The composition dependence of axial ratio c⁄a in L1_o-type ordered phase region of CuAu_1−yNi_y and AuCu_1−yNi_y was studied for various annealing temperatures by X-ray diffractometry. In CuAu_1−yNi_y (0≤y≤0.2), c⁄a increased with increase in composition y at each annealing temperature, and the c⁄a curve was shifted to larger side of c⁄a as the temperature became high. The increase in c⁄a with temperature was observed beyond about 523 K for each alloy, which just corresponded to a rapid fall in the measured long-range order parameter S. Further the increase in c⁄a became large as y increased, resulting in a change in shape of c⁄a vs y curve from convex to concave with increase in temperature. Similar behavior was observed also in AuCu_1−yNi_y (0≤y≤0.14). However, the tendency for increase in c⁄a to be large with increase in y was remarkable, i.e., c⁄a decreased with y in a low temperature range and then after showing a slight increase it increased rapidly in a high temperature range.
The experimental results were analyzed, according to the method applied previously, in terms of two energies; a repulsive interaction energy between ions and an electrostatic energy of ions. Numerical calculation interpreted qualitatively the observation in both systems.