CORRELATED CHEMICAL AND POSITIONAL FLUCTUATIONS IN THE GOLD-NICKEL SYSTEM - A SYNCHROTRON X-RAY DIFFUSE-SCATTERING STUDY

A quantitative analysis of the X-ray diffuse intensity scattered by an Au80Ni20 single-crystal was performed, in its metastable, homogeneous state, characteristic of the temperature of 520 K. An important featu re is that the intensity due to chemical short-range order is one orde r of magnitude smaller than the intensity due to atomic displacements. The solid solution is nearly random from the point of view of short-r ange order, but composition fluctuations along [100] directions are al ready present, of characteristic wavelength approximate to 1 nm, which are precursors of the composition modulations which develop when anne aling below the coherent spinodal decomposition boundary. The first-or der mean pair displacements were also obtained. Positional fluctuation s along [100] directions are found, which are correlated with composit ion fluctuations. These results are discussed in connection with the a nomalous low-temperature behavior of Au1-xNix Solid solutions.