Two-dimensional antiphase structure of Au₄Mg studied by high-voltage–high-resolution electron microscopy

A new structure model of Au₄Mg is proposed for the long-period superstructure which exists in the Au–Mg alloys with 16–22 at.% Mg at 570–470 K and gives electron diffraction patterns of the two-dimensional antiphase structure of Cu₃Pd-type (2d–Cu₃Pd), with the aid of high-resolution structure images taken with the 1000 kV electron microscope. The superlattice has monoclinic symmetry with the space group P2₁/m (No. 11), and the lattice parameters are A = 5a, B = b, C = 4c and β = 90°, where a(= b) and c(≃a) are those of the fundamental face-centred cell. The structure can be described as the two-dimensional antiphase structure having the first- and second-kind boundaries parallel to (001) and (100), respectively, and the domain size of 2.5a × b × 2c. Nearest-neighbour Mg atom pairs are not formed across the second-kind boundaries in this structure, in contrast with the ideal 2d-Cu₃Pd-type structure.