No abstract

Absolute measurements of the diffuse intensity in a volume in reciprocal space have been made at six temperatures ranging from 2°C above Tc 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 Tc 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 DO22-1ike fluctuations, whereas above 850°C, CuPt-like fluctuations develop. There are premonitory effects just above To; 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.

A brief review is given of the Boric-Sparks method of measuring and separating the diffuse scattering in alloys due to local displacements of atoms from lattice sites. Experimental techniques are reviewed with particular emphasis on simple computer controls, dead time, monochromators and specimens. After this, sources of error in the measurements are discussed and simple methods for assessing these errors presented. Equations for estimating higher-order terms than those included by Boric and Sparks are presented. It now appears that a thorough analysis of the errors needs to be made for each measurement prior to its publication. As this kind of experiment is readily automated and easily analyzed for errors with modern computers, this should now become standard procedure. It is also suggested that intensities in electron units be kept on file and made available when requested, as is done with structure factors.

No abstract

No abstract

No abstract

No abstract

No abstract