Short-Range Order in Cu–Mn Alloys: Spurious Contribution of MnO2 to the Diffraction Pattern

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Abstract
A weak diffraction line, sometimes observed in x‐ray diffraction just at the location corresponding to the center of a diffuse neutron scattering peak located between the {100} and the {110} positions in Cu‐rich Cu–Mn alloys, has been identified as that due to γ‐MnO2 which has a rutile structure. When the degree of oxidation is minor, the oxide grows epitaxially on the surface of Cu–Mn crystals. The crystallographic relation of the epitaxy was found to be (110)Cu–Mn ∥ (121)MnO2 and [111]Cu–Mn ∥ [111]MnO2. Neutron diffraction experiments on a well‐annealed polycrystalline alloy of 25‐at. % Mn show a number of diffuse peaks which are consistent with the short‐range ordering of the (M=1)‐type long‐period superlattice structure (DO22). However, only a part of the diffuse peak near the {100} reflection is due to the short‐range ordering of atoms. In view of the small difference in the scattering amplitude between Cu and Mn, the diffuse peak due to the short‐range ordering in Cu–Mn alloys would thus hardly be detectable in both x‐ray and electron‐diffraction patterns.

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