Mg-Cu-Si系に現れるMgCu₂型規則構造

抄録

A new MgCu₂ type ordered phase which we found in the Mg-Cu-Si ternary system belongs to the space group P4₁32 or P4₃32. The structure is characterized by the ordered 4-fold screw arrangement of Cu and Si atoms situated at 12d and 4a sites respectively, forming four small tetrahedra in the unit cell. Intensity data for a spherical single crystal of Mg(Cu_0.8Si_0.2)_2.4 were recollected and structural parameters together with the order parameter S were refined by a full-matrix least squares method. Twenty alloy specimes having different compositions in the ternary system were studied by the powder X-ray diffraction method, in order to determine the existence range of the new ordered structure. The order parameters for the specimens quenched from 473-893 K were estimated at room temperature by using Warren’s method. The order parameters were corrected by a successive approximation, taking into account of the atomic shifts due to ordering. The order parameter S thus obtained decreases continuously with increasing annealing temperature T, and the S-T curve obtained can be expressed by the formula, S=D(T_c−T)^β.