SHORT-RANGE ORDER IN CU-PT ALLOYS

The x-ray diffuse scattering intensity from disordered Cu-Pt alloys whose compositions were Cu-7.0, 10.0, 16.0, 24.5, 32.0 and 75.0 at.% Pt was measured at room temperature. Twofold and fourfold splittings of diffuse scattering due to short-range order (SRO) were observed at 100, 110 and their equivalent positions (X points), respectively, for Cu-16.0, 24.5 and 32.0 at.% Pt alloys, where the separation of split diffuse maxima increases monotonically with increasing Pt content. On the other hand, no splitting of diffuse scattering was observed at Cu-< 16.0 at.% Pt alloys. Extra diffuse maxima were observed at 1/2 1/2 1/2 and its equivalent positions (R points) for all samples, the intensity being related to the concentration. The SRO parameters were determined from all six alloys. The sign and magnitude of alpha200 strongly depend on the ratio of R- to X-point diffuse intensities. Synthesized data indicated that the first few SRO parameters dominate the R-point maxima, though the sign and magnitude of higher-order parameters play an important role in causing the appearance of split diffuse maxima. It is proposed that there are two types of correlation in the disordered state of the Cu-Pt alloy system, i.e. between the consecutive (111) atomic layers and due to Fermi surface imaging.