On the atomic structure of γ″ phase in Mg-Zn-Gd alloy

The atomic structure of γ″ phase in an Mg-Zn-Gd alloy has been determined by atomic resolution high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and first-principles calculation. The γ″ phase has a hexagonal structure with three atomic layers paralleling to basal plane. It is found that the atomic positions in the middle layer are not coincide with HCP structure but shifted 0.9 Å along 〈10-10〉_α. Moreover, most previously reported structures are unstable and will converge to present result after atomic relaxation, even though the atomic species and initial positions in the middle layer vary in those models.