A study of the ordered structures of the Au-Mn system by high-voltage-high-resolution electron microscopy. II. Two-dimensional antiphase structures based on the DO₂₂ structure

Two-dimensional antiphase domain structures existing in the composition range 20-23 at.% Mn were investigated by a high-resolution structure-imaging technique with a 1 MV electron microscope. The structures are based on the DO₂₂ structure and consist of parallelogram-shaped domains containing 4 × 3 columns of Mn atoms and lozenge-shaped domains with 4 × 4 and 3 × 3 columns, and the domains are separated by two-dimensional antiphase boundaries parallel to the (40) and (240) planes of the fundamental face-centred structure. The configuration of the domains changes delicately with a slight change of composition or annealing temperature, and the symmetry of the structure is lowered below about 670 K. The ideal structure models have compositions of about 22.7 at.% Mn. The images of about half of the specimen area of the 22.6 at.% Mn alloy annealed at 570 K do not correspond to these new structures, but bear a resemblance to the image expected from the two-dimensional antiphase structure proposed by Watanabe [J. Phys. Soc. Jpn (1960), 15, 1030-1040] for Au₃Mn, which is based on the L1₂ structure and has boundaries parallel to the (100) and (010) planes.