Structure of liquid alloys of the Au - Si and Au - Ge systems

A study of the liquid alloys of the simple eutectic systems Au-Si and AuCre at selected compositions has demonstrated a pattern of behaviour very similar to that seen in the related gold-based system Au-Sn. The sharp x ray diffraction profile of Au-Ge liquid alloys at the gold-rich end demonstrates that the structure, like that of pure Au. is simple, close-packed but with a mean closest distance for neighbouring atoms smaller than that for liquid Au or Ge. This result is also obtained for liquid Au-Si alloys. The packing in the liquid phase can only be interpreted in terms of A-B bonding, i.e. unlike atom interaction, a feature attributed to a high electrochemical factor. With higher Ge content. the x ray diffraction profile broadens in a manner consistent with a more open liquid structure. We account for the latter in terms of the increasing influence of G 4 e nearest neighbour packing. This model is straightforward in reconciling all observations of the liquid, but could not have been predicted from the solid phase structures of either Au-Si or Au-Ge where A-A and B-B bonding predominates. The liquid state has thus been shown to exhibit bonding behaviour distinct from the solid, and contrary to a commonly held assumption, its structure is not merely a reflection of solid state bonding. This disparity accounts for the anomalous solidification behaviour of the eutectic alloys, including expansion on solidification.