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

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.