Long-range and short-range order in NiPt

Experimental measurements were performed on a 48.4 at. % Ni/51.6 at. % Pt alloy. The order parameter S in equilibrium was measured as a function of temperature T by means of X-ray diffraction. Up to 592±3°C the sample is a single-phase ordered alloy with an order parameter above 0.98. Between 592 and 625±3°C there exists a two-phase region in which the ordered and disordered phases are in equilibrium. Above 625±3°C a disordered f.c.c. structure exists. Diffuse X-ray intensity measurements were performed at T = 1.1 T_c and both the short-range parameters α_i and the size-effect parameters β_i were calculated. From the α_i parameters so derived three ordering energies ν_i were determined. It was found that these energies were consistent with the symmetry of the ordered phase below the critical temperature.