Order-disorder phases of Ni_(1-x)Pt_x alloys with compositions of x = 0.14, 0.24, 0.25, 0.30, 0.35, 0.44, 0.50 and 0.57 were determined by X-ray diffraction (XRD) method. Lattice parameters of the eight disordered and five ordered Ni-Pt alloys were determined and found to increase with increasing Pt content due to expansion of the lattice by higher atomic radius of Pt atoms. The equiatomic alloy is fcc with a_o = 3.7511 A in the disordered state and fct in the ordered state with c/a ratio of 0.943, while for the alloys of x = 0.44 and 0.57, the c/a ratios were found to be 0.944 and 0.950, respectively. The average value of long-range order parameter S was found to be 0.95 for the equiatomic alloy after a prolonged annealing of 27 days at 525℃ indicating the alloy to be almost perfectly ordered. Present results reveal that the crystal is very near to perfectly ordered. It was also found that out of 1000 Pt sites in the crystal, 975 Pt atoms occupied their places and the rest 25 Pt sites were occupied by Ni atoms.
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