A structural study of the alloy Cu₃Au above its critical temperature

Absolute measurements of the diffuse intensity in a volume in reciprocal space have been made at six temperatures ranging from 2°C above T_c to 930°C. The Warren short-range order parameters were obtained after correcting for the intensity due to atomic displacements in a more complete manner than in earlier studies. As a result the short-range order parameters are considerably smaller than in these earlier investigations. The (previously disputed) specific heat anomalies in Cu₃Au above T_c and the L1₂ phase have been shown to be associated with unusual changes in diffuse X-ray scattering vs temperature. From studies of the scattering distributions, computer simulation and pair potentials obtained with the Warren short-range order parameters, the anomaly at 600°C appears to be due to the disappearance of DO₂₂-like fluctuations, whereas above 850°C, CuPt-like fluctuations develop. There are premonitory effects just above T_c; there is a large increase in the Debye-Waller factor, in the total intensity due to quadratic terms in atomic displacements, and an apparent change in the sign of average first-neighbor displacement. The long-range oscillations in the interatomic potentials determined from the diffuse-scattering data fit the Friedel potential only approximately. The electron-to-atom ratio in Cu₃Au was found to be ~ 0.97, in agreement with results on Cu alloys by other methods.