Interfacial properties of liquid alloys: An experimental study on Ga‐Bi and Ga‐Ge

Abstract

Interfacial phase transitions like wetting and prewetting transitions are of considerable interest in physics and chemistry of condensed matter since they represent phase transitions in reduced dimensionality. Besides this interfacial properties are of profound practical and technological interest. Most systems studied experimentally in this respect are characterized by Van der Waals intermolecular interactions. However, in the last few years it was shown that Coulomb liquids like liquid alloys or metal molten salt solutions exhibit interfacial phase transitions similar to those known in Van der Waals systems. In order to get more insight into these phenomena the fluid‐vapor interface of two different alloy systems have been studied using ellipsometry. Gallium‐bismuth is a binary alloy with large positive deviations from Raoult's law, exhibiting a distinct miscibility gap. Approaching liquid‐liquid coexistence a Bi‐rich film completely wets the fluid‐vapor interface. As can be estimated from the ellipsometric results the film thickness jumps at the monotectic temperature to a value of about 50 Å. In contrast, gallium‐germanium shows continuous miscibility and deviates much less from ideal mixing. As the Ge concentration in liquid Ga increases along the solid‐liquid coexistence curve the optical properties at the surface also vary continuously, which can be modelled within a simple effective medium approach.

Surface freezing in liquid Ga–Bi alloys: optical second harmonic and plasma generation study

“…This logarithmic dependence between thickness of the wetting film and chemical potential is consistent with experimental results of Chatain and Wynblatt 20 in Ga-Pb. The spectroellipsometric measurements 19 in the Ga-Bi system also indicate qualitatively an increase of the thickness of the Bi-rich wetting film approaching the monotectic point, whereas X-ray reflectivity data 22 between T eut and T mono were described using a monolayer model of pure Bi. More recent surface tension measurements 40 at similar conditions exhibit a surface excess of Bi at the liquid=vapour interface which can only be explained by a multilayer model in a similar way as has been done for the Ga-Pb system.…”

“…In discussing surface freezing in Ga-Bi alloys it is of interest to consider its relation to the wetting behaviour in this system. Formation of a macroscopic wetting film of the liquid Bi-rich phase on top of the bulk liquid Ga-rich phase at the monotectic point was found both by spectroellipsometry 19 and Xray reflectivity. 22 According to the idea of Dietrich and Schick concerning wetting at a liquid=liquid=solid=vapour tetra point, 39 the monotectic should be considered as the first experimentally accessible point where the wetting transition can be detected in a stable thermal equilibrium, whereas the true wetting transition temperature, T w , is located at the metastable extension of the liquid-liquid demixing curve.…”

“…A macroscopic wetting film of the liquid Bi-rich and Pb-rich phase, respectively, forms on top of the Ga-rich bulk liquid approaching the monotectic point. [18][19][20][21][22] In general, the origin of surface freezing independent of the type of interatomic interactions is an open question so far. If one considers surface freezing as a wetting transition whereby a quasi-solid film wets the liquid=vapour interface, the thermodynamic criterion for such a transition becomes similar to that for surface melting (Fig.…”

Surface freezing and wetting in Ga–Pb alloy: Second harmonic and plasma generation study

“…A more detailed relation of the SF transition to the bulk phase diagram at present is difficult because of large uncertainties of the liquidus line in this concentration range. Nevertheless, due to the close similarities in the bulk 9,10 and in the wetting behaviour in Ga-Bi 13,14 and Ga-Pb 15 similar features of the surface freezing transitions in both systems can be expected.…”

“…They show a liquid-liquid-vapour-solid tetrapoint wetting transition. [13][14][15] Furthermore, we report first observations of the wetting behaviour at the Ga-Pb alloy liquid/vapour interface during liquid-liquid decomposition.…”