SHORT-RANGE ORDER IN CU-PD ALLOYS

The x-ray diffuse scattering intensity was measured at room temperature from disordered Cu-Pd alloys containing 8.0, 9,5, 13.0, 21.8, 28.5 and 42.0 at.% Pd. Twofold and fourfold splittings of diffuse scattering due to short-range order (SRO) were observed at 100, 110 and their equivalent positions respectively from alloys with more than 13.0 at.% Pd, where the separation of the split diffuse maxima increases monotonically with increasing Pd content. No splitting of diffuse scattering was observed for alloys containing 13.0 at.% Pd or less. The diffuse intensities measured in Laue units increase with increasing Pd content except for 42.0 at.% Pd alloy, which shows a much lower intensity compared with that for 28.5 at.% Pd alloy. The SRO parameters were determined from all the six alloys. One representative set of the local atomic arrangements of Pd atoms in the FCC lattice which has been derived from computer simulation of the observed SRO parameters is shown. We have calculated the Pd-Pd atom pairs of the first- and second-nearest neighbours for all the six alloys, in both the random and the SRO case. Although the ordered structure for Cu-42.0 at.% Pd alloy is of the CsCl type, no different feature in the SRO structure was observed for this alloy in comparison with the other alloys.