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

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)

Keywords

metallic glasses, crystal structure, surface freezing, liquid alloys, surface crystals, phase transition, eutectics

Introduction

The surface structure 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 melts into a liquidlike (LL) surface that is typical of all other liquid metals. However, from an 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 quenching.

You may ask yourself what the connection is between glass formation and the 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 otherwise 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.

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).

Figure 1(a) shows how reflectivity of the liquid surface varies with temperature at a fixed angle <math>\alpha = \beta = </math>6.78°, which corresponds to a momentum transfer of <math>q_{z}</math> = 1.4 Å<math>^{-1}</math>. The abrupt drop in reflectivity is evidence of a first order transformation between two different surfaces phases. The authors attribute the hysteresis seen during heating and cooling to the temperature lag of the thermocouple.

Figure 1(b) displays GID curves of the sample above and below the surface phase transition temperature. These curves correspond to the two surface phases shown by the reflectivity measurements in Fig. 1(a). At higher temperature, the curve shows a broad diffuse maximum centered around <math>q_{z}</math> = 2.75 Å<math>^{-1}</math>, which is characteristic of a LL surface phase. On the other hand, at the lower temperature, two sharp Bragg reflection are superimposed on the diffuse maxima of the underlying liquid phase, indicating the presence of in-plane long range, crystalline order at the surface (LT phase).

Figure 1(c) is the intensity distribution within the <math>q_{xy}</math>- <math>q_{z}</math> plane of the Bragg reflection at <math>q_{z}</math> = 1.649 Å<math>^{-1}</math> for the LT phase. The truncation rod is aligned along the <math>q_{z}</math> direction, which indicates that the surface phase is a 2D phase. Furthermore, the structureless shape of the smooth portion of the rod intensity as a function of <math>q_{z}</math> demonstrates that the crystals are a monolayer phase, although the 2D unit cell is not necessarily confined to a single plane.

Conclusion

Currently <math>Au_{49}</math><math>Cu_{26.9}</math><math>Si_{16.3}</math><math>Ag_{5.5}</math><math>Pd_{2.3}</math> as well as glass forming <math>Au_{82}</math><math>Si_{18}</math> and <math>Au-</math><math>Si-</math><math>Ge</math> alloys are the only metallic liquids to exhibit surface freezing well above the melting temperature. This suggests that the phenomena of surface freezing in metallic liquids and glass forming ability are related and probably governed by similar physical properties. The authors propose that the high degree of icosahedral-like order in the bulk liquid, which is believed to be responsible for glass forming ability, also plays an important role in obtaining a high degree of layering at the liquid/vapor interface. Thus surface freezing should occur in many other metallic liquids that exhibit a high degree of order in the bulk phase, including strong glass forming liquids and icosaehdral quasicrystal forming liquids. Investigating these types of liquid/vapor interfaces and the role of surface forces is very relevant to soft matter physics.