Study of the ordered structures of the Au–Mn system by high voltage–high-resolution electron microscopy. III. Two-dimensional antiphase structure of Au₃Mn type

High-resolution structure images of the 23.5 and 24 at.% Mn alloys annealed at 573 K were studied by 1 MV electron microscopy. It was shown that the 24 at.% Mn alloy consists of two-dimensional antiphase structure (2d-APS) of Au₃Mn with the boundaries parallel to the (100) and (010) planes. The average domain sizes were measured as M₁ = 1.04 and M₂ = 2.41, and an orthorhombic structure model with M₁ = 1 and M₂ = 2.5 was proposed for the ideal structure of the 2d-Au₃Mn. From the 23.5 at.% Mn alloy, the structures corresponding to the intermediate state between the 2d-Au₃Mn and the 2d-APS(I), which consists of lozenge- and parallelogram-shaped domains and exists in the alloys with 21–23 at.% Mn, were observed. It was concluded that the statistical order of appearance of the stable ordered phases with increasing Mn content in the composition range of 20–24 at.% Mn is Au₄Mn → Au₂₂Mn₆ → Au₃₁Mn₉ → 2d-APS(I) → 2d-Au₃Mn (so called Watanabe structure).