X-ray-absorption fine structure ͑XAFS͒ and x-ray-diffraction ͑XRD͒ measurements of disordered alloys Au x Cu 1Ϫx and Au 0.5 Ag 0.5 prepared by melt spinning were performed. In the Au 0.5 Ag 0.5 alloy, no significant local deviations of the atoms from the average fcc lattice were detected while in Au x Cu 1Ϫx alloys, significant deviations of atoms from the average fcc lattice were found. Mean-square vibrations of the Cu-Cu distances revealed by the XAFS in Au x Cu 1Ϫx alloys indicate the weakening of contact between Cu atoms in the dilute limit. Our computer simulation for Au x Cu 1Ϫx clusters of 10 5 atoms reproduces the main features of both the XAFS and XRD data.
A correlation between precise x-ray powder diffraction patterns and atomic size mismatch in disordered mixed crystals (alloys and ionic crystals) is observed. The anisotropy of the elastic moduli has been taken into account for evaluation of the strain energy density of the mixed crystals revealed in x-ray powder diffraction measurements. The precursor of the phase transformation for a quenched disordered Au-Cu alloy is identified. § Present address:
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