Order-disorder phases of Cu-21.6 at. Pd alloy have been studied by in situ high temperature camera attached to a X-ray diffractometer. The experiment has been performed within the temperature range of 30 - 700℃. Two-step of ordering at 520 and 250℃ has been observed. In the temperature range from 520 - 300℃, complex superstructure of pseudotetragonal type has been identified and that of temperature range 250 - 30℃, face centered tetragonal (fct) structure of periodic one dimensional anti-phase domain (1D-APD) type has been observed. In the disordered state of the alloy above 520℃, fcc structure was observed. Lattice parameter of the disordered alloy a_o = 3.6856 A and that of ordered alloy a_o = 3.6904 A and c_o = 3.6760 A, where c_o/a_o ratio 0.9961 has been determined. The average value of long-range order parameter S was determined as 0.93. It was also found that out of 1000 Pd sites in the crystal, 945 Pd atoms occupied their places and the rest 55 Pd sites were occupied by Cu atoms in addition to the own sites of Cu atoms.
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