The present extended x-ray-absorption fine-structure (EXAFS) investigation aims at determining the atom-pair displacements on a local scale in the ${\mathrm{Au}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Ni}}_{\mathrm{x}}$ system, for the three types of pairs Au-Au, Au-Ni, and Ni-Ni. X-ray-absorption spectra have been obtained above both edges (Au $L_3$ and Ni K), over the whole composition range. For the purpose of the separation of the Au and Ni scattering contributions, a detailed analysis of EXAFS over a wide photoelectron wave-vector range beginning close to the edge has been performed. The amplitude and phase functions for unlike-neighbor pairs have been obtained in two ways: first by using empirically refined theoretical partial phase shifts, which enabled us to calculate the total phase functions, and secondly by making use of fluorescence recorded spectra of dilute alloys from which appropriate amplitude functions have been extracted. Many low-k amplitude effects such as the photoelectron wave curvature, the experimental energy resolution, the limited core-hole lifetime, and the many-electron excitation factors have been analyzed, and taken into account, as well as the difference between p- and d-symmetry photoelectron scattering.

Although the alloys are crystalline, a large positional disorder was found, resulting in a distribution of the three pair distances. An appropriate model for the EXAFS function was selected by use of several constraints relating the structural parameters determined by EXAFS to the ones of the regular fcc lattice, deduced from x-ray diffraction measurements. The three nearest-neighbor partial radial distribution functions are thus described in terms of mean distance and second and third momenta. They are broad and display a marked asymmetry, especially in concentrated alloys, these features increasing in the order Au-Au, Au-Ni, and Ni-Ni. Indeed, the Ni-Ni radial distribution function is very wide, at the extent that it is comparable to those encountered in amorphous metal-metal alloys. The three partial mean distances are distinct from each other by as much as 0.1 Å, and exhibit different dependence on composition. In dilute alloys, the relaxation around the impurity atoms was found to be appreciable. The most striking feature of the local structure, irrespective of the composition, is that the larger Au atoms are more confined (statically and dynamically) on the nodes of the regular lattice than the Ni ones. The positive bowing of the lattice parameter variations with composition is discussed in the light of the present results. The experimental work was completed by a computer simulation of the topological structure of these alloys, based on a minimization of the total elastic energy. The experimental results were quantitatively well reproduced, by use of a Morse interaction pair potential. Qualitative features were found to have small dependence on the particular pair potential chosen to model the interactions. In addition, as a further proof of the adequacy of the function chosen for EXAFS fitting, model EXAFS spectra calculated from the computed radial distribution functions were fitted in the same manner as the experimental ones, yielding an estimate of the results’ accuracy.

• Received 29 February 1988

DOI:https://doi.org/10.1103/PhysRevB.38.5944