Synchrotron Studies of Liquid Metal Surfaces
Final Report Abstract
The goal of the research proposal was to develop basic understanding of the atomic structure of the surfaces of liquid metals and alloys by means of synchrotron radiation. The combination of x-ray reflectivity, off-specular diffuse scattering and grazing incidence diffraction allows for a complete description of the atomic structure in the plane of the surface and normal to the surface. Special focus was laid on a systematic investigation of the physical origin of the existence of 2D crystalline phases at the surface of liquid eutectic Au82 Si18 that were discovered earlier and for which yet no theoretical explanation exists. Due to the availability of new instrumentation at the beamline ChemMatCARS at the Advanced Photon Source (APS) we have investigated details of the surface structure of liquid Au82 Si18 that could not previously be studied. These measurements allowed for a much more coherent and physically realistic understanding of the surface phases as well as their influence on the dynamic behavior of the surface of the liquid sample. For example, we were able to demonstrate that the surface of liquid Au-Si undergoes a transition from a 2D crystalline bilayer phase into a 2D crystalline monolayer phase slightly above the eutectic temperature. This monolayer phase eventually melts into a liquidlike surface at about 60 K above the eutectic temperature. Furthermore, we were able to measure bending rigidity of such thin 2D systems for the first time and derived a quantitative value for the bending rigidity. This finding was crucial for a more complete understanding of cryhstalline surface phases as this stiffness has the effect of quenching of the high frequency part of the capillary wave spectrum, i.e. it reduces the amplitude of the contribution of the capillary waves to the measured specular reflectivity. This resulted in a self-consistent picture of the surface phenomena of the technologically important liquid Au-Si alloy. One idea that we had considered is that crystalline surface phases arise from the large negative heat of mixing between Au and Si that leads to a high degree of chemical and topological short range order already in the bulk liquid Au82 Si18. For all metallic liquids investigated so far the surface induces atomic layering parallel to the surface and the thought is that for Au82 Si18 this might lead to in-plane 2D long-range crystalline order. A high degree of short range order in the bulk liquid is also known to be necessary for obtaining glass forming ability in metallic systems. Consequently, other glass forming systems like Pd-Si, Pd-Ge as well as the bulk metallic glass forming Au-Cu-Si-Ag-Pd system that show comparable mixing enthalpies and phase diagrams like Au-Si were investigated within the lifetime of the fellowship. For the Au-Cu-Si-Ag-Pd liquid alloy we were able to demonstrate the existence of a 2D crystalline monolayer phase for temperatures of up to 50 K above the eutectic temperature that has a different structure than the monolayer phase observed in Au-Si. This second example of surface freezing in a liquid metallic alloy indicates that it is very likely that many other of the known metallic glass forming systems show surface crystallization in the liquid phase as well as in the glass phase. Finally, the study of liquid metal surfaces was extended to new classes of material like pure Ge and Ge-rich Ge-Pd alloys that have a distinctly different bulk liquid structure than metallic liquids, i.e. liquid Ge seems to contain both metallic-like and covalent like character. Surprisingly, we found that for the surfaces of liquid Ge and liquid Ge64 Pd36 the kind of atomic layering normal to the surface that was found for all liquid metals up till now is absent. This is probably due to the unusual structure of bulk liquid Ge that persists even for the Ge-Pd alloy that contains 36 at.-% Pd. This result has significant impact on the basic understanding of the effect of surface layering in liquid metals.
Publications
- Surface structure of the liquid Au72 Ge28 eutectic phase: X-ray reflectivity, Phys. Rev. B, 80, 125414 (2009)
P. S. Pershan, S. E. Stoltz, S. Mechler, O. G. Shpyrko, A. Y. Grigoriev, V. S. K. Balagurusamy, B. H. Lin, and M. Meron
- Self-Consistent Interpretation of the 2D Structure of the Liquid Au82 Si18 Surface: Bending Rigidity and the Debye-Waller Effect, Phys. Rev. Lett., 105, 186101 (2010)
S. Mechler, P. S . Pershan, E. Yahel, S. E. Stoltz, O. G. Shpyrko, B. Lin, M. Meron, and S. Sellner
