X-ray diffraction study of short-range-order diffuse scattering from disordered Cu-29.8 at.% Pd alloy

The diffuse X-ray scattering from a single crystal of disordered Cu-29.8 at. % Pd alloy was measured at room temperature. A fourfold splitting of the diffuse scattering due to short-range order was observed at the 110 reciprocal lattice point, in good agreement with the result previously obtained by electron diffraction. With the use of the method proposed by Boric & Sparks [Acta Cryst. (1971), A27, 198-201] the short-range-order parameters were determined by separating the other components of the diffuse scattering, i.e. size effect, Huang and thermal diffuse scattering, from the observed intensity data. It was revealed from this analysis that the order parameters beyond the 15th neighbour shell play an important role in characterizing this short-range-order diffuse scattering, although their magnitudes are small compared with those of the lower order parameters. Based on the ordering theory valid above the critical temperature the atomic-pair-interaction potential ratios were also obtained. It is shown that the pair-interaction potential is of long range and oscillates with distance. A comparison is made between the present result and the form of long-range oscillatory potential predicted by the conduction-electron screening model, and then the form of the Fermi surface is discussed.