Crystalline monolayer surface of liquid Au–Cu–Si–Ag–Pd: Metallic glass former

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Entry by Emily Redston, AP 225, Fall 2011

Work in Progress

Reference

Crystalline monolayer surface of liquid Au–Cu–Si–Ag–Pd: Metallic glass former by S. Mechler, E. Yahel, P. S. Pershan, M. Meron, and B. Lin. Applied Physics Letters, 98, 251915 (2011)

Introduction

The surface structures of many liquid metals and alloys is well known. While each of these systems displays surface induced atomic layering, one mystery that still remains is why certain eutectic alloys form two-dimensional surface crystals at temperatures well above the eutectic melting temperature, <math>T_{e}</math>. In the <math>Au_{82}</math><math>Si_{18}</math> eutectic and in the ternary <math>Au-</math><math>Si-</math><math>Ge</math> eutectics, a low temperature (LT) 2D crystalline bilayer phase forms on melting and persists above <math>T_{e}</math>, where it eventually undergoes a first-order phase transformation to a 2D crystalline high temperature (HT) phase. As the temperature continues to increase, the HT crystal melt into a liquidlike (LL) surface that is typical of all other liquid metals. However, from the electron density model fit to the reflectivity, one can deduce that the LL surface of <math>Au_{82}</math><math>Si_{18}</math> has a more pronounced atomic layering than other liquid metals. Interestingly, the closely related <math>Au_{72}</math><math>Ge_{28}</math> eutectic alloy does not display surface freezing at all. The origin of this selective 2D surface crystallinity in liquid alloys like <math>Au_{82}</math><math>Si_{18}</math> remains unknown.

One clue may come from studies on glass formation in metallic alloys, a hot topic in research over the last few decades. <math>Au_{75}</math><math>Si_{25}</math>, an alloy only slightly off the eutectic composition <math>Au_{82}</math><math>Si_{18}</math>, was the first alloy successfully quenched from the liquid phase to an amorphous glass phase. The microscopic origin of glass formation is still intensily debated, however it is known that alloy systems with good glass forming abilities are characterized by the following general properties: (1) composition near a deep eutectic, (2) large differences in the atomic sizes of the components, and (3) large negative heat of mixing between the components. As a result of these properties, glass forming liquids show a high degree of short range order (SRO) in the liquid phase. It has been suggested that the icosahedral SRO often found in these liquids inhibits the formation of bulk crystalline phases during undercooling.

You may ask yourself what the connection is between glass formation and 2D crystalline phases found on the surface of certain liquids metals. It has been found that the binary <math>Au-</math><math>Ge</math> alloy (which did not exhibit any liquid 2D crystalline phases) cannot be quenched into an amorphous phase, even though it is otherqise very similar to <math>Au-</math><math>Si</math>. Thus it is hypothesized that surface freezing in the liquid phase and glass formation might have a common origin. This paper presented x-ray synchrotron studies of the surface properties of liquid <math>Au_{49}</math><math>Cu_{26.9}</math><math>Si_{16.3}</math><math>Ag_{5.5}</math><math>Pd_{2.3}</math>, which is known to have a very high glass forming ability.

Experimental Set-Up

The liquid <math>Au_{49}</math><math>Cu_{26.9}</math><math>Si_{16.3}</math><math>Ag_{5.5}</math><math>Pd_{2.3}</math> sample was prepared by melting its components in a crucible. It has a eutectic temperature <math>T_{e}</math> ≈ 625 K. Experiments were done at the Advanced Photon Source in Illinois, USA using an x-ray energy of 11.7 keV. The atomic scale surface structure of the liquid is characterized in the normal and in plane directions by x-ray reflectivity and grazing incidence diffraction (GID), respectively. Diffuse scattering is used to characterize thermal capillary waves.

Results

Figure 1(a) Reflectivity of the liquid <math>Au_{49}</math><math>Cu_{26.9}</math><math>Si_{16.3}</math><math>Ag_{5.5}</math><math>Pd_{2.3}</math>sample at a fixed <math>q_{z}</math> = 1.4 Å<math>^{-1}</math> during heating and cooling of the liquid sample at a rate of about 2 K/min. (b) GID patterns of the LT surface phase (T = 670 K) and of the LL surface phase (T = 685 K). (c) Crystal truncation rod within the <math>q_{xy}</math>- <math>q_{z}</math> plane of the Bragg reflection of the LT phase at <math>q_{z}</math> = 1.649 Å<math>^{-1}</math>taken by an area detector, showing that the rod is oriented along the surface normal, z. The dashed line indicates the shape of the Debye-Scherrer diffraction pattern expected for a 3D powder phase with <math>q_{xy}^2</math>+<math>q_{z}^2</math> = <math>(1.649)^2</math>Å<math>^{-1}</math>. (d) Integrated intensities of the truncation rod data shown in (c).

Discussion