AbstractÐThe tetrahedrally close-packed structure of Co 7 W 6 (m-phase) was studied by HRTEM. A massive presence of random sub-unit cell twins was detected. These planar defects were characterized in detail and the twinning probability at potential mirror planes was established. High-precision image simulation of the bulk structure and defect region was performed, using a simulated evolution optimization strategy to re®ne the imaging conditions. Ab initio calculations were carried out in order to determine the energy involved in twinning, as well as the distortion induced in the structure. 7

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