Structure of the liquid–vapor interface of a Sn:Ga alloy

Abstract

The atomic distributions along the normal to, and in the plane of, the liquid–vapor interface of a dilute (9:91 at. %) Sn:Ga alloy have been studied by x-ray specular reflectivity and grazing incidence x-ray diffraction at 57 °C, 41 °C, 26 °C, and 12 °C. Surface tensions, calculated from sample shape measurements, have also been determined at the same temperatures. The results of these experiments show that the atomic distribution in the liquid–vapor interface of the Sn:Ga alloy studied has both similarities and differences with the atomic distribution in the liquid–vapor interface of a dilute Bi:Ga alloy. In both alloys the atomic distribution along the normal to the liquid–vapor interface is stratified, and the composition of the outermost layer is, sensibly, 100% of the constituent with lower (pure liquid) surface tension, respectively, Sn and Bi. In both alloys the in-plane structure of the outermost layer is that of a liquid. In the homovalent Bi:Ga alloy the excess Bi segregates into a monolayer which forms the outermost layer of the stratified liquid–vapor interface, with no excess Bi in the second or deeper layers of the interface. In the heterovalent Sn:Ga alloy, the excess Sn segregates into both the outermost and the second layers of the stratified liquid–vapor interface. The outermost layer of the interface is found to be pure two-dimensional liquid Sn, in which the atomic diameter is smaller than that in normal liquid Sn by about 6%. In the second layer of the interface the Sn atomic concentration is found to be 22.3±1.6% at 57 °C and 22.7±1.7% at 41 °C. Within the limits of our experimental sensitivity, the Sn concentration in deeper levels of the liquid–vapor interface cannot be distinguished from that in the bulk liquid alloy. The surface tension of the (9:91 at. %) Sn:Ga alloy is found to be 671±42 dyn/cm at 57 °C, 573±58 dyn/cm at 41 °C, 587±50 dyn/cm at 26 °C, and 527±40 dyn/cm at 12 °C. The differences between the structures of the Sn:Ga and Bi:Ga liquid–vapor interfaces are interpreted, qualitatively, in terms of their electron density distributions.