Deviation from the virtual crystal approximation in disordered Au-Cu alloy nanocrystals: EXAFS and GIXRD investigation

The short-range order and the crystalline structure of AuCu and AuAg alloy nanoclusters obtained by sequential ion implantation in silica are investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy and by grazing incidence X-ray diffraction (GIXRD). While in the case of AuAg alloy the nearest neighbor interatomic distances correspond to the lattice parameter, in the case of chemically disordered solid solution AuCu alloy a significant deviation from the virtual crystal approximation is observed: the Au-Au, Cu-Cu and Au-Cu nearest neighbor distances are different, only their average corresponding to the fcc lattice parameter. This result, mainly related to the different atomic radius of the two alloy components, is for the first time reported for nanostructured systems.