We report on synchrotron scattering discoveries regarding the surface structure of the Au-Si liquid eutectic. A new result is the proof that one of these phases is an atomic bilayer that transforms on heating into a different monolayer phase. This second phase melts on further heating. Both surface crystalline phases exhibit bending rigidity that is similar to what is expected for monolayer graphene. This has the effect of reducing the amplitude of the Debye-Waller factor responsible for the loss of reflectivity due to thermally excited capillary waves. The latter is crucial for constructing a self-consistent explanation of the observed surface phenomena.
One speculation for the origin of the crystalline surface phase is the combination of strong interatomic forces between Au and Si, as expressed by their negative enthalpy of mixing and the deep lying eutectic temperature of the Au-Si eutectic when compared to the elemental melting temperatures. A negative enthalpy of mixing and a deep eutectic are also crucial for the formation of metallic glass alloys. The idea that the origin of surface crystallization is dependent on these properties is supported by the discovery that with the single exception of Au-Cu-Si-Pd-Ag surface crystallization has not been found in any of the other liquid metal alloys that we have studied. The difference is that Au-Cu-Si-Pd-Ag is a well known bulk metallic glass forming alloy. The crystalline surface structure in the liquid phase of this alloy is considerably different from Au-Si. Examples of surface structure of other simple glass forming alloys, like Pd-Ge are presented.
Supported by U.S. DOE grant DE-FG02-88-ER45379; ChemMatCARS: grantNSF/CHE-0822838.
Advanced Photon Source: U.S. DOE Contract No: W-31-109-Eng-38.