long range oricntational order despite the absence of periodic translational order, and their non-crystallographic symmetry. The early work in electron microscopy and diffraction assessed the degree of perfection of these materials; microdo-mains with slightly different orientations, phason strains and dislocations were found which masked the fundamental atomic structure. A defect free material was sought to deduce the atomic arrangement of these materials and this was found in Al-Cu-Fe. Preliminary examinations of these materials using 20 at.% Cu and 15 at.% Fe indicated they are almost entirely free of phason strains when viewed in a 200kV microscope.At the time of these investigations the microscopes employed were limited in resolution and thus provide limited information about the atomic arrangements of these quasicrystals. Recently, studies were made using a more powerful microscope operating at 400kV where the point resolution is improved to 1.7Å. In that case, a Al-Mn-Si specimen was viewed along the five-fold axis under apparent optimum imaging conditions. These prior studies have interpreted various image features as being due to dynamical scattering processes and therefore little about the atomic arrangement of the quasicrystal is obtained. Our interpretation of the image feature is different and is due to modification of the phase contrast transfer function. Here clarification is given of the information contained in high resolution micrographs of quasicrystals.